A company that makes all types of fireplaces and chimneys had a new press fabricated for forming top covers for chimneys. This press was built to handle all the custom designs requested by architects and builders. The tops, called chase covers, were normally made from SAE 304 stainless steel of various thicknesses. The press was inspected as it came out of the fabrication shop. After its speed and tonnage were confirmed, it was shipped to the customer’s assembly plant. The customer fitted the press with clamping and forming fixtures and began the fine tuning process of making good parts.
A problem surfaced that wasn’t present when the press was inspected at the fabrication shop; The clamp cylinder would “cog” on its down (extend) stroke and slightly deform the chase cover, leaving unwanted marks on its exposed surface.
The hydraulic circuit (see figure below) was reviewed, and technicians decided to reduce the pressure reducing valve setting. However, it didn’t help. They then adjusted the meter-out flow control on the rod side of the cylinder. Increasing the speed helped, but marked the part even deeper. Then, when they adjusted the clamp speed flow control to slow the cylinder down, the cogging problem increased. They could not find a happy medium adjustment between the flow and speed controls that would work.
When I first saw the circuit, I felt that the meter-out flow control, used with a pilot-operated check, should have prevented cogging.
Any idea as to what we had to do to fix their problem?
Cogging Cylinder Problem
Vertical cylinders with weights and pilot-operated (PO) check valves require special consideration. The cogging of the new press that formed top covers for chimneys is a common problem. Pressure builds in the cap end of the cylinder to extend the piston rod, and this same pressure pushes the PO check valve open. When the check valve opens, the cylinder can drop quickly, resulting in the cap-end pressure dropping low enough to allow the check to close, thus causing the cylinder to stop. The cycle keeps repeating, which causes the cylinder to cog.
To control cylinder speed, meter-out flow controls should maintain pressure on the cap end of the cylinder to prevent this loss of pilot pressure.
Several things were at play with this type of circuit: line sizes and their lengths, the pilot ratio of the PO check valve, the weight on the cylinder, the location of the flow control (flow controls were built into the cylinder end caps, so they could not be relocated), and the circuit pressure.
To fix the problem, CFC's Jon Rhodes removed the rod-end PO check poppet and added a counterbalance valve module to the stack. He was told that the PO check was designed to stop and hold the load from drifting down. The directional valve spool had the "P" and "B" ports blocked, with port "A" going to tank. This would cause the cylinder to drift up while the counterbalance valve prevented the cogging. (See figure below)
Cogging Cylinder Solution
Robert J. Sheaf, President CFC Industrial Training