By Clint West, Contributing Writer
Engineers learn that the most obvious solution to a problem isn’t always the right one.
Years ago when I was working for a company that manufactured backhoe type excavating machines, we encountered a problem with a new machine wherein the cylinders that hoisted the boom had failures on piston packing rings in less than one shift of operation.
The hoist cylinders had a 7-inch bore, 72-inch stroke, and the operating pressure was 1500 psi. There were orders for the new machines and a solution to the problem was of the highest urgency. I was assigned the project.
Chemical analysis of the failed packing, a square ring type made of nitrile rubber and cotton fabric, showed that it was burned as opposed to any other type of degradation. Most of my colleagues in the company thought then that the tight fit between seal and cylinder bore caused friction and heat. We placed small thermocouples in the piston close to the rings and measured temperatures during operation. The result was inconclusive. We saw two small temperature spikes in two hours of operation. When we disassembled the cylinder, we found severe damage to the top ring, as expected. I was no closer to a solution.
We then built a fixture in the lab that simulated field operation of the hoist cylinders. The fixture had a large “F” shaped arm with a cylinder pivoted at each tip of the “F” and a pivot at its bottom. The cylinders were anchored by pivot pins in a bracket at the end of a heavy old planer table. One cylinder was driven and the other provided resistance to the driven cylinder. An automatic reversing control using relays allowed the fixture to operate unattended. After one day’s operation, we disassembled the driving cylinder and found no damage at all to the seal. Friction in the cylinder was not a cause of burned packing.
We then disassembled the driven cylinder. The piston rings were badly burned. What was the difference between the driving and driven cylinders? The obvious difference was that the driving cylinder was always pressurized while the driven cylinder created a partial vacuum when filling with oil. We placed a centrifugal pump in the line to the driven cylinder so that no vacuum was created during filling and ran it for another day. During that day I shut off the centrifugal pump and bypassed it for only two cycles of the fixture. When disassembled, the driven cylinder had two small burned spots on the upper ring.
We then knew that a partial vacuum was a necessary condition for burning packing rings. The heat source then had to be the compression of the air and oil vapor that were brought out of solution by the vacuum. The machine boom would go down faster than the pumps could fill the hoist cylinders. When the bucket hit the ground the pumps would catch up and the pressure would rise fast, achieving a compression ratio much higher than any diesel engine and firing the oil vapor and air mixture.
The air bubble created had to rest against the packing ring in order to burn it. We did further testing and found that the oil temperature had to be above 170F to form enough vapor to ignite, a temperature commonly reached in our machine.
The emergency solution was to insert orifices in the hoist cylinder lines to restrict the downward speed, making sure the pumps could keep up. Some colleagues were not convinced, so we disassembled a cylinder from a different machine with hoist cylinder geometry reversed and found soot deposits on the end casting where the bubble would rest.
Author Clint West is semi-retired and works part time as an engineer at his company, Yuba City Steel Products. He has a BSME from Caltech, an MSME from Case Institute, and has been practicing engineering for 52 years. He had design responsibility for major components of the fueling system for the Atlas V booster motors made by Aerojet. He has been married for fifty years and has three children and five grandchildren.