Power screw

PRESENTED BY:- Roy raj jyoti

Power Screws

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

INDEXPower ScrewTypes of Power ScrewsLoad AnalysisEfficiency of power screwThreadsApplications of Power Screw

Types of power screws

Square ThreadAcme ThreadButtress Thread

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.

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.

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.

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 loadLowering the load.

Efficiency of power screw


Raising the load


lowering the load


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


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.

Efficiency of power screwtrapezoidal 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.


trapezoidal threads

Load Analysis

dp

L

N

f

P

F

LIFTING

y

x

pdL

tan

LdLdPd

Tp

ppSu

2

Threads

p pitch in./threadd diameter (major) in.

dp pitch diameter in.dr minor diameter in.

L Lead in.

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)




Thank you

Engineering

Power screw