A lifting gear is made up of several components for example a Derrick consists of a Derrick boom, blocks, shackles, wires etc.
SAFE WORKING LOAD – is the stress that a component of lifting apparatus can safely bear in normal use.
BREAKING STRESS OR BREAKING STRENGTH -It is the stress at which component will fracture.
FACTOR OF SAFETY – is a factor used to divide breaking stress to obtain safe working load generally it is 6.
FORMULAS OF BREAKING STRENGTH OF ROPES.
MANILA – (2D^2) / 300.
POLYPROPYLENE – (3D^2) / 300.
TERYLENE – (4D^2) / 300.
NYLON – (5D^2) / 300.
FORMULA OF BREAKING STRENGTH OF CHAIN.
GRADE I STUD – (20D^2 ) / 600
GRADE II STUD – (30D^2 ) / 600
GRADE III STUD – (43D^2 ) / 600
FORMULA BREAKING STRENGTH OF WIRE
6 stad , 12 wires per stand – (15D^2)/500
6 stad , 24 wires per stand – (20D^2)/500
6 stad , 37 wires per stand – (21D^2)/500
When a component of lifting gear is to be tested we cannot tested to its breaking strength as this would destroy the component, so which generally apply only 2 times to 4 times of safe working load.
A purchase is used to advantage when the load moves in the same direction as the effort.
When the load moves in the opposite direction to the effort it is rigged to be a disadvantage.
Mechanical advance = load/ effort = resistance / applied force.
Velocity ratio is the ratio of velocity of effort to velocity of load that is the distance moved by effort to the distance moved by load. It is n+1 for advantage and n for disadvantage.where n is number of sheaves.