During a recent conversation with a client, the issue of testing hydraulic pump rebuilds came up. As I explain in Insider Secrets to Hydraulics, it is essential that all hydraulic components are properly tested after rebuild. Ensuring a repaired component will perform the way it should when it’s installed on the machine BEFORE it leaves the shop, instills confidence in the customer and the repairer. And this is especially important for hydraulic pumps.
But this conversation was about testing BIG pumps. For example, Rexroth A7VSL1000 and A4VSO1000 (1000 cc/rev). Any purchasing officer who has been around a while understands that if she issues a purchase order for a pump rebuild, the expectation is the rebuilt pump should come with a test certificate, which ideally shows the pump has been tested to full flow and pressure prior to dispatch.
The conventional way of bench-testing a hydraulic pump is 100% energy loss. That is, the pump is loaded with a relief valve, which means all input power (pressure x flow) is converted to heat. While such heat generation is not ideal, it can be managed. But the more important implication of this is, the amount of power that can be converted to heat, can be no more than the power available to the test-bench. If the test-bench has 100kW available to the test pump’s input shaft, or 250kW, then that’s its limit.
This means, for hydraulic repair shops, how big their test-bench is often used as a point of difference over their competitors (size matters). But when the pumps get bigger and bigger, the reality is few if any repair shops have the necessary power to test them at full flow and full pressure at the same time. Consider a 1000 cc/rev pump which is capable of 1000 L/min at 300 bar. Working on a pump efficiency of 90%, 555kW (744HP) would be needed to test it at full power using the energy loss method-and that doesn’t include inefficiencies in the test-bench drive itself!
It’s quite possible that there is NO repair shop in the world that has this amount of raw power available to their hydraulic pump test-bench. At the last company I worked for we built a regenerative test-bench which could test these pumps to full flow and full pressure at the same time, but this was a different animal. Briefly, this involves connecting the 1000 cc/rev test pump to a 1000 cc/rev variable motor with both coupled to either end of an AC electric motor with thru shaft. The test pump is loaded by reducing the displacement of the variable motor, which then drives the test pump through the electric motor. The end result of this regenerative loop is the electric motor is only required to make up losses-and there’s minimal heat generation.
Going back to the conversation I had with my client, what it was really about was misinformation and erroneous belief about the level of testing actually being done on their large-displacement pumps after rebuild.
As I explained to this client, these pumps need to be tested at full flow to check and adjust displacement controls and set volume stops where applicable. AND they need to be tested at full pressure to check volumetric efficiency, set pressure cut-offs and to check for leaks. BUT to do all these things it’s actually NOT necessary to run the pump at full flow and full pressure at the same time. Plus, unless the repair shop doing the work has a regenerative test-bench, it’s unlikely they’ll have the necessary power to do this anyway.
This issue is one of many in the hydraulics biz where competitive tension results in the distribution of propaganda which confuses and misleads people with limited technical knowledge-purchasing officers in this case. It’s not their fault, but the bottom line is: if you don’t know or are not sure, do your homework and if necessary seek advice from unbiased sources. To do otherwise can be costly. And to discover 6 other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.
Thank you for talking about the importance of making sure you test the hydraulic pump after you get it repaired to make sure it has the right pressure. My parents just bought a small tractor that needs some repairs. I can see how anyone looking into this would want to make sure they shop around and find a professional that has the equipment to make sure these pumps are working properly.
It's interesting to read about some of the ways a hydraulic system is tested and how it can be done. It makes sense that testing a rebuilt hydraulic system is important to ensure there aren't any problems when it's used. I'll have to remember this because asking what sort of testing methods the repair company uses could be helpful.