Engineering Chamber

Cracks in Lifting Machinery.

By RT Vogt - Pr.Techni.Eng.

Problem Identification

The ores of most useful metals are seldom found on the surface of the earth – they are mostly found in soil or buried beneath the earth, even at times at considerable depth. If indications of their presence do not exist on the surface and are not discovered by accident, they are sought for by intelligent inquiry. The methods of extracting and raising them to the surface must be likely to produce a profit for the owners of the land. When found and made use of is another matter.

Technology is defined in the Oxford dictionary as “Science of the Industrial Arts”. Modern mining technology is highly involved and advanced as it relies to a marked degree on complex and sophisticated machinery.

These machines require vast amounts of finance to purchase, operate and maintain. It therefore makes sense for the customer to expect a long and trouble free service life.

The real world we live in has, however, taught us differently. Even the most expensive and robust machine can break down, usually at a time in direct proportion to the inconvenience it can cause.

On such occasions that the familiar comments are made:-

· Why does it always break in the same spot?

· The steel is still too weak, last time we doubled the thickness. This time we will use stronger steel.

· I give up! Every time we weld it, it breaks again – next to the weld.

· They always last for a few month and then all crack in the same area at the same time.

· These cracks must have some intelligence, as soon as the warranty expires; they appear as surely as head office cuts the budget.

The fundamental dilemma that has arisen can probably be found in the following argument: -

Modern steel-making practices such as vacuum arc degassing, controlled rolling, quenching and tempering have resulted in a family of steels with excellent properties, which can be guaranteed by the steel makers. Modern design aids such as CAD, finite element analysis and dynamic simulation on computers permit designers to push the steel to its limit. In designs for cranes and lifting machines allowable stress and limit state methods are used. It would seem that modern fabrication technology has not kept pace with steel-making and design improvements in the relevant industries. In addition it would be fair to say that designers do not always pay enough attention to small details and, at times, design errors inadvertently creep into a machine.

A very effective way of overcoming such problems is to systematically eliminate them by ”reverse engineering” i.e. “see where it went wrong, then fix it. This is of course not the ideal solution to problems. Such solutions should, however, be well engineered, fully documented and fed back to the original designer to be incorporated in present and future designs.

As far as cracks are concerned, the ideal situation would be to never have cracks in the first place. This is possible, but highly unlikely. If it is accepted that cracks do occur, it is necessary to understand why they occur so that effective corrective action can be implemented. Repairing cracks blindly or replacing cracked components with components of identical (non-original or pirate) design may simply postpone the problem and quite often will lead to an even worse situation than before.

Again many questions are asked:-

· How do cracks start and are all cracks bad?

· When does a discontinuity become a crack?

· Why are some cracks straight and some zigzag?

· Why do some cracks progress slowly and others propagate rapidly?

· Can cracks be welded up safely?

· Small cracks are harmless, big cracks are dangerous. Is this true?

· How good is a repaired crack?

· Can we use patch plates over cracks?

An attempt may be made to answer some of these, and many similar, questions in a further article on cracks and propagation thereof.

To summarize the identification of the problems:

Cracks in lifting machine structures, or in other lifting equipment components, can be precluded by extremely careful design and manufacture.

Cracks occur in service. The problem is what to do about them:

· Discard the item or lifting equipment/machine? Too expensive!!!

· Leave as is? May lead to unsafe operating conditions!!!

· Repair? Yes/may be, but will it last? Is it worthwhile?

· Tell the manufacturer/supplier about the problem and hope for the best? Not really acceptable since the supplier may have built the problem in – in the first place!

Methods and efficiency have improved extensively from the days of manual lifting machinery and equipment, with modern technology development. In recent years, requirements for greater structural efficiency and improved operational performance have inevitably led to situations where many components or structures are operating at virtually the mechanical limits of the material.

The demands imposed on such lifting equipment/machines by increased load capacity, speed of operations, as well as temperature extremes, corrosive environments and severely fluctuating operational loads have often increased the probability of deterioration and/or failure. These failures can often be attributed to the insidious undetected development of cracks in key components. This also frequently occurs through metal fatigue, leading to sudden unexpected fractures. Indeed it has been reported, and is also the view, that over 80% of failures that occur in major lifting equipment and other plant can be attributed to fatigue cracking and also incompetent use / operation of the equipment.

Such large scale lifting machinery/equipment is, however, very cost effective and efficient as long as it is utilized safely and proficiently by registered operators. Catastrophic failures of key components due to due to cracking could be avoided, if said equipment is correctly handled by qualified and competent persons.

There are several common factors affecting lifting equipment/machines, front end loaders, knuckle boom cranes (lorry loaders), mobile cranes, fork lift trucks etc. The common factors are those that lead to the problems of cracking and include the following:-

· All (or most) made of steel.

· Frequently comprise cast and forged metal components (and may thus have inherent defects).

· Almost all fabricated by welding (the quality of which is sometimes variable).

· All or most have complicated shapes, and changes in sections.

· Frequently no gentle transition in section changes; rarely perfectly smooth surface finish of components.

· Often machined to final size and shape (holes or sharp corners are often machined into a component).

· In service, all operate by carrying live loads, or carrying a product – resulting in significant load fluctuation (and hence cyclic stress).

· Exposure to extreme temperatures.

· Plant operated by people who are not infallible or competent.

· Lack of maintenance and compulsory 12 monthly inspection/testing as prescribed by the OHS Act – DMR 18.

· All go wrong at some time or another and need repairs.

Plant operation and recapitulation of some basic concepts: -

In view of the common features itemised above, it would perhaps be surprising if lifting machines/equipment did not exhibit defects or develop cracks during their service life. To extend this idea it is worthwhile to consider the operation of some large item of lifting machinery.

Such equipment would stand on foundations or feet or have some substantial ground connection. It would carry loads at various places, the most serious usually being the payload at some extremity and which is invariably a fluctuating load. Such a load causes both direct stress denoted by ό, and a cyclic stress, Λό. We recall the relationship between stress, ό, in Mpa as the force (in kN) per unit area (in mm sq.).

All formulation is covered in most engineering books or possibly in another technical article.

It is frequently from such stress concentration regions that the local stress exceeds the local strength on a very small localized area and fatigue cracks can initiate. With repetitive loading, such cracks can propagate by fatigue and may lead to catastrophic failures if not detected in time. Impact loading can lead to very large and very sudden stresses, which can further lead to unsuspected fast fractures.

Thus it becomes important to characterize the various parameters that will help us to understand how cracks develop in lifting machines/equipment and how we can safely “live with defects” (in certain cases only). In addition we need to recognise the importance of stress concentration factors; the effect of poor welding and weld defects; the true meaning of fatigue; what is the “toughness” of a steel (its resistance to crack propagation) and how do we measure it. More particularly, how big a crack can be tolerated and / or how long will it last before failure is catastrophic. Consequently how often should critical areas be thoroughly inspected to prevent such fatigue / stress failures?