POWER SCREWS

PPT Designed ByMohammedi Limdiwala

INDEX

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

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

TYPES OF POWER SCREWS

Square Thread Acme Thread Buttress Thread

SQUARE 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.

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.

29° included angle. Easier to manufacture. Common choice for loading in both

directions. Faster cutting. Longer tool life.

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

EFFICIENCY OF POWER SCREWRAISING THE LOAD

EFFICIENCY OF POWER SCREWLOWERING THE LOAD

EFFICIENCY OF POWER SCREW

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

EFFICIENCY OF POWER SCREWCONDITION 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.

EFFICIENCY OF POWER SCREWTRAPEZOIDAL THREADS

EFFICIENCY OF POWER SCREWBURSTING EFFECT ON THE NUT 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.

EFFICIENCY OF POWER SCREWCOLLAR FRICTION 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 -

LOAD ANALYSIS

dp

L

N

f

P

F

LIFTING

yx

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.

BALL SCREW

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)

APPLICATIONS OF POWER SCREWS

APPLICATIONS OF POWER SCREWS

APPLICATIONS OF POWER SCREWS

SUMMARY 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.

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