Why are hydraulic systems used in machinery such as bulldozers and backhoes, while pneumatic systems are used for things like opening and closing bus doors?Please provide a lot of detail, and examples!
Liquid is more dense than air; therefore it is easier to generate more power with hydraulic. This results in smaller systems to give you the equiv. power. It is also more reliable, as well as more forgiving.
Air is compressible whereas oil is not. If a back hoe or shovel on an excavator was driven by compressed air, then as you tried to make them do work, they would compress the air in the cylinders. You would have virtually no control over the positioning or movement. Hydraulics are much more controllable for position and as the oil is not compressible, the actuators will stay in position.
You get the detail. The reason is that hydraulics provide *lots* more power than pneumatics. You need power to move heavy machinery around, but not much to operate a bus door or even a jackhammer.
Where Are Hydraulic Systems Used
The biggest advantage is the amount of power in a given space. Hyd systems typically operate at 2000psi and up, compared to about 80psi for pneumatic. Also, since fluid is non compressible, the moement distance can be linear. Many aircraft, and industrial equipment now use electric servos instead of hydraulics. Biggest problem with hydraulics is leaks. 2nd biggest problem is continuous power consumption to the pump. Lots of air power is used in industry, so it is the easiest way to move or clamp anything, with nothing more than an air cylinder and a valve. Each system has it's place to do different things.
