VLC’s (vehicle loading cranes) are measured with the term Tonne Metre (T.M.). This is a measurement of force rather than weight.
The rated load x rated distance = the Tonne Metre capacity of that crane.
For example: You will notice that all the weights on the load chart x the distance will come up with a similar figure – just dropping slightly the further out you go.
ie. 0.95T x 2.26m = 2.15 Tonne Metre & 0.5T x 4.2m = 2.10 Tonne Metre
Why is this so?
A crane must lift the weight of its boom before it lifts the load. So the longer the boom, the greater the ‘fall off’ of Tonne Metre at reach.
Don’t buy more boom than necessary for your work. It costs money to buy, uses fuel and payload to carry and reduces the lift capacity of your crane.
One example of this is a typical 30TM crane with 3 boom sections reaching around 10.6m – such a crane has a capacity of 29TM.
With 6 boom extensions, reaching around 17m, the same crane is now down to 27.4TM – and weighs 440kg more.
Caution: Heavy (in close) lifts have a complication.
If we say that we need to lift 2 tonne at 3 metres, we would look for a 6 tonne metre crane. But with high or long slung loads, this calculation can lead to an embarrassment!
You would need a crane rated at 6 tonne metres or better to do the job BUT it is important to consider the physical size of the load you are lifting. For example, look at Figure 1 below.
To handle a high load of near crane capacity, the inner boom must be vertical (or near to vertical), and the outer boom pointing down, with the hook above the centre of the load.
In this configuration, only the outer boom can be raised. The operator may not get sufficient hook height to clear the base of the load over the tray body.
The resulting increase in the radius can be substantial and is often sufficient to over pressurize the luff rams and to activate the overload protection (if fitted) which may prevent the crane moving into the required position.
With our example of 2 tonne at 3 metres, if the load or sling length causes the hook to be so high that the inner boom cannot be used for the lift, then probably an 8 tonne metre crane would be the size required.
A winch rope crane will give a vertical lift as seen in Figure 2 (therefore no increase in radius), so this consideration is not applicable to UNIC & Cobra telescopic boom cranes fitted with winches.