A plating mill receives 6-8 ft. wide coiled steel and chemically treats it with a process similar to galvanizing. The rolls have about a 4-6 ft. OD and are unraveled like paper towels. The loose end of a sheet is fed into a pair of pinch rolls that direct the steel through several stations that wash, rinse, dry, and heat it. After it is coated and treated, the steel is then rolled back into 4-6 ft. coils and shipped to automotive stamping plants. An impressive mill like thid can be a city block long.
Feeding a new sheet into the pinch rolls and subsequent stations is time consuming. To reduce this bottleneck, plant personnel use a storage tower that accumulates a large amount of flat sheet. The lead end of a coil is clamped to hold it still as it starts to enter the first set of pinch rolls. A new coil is then loaded and butted up to the tail end of the previous one. Next, the two ends are welded together and the clamp is released to allow the new coil to thread through the machine. This keeps the process running continuously, 24/7.
The hydraulic circuit design shown is used in most of these mills. When the steel is unraveled off the roll, two long hydraulic cylinders extend, pushing two idler roll racks apart to fill the storage tower. A pressure reducing valve is used to create a force high enough to allow the steel to continue to run as the storage system moves and hits its maximum limit stops. The reducing pressure is just high enough to keep the rolls apart while the mill pulls on the sheet and feeds the different processing stations.
When the clamp engages to stop and hold the tail of the coil, the mill continues to run. When the sheet is clamped at the end for welding, the pinch rolls on the other side of the storage system continue to pull stored sheet from the storage tower. This extra force collapsing the storage tower causes the reducing valve’s outlet pressure to increase 75 psi, which relieves over the relief valve and allows the storage rack cylinders to retract as the stored sheet is removed.
Once the welding is complete, the clamps are released, and the pressure drops below the reducing valve setting. Then the rack starts extending to replenish the stored volume. The rack retracts during welding of a new roll and extends when the welding clamps are released.
An overheating problem developed when the relief valve stuck in the relieving position and did not close when the pressure dropped. The OEM, now out of business, had removed the valve’s nameplate, but a technician recognized the valve as a Vickers CT-10 and installed a new one. The heating problem persisted, so they replaced the pressure reducing valve, but still could not get it to work right.
The service manual instructed them to: plug both “vent ports” of both valves that were plumbed to the remote proportional control, set the reducing valve pressure to 1600 psi, and to adjust the relief valve pressure to 1500 psi. It also instructed them to lower the reducing valve pressure to 1425 psi, which they did, and all was fine at these pressures. However, when they reconnected the remote control, the problem came back. They then replaced the remote proportional valve, but to no avail.
They did not know what to do next. Do you?
The hydraulic circuit controlling the storage system relied on the relief valve to maintain a pressure 75 psi higher than the reducing valve. This worked fine when the remote proportional relief was isolated from the valves. However, when the two vent ports (remote control ports) were connected to the same pressure control, they would both be reducing and relieving at the same pressure, causing the system to overheat. What happened to the 75 psi differential they had when manually setting the maximum pressures?
When examining the illustration below, of a pilot-operated pressure control, the total set pressure is a result of the main poppet bias spring and the pilot spring. The pilot spring on the relief was set 75 psi higher than the reducing valve because the bias springs are normally the same low pressure — between 20 to 35 psi, depending on the manufacturer of the valves.
The original Vickers CT-10 relief valve had a bias spring that was 75 psi stronger than the bias spring in the reducing valve, allowing the proportional relief to control both — maintaining the 75 psi differential no matter where the remote pressure was set below the maximum controls. Jon Rhodes, a CFC-Industrial Training instructor, had the customer retrieve the old valve and reuse the stronger bias spring, which solved the overheating problem.
Rhodes also told them they could’ve installed a 75 psi check valve flowing from the vent port of the relief valve to the proportional control to accomplish the same 75 psi differential.
Robert J. Sheaf, President CFC Industrial Training