13.5.3 Operating Features

There is a wide choice of jack designs, each with its own operational advantages and disadvantages for certain applications. The operating features should be carefully considered when selecting the type of jack to be used.

Hydraulic jacks offer low to very high lifting capacities with comparatively low operator effort. During the lifting operation, the ram may be stopped accurately at any position, whilst lowering is normally by gravity and relies on the skill of the operative to control descending loads. A valve is manipulated to control the descent and must be completely shut when the required position is reached.

It must also be remembered that any loss of pressure within the hydraulic system will cause the ram to sink and even well maintained jacks in good order are liable to creep downward. A screwed ram with locking collar may be used to assist in accurate downward placement of the load and to sustain the load in position, but it is necessary to raise the ram to release the collar when the jack is to be removed.

Ratchet jacks offer medium lifting capacities, but operator effort can be very high, some operations needing more than one operative. The raising and lowering motions can only be halted after one complete stroke of the operating lever giving a height variation equal to one pitch of the rack teeth. They do however offer greater distances of travel than other types of jack. When lowering the load, the mechanism raises the load a short distance before it descends on each stroke of the operating lever.

Screw jacks offer medium lifting capacities but operator effort can be very high, some operations needing more than one operative. The raising and lowering motions can be accurately halted and the load sustained in any position. Lower operator effort can be achieved and placement of the load can be accurately controlled if a journal jack is selected.