Because gases compress more than liquids.
It is not only because a fluid doesn't compress, but it is also because hydraulic oil is a good lubricant so very tight seals can be made. Also the cylinders, hoses, pumps, and accumulators are all very heavily built to handle regular pressures of 4000 psi, whereas, pneumatics all are relatively light components because of typically being able to run off a regular air compressor of around 125 psi.
Being able to use a shop air compressor and repairs are relatively easy are what make pneumatics popular. But even with only 125 psi or so, you can still put in a large cylinder and get several tons of pressure. So some pneumatic cylinders will have a thick piston rod to handle whatever stresses are put on it.
Hydraulics use a fluid that doesn't compress, so hydraulics are more useful for precision motions, like construction equipment and cranes where people are in close proximity.
One final consideration is the question of stored energy. With a pneumatic system if you need pressures in the thousands of psi and there is an equipment failure the result can be equivalent to setting off a bomb.
Hydraulics uses a non compressable fluid.
Ive been searching for this explanation and I think i understand it. Why can a hydraulic system handle and work under much higher pressures then a pneumatic system?
