Speed Control Problem with Advancing Cylinder
An automotive assembly plant installed a simple two speed circuit, to advance a hydraulically driven cylinder at high speed, and shift to a slower speed for pressing. (See figure below, which the maintenance supervisor used for plumbing.)
Plant personnel were experiencing problems with the alignment of the steel stamping being out of line with the bored hole in the casting when shifting into slow speed. Shifting the 2-position directional valve to the slow pressing speed triggered a “bump” or “jolt” causing unwanted movement of the stamping.
In a recent hydraulic class, they were taught that fast shifting directional valves can cause sudden shock. So they purchased a “soft shift” directional valve with a 500ms shifting speed, but the problem became worse. They re-installed the original directional valve and installed a gage in the piping between the rod and the 2-position valve. The cylinder was a 5” bore with a 3 ½” rod. They were surprised that the pressures jumped from 350 PSI during fast advance, to 4000 PSI when the slow speed meter-out flow control was selected. The pump compensator was set at 2000 PSI.
Any idea as to what is causing the “bump/jolt”?
First, let’s review what the customer was taught in their recent hydraulics class: “Shock can be caused by fast shifting directional valves”. We contacted the instructor and he passed on the following: Cylinders can be extended and pressurized; and to retract, a typical directional valve is shifted, releasing the cap pressure to the reservoir. If the oil being used has a compressibility of 1½% per 3000 PSI, and the volume under compression is 6000 cubic inches, the discharge rate would be 1169 GPM with a typical valve having a shifting speed of 20ms. Slowing the shifting speed to 160ms would cut this to 146 GPM, thus reducing the shock effect.
The second item that caused concern was the 4000 PSI rod pressure. A 5” bore cylinder with a 3 ½” rod has an area ratio of approximately 2:1. All meter-out flow controls located on the rod side will cause intensification directly related to the area ratios and the load on the rod. Remember the old quote: “Meter-out when in doubt”, but I always add “Meter-out when in doubt, but watch out!” to remind me of this potential problem.
Now to the actual cause of the “bump/jolt” when shifting speeds: When a 2-position directional valve shifts, it moves to the transitional position and then to the final position. Most 2-position directional valves have a blocked center during transition. This was causing the problem. Flow was momentarily blocked and then opened to the flow control causing the bump/jolt problem. There are a couple of good solutions to the problem. One is to use a directional valve with a transitional center open to both ports. Another would require re-plumbing to parallel the flow control around the 2-position directional valve. (See figure below.) This solution is what was recommended and it did solve the problem.
Robert J. Sheaf, President CFC Industrial Training