Hydraulic cylinders: push, pull, speed and regeneration

In simple terms

Inside the steel tube (the barrel) is a piston with seals round it, fixed to a polished steel rod. Oil pumped into the closed end (the cap end) pushes the piston and the rod comes out. Oil pumped into the other end (the rod end) pulls it back in.

Pushing is stronger than pulling. When oil pushes from the cap end it presses on the whole face of the piston. When it pulls from the rod end, the rod covers part of that face, so the oil only presses on the ring around the rod. Try a thick rod in the animation and watch the pull force drop.

Pulling is faster than pushing. The same pump flow fills the smaller space on the rod side more quickly, so the rod comes in faster than it goes out.

Regeneration is a trick for speed. Instead of sending the rod-side oil back to the tank, the valve sends it round into the cap end. Now the cylinder is filled by the pump and by itself at the same time, and the rod shoots out. The catch: oil is pushing on both sides of the piston, so only the area of the rod itself gives force. Presses and log splitters use this for a fast approach, then switch back to normal for the hard push.

The faults show what we find on the bench. A bypassing piston seal lets oil slip from one side to the other, so the cylinder gets slow and the load creeps. A scored rod cuts the gland seals, so oil weeps out and dirt gets in. A bent rod pushes sideways on the bearings and seals and wears them out quickly.