Lifting beams and spreaders

The terms 'lifting beam' and 'spreader' are often used interchangeably but they do have specific meanings and different applications. Lifting beams are designed to be loaded in bending. A simple lifting beam will have an eye or link on the top side to connect to the lifting machine hook and two or more lifting points on the underside to connect to the load. They are ideal for lifting loads which are too weak or flexible to be lifted without support. For example they are often used in combination with vacuum lifters or lifting magnets to support long thin sheets or plates. They are also used when the headroom available is insufficient to accommodate slings, about which, more later.

A spreader is designed to be loaded as a strut in pure compression. It gets its name from the earliest designs which were used, quite literally, to spread the legs of a sling. Typically a simple spreader for a chain sling comprised a bar with a fork at each end. The fork engaged with the chain and held the bar in place. It could be used, for example, with a two leg sling to lift a cable reel by its axle and hold the legs apart to prevent the reel flanges being crushed.

Modern spreaders are generally a lot more sophisticated and usually have separate slings to connect to the lifting machine and the load. However the principle remains the same, that being that the spreader is loaded in pure compression. Spreaders generally need a lot more headroom than a lifting beam in order to accommodate the suspension slings but, relative to working load and dimensions, have the advantage of being lighter in weight.

There are also hybrids where the beam is suspended from a multi-leg sling which connects to the beam inboard of the lower attachment points. The tension in the sling legs exerts an inward compressive force on the section of the beam between the attachment points whilst the leverage from the lower attachment points exerts a bending force on the beam. There are several reasons why this design might be chosen. One is that it makes the beam more stable to lift when unladen. Another is that it can reduce the loads in the beam, particularly if there is one or more lower attachment points within the centre section. However it does increase the headroom required.

Lifting beams and spreaders can facilitate the lifting of loads not possible by any other means but there are some limitations and pitfalls to watch out for. One major consideration is load stability. Usually, with a sling, the connection to the load is made at or near the top of the load or by wrapping around or passing through the load. This generally means that the connections are above the load's centre of gravity, thus ensuring stability. Lifting beams and spreaders enable the connection to the load to be made to the sides or even the base of the load at points below the centre of gravity.

This might seem an attractive option for any load with upper parts which might be easily damaged, such as a machine tool or a skid mounted generator. However caution is required. First there needs to be at least three connection points and, when viewed in plan, the centre of gravity must lie within the area bounded by them. When viewed from either side, the load, the lifting beam or spreader and the slings which connect them form four sides of a rectangle. I mentioned earlier that lifting beams are often used when the headroom available is limited. Figure 1a illustrates the side view of just such a situation coupled with connection points below the centre of gravity. Essentially it is an unstable mechanism because the centre of gravity of the load will naturally try to be as far away from the crane hook as possible (as shown in figure 1b). Quite simply, the load will turn over. Figure 2 illustrates why. If A were greater than B it would be stable.

Another major consideration is load distribution. Whenever a load is supported at several points there is likely to be a degree of inequality in the share of load imposed on each. The likely variation should be taken into account when specifying or selecting the equipment. If the load is rigid, then some flexibility of the lifting beam may be desirable unless fine adjustment of the connections, such as may be achieved with rigging screws, is included.

In operation, spreaders have fixed dimensions, chosen to suit the particular application. They can either be made to measure or assembled from a kit of standard modules.

Lifting beams designed for multiple applications may have adjustable lifting points. The adjustment can be either in fixed increments or variable. Whichever method is used it should incorporate a suitable means of locking them into position when they are under load. If the locking is done manually, there should be a clear visible indication of whether the lock is engaged or not.

Lifting beams, and to some extent spreaders, may be designed to facilitate orientation of the load. It may be as simple as lifting upright a load stored flat or it may require an additional mechanism to generate the required movement. Whichever is the case, the design should allow for a tolerance on the intended orientation. Even a load intended to be lifted level cannot be expected to be perfectly level at all stages of the operation. As a general rule, an error of up to 6 degrees from the intended orientation should be allowed for. This is sufficient to be clearly visible to the operator in time to take corrective action before it is exceeded.

These and other design requirements for lifting beams are specified in the European standard, EN 13155 for non-fixed load lifting attachments. I strongly recommend that it is specified when purchasing them.

A few other points to consider. When lifting beams or spreaders are set down after use, they should be stable and it should be possible to disconnect them safely from the crane hook. Lifting beams with an integral lifting eye in particular are vulnerable to falling on their side during this operation unless supported. Depending upon the weight of the beam and its shape, special storage stands may be necessary.

Whilst considering the weight, remember that it is part of the total load the crane will lift. It is good practice that, as well as marking the beam with its working load, it is also marked with its own weight.

Lifting an unladen lifting beam or spreader can also bring its own risks, particularly if it is not balanced in the unladen state. Long beams in particular may need a tag line to keep them under control.

When using a lifting beam or spreader with an overhead crane, beware of fouling the crane bridge, especially on double girder cranes where the hook rises between the girders. It may be necessary to set the upper limit switch accordingly or fit a secondary switch. Similarly, when used with a mobile crane, if the load rotates, the beam may strike the jib if not restrained by a tag line.

Despite these few limitations, lifting beams and spreaders can provide an efficient and safe method of handling many loads which would otherwise prove impossible to lift.