7. Design of Machine Elements -Power screws.pptx

Design of Machine Elements
Dr. G. Praveen Kumar.
Assistant Professor
Mechanical Engineering Department
IIITDM Kurnool
27/07/22 Dr. G.Praveen Kumar, IIITDM Kurnool 1

Course Contents
Design philosophy, revision of failure theories, limits, fits and design under static load (4)
Design for variable loading - fatigue strength and design; design of shafts. (10)
Design of riveted, bolted and welded joints and Power Screws. (10)
Design and selection of belt drives. (4)
Design of Gears: spur and worm gears, Contact and bending fatigue strength, Gear accuracy. (8)
Tribology: Lubricant theories; Design of Journal bearings; Selection of ball and roller bearings (6)
25/07/22 Dr. G. Praveen Kumar, IIITDM Kurnool 2

Design
of
Power screws

• Introduction to power screws.
• Different forms of threads in power screws
• Load Analysis
• Efficiency of Square threaded screw
Contents

Introduction to power screws
 A power screw is a mechanical device used for
converting rotary motion in to linear motion and
transmitting power.
 A power screw is also called a translation screw. It
uses helical translatory motion of the screw thread in
transmitting power rather than clamping the
machine components.

• There are three essential parts of the power screw, viz., screw,
nut and a part to hold either the screw or the nut in its place.
• Depending upon the holding arrangement, power screws
operate in two different ways.
• In some cases, the screw rotates in its bearing, while the nut
has axial motion. The lead screw of the lathe is an example of
this category.
• In other applications, the nut is kept stationary and the screw
moves in an axial direction. A screw jack and machine vice are
the examples of this category.

The main applications of power screws are as follows:
(i) To raise the load, e.g., screw-jack;
(ii) To obtain accurate motion in machining operations, e.g., lead-screw of lathe;

The main applications of power screws are as follows:
(iii) To clamp a work piece, e.g., a vice;
(iv) To load a specimen, e.g., universal testing machine.

• A power screw has large load carrying capacity.
• The overall dimensions of the power screw are small, resulting
in compact construction.
• A power screw is simple to design.
• The manufacturing of a power screw is easy without requiring specialised
machinery. Square threads are turned on the lathe. Trapezoidal threads are
manufactured on a thread milling machine.
• A power screw provides large mechanical advantage. A load of 15 kN can be
raised by applying an effort as small as 400 N. Therefore, most of the power
screws used in various applications like screw-jacks, clamps, valves and vices
are manually operated.
Advantages of Power Screw

• A power screw provides precisely controlled and highly accurate linear
motion required in machine tool applications.
• A power screw gives smooth and noiseless service without any
maintenance.
• There are few parts in a power screw. This reduces cost and increases
reliability.
• A power screw can be designed with self locking property. In screw-jack
application, self-locking characteristic is required to prevent the load
from descending on its own.
Advantages of Power Screw

• A power screw has very poor efficiency, as low as 40%.
Therefore, it is not used in continuous power transmission in
machine tools, with the exception of the lead screw.
• High friction in threads causes rapid wear of the screw or
the nut. Therefore, wear is a serious problem in power screws.
Disadvantages of Power Screw

Terminology of Power Screw
•Nominal diameter(d)
•Core diameter(dc)
•Mean diameter(dm)
•Pitch (p)
•Lead (l)
•Lead angle(ƛ)
•Hand of threads

Nominal diameter(d)
•It is the largest diameter of an
external or internal thread.
•The screw is specified by this
diameter.

Core diameter(dc)
•It is the smallest diameter of an
external or internal thread.

Pitch (p)
• The pitch is defined as the distance
measured parallel to the axis of the
screw from a point on one thread to the
corresponding point on the adjacent
thread. It is denoted by the letter p.

Lead (l)
“The lead is defined as the distance
measured parallel to the axis of the screw
which the nut will advance in one revolution
of the screw. It is denoted by the letter l.
Lead=number of starts*pitch L=N*p
For a single-threaded screw, the lead is same as
the pitch. For a double-threaded screw, the
lead is twice of the pitch, and so on.

Lead angle(ƛ)
“It is an angle made by a helix or
thread with plane perpendicular to
an axis of screw.”

Hand of threads
When the axis of screw is vertical, if the thread slopes upward from left to
right. it is known as right hand thread; whereas if the thread slopes upward
from right to left, it is known as left hand thread.
.
Right Hand Threads. Left Hand Threads.

Thread profiles of power screws
1.Square Threads
• Asquare threads, shown in Fig. This threads is adopted for the transmission
of power in either direction. It is generally cut on lathe machine
with single point cutting tool.
• Applications:
Used in Screw jacks, presses & clamping devices
• Advantages
1. Square threads have maximum efficiency of all thread forms.
2. They exert minimum radial pressure on nut.
3. They can transmit power in either direction.
• Disadvantages
1. Strength of the square threads is lowest of all the thread forms.
2. These threads cannot be used conveniently with split nut because:
engagement and disengagement is difficult

2.ACME/Trapezoidal Threads
• The Acme threads, shown in Fig. have thread angle
equal to 29°.
• Applications:
Used for lead screws of machine tools, bench vices
• Advantages
1. Acme threads permit the use of split nut which can compensate the wear.
2. Acme threads are stronger than the square threads in shear because of the larger cross-
section at the root.
3. Acme threads can transmit power in either direction.
• Disadvantages
1. Because of slope given to the sides the efficiency of acme threads is lower than that of
square threads.
2. Slope on the sides introduces some bursting pressure on the nut.

3. Buttress Threads
• Buttress threads, as shown in Fig. are used where force or power
is required to be transmitted only in one direction
• Applications:
• Used in screw jacks & vices where force is to be applied in only one direction.
• Advantages
1. Buttress threads are stronger in shear than any other power threads
because of the largest cross section at the root.
2. Buttress threads combine the high efficiency of square threads and high
strength of V- threads.
• Disadvantages
1. Buttress threads are used to transmit power in only one direction.
2. Theses threads are difficult to manufacture.

Torque requirement –Lifting Load
Torque required to raise the load against thread friction
• The advancement (motion) of the screw or nut in the direction of load is
equivalent to raising the load, as shown in fig.1 the force diagram of an
equivalent inclined plane for raising the load is shown in fig.2

Torque requirement –Lowering load
Torque required to lower the load against thread friction
• The advancement (motion) of the screw or nut in the direction of load is
equivalent to lowering the load, as shown in fig.1 the force diagram of an
equivalent inclined plane for lowering the load is shown in fig.2

Self locking screws
Self locking Screws
Torque required to lower the load against thread friction is given by
In this equation if the torque required to lower the load Tt will be
positive. Such screw is known as self-locking screw.
For self-locking screw, friction angle is greater than lead angle and
torque required to lower the load Tt will be always positive.
Applications:
Self locking screw is used in screw-jack & C-Clamps.

Over hauling screws
Over hauling Screws
Torque required to lower the load against thread friction is given
by
In this equation if the torque required to lower the load Tt will
be negative. i.e. load will start moving downward without the application
of any torque causing the screw to rotate. Such screw is known as over
hauling screw.
For over hauling screw, friction angle is less than or equal to lead
angle and torque required to lower the load Tt will be zero or negative.

Screw efficiency of square threads
Screw efficiency: it is the ratio of zero friction input torque to the actual
input torque
Expression for Screw efficiency:
zero friction input torque to the actual input torque is given by,

λ
λ

Collar friction torque
Collar friction Torque
•In many applications, load does not rotate
with screw, and hence some additional torque
must be applied to overcome the friction at
collar.
•Fig. shows the power screw with the collar
and cup. The collar of the power rotates with
screw while cup remains stationary due to
load W. this results in friction at the annular
surface between the collar and the cup.

Collar friction torque
Expression for collar friction Torque
•The torque required to overcome the collar friction is given by,
According to uniform pressure theory
According to uniform wear theory
In general can be written in this form

•Overall efficiency :- ratio of total zero friction input torque to total
actual input torque.
Expression for overall efficiency
Total actual input torque is given by
Total zero friction input torque is given by
overall efficiency

Design of power screws
The nominal diameter of a triple threaded square screw is 50 mm, while the pitch is 8
mm. It is used with a collar having an outer diameter of 100 mm and inner diameter as 65
mm. The coefficient of friction at the thread surface as well as at the collar surface can be
taken as 0.15. The screw is used to raise a load of 15 kN.
Using the uniform wear theory for collar friction, calculate: (i) torque required to raise the
load; (ii) torque required to lower the load; and (iii) the force required to raise the load, if
applied at a radius of 500 mm.

Ex.4.1 The following data refers to a screw jack:
• Nominal diameter of screw=40mm
• Pitch of threads=7mm
• Type of screw=single start square threaded
• Coefficient of thread friction=0.15
• Coefficient of collar friction=0.1
• Effective mean diameter of collar=70m, if operator can comfortably
exert a force of 150N at a radius of 1.2m to raise the load, calculate
i) The maximum load that can be lifted
ii) The efficiency of the screw
iii) The overall efficiency.

To find Lead angle
To find friction angle
Torque required to raise the load

Torque required to raise the load

Torque applied by the operator

Maximum load that can be lifted
Torque reqd to raise the load=torque applied by operator

Efficiency of Trapezoidal & ACME threads

A double-threaded power screw, with ISO metric trapezoidal threads is used to raise a load of 300 kN. The nominal diameter
is 100 mm and the pitch is 12 mm. The coefficient of friction at the screw threads is 0.15. Neglecting collar friction, calculate
(i) torque required to raise the load; (ii) torque required to lower the load; and (iii) efficiency of the screw.

The lead screw of a lathe has single-start ISO metric trapezoidal threads of 52 mm nominal diameter and 8 mm pitch.
The screw is required to exert an axial force of 2 kN in order to drive the tool carriage during turning operation. The
thrust is carried on a collar of 100 mm outer diameter and 60 mm inner diameter. The values of coefficient of friction
at the screw threads and the collar are 0.15 and 0.12 respectively. The lead screw rotates at 30 rpm. Calculate (i) the
power required to drive the lead screw; and (ii) the efficiency of the screw.

Thank you

Classification of joints
The right hand or left hand threads
satisfy the respective hand rule, as
shown in fig.
For the right hand threads, if the
right hand fingers are kept in the
direction of rotation of the nut. the
thumb will indicate the direction of
an advancement of the nut fig.
For the left hand threads, if the left
hand fingers are kept in the direction
of rotation of the nut, the thumb will
indicate the direction of
advancement of the nut .
In power screws right hand threads
are most common, especially in
lifting devices and clamping devices.

DESIGN OF SCREW AND NUT
There are three basic components of a power screw, viz., screw, nut and frame. The desirable properties of
screw material are as follows:
(i) It should have suffi cient strength to withstand stresses due to external load and applied torque
(ii) It should possess high wear resistance.
(iii) It should have good machinability.
Screws are made of plain carbon steel such as 30C8, 40C8 and 45C8 or alloy steels like 40Cr1. The screws
are case hardened, e.g., the lead screw of a lathe is case hardened by the nitriding process

The body of the screw is subjected to an axial force W and
torsional moment (Mt )t , as shown in Fig. 6.11. The direct
compressive stress sc is given by,
The torsional shear stress is given by,
The principal shear stress is given by,

The threads of the screw, which are engaged with the nut, are subjected to
transverse shear stress. The screw will tend to shear off the threads at the
core diameter under the action of the load W. The shear area of one thread
is (Πdct). The transverse shear stress in the screw is given by

7. Design of Machine Elements -Power screws.pptx

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