6.A1.3 THE SELECTION OF PNEUMATIC POWER OPERATED HOISTS

The choice of the control option will to some extent be a matter of personal choice. The cord control is the most basic and cheapest option. 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. It should be borne in mind that cord control is more easily operated by accident than the other types of control. The pendant control is the most expensive option, but the operative is given two switch type valves clearly defining the direction of operation and these are protected from inadvertent operation by the design of the hand held casing.

The choice of the lifting medium will to a large degree be the same as for electric power operated hoists but a third option commonly exists for pneumatic power operated hoists, that of roller chain. This is only flexible in one plane and as a result tends to offer smoother operation with less load oscillation, it is therefore ideal for handling delicate loads. The lack of articulation can be restrictive in use when connecting to loads which do not incorporate flexible slinging arrangements and storage of the slack chain is difficult.

Standard pneumatic power operated hoists exhaust the used air to atmosphere via a series of vents. This can be unduly noisy in some circumstances and the release of air under pressure may also be hazardous in some applications. Noise can be reduced by the provision of a baffled exhaust vent. It is also possible to duct the exhaust air away from the unit. In both cases, it is necessary to discuss this with the supplier as the provision of unsuitable equipment may affect the hoist’s performance. Further the design of such equipment will depend to some extent on the particular installation.

A vital consideration when selecting pneumatic power operated hoists is the provision of a suitable air supply. The working capacity and operational speed of the hoist rely on the air supply being at the correct pressure and delivery rate. In the event of the pressure being below that for which the hoist is designed, the actual lifting capacity will be lower than that for which the hoist is rated. Equally, if the pressure is higher than intended, it would be possible to raise a higher load than that for which the hoist is designed. The latter is a danger which must be averted and this can be done by the provision of a pressure regulator in the air line near to the hoist entry port. Manufacturer’s specific recommendations should be followed in respect to the supply pressure and delivery rate although these are usually based on 6 bar (90 psi). If the delivery rate of the air supply falls below that for which the hoist is designed, the operating speed will be slower than intended.

Air motors are self purging and as a result, any lubrication is lost with the exhausting air. Oil mist lubricators are available which allow the incoming air to collect lubricant and pass this directly through the motor.

Compressed air is moisture laden. Condensing water will cause corrosion and therefore steps should be taken to prevent this. Similarly, other contaminants should be removed from the air feed. Consideration should therefore be given to the provision of suitable filter/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, which are usually of a coiled hose design allowing a 3:1 extension. The length of coiled hoses can be restrictive in that the loops of hose occupy considerable space and place 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. A variation of the coiled hose may be suspended from a festoon wire to allow longer travel distances and hose reeling drum systems may be used for some applications.

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.