The choice between a hydraulic and pneumatic system is a very complicated question which each and every system design engineer will have to face at-least once in their lifetime. A knowledge of hydraulic & pneumatic systems would benefit most design engineers in their professional journey. This article will help you to understand the situations in which you should a prefer pneumatic system over a hydraulic system.
Comparing a pneumatic system to a hydraulic system
Situations favouring pneumatic systems are:
i) A clean system. In a consumable goods or medicine plant, it is unacceptable to have oil leaking out from a hydraulic system. Pneumatic systems can work with nitrogen/inert gas as their medium and hence they are preferred in industries where the risk of contamination is high.
ii) Regions of intermittent power supply. Hydraulic systems stop functioning as soon as the power fails. This can be catastrophic for certain industries (i.e. in the glass industry, the liquid glass will start solidifying inside the equipment and can lead to a change of equipment. It is the same with food industries dealing with semi-solids like wheat mold). Pneumatic systems have a compressed air reservoir and in the event of a power failure the air reservoir can be used to power the system.
iii) Hydraulic systems are expensive. In a hydraulic system, small oil leaks can lead to severe pressure loss and the system may not give the desired output. To avoid oil leakage, precision manufacturing methods are used to manufacture hydraulic components with a high surface finish – this increases the cost of the components. A pneumatic system can handle leakage, as air leakage won’t create environmental damage/hazard and will go directly into the atmosphere. Additionally, a small leakage will not lead to drastic pressure drop and the system should still be able to perform the required operation.
iv) Pneumatic systems are more prone to wear compared to hydraulic systems because hydraulic systems are self-lubricating.
v) Response time. This is the time taken by the system to perform a desired function. In a pneumatic system the air compressor takes considerable time to compress gasses from atmospheric pressure to working pressure (due to the high comprehensibility of gas). As a consequence the response time can be high whereas a hydraulic system develops working pressure as soon as the power is supplied to the pump.
vi) Operating speed. Pneumatic systems can be used for high speed applications as the to-and-fro motion of gas consumes very less power (less than 2%). On the other hand liquid has more inertia compared to gas and hence the power consumed (or power lost) in acceleration & deceleration is high, which makes it economical for comparatively low speeds. This is the prime reason behind the development of certain high speed pneumatic systems – for example a pneumatically operated dentist’s drill which rotates at 400,000 RPM. In this situation the cushioning provided by the compressed air tends to dampen the vibrations.
vii) Lifting capacity. Pneumatic systems cannot be used to lift heavy loads because general the operating pressure rarely exceeds 50 bar. Lets understand the reason behind this. Consider an application where we have to move a block along a surface by pneumatic cylinder.
Unlike a hydraulic system, the pressure of gas inside the cylinder is never uniform.
Because of this uneven pressure, a pneumatic system will move the load as the pressure increases to the required level. As the load moves, the volume inside the cylinder increases and the pressure drops. When the pressure inside the cylinder falls below the required pressure to move the load, the piston stops and the system waits for the pressure to build up again. This leads to uneven motion and jerking. The prime reason behind attaching a compressed air reservoir is to minimize the jerks by providing the required pressure all the time.
ix) Weight of return line. Pneumatic systems are open loop and hence they directly eject the air into the atmosphere after the operation. This saves the system from return lines and hence, leads to weight reduction.