13.10 SAFE USE OF JACKS
In addition to any specific instructions relating to the safe use of the jack issued by the manufacturer, the following points should be observed:
Most jacking operations involve multiple jacks, requiring special care since it is not possible to raise or lower them in perfect unison. This results in an uneven loading condition with the weight distribution being transferred from one jack to another. Any one jack can therefore be carrying a greater load than the other jacks in the arrangement at any time. The capacity of the jacks should be adequate to account for this and the operation must be supervised in such a way that this condition is limited. (See note at end of this subsection)
In many cases, it will not be possible for the operatives to see all of the load and it will be necessary to have a Responsible Person in charge of the operation to ensure the load is kept level and that each jack carries its correct share of the load within the limits of operation. An agreed system of instruction and signals should be used so as to avoid any confusion between the operatives.
The floor must withstand the load imposed by the jacks. Use floor plates if necessary to distribute the load over a wider area. Caution must be exercised to avoid hidden dangers such as buried cables, pipes and ducts which may affect the load bearing capability of the floor on which the jacks are to be used.
The surface on which the jack is to be placed should be level and even, allowing the base of the jack to sit firmly without any tendency to rock or tilt. If necessary, suitable floor plates or packers should be used to ensure this condition.
The jacking point on the load must be carefully selected so as to ensure the jacking operation does not damage the load. Precautions must be taken to ensure the head of the jack is in full, firm contact with the jacking point and to prevent the head of the jack slipping. If necessary, suitable packing should be placed between the jack head and the load.
Never raise a load higher than is necessary. Use suitable packers to ensure the load will not tilt or fall in the event of a jack failure. A process of lifting and packing in short stages may be necessary to ensure that in the event of a jack slipping or failing the load will only drop a short distance onto the packing. Where fitted, screwed collars should be regularly adjusted for this purpose.
Never leave a load supported entirely by jacks; ensure the load is supported by suitable packing or by the use of screwed collars. In some cases, purpose made axle stands or trestle supports may be used to advantage.
Only use packing which is capable of withstanding the crushing effect of the load and which itself will not kick out or topple due to the downward thrust of the load.
Care must be taken to prevent the jack kicking out or toppling due to an oblique thrust caused by incorrect packing, unstable load, uneven operation or unsuitable mounting. Special care is necessary if one jack only is being used.
In cases where independent cylinders and pumps are employed, additional measures may be necessary to ensure stability. Cylinders tend to be of small diameter with no enlarged base so the stability of the arrangement must be guaranteed before commencing the operation.
Jacking operations are often carried out in confined spaces as no other means of lifting can gain easy access. Care must be taken to ensure operatives do not become trapped or pinned by the movement of the load and that they always have a clear exit.
Never reach, or allow anyone else to reach, under a load supported by jacks. Keep fingers and toes clear at all times, taking special care when placing or removing packing, or when positioning machinery skates, slings or other lifting or handling equipment.
Under no circumstances should anyone be permitted to work or climb on a load which is supported by jacks.
Never over-extend a jack. It is unwise to use any jack in its fully extended position. In the case of hydraulic jacks, many are fitted with a mechanical stop which prevents the ram from extending too far. Others are merely marked with an indicator when the maximum ram extension is achieved. Continued operation will result in the ram being ejected from the body causing the load to fall. Screw jacks are provided with a viewing port in the body so that the operative is able to monitor the screw movement. The screw must be visible through the port at all times or the screw may be ejected from the body thus dropping the load. On some models, the screw may be pegged or spot welded to prevent it being raised too far. Continued operation can cause this to nip and lock the screw making lowering difficult. In the case of ratchet and journal jacks, racks and journals are usually captive within the body. Continued operation after the jack has reached its maximum extension may cause excessive strain on the mechanisms and in severe cases may result in them becoming jammed or failing.
In the case of ratchet jacks, whilst in operation the load is carried by the operative via the operating lever. At the end of each stroke of the lever, the load is sustained by a pawl. On no account should any attempt be made to balance and hold the load by applying effort to the operating lever.
In all operations care must be taken to ensure that the jack is not overloaded inadvertently, as may occur if the load tips shifting the balance of the load. In some cases, the equipment may have built-in overload protection or the operational features may limit the ability to lift loads greater than the rated capacity. Even so, damage or failure may still be caused by overloading due to shifting loads. Whilst this can occur during the lifting operation, a greater danger exists when loads are being lowered and this is particularly the case with hydraulic jacks. Lowering is controlled by the skill of the operative opening a valve allowing the jack to retract by gravity. In multiple jack applications, it is very difficult to maintain even control when lowering. (See section 13.5.3 Operating Features)
In the case of hydraulic jacks, a pressure relief valve may be fitted in the system so that once a predetermined pressure is achieved, the valve will open preventing a further increase in pressure. Alternatively, the system may be designed so that it is impossible to exceed a predetermined pressure by normal operation. However, to ensure the jacks operate correctly up to and including their rated capacity this maximum pressure will be in excess of the normal working pressure. If dirt is present in the oil or if the oil has become emulsified, relief valves and non-return valves may not function correctly. In these circumstances, excessive pressure may be developed, this can lead to failure of seals or damage to the hydraulic system, allowing the load to fall.
In the case of mechanical jacks, operating levers may be fitted with shear pins so that if a greater effort is applied than that for which the mechanism is designed, the pins will shear preventing further operation. However, in the majority of cases, mechanical jacks are limited only by the length of lever and the effort that the operative can apply. The use of longer levers or more than the recommended number of operatives can result in the mechanism becoming over-stressed and failing, allowing the load to drop.
Whilst the above measures will protect the equipment if attempts are made to raise loads greater than the design maximum, if a greater load is accidentally placed on a raised jack, e.g. as a result of the load tipping, the jack will fail thus dropping the load.