# Engineering Mechanics

(Joyce) #1

Chapter 11 : Simple Lifting Machines  213

11.17. WORM GEARED SCREW JACK

It is a further improved form of differential screw jack, in which the velocity ratio is further
intensified with the help of a geared screw jack.

Fig. 11.16. Worm geared screw jack.
In Fig. 11.16. is shown a jack with worm geared screw. It is also an improved form of screw
jack, in which the screw is lifted with the help of worm and worm wheel, instead of effort at the end
of a lever. Now consider a worm geared screw jack.
Let l = Radius of the effort wheel (or length of the handle).
p = Pitch of the screw,
P= Effort applied to lift the load, and
T= No. of teeth on the worm wheel.
We know that distance moved by effort in one revolution of wheel (or handle)
= 2πl ...(i)
If the worm is single threaded (i.e. for one revolution of the wheel A, the screw S pushes the
worm wheel through one teeth) then the worm wheel move through 1/T revolution. Therefore distance
p
T

``= ...(ii)``

``∴``

``````Distance moved by the effort 2 2
Velocity ratio =

``````llT
p p
T``````

``````ππ
==``````

``Now M.A.``

``````W
P``````

``= ...as usual``

and efficiency,

``````M.A.
V.R.``````

``η= ...as usual``

Notes : 1. If the worm wheel is double threaded i.e. for one revolution of wheel A, the screw S pushes
the worm wheel through two teeths then

``````2
V.R.
2``````

``````lT
p``````

``````π
=``````