Heinz-Gert Kessel, tower crane correspondent, Cranes Today, Germany

Unlike mobile cranes — where the maximum capacity without correspondence to any working radius is used — for tower cranes, the load moment related tm class is used. This factor simply multiplies the maximum lifting capacity with the associated radius. For example, the Wolff 100B is placed in the 100tm class, as it lifts a maximum of 2.5t at 40m radius.

This does not necessarily indicate the maximum load moment provided by the crane, which is made up of the maximum radius at which the maximum capacity can be lifted with the shortest jib and maximum falls. For example, the Wolff 100B luffing jib crane can handle 6t at a maximum radius of 24m. Therefore it provides a maximum load moment of 144tm or 1440kNm.  However as the name 100B indicates, many tower crane manufacturers choose type classifications related to the load moment for their crane range, giving a rough orientation about the capacity of the crane.

Carsten Bohnenkamp, technical director, Potain, France 

There is no industry standard for the tonne-metre ratings! 

The only and best way to really compare cranes, is to compare their load charts and judge if the crane will fit to the customer needs. Fifty years ago some German crane manufacturers created a definition that made it possible for them to compare tower crane performance. 

It rates a tower crane based on the intersection of the load moment curve with an arbitrary chosen logarithmic line, defined as A=20*LOG(M-2) where M equals the nominal load moment, and A is the load radius.

The names and numbers chosen for tower cranes, such as the MDT88, do not refer to these methods. They are chosen for marketing reasons and to position a new product in our product range.