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M Rating

A crane can obviously lift the weight it is rated for. Does this mean that, when choosing a crane of a particular rating, the only difference between one crane and another is the price?

Far from it. There are many other factors to be considered: load weight, load size, typical travel distance heights of lift, frequency of use, shift patterns and special environmental consideration (eg. the weight of magnet permanently attached to the crane).

If any of these factors is not taken into account, the penalty can be a huge increase in maintenance costs and greatly shortened operating life.

Take a ten-tonne safe working load crane in a power station, used for initial plant assembly and subsequently only for occasional maintenance. Compare this with a ten-tonne crane as used by a steel merchant. The steel merchant would require a much heavier duty crane than the power station.

 

Free Consultancy Service

 

As the New Zealand Agency for Street Crane Company, New Zealand's and Britain's leading crane manufacturer, our company provides a free consultancy service applying a computer model to explore the effects of differing loads and patterns of use. Under the British standard BS466 these requirements are expressed as an "M rating".

M ratings range from M3 to M8, with service life doubling with each rating increase. For example if a crane with an M4 rating for a specified application has a projected service life of 20 years, a crane with an M3 rating would have a projected service life of only 10 years in the same application. This is why the selection of a crane with the right rating is an absolutely critical decision.

The power station crane mentioned above would normally have an M rating of three, whereas the steel merchant's crane, with a magnetic lifting device attached, would normally have a rating of six or seven. Physical differences between these cranes would be evident in the size of motors and mechanical components. In fact the M rating may affect virtually all of a crane's components: the size of the wire rope, the drum, bearings, gears, motors, electrical componentry, beams, carriage wheels, etc.

 

Software for establishing M rating

 

Street has developed a software package which analyses the various parameters to be taken into account and from this data determines the optimum M rating for a long, reliable and safe service life. You don't use a Mini to tow a 20' boat, and you don't use an M3 to M5 rated crane in a steel merchant application. We can help you choose the right tool for the task.

The Street software may in fact determine different M ratings for different components of the same crane. For example, a steel merchant may be lifting weights that are generally well below the crane's maximum capacity, but the crane may be travelling long distances to pick up steel from all areas of the warehouse. Thus the M rating for the hoist may be lower, while the M rating for the carriage wheels may be significantly higher.

A complication to be aware of is that cranes of European origin have an 'FEM rating'. This is not the same as the British 'M rating' but the terms used are similar and can lead to confusion.

Overhead Travelling Cranes

Single Girder Cranes

Double Girder Cranes

Goliath / Semi-Goliath Cranes

Jib Cranes

 

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