Have you ever wondered why the landing gears of Boeing 787 Dreamliner (and in fact all commercial aircrafts) use hydraulics and not electronic or mechanical power transmission? The reason is that unlike most mechanical and electrical systems, “fluid power systems” (hydraulics and pneumatics) are more reliable and are better able to withstand shock and dampen vibrations.
So what are the distinguishing factors that give hydraulics an upper hand from other mechanical and electrical systems and under what circumstances?
The most important factor that gives fluid power system an upper hand is the high power to weight ratio (a 5hp hydraulic motor can be easily hold in the palm of your hand, but a 5-hp electric motor weights around 20kg!)
This difference in power to weight ratio is key to the efficiency of a system.
There are applications where weight is incredibly important. Aircraft is a good example, as well as earth moving machinery, submarines, speed boats, etc.
Earth moving equipment mostly operates on rough terrain; submarines and speed boats have to deal with rolling ocean waves. Such movements demand frequent acceleration and deceleration.
With increase in weight, the force (force=mass x acceleration) required also increases proportionately and leads to higher fuel consumption. In these situations fluid power is the preferred option, as it makes the system work quickly with minimum machine weight.
Additionally, many electrical systems require heavy electric motors to be mounted on the axis of the motion, which requires rigid holding linkages and increases size and weight of linkages. For example, adding a heavy motor on a crane (as shown in the figure above) far from the center of gravity increases the chances of topple.
Compare that to a hydraulic crane system where a hydraulic power pack can be mounted on the opposite side of the hoisting boom and fluid can be transferred to the output link easily. This balances out the couple generated by the lifting force and makes the system rigid. It also keeps the center of gravity as low as possible, which increases stability.
Fluid power systems can be divided into two systems:
1) Hydraulics systems, which uses nearly incompressible liquid as a medium.
2) Pneumatic systems, which use highly compressible gas as a medium.
Hydraulic systems work by compressing liquids and utilize that pressure to generate mechanical force.
A change in pressure from atmospheric pressure to 50 bar gauge pressure in a closed system containing 20 liter of water requires adding an additional 0.05 liters of water inside the system to keep the volume of the system constant. That also means that at 50 bar pressure water compresses from 20 ltr to approximately 19.95ltr. This contraction of volume is extremely small; hence liquids are generally considered “nearly” incompressible.
Another important point to consider is that hydraulic systems offer quick response (very high acceleration). A hydraulic motor can pump 50ml water in milliseconds if not nanoseconds!
Pneumatic systems work by compressing highly compressible fluids (gases) and utilizing that pressure to generate mechanical force. Compared to hydraulics, pneumatics is slow. Gases take considerable time to build up pressure (the most commonly used air compressor (for speed saw) takes more than 3 minutes to fill a 120 gallon cylinder, at the rate of 5 CFM (cubic feet per minute) to a pressure up to 6 bar. This is why you have to wait for at least 3 minutes for the compressor system to be ready to use.
But, you’ve likely seen race car crews remove tires with pneumatic drills in just seconds. So if pneumatics are so slow, how can this process be so fast? The answer is that the acceleration is very low but the velocity can be pretty high. The system takes a long time to be ready, but once the pressure is built up, it can be transferred quickly.
What matters most when selecting a fluid power system?
One of the most important factors is power loss.
Moving hydraulic fluid at high speeds (back and forth inside a double acting cylinder for example – see image below) creates considerable wastage of energy as liquids have more inertia than gases.
This inertia opposes acceleration & deceleration, leading to heating of the liquid.
Gases have much lower inertia, and so power loss will be much smaller applications with frequent acceleration and deceleration. This enables pneumatics to be used at extremely high speeds.