Time for an air lift?
Pneumatic power operated hoists have advantages over their electric counterparts in particular applications but while they have similar safety requirements, there are some areas requiring special attention, writes Ben Dobbs, Head of Global Standards and Legislation at the Lifting Equipment Engineers Association (LEEA).
Where a suitable air supply is available pneumatic power-operated hoists have their advantages over electric-powered hoists, resulting in their wide use, particularly in industries where electrical equipment could create safety risks. This includes hazardous and explosive environments because they do not generate electrical sparks. They can operate continuously with minimal heat buildup, which is useful in high-volume manufacturing. Without electrical components at the point of operation, pneumatic hoists perform in areas exposed to water, steam, salt spray and corrosive chemicals. Operators can also achieve smooth starts and stops with excellent load positioning, which is particularly useful for equipment assembly.
A pneumatic hoist works by directing compressed air from a plant air supply through control valves into an air motor. The motor drives a gearbox and lifting mechanism, such as a chain or wire rope, enabling the hoist to lift, lower or even traverse loads safely and precisely – without the need for hydraulics or electricity. While they have generally similar safety requirements to their electric counterparts, there are some areas requiring special attention.
Lower capacity units generally use vane motors for drive, resulting in a more compact hoist than its electric-powered equivalent. Piston motor versions are better suited for heavier duties. Like electric hoists, either link chain, roller chain or wire rope can be used as the lifting medium.
Control options
Controls are generally a matter of personal choice. Cord control is the most basic and cheapest option. Here, the hoist control valve is fitted with a double lever arm from which rope pull cords are suspended. These normally connect to a common control handle. Pulling the appropriate cord opens the valve so that the air flow is in the required direction to raise or lower. With experience, an operative can learn to control the hoist and lower motion speeds within fine limits, but this is more easily done with the twist rod control. Fitting a hoist control valve with a suspended twist rod directly opens the valve, allowing the air to flow in the required direction. Cord control is more easily operated by accident than the other types of control.
The pendant control is the most expensive. Supply hoses, suspended direct from the motor supply ports, connect to a hand-held control valve unit. By opening the valve in the required direction, air is supplied to the selected motor supply port. Two switch-type valves clearly define the direction of operation. Hand-held casing protects against inadvertent operation.
Exhaust
Standard pneumatic hoists exhaust used air via a series of vents. This can be noisy, though a baffled exhaust vent will reduce the sound, and the release of air under pressure may also be hazardous in some applications. It is also possible to duct the exhaust air away from the unit. Discuss this with the supplier, as the provision of unsuitable equipment may affect the hoist’s performance. The equipment design will depend to some extent on the installation.
Air supply
The manufacturer’s specific recommendations should be followed in respect to the supply pressure and delivery rate – these are usually based on 6 bar (90 psi). A pneumatic hoist’s working capacity and operational speed relies on the correct pressure and delivery rate of air supply. If the pressure falls below the hoist’s design specification, the operating speed will be slower than intended and the actual lifting capacity will be lower than the hoist’s standard rated capacity. If the air pressure is increased above the manufacturer's recommendation, while it can lift more weight than its rated capacity, this creates a potential danger and must be averted. A pressure regulator in the air line near to the hoist entry port will achieve this.
Air motors are self-purging and as a result, any lubrication is lost with the exhausting air. Oil mist lubricators are available, and they allow the incoming air to collect lubricant and pass it directly through the motor.
Given that compressed air is moisture laden, condensing water will cause corrosion so steps should be taken to prevent this. Similarly, other contaminants should be removed from the air feed. It is important to consider suitable filters/drainers, which should be placed in the air line before the oil mist lubricator.
Fixed suspension hoists may be connected directly to the air line with suitable rigid tubing, but it may be desirable to have a short flexible hose and bayonet connector feeding to the hoist to enable it to be readily removed for maintenance or storage purposes. Travelling hoists require flexible hose supply lines – often coiled hoses. These can be restrictive due to hose loops occupying considerable space and placing a drag on the hoist, which in severe cases can cause the travel to be arrested or even cause the hoist to be pulled backward.
Some applications can suspend a coiled hose from a festoon wire to allow longer travel distances and hose reeling drum systems. However, due to pressure loss in long supply systems, the only option for long travel runs is to use a supply line with bayonet connectors at regular intervals and a flexible hose for connection to the hoist. This latter option is restrictive in that the hoist has to be halted at stages throughout the travel operation to enable the supply to be reconnected to the next available supply point.
By paying attention to these areas, pneumatic power operated hoists will provide the ideal tool in many jobs. Due to the wide range of manufacturers and models available, consult with specific manufacturers to establish the range of designs, capacities and lifts they are able to offer.
