Discard criteria

A load-bearing element is not replaced when it fails but when it reaches a defined limit – well before fracture. That limit is the discard criterion. It is set so that a margin remains between it and failure, even if the damage continues to grow between two inspections.

“It is still holding” is therefore no argument. The question is not whether the element carries the load today but whether the margin to failure is still the one the hoist was designed with.

A load chain wears where the links bear on each other. Each link becomes thinner at the bearing point, and the chain becomes longer. Two things are therefore checked: the pitch – measured on a single link and over several links – and the mean link diameter at the worn point.

For link diameter, a common limit is a reduction of the mean diameter by ten per cent against the nominal size. The permitted elongation is stated in the operating manual of the hoist: it has to match the chain wheel, and an elongated chain no longer seats properly in its pockets. Regardless of any measurement, a chain must be discarded if it shows cracks, bent or twisted links, notches or severe corrosion.

A wire rope announces its end: individual wires break long before the rope parts. ISO 4309 specifies how many visible broken wires are permitted over a reference length – six times and thirty times the rope diameter. The number depends on the rope construction and is not a single fixed value.

In addition, the reduction in rope diameter, corrosion and deformation are assessed. A broken strand, birdcaging, a kink or a bend means immediate discard without any counting.

An overloaded hook opens up. The throat opening is therefore measured between two marks and compared with the original dimension; an increase of more than ten per cent means discard. Wear in the hook saddle, cracks and notches and the function of the safety latch are checked as well.

Discard is established at the periodic inspection by a competent person. Obvious damage, however, is already noticed by the operator during the visual check before starting work – and the same applies then: the hoist stays out of service until the element has been replaced.

Replacement is by an element of the same type and grade as specified by the manufacturer; with chains, normally together with a check of the chain wheel and chain guide. The finding and the replacement are recorded in the inspection records of the equipment.