12.5.2 Selection of Mobile Supporting Structure

Goalpost mobile supporting structures are intended for light duty and are ideal for occasional maintenance work or similar applications. Their structure is less bulky than other designs, allowing the operative more freedom of movement around the structure than with other designs. The structure is however less robust than ‘A’ frame designs. They should not be considered where regular or heavy duty lifts are to be made.

For light duty applications which require regular dismantling of the mobile supporting structure, either for storage or for transportation from site to site, a foldaway mobile supporting structure should be considered.

‘A’ frame mobile supporting structures are intended for heavier duty work and applications where the structure will be left in its erected condition. They are ideal for applications which regularly require a mobile lifting facility such as loading and unloading in yards and open areas or for use in workshops. The structure is more robust than goalpost designs. They should be considered for all general purpose applications.

For heavier duty applications which require the mobile supporting structure to be dismantled between periods of use and where no other lifting aids are available to assist in the erection of the mobile supporting structure and sufficient space is available, a self-erecting mobile supporting structure should be considered.

For applications where limited variation of the height of the runway beam is required, an adjustable height mobile supporting structure should be considered. Designs vary and it should be noted that in some cases additional stays or guys are necessary to provide stability. Consideration must be given to the space occupied by such stays.

For applications where the structure will be left in its erected condition for long periods of service, but occasional dismantling for transportation or storage is required, a demountable mobile supporting structure should be considered. Consideration must also be given to other lifting facilities that will be necessary to assist in the dismantling and erection operations.

Heavy duty mobile supporting structures should be considered for applications where:

Light duty mobile supporting structures may be considered for applications where:

The selection of the bracing method will largely depend on the application.

For applications which require maximum internal clearance dimensions and where there are no external obstructions, external bracing should be considered. This option allows for the maximum amount of travel along the runway or for wide loads to be lifted to the maximum height.

For applications where external obstructions exist, such as use over gangways or between machines, internal bracing should be considered. This option limits the amount of travel along the runway.

For applications where the maximum travel along the runway is required but external clearances are limited, a box braced, rigid or spliced braced mobile supporting structure should be considered. It should be borne in mind that in addition to providing rigidity to the structure, bracing may also be used to give support to the runway beam, enabling a smaller section of beam to be used. This is important in areas of limited clearances where maximum lifting height is required.

Variations in the design of the bracing and method of attachment are possible in some cases to suit particular applications. An example of this would be an internally braced mobile supporting structure where the trolley and hoist is required to travel up to the supporting member. This may be done using twin bracing members suitably spaced either side of the runway beam.

Where special bracing requirements exist, they should be fully discussed with the supplier and his advice sought.

The selection of the type of castor will largely depend on the surface on which the mobile supporting structure is to be moved and the duty for which it is intended. Careful consideration must also be given to the need for the wheels to be fitted with brakes and for castors to be fitted with rotational locks.

Consideration should be given to the need for jacks. In the case of mobile supporting structures fitted with light duty castors, jacks are essential to ensure the load is not supported on the castors. They should also be considered for applications where the mobile supporting structure is to be used in one position for long periods of time. Jacks should also be considered for runway levelling purposes.

Careful consideration must be given to the type of jack to be selected to ensure it is suitable for the purpose for which it is intended. Jacks intended for parking purposes, i.e. preventing movement and taking the weight off the wheels, may not be suitable for levelling purposes.

The use of brakes, wheel locks and castor rotational locks should always be considered where inadvertent movement might occur, e.g. outdoor due to high winds. They may be fitted to most types of mobile supporting structures and used in addition to, or as an alternative to, jacks to prevent movement.