CHAPTER m.
STRENGTH OP ROPE,
THE teim stress denotes the load put on material, and strain is the
molecular disturbance made evident by a change of shape or a fracture
of the material due to the stress which has been applied.
Stress comes before strain and the transition from stress to strain '
introduces another factor called the "modulus of elasticity," Young's
modulus=stress divided by strain, within the limits of proportionality.
The term breaking or ultimate strength is the load or weight applied
to material when testing it to destruction.
Rope is made of hemp, manila and coir, their relative strengths
being in the order named. Splicing a rope reduces its strength about
one-quarter, and three stranded ropes are stronger than the corres-
ponding size of four stranded ropes.
No rigorous rule can be laid down to arrive at the ultimate breaking
strengths of different sized ropes as so much depe&ds upon the quality
of the natural fibre and the process adopted in its manufacture. The
size of a rope is expressed in terms of its circumference given in inches,
and a fair estimate of the breaking strength of good, honest hemp or
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manila is obtained from the formula —, where G is the size of the rope,
o
Nor can a hard and fast rule be laid down to estimate the safe
working load for a given size of rope, but one-sixth of its ultimate
strength offers a good factor of safety in order to resist excessive stresses
due to sudden jerks on the fall. When an occasional lift is made there
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is not so much wear and tear on the gear and — may *be accepted as
giving a safe margin. /
Ewmpk—-Given a 3-inchmanila rope, estimate its ultimate strength
and. safe working loads.
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Ultimate strength=—=1=5 3 tons.
3 3
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