power screw

1. PRESENTED BY:-
Roy raj jyoti
Power
Screws

2. Power screw
 Convert a rotary motion into a linear motion for power
transmission.
 Produces uniform motion and the design of the power screw
may be such that:
 (a) Either 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

3. INDEX
Power Screw
Types of Power Screws
Load Analysis
Efficiency of power screw
Threads
Applications of Power Screw

4. Types of power screws
Square Thread
Acme Thread
Buttress Thread

5. Square Thread
 Strongest thread.
 No radial load.
 Hard to manufacture.
 Much higher intrinsic efficiency
than acme threads.
 This thread is difficult in machining.
 It is the lowest friction and most
efficient thread form, but it is difficult
to fabricate.

6. Acme thread
 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.

7. Buttress Thread
 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.

8. Efficiency of power screw
 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.

9. Efficiency of power screw

10. Efficiency of power screw
Raising the load

11. Efficiency of power screw
lowering the load

12. Efficiency of power screw
Here work output = F. L
Work input = p. πdm

13. Efficiency of power screw
Condition for self locking
• 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.

14. Efficiency of power screw
trapezoidal threads
 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.

15. Efficiency of power screw
trapezoidal threads

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

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

18. Applications of power screws
 Jack screws
 Lead screws of a lathe
 Screw for vices
 Presses
 C-clamps
 Instron material testing machines
 Machine tools (for positioning of table)

19. Applications of power screws

20. Applications of power screws

21. Applications of power screws