Power operated hoist inspection29 September 2004
Derrick Bailes explains how to perform in-service inspections
Power operated hoists are available in a wide range of working loads. In the lower capacities they have largely replaced hand operated blocks for routine repetitive lifting operations, particularly on production lines. They are also used increasingly as portable or semi-portable tools, and are offered by a wide variety of hire outlets or kept available in maintenance stores. Portable equipment is vulnerable to accidental damage during transportation and additional checks are necessary to combat this danger.
As with any equipment about to be inspected, it must be reasonably clean. In a workshop environment this is unlikely to be a problem, but portable hoists used in conditions such as those encountered on a construction site may well be contaminated. Be cautious about the cleaning process used. In general, if the dirt will not wipe off or brush off, consider sending the unit for more thorough maintenance.
Ensure that the local lighting is adequate, particularly for permanently installed equipment. If available, check whether the manufacturer’s instructions make specific recommendations. Be methodical and check the following:
(1) On electrically powered hoists, isolate the hoist and check cable glands for signs of pulling, and cables for wear and damage. If pendant controlled, check that the support cable takes the weight of the pendant box and that the box is secure, undamaged and the button labels are legible.
(2) On portable hoists the power lead usually has a plug connection. However, because the phasing of a socket cannot be guaranteed, the last user may have rewired the plug to make the hoist work in the correct direction. Alternatively, they may have tampered with the pendant. Therefore check both plug and pendant for tampering and, if in doubt, refer to an electrician. It is always advisable to do a regular portable appliance test (PAT) on the electrical system; all reputable hire companies routinely do so.
(3) On pneumatic hoists, check hoses and connectors for wear, damage and security. Check that the supply pressure is in the correct range. This is important as too low a pressure will prevent the hoist lifting its working load and, for hoists without a load limiter, too high a pressure will allow it to be overloaded. Check that there is a water filter/drain and lubricator in place and that both are maintained. Check that the controls are secure and undamaged and that the direction indicators are legible.
(4) Starting at the top, as appropriate check the following for obvious signs of damage, excessive wear, looseness of fixings etc:
(a) The connection to the supporting structure. If the hoist is trolley mounted check the fit of the trolley to the track.
(b) The body of the hoist including casings and covers.
(c) The lifting medium. For wire rope, check for increase or decrease in diameter, opening of strands, kinks, broken wires, corrosion and signs of mechanical damage from external sources such as crushing, cuts or heat damage. Faults are most likely to occur at the terminations and where the rope passes over sheaves, particularly any compensating sheaves. Check the condition of the rope guides and ensure that the rope is laying correctly in the drum grooves. If a lower limit switch is fitted check that, with the hook fully lowered, there are at least two turns of rope left on the drum and that the rope is not slack. If a lower limit switch is not fitted check that when the hook reaches the floor there are still at least two turns of rope left on the drum. Whilst the hook is lowered, check that the falls of rope are not cabling, ie the falls are not twisted together.
For chain, check for wear on the bearing surfaces on the inside of the crown of the links. Chain for power operated hoists is usually surface hardened to give good wear resistance. However, once the hard casing is worn through, rapid wear occurs and this is characterised by a rough appearance of the bearing surfaces. Check also that the chain articulates freely, hangs without twist and is free from kinked or notched links. Damage to links can occur if the chain is misused by wrapping it around the load or loading it across a corner. Check that the chain terminations are secure. In particular check the slack end anchorage. Several designs of hoist utilise a slipping clutch mechanism both to prevent overload and act as a hoist/lower limiter. If the chain is fully run out, it is vital that the slack end anchor is strong enough to cause the clutch to slip. If not the chain will simply run through and the load will drop. If the hoist has a load chain collecting box, ensure that when the hook is fully raised it can accommodate all the chain without spillage and with enough space above to allow the chain to feed smoothly into the chain guide when lowering.
(d) The bottom hook and sheaves. Check the hook for signs of opening out, ensuring the safety catch is present and fits correctly. Check the swivel for wear. On outdoor hoists, if the bottom hook has a threaded shank and nut, check for fretting corrosion which can occur between the nut and shank due to ingress of water. This can lead to the thread stripping and the load dropping.
On a wire rope hoist, check the sheaves for security, damage and excessive wear. On a multi-fall chain hoist, check that the chains are not twisted. This can occur if the bottom block is turned over between the falls of chain.
(5) Operate the hoist under load, preferably close to the working load but not an overload. Check that the controls and limit switches function correctly.
Observe whether the hoist operates smoothly. On a chain hoist, operate it through the entire range of lift because, if the hoist is regularly used within only a small part of the range, the chain may be more worn at that point. For smooth operation, the chain must mate accurately with the loadwheel. Over time they tend to wear together and retain a reasonably accurate fit. However the unworn portion of chain will not run smoothly in a worn loadwheel and will show similar symptoms to a worn chain in an unworn loadwheel. The symptoms are a cracking noise as each link enters or leaves the loadwheel.
Observe the operation of the brake. On release of the control button, the brake should promptly arrest a load being lowered without visible slip. When restarting to hoist a suspended load there should be no visible slip back.
A particular problem is known to occur with a type of wire rope hoist called a scaffold hoist. As the name implies, these are generally installed on scaffolding at building sites to lift materials etc. The problem is that they can fail to restart hoisting a suspended load and may even allow it to slip down. This is because they are single phase 110 Volt units and consequently require a high current to start. They are usually supplied via a transformer from a 220 Volt supply and the transformer is not of adequate capacity or the supply cable is too long with a consequent voltage drop. When lifting from a surface, the initial slack allows the motor to start turning before the load is lifted. Once suspended, although there is sufficient power to release the brake, there is insufficient to provide the necessary torque. The solution is to ensure an adequate power supply.
(6) Finally, check that the SWL or Working Load is clearly displayed and the ID mark is legible.
If any of the above faults are found the hoist should be withdrawn from service.