TENSION ON PURCHASE PALLS 39

Safe working load about Jxl2D2=6D2, D being the number of
incnes in the diameter of the iron forming the common links, quoted in
commerce in eighths and sixteenths of an inch.

Example —Required the breaking strength, the proof load and the
working load of a f-inch iron chain.

Breaking strength 30D2=30x|xf=17 tons.
Proof load 12D2=12xf Xi=6J tons
Working load 6Z)2=6xf x|=34 tons.

d2
Another rule to find the approximate load for iron chain is— where

d is the size of the chain in eighths of an inch.

d2 4x4
Thus J-inch chain is four-eighths and — =------= 1-6 tons as a

safe working load.

To find the smallest size of chain to lift a given load we can apply the

,________
same rule, namely, load = —.'. d = V 10 X load
Example —Required the smallest size of chain for lifting loads of 2|
tons and 5 tons
d*= 10xload=10x2-5 tons=25 c?=5-eighths
Use a |.-inch chain for 2i-ton loads.
d*= 10xload=10x5 tons = 50 <Z=7-eighths.
Use a |-inch chain for 5-ton loads.
The Frictional Resistance of a purchase increases with the number
of sheaves, and an allowance of about one-tenth of the weight to be
lifted for every sheave in the purchase is usually added to the weight
when estimating the additional force that must be exerted to raise the
weight. The theoretical advantage is found by. counting the parts
of the fall at the moving block. When the single block in a luff tackle
is the moving one it is said to be rove to disadvantage and the power
gained is three, but if the double block is the moving one it is rove
to advantage and the power gained is four; that is to say, a pull, or
force, of 1 ton should balance a weight of 4 tons. But in a lufi
tackle rove to advantage there is the weight of the rope and blocks,
4
also the friction of three sheaves to overcome, so 3 X — tons = -1*2
tons for friction and this should be added to the 4-ton weight, making the
toi#l load on the purchase about 5*2 tons. The weight, however,
will be distributed almost equally amongst the several parts of rope ID