hydraulic subtopic

2.  Water was the first fluid used for the
transmission of fluid power.
 The main advantages of water as a hydraulic fluid
its availability, low cost, and fire resistance.
 Disadvantages of water:
 corrodes,
 evaporates,
 support growth of bacteria,
 contaminates,
 poor lubricity,
 low viscosity

3.  Mineral oils were readily available at the beginning
of the 20th century, but were not practically used in
hydraulic systems.
 1940s: First additives were used to improve the
physical and chemical properties of hydraulic
mineral oils. The first additives were developed to
counter rust and oxidation.
 But, mineral oils are highly flammable, and fire risk
increases when operating at high temperatures.
 This has led to the development of fire-resistant
fluids that are mainly water-based, with limitations
on the operating conditions. The need for extremes
of operating temperatures and pressures led to the
development of synthetic fluids.

4.  Power transmission
 Lubrication
 Sealing – thin hydraulic film act as sealing
that reduce leakage
 Cooling – capable to absorb heat generated
by moving part
 Cushioning of oscillations caused by pressure
jerks
 Corrosion protection

5.  Viscosity
 Oil density
 Oil compressibility
 Thermal expansion
 Vapor Pressure
 Lubrication and Anti-Wear Characteristics
 Compatibility
 Chemical Stability
 Oxidation Stability
 Environmentally Acceptable Hydraulic Oils

6.  Viscosity describes the resistance to the
laminar movement of two neighboring fluid
layers against each other.
 Simply, viscosity is the resistance to flow. It
results from the cohesion and interaction
between molecules
E.g. The lower plate
is fixed, while the
upper plate is
moving at a steady
speed v

7.  Important to keep the oil viscosity within a
certain range during the system’s operation;
otherwise, the operating conditions will change
with temperature.
 The viscosity index (VI) of oil is a number used in
industry to indicate the effect of temperature
variation on the viscosity of the oil.
 A low VI signifies a relatively large change of
viscosity with temperature variation.
 A high VI means relatively little change in
viscosity over a wide temperature range.
 The best oil is the one that maintains constant
viscosity throughout temperature changes.

8.  Effect of Viscosity on Hydraulic System
Operation
 The oil viscosity influences the function of
hydraulic power systems as it introduces
resistance to fluid flow and to the motion of
bodies moving in the fluid. Herein, the
following effects are studied:
◦ Hydraulic losses in transmission lines
◦ Resistance to fluid flow in narrow conduits
◦ Viscous friction forces and damping effect

9.  Density - the mass per unit volume: ρ=m/V.
 The hydraulic oils are of low compressibility
and volumetric thermal expansion. Therefore,
under ordinary operating conditions, the oil
density is practically constant.
 The density of mineral hydraulic oils ranges
from 850 to 900 kg/m3
 The oil density affects both the transient and
steady state operations of the hydraulic
systems.

10.  Local Losses
◦ Result from a rapid variation in the magnitude or
direction of the velocity vector. E.g. throttling
elements, elbows, and T connections area
◦ Directly proportional to the fluid density
 Hydraulic Inertia
◦ Occurs during the early running stage
◦ Affects the transient response of the hydraulic
transmission lines, but it has no significant effect
on its steady state behavior

11.  Defined as the ability of liquid to change its volume
when its pressure varies.
 Liquids are of very low compressibility, compare
with gas. Therefore, liquids are usually assumed
incompressible.
 But this is applied when the liquid compressibility
has no significant effect on the performance of the
studied system.
 The hydraulic oil compressibility has a direct
impact on the transient behavior of the hydraulic
system.
 Generally, the reduction of oil volume by 1%
requires an increase of its pressure by 10 to 20
MPa.

12.  The hydraulic liquids are subjected to
volumetric thermal expansion. Generally, the
volume of liquids changes with temperature
as follows:

13.  The fluid must be capable of covering the
contact surfaces of all moving parts with a
thin and continuous lubricating film.
 The lubricating film may be destroyed, as a
result of high loading forces, insufficient oil
delivery, and low viscosity.
 The lubricating power and film strength of a
liquid are directly related to its chemical
nature and can be improved by the addition
of certain chemical agents.

14.  The fluid must be fully compatible with other
materials used in the hydraulic system, such as
those used for bearings, seals and paints.
 It should not react chemically with any of these
materials, nor change their physical properties.
 Moreover, the fluid leaks out from the hydraulic
system and encounters other system parts, such
as electrical lines, mechanical components, and
others, so the fluid must also be compatible with
the materials of these parts.

15.  The hydraulics fluid must be stable in term of
chemical and oxidation.
 Some metals, such as zinc, lead, brass, and
copper, have undesirable chemical reactions
with certain liquids.
 Can be improved by the addition of oxidation
inhibitors

16.  An oil leak may result in considerable pollution of
the surroundings and ground water.
 Mineral oils are composed of relatively stable
hydrocarbon compounds, and are only very slowly
broken down by microorganisms in the
environment.
 Thus, pollution by conventional mineral hydraulic
oils can disturb the ecological balance for long
periods.
 Led to a growing interest in biodegradable
products including hydraulics fluid.

17. Mineral Oils
 The most widely used hydraulic fluids.
 Relatively inexpensive, widely available, and can be
offered in suitable viscosity grades.
 Good lubricity, noncorrosive, and are compatible with
most sealing materials with the exception of butyl
rubber.
 Chemically stable for reasonable operating
temperatures. At higher temperatures, however, they
suffer chemical breakdown.
 Premium grade mineral oils contain a package of
additives to combat the effects of wear, oxidation,
and to improve viscosity index and lubricity.

18.  Disadvantages of mineral oils:
◦ Cannot be remedied by incorporating additives.
◦ High flammability
◦ Increase in viscosity at high pressures.
 Fire risk excludes the use of mineral oils in
hazardous areas such as injection and plastic
molding machines, coal mines, and near
furnaces.
 The viscosity pressure characteristics limit
their use to pressures below 1000 bar

19.  This hydraulic fluid consists of tiny droplets
of oil dispersed in a continuous water phase.
 The dilution is normally between 2% and 5%
oil in water, and the characteristics of the
fluid are more similar to water than oil.
 It is extremely fire-resistant, is highly
incompressible, and has good cooling
properties.
 Nain disadvantages - poor lubricity and low
viscosity.

20.  The water-in-oil emulsions are the most popular
fire-resistant fluids.
 They have a continuous oil phase in which tiny
droplets of water are dispersed. Their lubrication
properties are very much reduced.
 For optimum life, the operating temperatures should
not exceed 25°C, but intermittent operation up to
50°C is permissible.
 At the higher temperature, water content is affected
owing to evaporation, which decreases the emulsion’s
fire-resistance properties.
 When the system has been idle for long periods,
there is a tendency for the oil and water to separate.
However, during running, the pump will re-emulsify
the fluid.

21.  These fluids were developed primarily for use in
aircraft because of their very low flammability
characteristics.
 However, their application is limited since they
cannot be used at high temperatures because of
their water content.
 Their lubricating ability is inferior to that of
mineral oils; they attack most paints; they are
very stable with respect to shear because of the
low molecular weight of their constituents;
 Good anti-freeze properties make them
particularly suitable for low-temperature
applications.

22.  Synthetic oils, such as phosphate esters, have remarkably
good fire resistance properties. They are used in industries
such as plastic molding and die-casting, where unusually
great fire risks occur.
 Their lubricating ability is similar to that of mineral oil.
 Synthetic oils are superior when compared with mineral-
oil–based fluids in term of Thermal stability, Oxidation
stability, Viscosity-temperature properties (VI), Low
temperature fluidity, Operational temperature limits and
Fire resistance
 Disadvantages:
◦ Elastomers used in conjunction with phosphate esters must be
chosen carefully.
◦ Certain metals, particularly aluminum, and most paints are
susceptible to attack.

23. The following are the main requirements imposed on
hydraulic liquids:
 Satisfactory flow properties throughout the entire
range of operating temperatures.
 A high viscosity index that ensures moderate
viscosity variation in relation to the temperature
fluctuations.
 Good lubricating properties are a prerequisite to
reduce the wear and increase the service life of the
system.
 Low vapor pressure to avoid cavitation.
 Compatibility with system materials since the fluid
should not react chemically with any of the used
materials or deteriorate their physical properties.

24.  Chemical stability is necessary to increase the
service life of liquid and avoid performance
deterioration.
 Corrosion protection by adding effective
corrosion inhibitors.
 Rapid de-aeration and air separation.
 Good thermal conductivity is required to rapidly
dissipate the heat generated due to friction
between elements and due to hydraulic losses.
 Fire resistance is essential in some applications.
 Electrically insulating properties can be
significant in a number of modern designs.
 Environmental acceptability.