38 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
where C is the size of the wire and this fits in quite well with the ultimate
breaking stresses given for rope having 12 wires in each of the six
strands as given in Bullivant's table, but 3C2 gives a nearer approach
to its breaking strength when there are 24 wires per strand and 3JC2
for the extra special rope with 37 wires per strand as indicated in the
table.
Q What would be the breaking stress of 2-inch wire ropes having
12, 24 and 37 wires in each strand?
Ans (12 wires) 2C2, 2x2x2=8 tons.
(24 wires) 3C2, 2x2x3=12 tons.
(37 wires) 3JC2, 2x2x3|=13 tons.
When referring to the breaking strength of wire rope we 'shall
assume the rule 2C2.
Example—Find the ultimate breaking strength, also the safe
working loads, of (a) 3-inch and (b) 3J-mch wire ropes.
(a) Breaking strength 2C2=2x3x3=18 tons.
2C2 18
Working load -—— = — =3 tons.
6 6
(b) Breaking strength 2C2=2x3|x3£=24£ tons.
2C2 49
Working load —- = —-- = 4-373- tons.
6 12
Example.—Find (a) ultimate breaking strengths, also (b) the safe
working loads of 2-inch, 4-inch, and 4J-mch steel flexible wire rope for
continuous working.
Ans. 2-inch wire (a) 8 tons, (b) 1J tons.
4-inch wire (a) 32 „ (b) 5J „
4t-inchwiie(a)40i „ (b) 6| „
Chain is made from steel or iron bars, forged or cast, and built tip
link by link, every part of guaranteed chain being tested as there is
always the possibility of a chain having a link improperly welded*
burnt or otherwise defective, and this can be detected only by testing. <
The breaking strength of close link cargo chain is about twice its
proof load and the proof load is from 2 to 2J times the average working*
load. The proof load is the stress applied to the chain when testing it
in a Proving House. The size of chain is the diameter of the iron
bar forming the link.
Breaking strength is about 30 D2
Proof load about 12D2