Common situation - a hydraulic machine has a problem, usually the good old "lack of force". A series of troubleshooting attempts is made, leading to the final "it's probably the pump/it must be the pump/there's a chance it is the pump" conclusion (which can also come in the "motor" flavor). Then, as no "advanced" testing gear is around, the aforementioned pump/motor is dismounted from the rig and is brought over to a hydraulic workshop for a "check-up" on a pump test stand.
Sounds logical, doesn't it? Indeed, testing a pump on a proper bench seems like a quick way to verify its operation and confirm the "diagnosis". Plus, if the pump turns out to be OK, having it on a test bench is a way to verify and correct the adjustments, if necessary. So... win-win?
I have two things to say about this.
First - there aren't many hydraulic shops that would agree to test a "foreign" pump just because you want to check your troubleshooting theory - the risk that a pump in an unknown condition could damage or contaminate the test bench is very real, and affects most shop's willingness to test pumps and motors without overhauling them first.
Second - even if you can get a shop to test your component, or maybe, there's a test stand in your shop and you don't care about filling the lines with metal shavings because you only live once anyway - there still is a simple step, well, three steps, actually - which you can perform in under a minute, and which will immediately tell you that a bench test of this particular pump or motor is nothing but a waste of time, and the only correct way to proceed would be to open it and assess if it can still be repaired.
For as simple as it sounds, I have seen more than once mechanics blindly decide to shove a pump into a test rig without much thought or verification. And although in my mind this practice is reckless and dangerous, I still believe that things can be "fixed" if you master the following "three simple checks" every time you consider running an unfamiliar unit on a bench.
Simple Test Number One - the "rotate the shaft" test.
You take a suitable tool, most of the times a pipe wrench, and rotate the shaft a couple of turns. An experienced mechanic will easily detect jerky torque resistance or excessive play/vibration - sure symptoms of serious failure in the rotary group or the bearings. No further testing is needed.
Simple Test Number Two - the "air in the pressure line" test.
You take an air gun, improvise a sealing accessory from a rag or a shop towel, and inject compressed air into the pump's pressure port/ports. If there is a serious damage to the rotary group, it will be exposed by the generous amount of air passing through to the case drain or the other port. Some previous experience might be needed to determine how much exactly is "generous" for a certain pump model. Using common sense is not a bad thing, too - for example, in case of a vane pump, this test is useless.
Simple Test Number three - the "flashlight test".
You take a small LED flashlight, and peek inside the casing through the drain ports. Any signs of wear, like metal shavings or chips or even shamelessly broken parts staring back at you, are all tell signs of a busted pump. Sometimes you can combine the flashlight with a magnetic pickup tool, which you can use to poke about the insides of the pump and see if you can fish out something magnetic. See the shiny Santa's beard at the end of it? No more testing for you! Just make sure you use a quality tool that won't leave the magnet behind.
Anyhow - these checks take under a minute to perform, and are a fast way of detecting damage without performing "wet" tests.
An example. Imagine that the pump you are about to test has a valve plate that looks like this. You wouldn't detect any uneven or excessive torque resistance on the shaft, nor would you find wear debris inside the casing, but you would definitely see a large amount of air passing through the pressure line to the casing, something that good pumps don't do.
As you can imagine, skipping these three one-minute checks can cause easily avoidable downtime, which, by definition, is a shame...
This is just another reminder that you should always perform simple checks first when you troubleshoot.
Of the above three, the air test is my favorite.
In fact, knowing how to correctly use compressed air to analyze hydraulic components is an invaluable skill for a hydraulic tech. If you know exactly where to blow and what to expect, you can easily detect malfunctions on even disassembled components - way before the "wet testing" stage. Checking the correct operation of check-valves, shuttle-valves, control spools, existence of oil passages or hidden plugs and orifices and much more - can all be done if you "master the blow gun!" (The shop kind, not the dart kind).