40 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
the purchase at the moving block. There are four parts in the lufi
5-2
tackle when rove to advantage so — tons gives 1 & tons as the load on
4
each part of the purchase.
nW
The weight + friction may be expressed as W -\—— where W is
the weight to be lifted and n the number of sheaves, thus in the above
nW 3x4
example, W + — == 4 H-----— = 52 tons.
Example.—A weight of 4 tons is to be lifted with a gun tackle, find
approximately the pull on the hauling part if rove to disadvantage.
Ans. There are 2 sheaves and 2 parts of rope (Fig. 1). Total
load=weight+friction=TF+—-=4H—~- =4-&tons. Pull on hauling
4-8
part = —— = 2*4 tons.
2
"There is less friction with sheaves of laiger diameter than oi
smaller diameter and with thin rope than with thick rope, so
the maximum advantage is gained by using large sheaves and
strong small sized rope. Fast winding adds to the tension on
each part of rope and there is less tension when lowering the
weight than when it is merely hanging on the purchase."
' Exercises.
1. A weight of 12 tons is being lifted with a three-fold
purchase, find the total load and the pull on the hauling part, when
(a) rove to disadvantage, (b) rove to advantage.
Ans 19-2 tons, (a) 3-2 tons, (&) 2-75 tons.
2. Find the total load on a double purchase and the pull on the
hauling part of the fall when rove to advantage and lifting 20 cwt.
Ans. 28 cwt. and 5*6 cwt.
3. A treble purchase rove to advantage^ lifting 40 tons. Find the
total load and the pull on the hauling part,
Ans. 64 tons and 9y tons.
4. Lifting a weight of 15 tons with & lufE tackle rove to dis-
advantage, find the total load and the pull on the hauling part.
Ans. 19-5 tons and 6-5 tons.