Power Screw and its application

PPT Designed By
Mohammedi Limdiwala

Power Screw
Types of Power Screws
Load Analysis
Efficiency of power screw
Threads
Applications of Power Screw

A power screw is a drive used in machinery
to convert a rotary motion into a linear
motion for power transmission.
It produces uniform motion and the design
of the power screw may be such that:
(a) Either yhe screw or the nut is held at rest
and the other member rotates as it moves
axially. Ex: screw clamp
(b) Either the screw or the nut rotates but
does not move axially. Ex: press

Square Thread
Acme Thread
Buttress Thread

Strongest thread.
No radial load.
Hard to manufacture.
Square threads have a much higher intrinsic
efficiency than acme threads.
Due to the lack of a thread angle there is no
radial pressure, or bursting pressure, on the
nut. This also increases the nut life.
This thread is difficult in machining.

The square thread form is a common screw
thread form, used in high load applications such
as lead screws and jack screws.
 It gets its name from the square cross-section of the
thread.
It is the lowest friction and most efficient thread
form, but it is difficult to fabricate.

Power Screw and its application

The Acme thread form has a 29° thread
angle with a thread height half of the pitch.
The apex and valley are flat.
 This shape is easier to machine than is a
square thread.
 The tooth shape also has a wider base which
means it is stronger than a similarly sized
square thread.
 This thread form also allows for the use of
a split nut, which can compensate for nut wear.

29° included angle.
Easier to manufacture.
Common choice for loading in both
directions.
Faster cutting.
Longer tool life.

In machinery, the buttress thread form
is designed to handle extremely high
axial thrust in one direction.
 The load-bearing thread face is
perpendicular to the screw axis or at a
slight slant (usually not greater than 7°)
 The other face is slanted at 45°.

 Great strength.
 Only unidirectional loading.

 A square thread power screw with a single start
is shown in figure.
 Here p is the pitch, α the helix angle, dm the
mean diameter of thread and F is the axial load.
 In order to analyze the mechanics of the power
screw we need to consider two cases:
 Raising the load
 Lowering the load.

 Here work output = F. L
 Work input = p. πdm

 The load would lower itself without any
external force if
πµdm < L
 And some external force is required to lower
the load if
πµdm >= L
 Therefore this the condition of self locking.

 The above analysis is for square thread and
for trapezoidal thread some modification is
required. Because of the thread angle the
force normal to the thread surface is
increased.
 The torque is therefore given by
 The increased friction is due to the wedging
effect. But they are not preferred due to
greater losses due to friction.

 Bursting effect on the nut is caused by the
horizontal component of the axial load F on
the screw
Fx = FtanØ
 For isometric nut 2Ø = 60º and Fx = 0.5777F.

 If collar friction μc is considered then another
term μFdc/2 must be added to torque
expression.
 Here dc is the effective friction diameter of
the collar.
 Therefore we may write the torque required
to raise the load as -

dp
L

N
f
P
F
LIFTING
y
x
pd
L

 tan  
 Ld
LdPd
T
p
pp
Su 




2

p pitch in./thread
d diameter (major) in.
dp pitch diameter in.
dr minor diameter in.
L Lead in.

Jack screws
Lead screws of a lathe
Screw for vices
Presses
C-clamps
Instron material testing machines
Machine tools (for positioning of
table)

 Power screw drive in machinery is firstly
discussed and some details of the thread forms
used in such drives are given. The force system
at the contact surface between the screw and
the nut is analyzed and the torque required to
raise and lower a load, condition for self locking
and the efficiency of a power screw are derived.

Power Screw and its application

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