How to tackle buying lifting equipment? (I)
How to tackle buying lifting equipment? The decisions a purchaser has to make are similar to those of someone wishing to purchase a car or a washing-machine, having similar questions to be answered.
The purchaser has to choose the product that best suits their needs, in terms of their expectations of quality, features and so on.
Leaving aside the matters of marketing, widely studied and tackled in plentiful bibliography, we will focus on the technical part, always tougher (and obviously, infinitely more boring).
There are several criteria which should be taken into account when specificing the main features of lifting equipment (be it a crane , hook, under hook equipment or auxiliary).
Two of the criteria should be emphasized among them all are :
- The lifiting capacity.
- The classification or group.
The first one looks obvious but on the other hand, the second and often used by manufacturers is not.
Cranes and devices for lifting are used in general industry for lifting and manipulating loads whose mass is in its nominal capacity. It would be easily understood that for machines of equal capacity, according to the sort of utilization or service it will offer, its dimensions will differ significantly. For example: a crane of 120 tons that will work occasionally with a specific capacity, or another one with the same specification that will be used far more intensively.
The design and conceptualization of this kind of device, in general like any other kind of machinery, has to take into account these conditions of work in order to offer an appropriate level of safety and expected life which satisfies customers both in technical and economical terms.
Generally speaking, a large part of existing rules in the industry related to this kind of machinery, work under some type of classification, called ‘Group or Class’.
In practice, these groups are very similar to one another, hence it is usually not difficult to achieve the conversion. This helps the engineeers have a clear idea and a reference that allows them manufacture the device for the conditions that it will be required for.
There are several standard of reference: DIN, FEM, EN, CMAA, OCT, etc. The first two have been used throughout the years with an outstanding success by the European manufacturers. They have been proved very effective tools for the design and manufacture of bridge cranes.
It is important to define these parameters before designing the crane or equipment, as far as it will allow to offer to the customer the best product from a technical and economical point of view. In practice, this classification has got two applications, even though they are intimately related, they have to be considered as different objets:
- Firstly, the classification is used by the purchaser and the manufacturer of the device, who are interested in reaching an agreement on the conditions of service of the device. The agreed classification makes the general classification of the whole device and it serves of contractual and technical reference; but it does not concern to the design.
- Secondly, the classification allows any manufacturer to have a base from which to carry out the design and to ensure that the equipment will have the required life expectancy, for the given conditions of service, both from a structural and mechanical point of view.
The aim of this classification as we mentioned before, is to determine and agree the conditions of the crane’s service and/or individually designed and manufactured accesories, that allows the selection of these machines according to its usage forecast.
The conditions of this service are seen from a general point of view, independently of the type of equipment and its operation. These kind of classifications have proved to be very useful from all 3 aspects: technical, economical and safety. The conditions of the service come mainly determined by the following parameters:
- The number of work cycles during serviceable life of the machine.
- The frequency related to the load to be handled (spectrum load).
Users hope the device will achieve a certain number of cycles of manoeuvres during its serviceable life and this number of cycles constitutes one of the basic parameters of this classification. Sometimes, due to the inner nature of the work to achieve, it is very simple to determine the number of the cycles. For example, in the case of automated cranes and repetitive tasks. Although, in others, it will be necessary to realize some valuations.
Spectrum load is one of the basic parameters of the classification of a crane. It is basically about the number of times a load is raised, of a first-rate order that matches up with the capacity of the device. As it is, it describes the variable load of lifting mechanisms during the movements of work and this has to be calculated and and verified.
Once the types of cycles and spectrum loads are defined, the group is selected according to the appropriate parameters. The exact calculation is often quite complex (to the enjoyment of engineers, computer programmers and alike who like working out these calculations)
The salesmen on the other hand do not tend to spend their precious time in working out these calculations. Instead, they use what generally works, common sense and obviously the manufacturer’s experience, who will advise with good judgement on which type of classification is required in each case.
FEM – 1001 – 101998. Règles pour le calcul des appareils de levage, cahier 2.
FEM – 9511-061986. Classement des mecanismes.
CMMA Specification 70, Revised 1988. Specifications for Electric Overhead
DIN EN 13001-1 -Cranes – General Design – Part 1: General principles and
Group of work: Classification in which a part or the whole crane according to its design and inspection, meeting the engineering specifications. It also serves to determine the PFS (period of sure functioning).
Work cycle: Sequence of movements that starts when the crane is ready to lift the useful load ans finishes in the moment when the crane is ready to lift the following useful load during the same task.
Spectrum load: Basically it is the number of times, that a load of a determined magnitude is lifted, according to the capacity of the device.