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Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 1
Electric rod actuators vs. hydraulic
cylinders: A comparison of the pros and
cons of each technology
By Aaron Dietrich, Director of Marketing
Tolomatic, Inc.
Hydraulic cylinders, known for their high force at an affordable cost, have
been widely used in factory automation equipment and other special
automation equipment for decades. Hydraulics are rugged, relatively simple
to deploy and provide a low cost per unit of force. In recent years, electric
rod actuator (cylinders) have become more flexible, precise and reliable
with increasingly larger force capacities. These advancements in electric
rod actuators have created an ongoing debate over which technology—
hydraulic cylinder or electric actuator—offers the best overall solution for
the same application. This paper will consider a variety of factors affecting
the performance and cost of each technology, including: motion control
capabilities; system components and footprint; force capabilities; speed
capabilities; temperature; life and maintenance of devices; data collection;
efficiency/utility costs; leaks and environmental concerns; as well as a few
additional factors.
Motion control capabilities
The main reason engineers select an electric actuator system over a hydraulic
cylinder system is the flexibility of its motion control capabilities: position
control (multiple positions, accuracy); velocity control; control of acceleration/
deceleration; control of output force; and complex control of all these
motion variables on the fly. Electric actuators, coupled with a servo drive and
motor system, have infinite control over position; accuracy and repeatability
levels are far beyond the capabilities of a hydraulic system.
Standard hydraulics are great for end-to-end position applications, but
mid-stroke positioning is more complicated, requiring a control valve and
operator assistance. Mid-stroke positioning is open loop and requires an
operator to decide which position is acceptable. Additionally, speed control
is monitored through a control valve and again requires an operator to dial
in the acceptable speed for an application, but reaching an exact speed
setting is often difficult to achieve. Once the speed setting is adjusted,
the pressure force output required from a hydraulic cylinder is regulated
About the Author
Aaron Dietrich
is Director of
Marketing for
Tolomatic. He
has an extensive
background
in the motion
control industry
and has several years of experience
working specifically in the drive
and controls area as a design and
application sales engineer. Aaron has
a B.S. in electrical engineering from
the University of North Dakota and his
MBA from the University of St. Thomas
in Minnesota.
About Tolomatic	
Tolomatic is a leading supplier of linear
actuators, both electric and pneu-
matically driven. Tolomatic’s expertise
includes linear actuators, servo-driven
high-thrust actuators, servo and step-
per motors, drives, and configured
linear systems. Standard pneumatic
and power transmission products are
built to order and shipped in five days
or less, electric actuators in 15 days
or less. Tolomatic also manufactures
right-angle gear drives, caliper disc
brakes, and clutches.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 2
through the pressure valve. Again, this typically requires an operator to dial
in the desired force. Finally, the repeatability of position, speed and force
of a hydraulic cylinder are subject to worn seals, leaks, pressure drops and
spikes from the pump and other maintenance factors. It is difficult to get
repeatable performance from day-to-day, month-to-month or year-to-year
in a production environment when oil quality and viscosity change due to
temperature variations. Obtaining the desired performance level will require
constant operator intervention.
More advanced hydraulic systems, called “servo-hydraulic,” can
precisely control position, velocity and force, but they require additional
components—a servo controller, an electrohydraulic servo valve, and a
position feedback device such as a linear transducer—which adds significant
complexity, space and cost to a hydraulic system. These components control
the pressure and flow into the hydraulic cylinder, similar to how a servo drive
controls current to a servo motor. Hydraulics also have even more advanced
controllers, which allow multiple axes to be coordinated together. However,
this is rare in hydraulics system implementation, and these controllers add
extreme complexity and cost to the overall system. In addition, they can
be quite sensitive and need regimented maintenance to ensure desired
performance.
MOTION CONTROL COMPARISONS
Position Velocity Accel/Decel Force
Low
Very High
High
Medium
CONTROLCAPABILITIES
ELECTRIC SERVO HYDRAULIC HYDRAULIC
CONTROL VARIABLES
Figure 1: Control over all aspects of motion can be extremely important in critical applica-
tions where precise position, speed and force come into play. This chart shows the control
capabilities of the three technologies.
When combined with a servo control system, electric actuators offer more
than infinite control and superior accuracy and repeatability. Multiple-axis
servo controllers are readily available off-the-shelf on most modern control
systems today. Controllers and electric actuators can be easily and cost
effectively coordinated together in complex configurations. Velocity of one
Standard hydraulic systems
require constant operator
intervention in order
to achieve the desired
performance level.
Servo-hydraulic systems can
precisely control position,
velocity and force, but
they require additional
components and are quite
complex and costly.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 3
Electric servo systems provide
infinite control, superior
accuracy and repeatability
and, once programmed,
require little intervention or
maintenance.
An important feature
of electric actuators is
their ability to provide
programmable control of all
profile variables.
or multiple electric actuators is precisely and accurately controlled at all
times and can easily blend from one speed to another without stopping
or overrunning position. Acceleration and deceleration control means that
electric actuators will not “bang” into hard-stops or jolt into action. This
eliminates stress on frame elements and the need to over-engineer structures
to withstand shock loads. All movements will be smooth—allowing electric
actuators to be used in mission-critical processes where machine vibrations
are not acceptable or process speed is affected. Force is controlled through
current to the servo motor. Since servo controllers have precise control over
current, almost all electric actuators provide accurate and repeatable control
of force output at the work point.
Figure 2: This chart shows different motion profile positions at different velocities with dif-
ferent accel/decel rates, all under full and precise control.
Finally, an important feature of electric actuators is their ability to provide
programmable control of all the motion profile variables. As a result, the
only operator interaction required is the up-front design time to build desired
performance variables into a PLC or another controller’s programming
environment. Once set, the operation repeats from day-to-day, month- to-
month and year-to-year. Furthermore, with the use of HMI (Human Machine
Interface) screens, the variables of position, velocity, acceleration/deceleration
and force can be easily changed at any time, providing maximum flexibility.
In an OEM environment, system performance will be easier to control due to
the increased consistency of an electric vs. a
fluid system.
POSITION
VELOCITY
ACCEL / DECEL
Time
Position
#1
SPEED 1
ACCEL1 ACCEL2
DECEL1
DECEL2
DECEL3
SPEED 2
SPEED 3
#2
#3
#4
DWELL
DWELL
Time
Velocity
Time
Accel/Decl
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 4
System components and footprint
The number of
components and overall
space required for a
hydraulic system is much
greater than an electric
system. Hydraulic systems
require: a cylinder; a
power unit to provide
oil pressure; control and
accessory valves; filters;
hoses; fittings; and
additional components.
Hydraulic cylinders offer a
compact footprint at the
work point (where the
power density is required),
but the hydraulic power
unit (HPU) which regulates
flow and pressure to
these actuators and other
components, can require a large footprint in floor space. HPUs are not small
and typically are placed near the actual cylinder itself, further increasing
the system footprint. In very large systems with remote mounted HPUs,
long lengths of hose can decrease the overall rigidity and efficiency of the
hydraulic system.
Figure 4: Electric system components
Electric rod actuators provide a smaller overall footprint. These systems
utilize a mechanical actuator; a motor (servo, or other); an optional gearbox;
cables; and a drive/amplifier, which is usually mounted in a control cabinet.
Although the electric actuator—due to its design of integrating a power
The number of components
and overall space required
for a hydraulic system is
much greater than an electric
system.
Electric rod actuators provide
a smaller overall footprint.
Mechanical Actuator
Motor (Optional Gearbox)
Control Cabinet
Drive
Motor Cables
Power
Unit
Control
Valves
Heat
Exchanger
Oil Tank
Heater
Manual
Override
Pump
Flow
Control Valve
Suction
Screens/
Filters
Accumulator
Pressure
Gauge
Shut Off
Valve
Figure 3: Hydraulic system components
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 5
screw and bearing system—does require additional length over a hydraulic
cylinder, when considering the overall system footprint, this additional
length is more than compensated for by the much smaller footprint of the
servo drive—the functional equivalent of the HPU. Typically, automation
equipment utilizes a control cabinet and can be designed for an additional
drive. Size requirements for a servo drive are normally a fraction of the size
requirements of a HPU.
In new system designs, if the additional space at the work point can be made
to accommodate the electric actuator’s working stroke and overall length,
the overall footprint of the machine can be greatly decreased by eliminating
the need for large power units.
Force capabilities
Due to their high operating pressures, hydraulic cylinder systems are great at
producing extremely high forces. Typical pressures range from 1800 to 3000
psi (124.1 to 206.8 bar). In some high-pressure hydraulic systems, pressure
ratings up to 5000 psi (344.7 bar) are used to further emphasize power
density. Since hydraulic cylinders operate on the Force = Pressure x Area
fluid power principle, the high pressures allow smaller cylinders to reach very
large forces. For example, 3-inch and 5-inch bore cylinders at 2200 psi could
achieve approximately 15,000 lbf (66,723.3 kN) and 43,000 lbf (191,273.5
kN), respectively. However, hydraulic cylinders are not usually used to their
full output force capability; they are typically oversized to improve control.
When considering electric actuators, it is important to determine the working
force required. To estimate the approximate force required for an electric
actuator, the typical approach is to adjust the hydraulic work port or system
pressure until the operation can no longer be performed. Electric actuator
systems rely on current through the servo motor to produce torque to the
mechanical system, which drives the power screw to turn and generate
force. This is a huge advantage; force is instantaneous. In hydraulic systems,
where system rigidity is not optimized, the hydraulic actuator must wait for
pressure to build until force is achieved. Another big advantage with electric
systems is that the servo controller automatically regulates the current.
The electric actuator system essentially uses current “on demand.” Any
adjustment happens automatically. A hydraulic power unit must always keep
pressure in the system for the hydraulic cylinder to actuate, which can be
highly difficult.
When selecting an electric actuator system, it is important to consider
the motor’s RPM and torque capabilities, coupled with the screw lead in
the electric actuator. Matching speed and torque from the servo motor
with the lead screw’s mechanical output can be complex. Achieving the
extreme forces that hydraulics can produce is entirely possible with electric
Size requirements for a servo
drive are normally a fraction
of the size requirements of a
HPU.
Achieving the extreme forces
that hydraulics can produce is
entirely possible with electric
technology, but typically, the
electric actuator deployed
will have a larger body
diameter and the electric
actuator system will have a
velocity maximum that can’t
be exceeded.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 6
technology, but typically, the electric actuator deployed will have a larger
body diameter and the electric actuator system will have a velocity maximum
that can’t be exceeded. The complexity in sizing a system can easily be
overcome as actuator and servo component manufacturers provide easy-to-
use motion control sizing software packages that factor in all these variables.
Velocity capabilities
Achieving high velocities at high forces presents challenges for both hydraulic
and electric technologies. Hydraulics require pressure for force and flow for
speed. To achieve higher speeds at higher forces, there must be enough
pressurized oil in the system to basically push the required volume of oil into
a cylinder in the required amount of time (defined as flow). This typically
requires an accumulation system to hold the pressurized volume. The
problem can multiply with long stroke cylinders; uncharged accumulators
may starve the system of oil. In the end, deploying additional capacity in
hydraulic accumulator systems allows them to achieve high speeds at high
forces. The downside of this practice is that without servo-hydraulic control
on the hydraulic system, excess energy (force x velocity) is essentially being
utilized in an open loop control scheme.
As stated earlier, the force capabilities of an electric actuator system depend
on the right combination of servo motor RPM, servo motor torque and
mechanical advantage from screw lead and possibly a gearbox. As servo
motors increase in size, torque typically increases significantly but RPMs
decrease. In extreme applications of force and speed, the only way an electric
actuator system may be able to achieve the desired performance is to grossly
oversize the system—which can be cost prohibitive. Alternately, remember
that an electric actuator system has complete control over the motion profile;
it doesn’t have to stroke the entire length of each cycle. Plus, with control
of acceleration and deceleration, the electric system can settle more quickly,
which reduces cycle time and increases efficiency. Finally, electric actuator
technology may alleviate some of the peak velocities required as more
intelligent, shorter moves can be executed.
Temperature (hot and cold)
Heat
A major problem for hydraulic systems is heat management. The inefficiencies
of hydraulic systems cause overheating. Hydraulic fluid temperatures above
180°F (82°C) damage most seal compounds and accelerate degradation of the
oil. Circuit design can play a big part in generating heat. Hoses, fittings and
other components can be a factor if they are inaccurately sized for the flow
requirements. Sophisticated load-sensing systems can mitigate most heat issues,
but can be costly. Heat exchangers are an economical way to manage this
without significantly over sizing your reservoir and ultimately your HPU footprint.
An electric actuator system
has complete control over the
motion profile and can settle
quicker for reduced cycle
time and increased efficiency.
A major problem for
hydraulic systems is heat
management.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 7
HEAT RESISTANCE
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Figure 5: Compared to hydraulics, electric systems are better at managing heat resistance.
There are two ways to solve overheating problems in hydraulic systems:
decrease heat load or increase heat dissipation. Hydraulic systems dissipate
heat through the reservoir, by having the correct level of oil or use of a heat
exchanger. Keeping a hydraulic system in proper form to manage heat will
require constant attention and maintenance.
Electric actuators can overheat at times, but the cause is usually due to
extreme changes in duty cycle or running the actuator harder than it was
designed or sized for. Extreme changes in the ambient temperature not
accounted for in the design may also cause overheating, but these conditions
are exceptions and typically would not occur in most factory automation
environments. Due to their higher efficiency and sizing accuracy, electric
actuator systems can be selected to run at a desired temperature for the
given amount of work required. Accurately predicting temperature allows the
electric actuator system to perform consistently without affecting the life of
the device.
Cold
Cold temperatures present other problems for hydraulic systems. Cold oil
causes sluggish and inconsistent operation until the oil is warm—which in
turn causes large swings in force and speed. Hot to cold temperature swings
can also affect the integrity of the rod seal—a critical component that helps
to prevent leaks and contamination. Oil tank heaters can manage start-up
temperatures, but they are an extra component and expense.
Electric actuators, on the other hand, can be deployed with extreme
temperature grease which allows for quick, effective starts in cold
temperatures. There is usually a small performance difference in force
repeatability from a cold to a warm temperature. This difference is acceptable
in most applications.
Electric actuators can
overheat at times but the
cause is usually due to
extreme changes in duty
cycle or exceeding its design
specifications.
Cold oil in hydraulic
systems causes sluggish and
inconsistent operation which
can affect force and speed.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 8
COLD RESISTANCE
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Figure 6: Electric systems are not susceptible to fluid thickening in cold temperatures and
perform consistently despite fluctuations in temperature swings.
Service life and maintenance
Hydraulics are rugged, widely used devices that can have a long service life.
The trade-off is that they require frequent maintenance and attention to
achieve the desired performance over the life of the system. Maintaining the
integrity of the rod and piston seals is key to repeatability because these are
the main elements that contain the pressure required for motion and force. If
these components wear or are damaged, they must be replaced. Otherwise,
force will be reduced at initial pressure or “blow by” will affect speed.
Changing oil filters and oil on a periodic basis are additional maintenance
tasks. Contaminated or degraded oil can also severely affect the operation of
the system. Neglecting maintenance items will lead to leaks or contamination
and premature failure of components.
Figure 7: Electric systems require little or no maintenance compared to hydraulic systems.
On the other hand, electric actuators sized for the life of an application
require no maintenance. The main power elements of an electric actuator
MAINTENANCE
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Hydraulics require frequent
maintenance and attention
to achieve the desired
performance over the life of
the system.
Electric actuators sized for
the life of an application
require no maintenance.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 9
are the power screw and other thrust-bearing elements. These elements
have a Dynamic Load Rating (DLR) specification that can be utilized to
estimate the amount of work (force over distance) an electric actuator can
achieve. Utilizing an industry standard L10 life estimation (see our guide
on estimating actuator life), electric actuators can be estimated with 90%
reliability to last in an application. These power elements are typically greased
for life, but easy, in-field greasing methods can be applied if necessary. The
other wear element on an electric rod-style actuator is the rod seal, designed
to hold out water, dust and other environment contamination from the
actuator’s internal components. Unlike hydraulic cylinder seals, there is no
pressure to hold in the actuator for proper operation. Even if the seal fails,
the actuator will continue to operate. Rod seals on most electric actuators
are easily and inexpensively replaced if damage occurs. Misuse is the primary
reason that electric actuators fail. Exceeding the actuator’s performance
specifications for extended periods of time or gross neglect are the most
common misuse factors.
Data collection
In the never-ending quest to improve manufacturing processes, data
collection in critical areas is becoming more and more prevalent in today’s
manufacturing environment. Again, hydraulic cylinders require expensive,
complex servo-hydraulic systems with additional sensors to track and monitor
position, velocity, force and other factors happening at the work point. These
factors are all built into an electric actuator’s servo system. Monitoring
the motor current makes it easy to track force and repeatability. The
feedback device on the motor registers accurate position and velocity.
Figure 8: Electric systems provide the most data collection capabilities over servo hydraulic
and conventional hydraulic systems.
Efficiency and electric utility costs
Electric actuator systems typically operate in the 75-80% efficiency range
for the work they complete; hydraulic actuator systems typically operate in
the 40-55% efficiency range. An additional factor in the electric utility cost
DATA COLLECTION
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Hydraulic cylinders require
expensive, complex servo-
hydraulic systems with
additional sensors to track
and monitor position,
velocity, force and other
factors happening at the
work point.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
www.tolomatic.com 763-478-8000	 9900-4412_00 Page 10
equation is that electric actuators only demand current to the motor when
it is required. When electric actuators are at rest, they require no current or
very low amounts of current to hold their positions. Hydraulics require that
the power unit keep the hydraulic system pressurized at all times when the
system is turned on—resulting in inefficient use of power.
Figure 9: Compared to servo hydraulic and conventional hydraulic systems, electric systems
operated far more efficiently.
Below are three application examples that show the power usage of electric
and hydraulic actuator systems.
Figure 10: The above charts show the difference in electric utility costs for a hydraulic and
an electric cylinder. The blue arrow depicts cylinder operation at 50% duty cycle and the
resulting estimated utility costs.
Electric actuator systems
typically operate in the
75-80% efficiency range for
the work they complete;
hydraulic actuator systems
typically operate in the 40-
55% efficiency range.
OPERATION EFFICIENCY
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Electric vs. Hydraulic Power Costs
POWER-OUT (kW) Ã = Velocity (m/sec) x Force (N) ÷ 1,000 (converted to kN)
POWER-IN (kW) Ä = Power-Out (kW) ÷ Efficiency (%)
COST OF APPLICATION $ = (Power-In) x (Hours/year) x (Electricity Cost)
1
2
3
3IN(76MM)BOREHYDRAULICCYLINDER
• POWEROUT:2kw
• SPEED:1.8in/sec(45mm/sec)
• FORCE:10,000lbf(44.5kN)
4IN(102MM)BOREHYDRAULICCYLINDER
• POWEROUT:10kW
• SPEED:2.9in/sec(75mm/sec)
• FORCE:30,000lbf(133.5kN)
6IN(152MM)BOREHYDRAULICCYLINDER
• POWEROUT:20kw
• SPEED:3.5in.sec(90mm/sec)
• FORCE:50,000lbf(222.5kN)
ASSUMPTIONS: Electric Efficiency 80%; Hydraulic Efficiency 45%; 2000 PSI; Cost kW/hr $0.07
BLUE ARROWS DESIGNATE COST SAVINGS SHOWN AT 50% DUTY CYCLE
$0
$500
$1,000
$1,500
$2,000
$2,500
$3,000
ENERGYCOST($)
ENERGYCOST($)
ENERGYCOST($)
DUTY CYCLE (%) DUTY CYCLE (%) DUTY CYCLE (%)
Energy Cost: 2 kW Energy Cost: 10 kW Energy Cost: 20 kW
HYDRAULIC
ELECTRIC
HYDRAULIC
HYDRAULIC
ELECTRIC
ELECTRIC
$0
$2,000
$6,000
$8,000
$10,000
$12,000
$14,000
$0
$5,000
$10,000
$15,000
$20,000
$25,000
$30,000
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 1000 10 20 30 40 50 60 70 80 90 100
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
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By dividing the efficiencies of the two technologies into one another, an
approximate factor can be determined on electric utility cost based on when
the actuators/cylinders are actually moving.
For example: 80% (electric) / 40% (hydraulic) = 2. This results in electric
being twice as efficient or hydraulic taking twice as much power to do the
same work. This is an approximation, but it is not far from reality. In fact,
unless a hydraulic system is properly maintained, the overall efficiencies of
the system can go to levels of 20% making electric technology up to 4 times
as efficient (or hydraulic utility costs up to 4 times higher).
Although electric actuator systems are twice as efficient as hydraulic systems,
an electric servo solution can be more expensive to implement initially. But
the cost over the life of the system can be substantially less when considering
the increased performance, system flexibility and decrease in utility costs.
Leaks and environmental concerns
Many hydraulic industry professionals have said, “It is not a matter of IF
hydraulics will leak, but a matter of when and by how much.” Leaks create
messes in the manufacturing environment and also present a safety hazard
if someone slips on the spill. More alarming is leaks could cause serious
contamination to key processes and products—such as food processing,
pharmaceuticals, medical device and others. A contamination event can
be costly both in terms of clean-up and scrapped product—and even more
costly if the contaminated product reaches consumers and is then recalled.
Additionally, many hydraulic systems are deployed outdoors and in the
vicinity of living spaces and fresh water. Leaking oil into the surrounding
environment can be a major concern for communities.
Figure 11: Unlike hydraulic systems, electric systems present minimal environmental risk.
The grease on the lead screw of an electric actuator is the only potential contaminant.
However, high-quality actuator seals on the actuator rod virtually prevent contamination.
By dividing the efficiencies
of the two technologies into
one another, an approximate
factor can be determined on
electric utility cost based on
when the actuators/cylinders
are actually moving.
A contamination event
can be costly both in terms
of clean-up and scrapped
product—and even more
costly if the contaminated
product reaches consumers
and is then recalled.
ENVIRONMENTAL RISKS
Electric Servo Hydraulic Hydraulic
Low
High
Medium
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
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Electric actuator technology is one of the cleanest linear motion technologies
that can be actively and easily deployed. Grease on the lead screw is the only
potential contaminant; special greases (food grade, clean room, etc.) can
be applied if required. The high-quality seals used on the rod of the electric
actuator keep grease inside the actuator, virtually eliminating contamination
issues.
Other factors
Noise: A noisy power unit running a hydraulic cylinder can be a noise
polluter for any operator near the machine.
Shock loads: Hydraulics are good at handling shock loads. Hose compliance
(unless hard plumbed) and oil bypassing along the main or work port relief
valve absorb shock loads. In addition, internal leakage within the cylinder
and valves can also offer compliance with shock load conditions. A large
shock load in-line with the rod would be absorbed in most cases by the
piston against the compressed oil in the body of the hydraulic actuator. On
the other hand, shock loads on an electric actuator’s lead screw or bearing
system may affect performance. Electric actuators may have to be oversized
in order to handle shock loads or use external shock absorbing mechanisms
to prevent damage.
Side loads: Side loading—due to misalignment or pressure from a moment
arm—places stress on the rod and piston seals of a hydraulic cylinder. The
seals may wear prematurely or fail completely, resulting in poor performance
and eventually premature failure of the cylinder. Electric actuators don’t
like side loading either, but for different reasons. Side loading puts stress
on the actuator’s front rod seal and on the
nut of the lead screw. The rod seal may fail,
allowing contaminants into the actuator
and causing premature failure of the electric
actuator. Side loading the lead screw nut can
diminish the estimated life of the ball or roller
screw. However, unlike hydraulic actuators,
electric actuators typically deploy an anti-
rotate mechanism on the lead screw nut of the
actuator, which allows it to absorb some side-
loading due to mounting misalignment. Finally,
both hydraulic and electric cylinders require care
and consideration when mounting to make sure the cylinder is in alignment
with the system it is intended to move. (See our white paper on actuator
alignment for more information.)
Hydraulics are good at
handling shock loads. Electric
actuators may have to be
oversized or use external
shock-absorbing mechanisms
to prevent damage.
Neither hydraulic cylinders
or electric actuators are well
suited for side loading.
Figure 12: Side loading of rod
actuators is not recommended
because it places stress on
internal actuator components.
Copyright © 2016 Tolomatic, Inc.	 Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology
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Summary
Hydraulic actuators have an industry reputation for delivering high force.
Hydraulic systems are usually less expensive from a purchased cost standpoint
to implement than an electric servo system. Their drawbacks include a
larger space footprint, regular maintenance and manual system adjustments
for optimal system performance. Hydraulic systems are susceptible to
fluctuations in temperature, prone to leaks and operate in an open loop
environment, posing challenges for data collection. Although servo-
hydraulics can be implemented to alleviate some of these issues, they result
in a larger space footprint and are very costly to employ. Hydraulics do offer a
long service life, but they are not as efficient operators as electric systems.
Electric systems offer smaller overall space requirements than hydraulic
systems. They also provide precise position, velocity and speed control with
more efficient operation. They operate in a closed loop environment for easy
data collection, and they are virtually maintenance free. Preferred for their
higher level of accurate performance, electric servo systems can be more
costly than hydraulic to initially implement. However, the increased efficiency
of operation with little or no maintenance over the life of the system makes
their total cost of ownership lower over the life of the equipment and an
attractive alternative to hydraulics.
As electric rod-style actuators become capable of achieving high-end
hydraulic forces, they will continue to be viable candidates for replacing
hydraulic systems in many applications. Evaluating capabilities and limitations
discussed in this paper and aligning them with system goals and objectives
will help determine the best choice for the application.
Electric systems offer smaller
space requirements and
provide precise position,
velocity and speed control
with more efficient
operation.

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9900-4412_00_electric-vs-hydraulic-actuators-wp

  • 1. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 1 Electric rod actuators vs. hydraulic cylinders: A comparison of the pros and cons of each technology By Aaron Dietrich, Director of Marketing Tolomatic, Inc. Hydraulic cylinders, known for their high force at an affordable cost, have been widely used in factory automation equipment and other special automation equipment for decades. Hydraulics are rugged, relatively simple to deploy and provide a low cost per unit of force. In recent years, electric rod actuator (cylinders) have become more flexible, precise and reliable with increasingly larger force capacities. These advancements in electric rod actuators have created an ongoing debate over which technology— hydraulic cylinder or electric actuator—offers the best overall solution for the same application. This paper will consider a variety of factors affecting the performance and cost of each technology, including: motion control capabilities; system components and footprint; force capabilities; speed capabilities; temperature; life and maintenance of devices; data collection; efficiency/utility costs; leaks and environmental concerns; as well as a few additional factors. Motion control capabilities The main reason engineers select an electric actuator system over a hydraulic cylinder system is the flexibility of its motion control capabilities: position control (multiple positions, accuracy); velocity control; control of acceleration/ deceleration; control of output force; and complex control of all these motion variables on the fly. Electric actuators, coupled with a servo drive and motor system, have infinite control over position; accuracy and repeatability levels are far beyond the capabilities of a hydraulic system. Standard hydraulics are great for end-to-end position applications, but mid-stroke positioning is more complicated, requiring a control valve and operator assistance. Mid-stroke positioning is open loop and requires an operator to decide which position is acceptable. Additionally, speed control is monitored through a control valve and again requires an operator to dial in the acceptable speed for an application, but reaching an exact speed setting is often difficult to achieve. Once the speed setting is adjusted, the pressure force output required from a hydraulic cylinder is regulated About the Author Aaron Dietrich is Director of Marketing for Tolomatic. He has an extensive background in the motion control industry and has several years of experience working specifically in the drive and controls area as a design and application sales engineer. Aaron has a B.S. in electrical engineering from the University of North Dakota and his MBA from the University of St. Thomas in Minnesota. About Tolomatic Tolomatic is a leading supplier of linear actuators, both electric and pneu- matically driven. Tolomatic’s expertise includes linear actuators, servo-driven high-thrust actuators, servo and step- per motors, drives, and configured linear systems. Standard pneumatic and power transmission products are built to order and shipped in five days or less, electric actuators in 15 days or less. Tolomatic also manufactures right-angle gear drives, caliper disc brakes, and clutches.
  • 2. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 2 through the pressure valve. Again, this typically requires an operator to dial in the desired force. Finally, the repeatability of position, speed and force of a hydraulic cylinder are subject to worn seals, leaks, pressure drops and spikes from the pump and other maintenance factors. It is difficult to get repeatable performance from day-to-day, month-to-month or year-to-year in a production environment when oil quality and viscosity change due to temperature variations. Obtaining the desired performance level will require constant operator intervention. More advanced hydraulic systems, called “servo-hydraulic,” can precisely control position, velocity and force, but they require additional components—a servo controller, an electrohydraulic servo valve, and a position feedback device such as a linear transducer—which adds significant complexity, space and cost to a hydraulic system. These components control the pressure and flow into the hydraulic cylinder, similar to how a servo drive controls current to a servo motor. Hydraulics also have even more advanced controllers, which allow multiple axes to be coordinated together. However, this is rare in hydraulics system implementation, and these controllers add extreme complexity and cost to the overall system. In addition, they can be quite sensitive and need regimented maintenance to ensure desired performance. MOTION CONTROL COMPARISONS Position Velocity Accel/Decel Force Low Very High High Medium CONTROLCAPABILITIES ELECTRIC SERVO HYDRAULIC HYDRAULIC CONTROL VARIABLES Figure 1: Control over all aspects of motion can be extremely important in critical applica- tions where precise position, speed and force come into play. This chart shows the control capabilities of the three technologies. When combined with a servo control system, electric actuators offer more than infinite control and superior accuracy and repeatability. Multiple-axis servo controllers are readily available off-the-shelf on most modern control systems today. Controllers and electric actuators can be easily and cost effectively coordinated together in complex configurations. Velocity of one Standard hydraulic systems require constant operator intervention in order to achieve the desired performance level. Servo-hydraulic systems can precisely control position, velocity and force, but they require additional components and are quite complex and costly.
  • 3. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 3 Electric servo systems provide infinite control, superior accuracy and repeatability and, once programmed, require little intervention or maintenance. An important feature of electric actuators is their ability to provide programmable control of all profile variables. or multiple electric actuators is precisely and accurately controlled at all times and can easily blend from one speed to another without stopping or overrunning position. Acceleration and deceleration control means that electric actuators will not “bang” into hard-stops or jolt into action. This eliminates stress on frame elements and the need to over-engineer structures to withstand shock loads. All movements will be smooth—allowing electric actuators to be used in mission-critical processes where machine vibrations are not acceptable or process speed is affected. Force is controlled through current to the servo motor. Since servo controllers have precise control over current, almost all electric actuators provide accurate and repeatable control of force output at the work point. Figure 2: This chart shows different motion profile positions at different velocities with dif- ferent accel/decel rates, all under full and precise control. Finally, an important feature of electric actuators is their ability to provide programmable control of all the motion profile variables. As a result, the only operator interaction required is the up-front design time to build desired performance variables into a PLC or another controller’s programming environment. Once set, the operation repeats from day-to-day, month- to- month and year-to-year. Furthermore, with the use of HMI (Human Machine Interface) screens, the variables of position, velocity, acceleration/deceleration and force can be easily changed at any time, providing maximum flexibility. In an OEM environment, system performance will be easier to control due to the increased consistency of an electric vs. a fluid system. POSITION VELOCITY ACCEL / DECEL Time Position #1 SPEED 1 ACCEL1 ACCEL2 DECEL1 DECEL2 DECEL3 SPEED 2 SPEED 3 #2 #3 #4 DWELL DWELL Time Velocity Time Accel/Decl
  • 4. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 4 System components and footprint The number of components and overall space required for a hydraulic system is much greater than an electric system. Hydraulic systems require: a cylinder; a power unit to provide oil pressure; control and accessory valves; filters; hoses; fittings; and additional components. Hydraulic cylinders offer a compact footprint at the work point (where the power density is required), but the hydraulic power unit (HPU) which regulates flow and pressure to these actuators and other components, can require a large footprint in floor space. HPUs are not small and typically are placed near the actual cylinder itself, further increasing the system footprint. In very large systems with remote mounted HPUs, long lengths of hose can decrease the overall rigidity and efficiency of the hydraulic system. Figure 4: Electric system components Electric rod actuators provide a smaller overall footprint. These systems utilize a mechanical actuator; a motor (servo, or other); an optional gearbox; cables; and a drive/amplifier, which is usually mounted in a control cabinet. Although the electric actuator—due to its design of integrating a power The number of components and overall space required for a hydraulic system is much greater than an electric system. Electric rod actuators provide a smaller overall footprint. Mechanical Actuator Motor (Optional Gearbox) Control Cabinet Drive Motor Cables Power Unit Control Valves Heat Exchanger Oil Tank Heater Manual Override Pump Flow Control Valve Suction Screens/ Filters Accumulator Pressure Gauge Shut Off Valve Figure 3: Hydraulic system components
  • 5. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 5 screw and bearing system—does require additional length over a hydraulic cylinder, when considering the overall system footprint, this additional length is more than compensated for by the much smaller footprint of the servo drive—the functional equivalent of the HPU. Typically, automation equipment utilizes a control cabinet and can be designed for an additional drive. Size requirements for a servo drive are normally a fraction of the size requirements of a HPU. In new system designs, if the additional space at the work point can be made to accommodate the electric actuator’s working stroke and overall length, the overall footprint of the machine can be greatly decreased by eliminating the need for large power units. Force capabilities Due to their high operating pressures, hydraulic cylinder systems are great at producing extremely high forces. Typical pressures range from 1800 to 3000 psi (124.1 to 206.8 bar). In some high-pressure hydraulic systems, pressure ratings up to 5000 psi (344.7 bar) are used to further emphasize power density. Since hydraulic cylinders operate on the Force = Pressure x Area fluid power principle, the high pressures allow smaller cylinders to reach very large forces. For example, 3-inch and 5-inch bore cylinders at 2200 psi could achieve approximately 15,000 lbf (66,723.3 kN) and 43,000 lbf (191,273.5 kN), respectively. However, hydraulic cylinders are not usually used to their full output force capability; they are typically oversized to improve control. When considering electric actuators, it is important to determine the working force required. To estimate the approximate force required for an electric actuator, the typical approach is to adjust the hydraulic work port or system pressure until the operation can no longer be performed. Electric actuator systems rely on current through the servo motor to produce torque to the mechanical system, which drives the power screw to turn and generate force. This is a huge advantage; force is instantaneous. In hydraulic systems, where system rigidity is not optimized, the hydraulic actuator must wait for pressure to build until force is achieved. Another big advantage with electric systems is that the servo controller automatically regulates the current. The electric actuator system essentially uses current “on demand.” Any adjustment happens automatically. A hydraulic power unit must always keep pressure in the system for the hydraulic cylinder to actuate, which can be highly difficult. When selecting an electric actuator system, it is important to consider the motor’s RPM and torque capabilities, coupled with the screw lead in the electric actuator. Matching speed and torque from the servo motor with the lead screw’s mechanical output can be complex. Achieving the extreme forces that hydraulics can produce is entirely possible with electric Size requirements for a servo drive are normally a fraction of the size requirements of a HPU. Achieving the extreme forces that hydraulics can produce is entirely possible with electric technology, but typically, the electric actuator deployed will have a larger body diameter and the electric actuator system will have a velocity maximum that can’t be exceeded.
  • 6. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 6 technology, but typically, the electric actuator deployed will have a larger body diameter and the electric actuator system will have a velocity maximum that can’t be exceeded. The complexity in sizing a system can easily be overcome as actuator and servo component manufacturers provide easy-to- use motion control sizing software packages that factor in all these variables. Velocity capabilities Achieving high velocities at high forces presents challenges for both hydraulic and electric technologies. Hydraulics require pressure for force and flow for speed. To achieve higher speeds at higher forces, there must be enough pressurized oil in the system to basically push the required volume of oil into a cylinder in the required amount of time (defined as flow). This typically requires an accumulation system to hold the pressurized volume. The problem can multiply with long stroke cylinders; uncharged accumulators may starve the system of oil. In the end, deploying additional capacity in hydraulic accumulator systems allows them to achieve high speeds at high forces. The downside of this practice is that without servo-hydraulic control on the hydraulic system, excess energy (force x velocity) is essentially being utilized in an open loop control scheme. As stated earlier, the force capabilities of an electric actuator system depend on the right combination of servo motor RPM, servo motor torque and mechanical advantage from screw lead and possibly a gearbox. As servo motors increase in size, torque typically increases significantly but RPMs decrease. In extreme applications of force and speed, the only way an electric actuator system may be able to achieve the desired performance is to grossly oversize the system—which can be cost prohibitive. Alternately, remember that an electric actuator system has complete control over the motion profile; it doesn’t have to stroke the entire length of each cycle. Plus, with control of acceleration and deceleration, the electric system can settle more quickly, which reduces cycle time and increases efficiency. Finally, electric actuator technology may alleviate some of the peak velocities required as more intelligent, shorter moves can be executed. Temperature (hot and cold) Heat A major problem for hydraulic systems is heat management. The inefficiencies of hydraulic systems cause overheating. Hydraulic fluid temperatures above 180°F (82°C) damage most seal compounds and accelerate degradation of the oil. Circuit design can play a big part in generating heat. Hoses, fittings and other components can be a factor if they are inaccurately sized for the flow requirements. Sophisticated load-sensing systems can mitigate most heat issues, but can be costly. Heat exchangers are an economical way to manage this without significantly over sizing your reservoir and ultimately your HPU footprint. An electric actuator system has complete control over the motion profile and can settle quicker for reduced cycle time and increased efficiency. A major problem for hydraulic systems is heat management.
  • 7. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 7 HEAT RESISTANCE Electric Servo Hydraulic Hydraulic Low High Medium Figure 5: Compared to hydraulics, electric systems are better at managing heat resistance. There are two ways to solve overheating problems in hydraulic systems: decrease heat load or increase heat dissipation. Hydraulic systems dissipate heat through the reservoir, by having the correct level of oil or use of a heat exchanger. Keeping a hydraulic system in proper form to manage heat will require constant attention and maintenance. Electric actuators can overheat at times, but the cause is usually due to extreme changes in duty cycle or running the actuator harder than it was designed or sized for. Extreme changes in the ambient temperature not accounted for in the design may also cause overheating, but these conditions are exceptions and typically would not occur in most factory automation environments. Due to their higher efficiency and sizing accuracy, electric actuator systems can be selected to run at a desired temperature for the given amount of work required. Accurately predicting temperature allows the electric actuator system to perform consistently without affecting the life of the device. Cold Cold temperatures present other problems for hydraulic systems. Cold oil causes sluggish and inconsistent operation until the oil is warm—which in turn causes large swings in force and speed. Hot to cold temperature swings can also affect the integrity of the rod seal—a critical component that helps to prevent leaks and contamination. Oil tank heaters can manage start-up temperatures, but they are an extra component and expense. Electric actuators, on the other hand, can be deployed with extreme temperature grease which allows for quick, effective starts in cold temperatures. There is usually a small performance difference in force repeatability from a cold to a warm temperature. This difference is acceptable in most applications. Electric actuators can overheat at times but the cause is usually due to extreme changes in duty cycle or exceeding its design specifications. Cold oil in hydraulic systems causes sluggish and inconsistent operation which can affect force and speed.
  • 8. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 8 COLD RESISTANCE Electric Servo Hydraulic Hydraulic Low High Medium Figure 6: Electric systems are not susceptible to fluid thickening in cold temperatures and perform consistently despite fluctuations in temperature swings. Service life and maintenance Hydraulics are rugged, widely used devices that can have a long service life. The trade-off is that they require frequent maintenance and attention to achieve the desired performance over the life of the system. Maintaining the integrity of the rod and piston seals is key to repeatability because these are the main elements that contain the pressure required for motion and force. If these components wear or are damaged, they must be replaced. Otherwise, force will be reduced at initial pressure or “blow by” will affect speed. Changing oil filters and oil on a periodic basis are additional maintenance tasks. Contaminated or degraded oil can also severely affect the operation of the system. Neglecting maintenance items will lead to leaks or contamination and premature failure of components. Figure 7: Electric systems require little or no maintenance compared to hydraulic systems. On the other hand, electric actuators sized for the life of an application require no maintenance. The main power elements of an electric actuator MAINTENANCE Electric Servo Hydraulic Hydraulic Low High Medium Hydraulics require frequent maintenance and attention to achieve the desired performance over the life of the system. Electric actuators sized for the life of an application require no maintenance.
  • 9. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 9 are the power screw and other thrust-bearing elements. These elements have a Dynamic Load Rating (DLR) specification that can be utilized to estimate the amount of work (force over distance) an electric actuator can achieve. Utilizing an industry standard L10 life estimation (see our guide on estimating actuator life), electric actuators can be estimated with 90% reliability to last in an application. These power elements are typically greased for life, but easy, in-field greasing methods can be applied if necessary. The other wear element on an electric rod-style actuator is the rod seal, designed to hold out water, dust and other environment contamination from the actuator’s internal components. Unlike hydraulic cylinder seals, there is no pressure to hold in the actuator for proper operation. Even if the seal fails, the actuator will continue to operate. Rod seals on most electric actuators are easily and inexpensively replaced if damage occurs. Misuse is the primary reason that electric actuators fail. Exceeding the actuator’s performance specifications for extended periods of time or gross neglect are the most common misuse factors. Data collection In the never-ending quest to improve manufacturing processes, data collection in critical areas is becoming more and more prevalent in today’s manufacturing environment. Again, hydraulic cylinders require expensive, complex servo-hydraulic systems with additional sensors to track and monitor position, velocity, force and other factors happening at the work point. These factors are all built into an electric actuator’s servo system. Monitoring the motor current makes it easy to track force and repeatability. The feedback device on the motor registers accurate position and velocity. Figure 8: Electric systems provide the most data collection capabilities over servo hydraulic and conventional hydraulic systems. Efficiency and electric utility costs Electric actuator systems typically operate in the 75-80% efficiency range for the work they complete; hydraulic actuator systems typically operate in the 40-55% efficiency range. An additional factor in the electric utility cost DATA COLLECTION Electric Servo Hydraulic Hydraulic Low High Medium Hydraulic cylinders require expensive, complex servo- hydraulic systems with additional sensors to track and monitor position, velocity, force and other factors happening at the work point.
  • 10. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 10 equation is that electric actuators only demand current to the motor when it is required. When electric actuators are at rest, they require no current or very low amounts of current to hold their positions. Hydraulics require that the power unit keep the hydraulic system pressurized at all times when the system is turned on—resulting in inefficient use of power. Figure 9: Compared to servo hydraulic and conventional hydraulic systems, electric systems operated far more efficiently. Below are three application examples that show the power usage of electric and hydraulic actuator systems. Figure 10: The above charts show the difference in electric utility costs for a hydraulic and an electric cylinder. The blue arrow depicts cylinder operation at 50% duty cycle and the resulting estimated utility costs. Electric actuator systems typically operate in the 75-80% efficiency range for the work they complete; hydraulic actuator systems typically operate in the 40- 55% efficiency range. OPERATION EFFICIENCY Electric Servo Hydraulic Hydraulic Low High Medium Electric vs. Hydraulic Power Costs POWER-OUT (kW) Ã = Velocity (m/sec) x Force (N) ÷ 1,000 (converted to kN) POWER-IN (kW) Ä = Power-Out (kW) ÷ Efficiency (%) COST OF APPLICATION $ = (Power-In) x (Hours/year) x (Electricity Cost) 1 2 3 3IN(76MM)BOREHYDRAULICCYLINDER • POWEROUT:2kw • SPEED:1.8in/sec(45mm/sec) • FORCE:10,000lbf(44.5kN) 4IN(102MM)BOREHYDRAULICCYLINDER • POWEROUT:10kW • SPEED:2.9in/sec(75mm/sec) • FORCE:30,000lbf(133.5kN) 6IN(152MM)BOREHYDRAULICCYLINDER • POWEROUT:20kw • SPEED:3.5in.sec(90mm/sec) • FORCE:50,000lbf(222.5kN) ASSUMPTIONS: Electric Efficiency 80%; Hydraulic Efficiency 45%; 2000 PSI; Cost kW/hr $0.07 BLUE ARROWS DESIGNATE COST SAVINGS SHOWN AT 50% DUTY CYCLE $0 $500 $1,000 $1,500 $2,000 $2,500 $3,000 ENERGYCOST($) ENERGYCOST($) ENERGYCOST($) DUTY CYCLE (%) DUTY CYCLE (%) DUTY CYCLE (%) Energy Cost: 2 kW Energy Cost: 10 kW Energy Cost: 20 kW HYDRAULIC ELECTRIC HYDRAULIC HYDRAULIC ELECTRIC ELECTRIC $0 $2,000 $6,000 $8,000 $10,000 $12,000 $14,000 $0 $5,000 $10,000 $15,000 $20,000 $25,000 $30,000 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 1000 10 20 30 40 50 60 70 80 90 100
  • 11. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 11 By dividing the efficiencies of the two technologies into one another, an approximate factor can be determined on electric utility cost based on when the actuators/cylinders are actually moving. For example: 80% (electric) / 40% (hydraulic) = 2. This results in electric being twice as efficient or hydraulic taking twice as much power to do the same work. This is an approximation, but it is not far from reality. In fact, unless a hydraulic system is properly maintained, the overall efficiencies of the system can go to levels of 20% making electric technology up to 4 times as efficient (or hydraulic utility costs up to 4 times higher). Although electric actuator systems are twice as efficient as hydraulic systems, an electric servo solution can be more expensive to implement initially. But the cost over the life of the system can be substantially less when considering the increased performance, system flexibility and decrease in utility costs. Leaks and environmental concerns Many hydraulic industry professionals have said, “It is not a matter of IF hydraulics will leak, but a matter of when and by how much.” Leaks create messes in the manufacturing environment and also present a safety hazard if someone slips on the spill. More alarming is leaks could cause serious contamination to key processes and products—such as food processing, pharmaceuticals, medical device and others. A contamination event can be costly both in terms of clean-up and scrapped product—and even more costly if the contaminated product reaches consumers and is then recalled. Additionally, many hydraulic systems are deployed outdoors and in the vicinity of living spaces and fresh water. Leaking oil into the surrounding environment can be a major concern for communities. Figure 11: Unlike hydraulic systems, electric systems present minimal environmental risk. The grease on the lead screw of an electric actuator is the only potential contaminant. However, high-quality actuator seals on the actuator rod virtually prevent contamination. By dividing the efficiencies of the two technologies into one another, an approximate factor can be determined on electric utility cost based on when the actuators/cylinders are actually moving. A contamination event can be costly both in terms of clean-up and scrapped product—and even more costly if the contaminated product reaches consumers and is then recalled. ENVIRONMENTAL RISKS Electric Servo Hydraulic Hydraulic Low High Medium
  • 12. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 12 Electric actuator technology is one of the cleanest linear motion technologies that can be actively and easily deployed. Grease on the lead screw is the only potential contaminant; special greases (food grade, clean room, etc.) can be applied if required. The high-quality seals used on the rod of the electric actuator keep grease inside the actuator, virtually eliminating contamination issues. Other factors Noise: A noisy power unit running a hydraulic cylinder can be a noise polluter for any operator near the machine. Shock loads: Hydraulics are good at handling shock loads. Hose compliance (unless hard plumbed) and oil bypassing along the main or work port relief valve absorb shock loads. In addition, internal leakage within the cylinder and valves can also offer compliance with shock load conditions. A large shock load in-line with the rod would be absorbed in most cases by the piston against the compressed oil in the body of the hydraulic actuator. On the other hand, shock loads on an electric actuator’s lead screw or bearing system may affect performance. Electric actuators may have to be oversized in order to handle shock loads or use external shock absorbing mechanisms to prevent damage. Side loads: Side loading—due to misalignment or pressure from a moment arm—places stress on the rod and piston seals of a hydraulic cylinder. The seals may wear prematurely or fail completely, resulting in poor performance and eventually premature failure of the cylinder. Electric actuators don’t like side loading either, but for different reasons. Side loading puts stress on the actuator’s front rod seal and on the nut of the lead screw. The rod seal may fail, allowing contaminants into the actuator and causing premature failure of the electric actuator. Side loading the lead screw nut can diminish the estimated life of the ball or roller screw. However, unlike hydraulic actuators, electric actuators typically deploy an anti- rotate mechanism on the lead screw nut of the actuator, which allows it to absorb some side- loading due to mounting misalignment. Finally, both hydraulic and electric cylinders require care and consideration when mounting to make sure the cylinder is in alignment with the system it is intended to move. (See our white paper on actuator alignment for more information.) Hydraulics are good at handling shock loads. Electric actuators may have to be oversized or use external shock-absorbing mechanisms to prevent damage. Neither hydraulic cylinders or electric actuators are well suited for side loading. Figure 12: Side loading of rod actuators is not recommended because it places stress on internal actuator components.
  • 13. Copyright © 2016 Tolomatic, Inc. Electric Rod Actuators vs. Hydraulic Cylinders:A comparison of the pros and cons of each technology www.tolomatic.com 763-478-8000 9900-4412_00 Page 13 Summary Hydraulic actuators have an industry reputation for delivering high force. Hydraulic systems are usually less expensive from a purchased cost standpoint to implement than an electric servo system. Their drawbacks include a larger space footprint, regular maintenance and manual system adjustments for optimal system performance. Hydraulic systems are susceptible to fluctuations in temperature, prone to leaks and operate in an open loop environment, posing challenges for data collection. Although servo- hydraulics can be implemented to alleviate some of these issues, they result in a larger space footprint and are very costly to employ. Hydraulics do offer a long service life, but they are not as efficient operators as electric systems. Electric systems offer smaller overall space requirements than hydraulic systems. They also provide precise position, velocity and speed control with more efficient operation. They operate in a closed loop environment for easy data collection, and they are virtually maintenance free. Preferred for their higher level of accurate performance, electric servo systems can be more costly than hydraulic to initially implement. However, the increased efficiency of operation with little or no maintenance over the life of the system makes their total cost of ownership lower over the life of the equipment and an attractive alternative to hydraulics. As electric rod-style actuators become capable of achieving high-end hydraulic forces, they will continue to be viable candidates for replacing hydraulic systems in many applications. Evaluating capabilities and limitations discussed in this paper and aligning them with system goals and objectives will help determine the best choice for the application. Electric systems offer smaller space requirements and provide precise position, velocity and speed control with more efficient operation.