Ship Winch Project

At Bond Instrumentation - Bond were involved in the HMS Diligent refit and needed to order 4 winch pins at approximately 2000 each. For peace of mind they decided to verify that the winch pins were of the correct size to withstand the forces expected. The pin is shown in the diagram below.

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Analysis.

The winch is a winding system with two diameter cylinders one which is used as the brake, with a brake strap around it tightened by means of a hydraulic ram. This belt provides a frictional resistance to any rotation of the winch when the ram is switched on.

The winch it'self is a cylinder on which the mooring (steel) 'rope' is wound. The maximum wound up diameter (which will produce the greatest mechanical advantage on the winch), is the diameter used to calculated the maximum stresses on the pin.

Engineering analysis to follow... But basically there are two bending regeimes and a cross-sectional sheer stress system to take into account when analysing the maximum stresses in the pin.

The force on the winch pin will be a maximum when there are no forces acting on the ship pulling it away from it's mooring. These would otherwise be forces caused by the wind (windage), tides and a rouge captain placing the ship into full astern drive (very unlikely but you never know). With no load placed on the winch the hydraulic ram will be tightening the brake around the winch drum with no resisting forces and the entire load will be born by the pin.

The point at which the brake belt is about to slip will be the time when the force on the pin is zero.

Considering the first two only (windage and tide): The maximum specification for the winch was that it should withstand 100 tonnes of weight.

The maximum shear stress on the pin will be 100T / A (Area of the pin).