Pneumatics offer rapid movement of cylinders and have the great advantage of availability in very small sizes. Why do pneumatics offer a higher operating speed of its components? This is mainly due to air compressor flow rates, air is very agile and can flow through pipes very quickly and easily with little resistance, while hydraulic oil is a viscous substance and requires more energy to move. Also in pneumatics, cylinders and valves can dump their compressed air straight to the atmosphere when they need to change direction or alter their state quickly, compared with hydraulics where the oil must be routed back to the reservoir.
In applying the above we can say that a pneumatically operated play detector system is superior to a hydraulic one since the air has little resistance throughout the closed system and when exhausting can do so straight to atmosphere. However, that said hydraulic systems also meet the MoT approval criteria.
Pneumatics does not have the potential force that hydraulics has to offer. The lifting or moving of heavy loads is not best suited to pneumatics. Hydraulics can smoothly lift and move loads because the hydraulic oil is not compressible, compared to air which can become jerky and spongy as the air pressure fluctuates with cylinder movement or load changes. In general a much larger pneumatic cylinder is needed to obtain the same force that a hydraulic ram can produce.
Here hydraulic systems win hands down. A pneumatic system is just not suited to moving loads and so therefore is not the ideal solution to investigate suspension systems or steering components, Again, we can say that a pneumatic system meets the minimum load and movement criteria to be approved for scheme use but with the movements being subject to the fluctuations in pressure caused by the compressibility of air a detailed totally repeatable inspection is more difficult.
So what can we conclude from this straightforward engineering analysis of the 2 systems?
It seems that both methods of play detector operation offer suitability for use on vehicle lifts, albeit for a more repeatable test a hydraulic system is superior. But there is also one other important factor to consider that overwhelmingly makes a hydraulically powered play detector system a more attractive and reliable tool for vehicle inspections.
The operation to raise the platforms of most MoT vehicle lifts is by means of a hydraulic circuit. Therefore with such a system already on-board to power the lift, it makes sense to utilise that same system for operating the play detectors. This immediately decreases the potential failure modes by 50%! With a pneumatic system you are not only exposed to the potential failure modes that a hydraulically driven lift is succeptable to i.e;
- Power Failure
- Hydraulic Circuit Failure (Hose Burst / Pump Failed)
but also failures pertaining to;
- Compressor Failure
- Air Circuit Leaks
As well as the above, hydraulic systems make it easier to construct a totally flexible modular system that can allow installation of a “flat platform lift” today, whilst allowing a very quick ‘retro-fit’ at a later date of hydraulically powered features to the already in-situ hydraulic system!
If you consider all the above factors it becomes apparent that a hydraulically operated system offers many advantages over dual hydraulic/pneumatic based lift systems for use on vehicle lifts. It is for this reason that we use such systems for all our mot and service lifts.Quoted Source: The Engineering Student