A specialty machine builder was having problems getting a vertical cylinder with a regenerative circuit to lift a hinged member. When the hinged member was lowered, an attached drill would drill a hole and lift it out of the way so the work piece could move horizontally and then have another hole drilled. It looked somewhat like a band saw that would cut horizontally fed bars and angles.
When the cylinder was extended to lift the pivoting fixture up, it would extend about halfway up slowly and then pick up speed reaching the designed regen speed. You could hear oil blowing over the relief valve when it was extending slowly and stop when the regen speed was reached. Adjusting the counterbalance valves did not help.
What was the problem?
The regen circuit that did not work properly until the hinged fixture arm was about halfway up was caused by the spring drain line connected internally. Any pressure in the spring chamber of pressure controls is additive to the valve’s pressure setting. The pressure needed to lift the hinged fixture continually decreased as the fixture was raised. Externally draining the rod side counterbalance valve solved the problem.