(^196) „„„„„ A Textbook of Engineering Mechanics
Velocity ratio of the machine
We know that velocity ratio of a worm and worm wheel,
250 60
V.R. 75
22250
DT
nr
×
== =
××
Ans.
Effort required to lift the load
Let P= Effort required to lift the load.
We also know that mechanical advantage,
300
M.A.
W
PP

and efficiency,
300
M.A. 4
0.5
V.R. 75
P
P

or
4
8N
0.5
P== Ans.
11.8.WORM GEARED PULLEY BLOCK
Fig. 11.6. Worm geared pulley block.
It is an improved form of worm and worm wheel, in which the velocity ratio is intensified with
the help of a pulley block. It consists of a square threaded screw S and a toothed wheel. One end of the
string is fastened to the frame. This string passes round the load drum B and then over the pulley A
(which is keyed to the wheel). The other end of the string is also fixed to the frame. The effort is
applied to the wheel C by a rope as shown in Fig. 11.6.
Let D= Diameter of effort wheel,
r= Radius of pulley A,
W= Load lifted,
P= Effort applied to lift the load, and
T= No. of teeth on the worm wheel.