We have the advantage of force density with hydraulics. The issue is the compressibility of air in this case. When it comes to pneumatic applications, the most inefficient part is actually creating the compressed air. Because of gas laws, the fact of compressing it creates so much heat, it’s really hard to get a high volume of really high-pressure air without creating excessive amounts of heat. If you take a one cubic foot amount of air and compress it down to one cubic inch, the heat that was in that one cubic foot, is now in one cubic inch. That can be multiplied by a thousand times. The heat is one of the issues. It’s hard to create a lot of high pressure pneumatic applications. But mostly it’s safety.
3,000 psi of compressed air is literally a potential explosive—which is why accumulators, air compressors, and similar pressure vessels have very strict safety guidelines and standards.
If you were to have a failure in a 3,000-psi air cylinder, it would be absolutely catastrophic. You would have shrapnel going in every direction for a mile. You would be shooting rods through the roof of a plant. It’s just not safe. It’s catastrophic when there is a pneumatic failure. Whereas with a hydraulic failure, even at 3,000 psi, you might blow an end seal and get some leakage of oil and pressure quickly drops to zero.