Double Purchase Crab Winch
A double purchase crab winch is an intensified design of a single purchase crab winch, to obtain a higher value of velocity ratio.
In a double purchase crab winch, there are two spur wheels of T1 and T2 as well as two pinions of teeth T2 and T4. The arrangement of spur wheel sand pinions are such that the spur wheel with T1 gears with the pinion of teeth T2. Similarly, the wheel with teeth T3 gears with the pinion of teeth T4. The effort is applied to a handle as shown in figure.
Let T1 and T2 = No. of teeth of spur wheels,
T2 and T4 = No. of teeth of the pinions,
l = Length of the handle
r = Radius of the load drum
W = Load lifted, and
P = Effort applied to lift the load at the end of the handle.
In one revolution of the handle, distance moved by the effort
= 2 π l
∴ Number of revolutions made by the pinion 4
= 1
And number of revolutions made by the wheel 3
= T4 / T3
∴ number of revolutions made by the wheel 2
=T4 / T3
And number of revolutions made by the wheel 1
Distance moved by the load
Now M.A. = W / P
and efficiency, η = M.A. / V.R.