Check out these spectacular pictures:
What we have here is a brand spanking new variable displacement motor that failed catastrophically as soon as the drilling rig started, and aside from generating a spectacular set of textbook over-speed damage pictures, it also teaches a lesson to never "trust" a new hydraulic unit, be it a pump or a motor. Allow me to explain what I mean by that.
Let us start with the over-speed. When an axial piston unit exceeds certain rpm, the centrifugal force that pushes the pistons against the outer side of the barrel bores becomes so great, that it squeezes the lubricating oil film from between the sliding surfaces until they touch each other and all sorts of bad things start to happen.
When over-speed is "mild", so to speak, like a 4000 rpm unit running at 5000 rpm on "special occasions", the wear pattern may actually resemble normal wear (when such a unit does fail prematurely), and may not clearly point to the fact that the motor was running at higher than intended rpm, especially in systems that run with quality mineral oil at optimal viscosity.
However, when over-speed is severe - read 2x max rpm and more - the failure is instant and the resulting wear pattern always contains characteristic attrition spots located on the outer side of the piston bores. Usually, the marks are deep scores surrounded with heat-tinted zones, because when adhesive wear stars to happen at such high relative speeds, the parts literally weld together and then tear chunks of material from each other and use these very chunks to "cut, scrape and gouge".
Just look at these beautiful images. You can clearly see the marks, the chunks of cylinder block material that got welded onto the pistons, and the final "coup de grâce" - when the newly welded pistons yanked the retainer plate up from the shaft. Here are some high resolutions close-ups for your admiration, check out the the heat colors on the piston side. Worth zooming in!
Classic!.. Now, let us talk about the perfect lesson, and begin with the cause of this failure. Not one, not two, but three things contributed to this spectacular explosion.
The hydraulic motor was an A6VM55 series 63 Rexroth equipped with the HD1D control - which is a positive proportional hydraulic displacement control with high-pressure override. The motor was supplied to a core drilling company as a "turnkey replacement", and its application was supposed to be the case of "shove it in and push the start button", however it had three adjustments that were a tiny bit off...
First - the minimum displacement limiting screw was set to a very low displacement (for that particular drilling rig), second - the proportional hydraulic control was set to a very low pilot pressure threshold, and third - the high-pressure override was set to an abnormally high level. When the mechanic (a very experienced guy, by the way) installed the motor and tried to check the max. speed of the drilling head, like he'd done countless times before, the motor went z-z-z-z-i-i-i-i-i and then seized and spat the shaft seal out before the operator had any time to react!
This once again proves that you can never, but never, "trust" a new unit, especially when it comes from a new supplier. But even if it comes from the best and most OEM supplier in the world, you should still treat it as if it has its setting screwed up in the worst imaginable fashion (for the intended application), and exercise the respective cautions, always assuming the worst-case scenario.
If the pump has a pressure compensator - assume that it is set at abnormally high pressure, if it has an LS adjustment - check if the adjusting screw isn't bottomed out and blocking the LS spool in the "always-on" position, if there's a minimum displacement limiter on a motor - assume incorrect setting, if it's a closed-loop pump - assume that the null will be off, etc...
This doesn't mean that it's all and always bad, but when you replace an old unit with a new one, you should bear in mind the consequences of possible incorrect settings and be prepared for them before pushing the "start" button.
All hydraulic systems are different, so your actions should be trimmed to the system that's in front of you. Checking a few things and taking simple precautions like lowering the relief setting, not using full command inputs, and have the "emergency stop" button handy is a good place to start.
I have to warn you about one very important thing now - Murphy's law is real! If you start applying the "don't trust the new units" philosophy to your hydraulic practice, I guarantee you that the first several hundred times you take all the precautions expecting a new unit to be off - the settings will be OK, however if you decide to skip a few checks "just this one time" - the unit will be so badly set that it will explode and take the rest of the system with it. Just something for you to bear in mind - this only works under the "100 percent commitment" condition!