A variety of safety devices exist to limit the amount of movement of a lifting appliance, protecting it from the worst effects of overloading or to protect the appliance itself. If a device is essential for safe use, it will be fitted as standard. But if it provides additional features for safety, it may only be an optional extra. If so, check the manufacturer’s specification.
Motion limits
Power operated wire rope hoists are required to have an upper (hoisting) limit. This prevents the bottom hook travelling up into the hoist body, resulting in the wire rope breaking and the load being dropped.
As for the bottom limit, prior to modern requirements it was not a mandatory requirement, but if a hoist without a bottom limit is over-lowered, the rope can wind onto the drum in the reverse direction. This will result in the reversal of motion control sense and the non-operation of the upper limit. Similar limits are advisable on power operated chain hoists.
Most in-service power operated wire rope and chain hoists have both upper and lower limits as standard, but in the rare case of a power operated hoist found in-service without a bottom limit, it is advisable to fit one. However, unless the power operated hoist is modified or taken into a new undertaking, this advice is not mandatory and for winches not normally fitted with motion limits, additional precautions are necessary.
To prevent over-winding, ensure that the bottom hook reaches the floor before the rope is fully paid out and that the rope is marked with a visual indicator, such as a coloured marker, to warn the operative that the maximum travel has been reached.
Safety devices are becoming more advanced and in many cases they can be retrofitted to existing lifting appliances. It is worth continually assessing older in-service equipment and fitting such devices appropriate to any identified risk.
Hoisting and lowering limits are a form of safety device intended for occasional use only. In some cases, limits may require manual resetting before further operation is possible. They must not be used as a means of positioning the bottom hook. In applications where this is necessary, such as a production lines, additional limits – known as working limits – should be considered. These are of a heavy-duty type intended for regular use and are available as an optional extra.
For power operated travelling appliances, the runways and tracks to which they are fitted must be provided with positive end stops to prevent the appliance running off the end of the track or colliding with the supporting structure.
Anti-collision and approach limits
Where more than one travelling appliance is fitted to the same track, it is necessary to prevent collision or it may be necessary to prevent more than one of the appliances entering a specified area of the track. This can be achieved in several ways, the choice of which will to some extent depend on the application.
Manually operated travelling appliances, and indeed certain power operated appliances, can be fitted with extended arms and simple buffers that limit the approach of the appliances. To prevent collision or limit the approach of power operated travelling appliances, sensor switches may be fitted to the appliances. Various types are available ranging from simple trigger switches, which are mechanically operated to disconnect the power and isolate selected areas of the track, to infrared and microwave sensors that disconnect the approach travel motions when the set distance between the units is reached. These devices permit the movement of the appliances in the reverse direction and automatically reset once the distance between the units exceeds that for which they are set.
Overload protection
The simplest form of overload protection used on manually operated appliances is the shear pin. This is used on lifting and pulling machines that use a gripping action on the wire rope and similar appliances to prevent excessive effort being applied to the operating lever. Slipping clutches are sometimes used in power operated chain hoists and may also be found on some manually operated equipment. They are set to slip when the load increases beyond a predetermined amount. Slipping clutches are also used in some designs of lifting appliances as the upper (hoisting) limit, thereby serving a dual purpose.
A wide variety of overload protection devices are used to protect the electrical systems of electric power operated appliances. They operate to disconnect the supply if physical or electrical overloading occurs. In some cases, it may be necessary to reset the device manually before further operation is possible whilst others automatically reset after a period of time or when the overload is removed. The primary purpose of electro-magnetic and thermo-electric devices is to prevent damage by electrical overload. They will, therefore, permit an excessive load to be lifted for some distance before they operate.
Load measuring or sensing devices are used to prevent physical overload by stopping the appliance operating if the load exceeds that intended. Since improvements to safety legislation and standards, they have become a standard feature of many appliances.
In all cases of electric power operated appliances, fuse protection must be provided in the supply system.
Safety in lifting operations is paramount and these devices are key allies, together with adhering to regulations and guidance, in ensuring the prevention of accidents, injuries and equipment damage during the handling of heavy loads. For further guidance consult the LEEA Code of Practice for the Safe Use of Lifting Equipment (COPSULE), which can be accessed for free at www.leeaint.com.
