A couple of weeks ago I serviced this little puppy here - a Rexroth A6VM355 variable displacement motor. A severe case pressure spike.
The front cover that houses the shaft seal created enough force to shear the metal over the snap-ring that secures it in place! When I pulled the cover out, I was surprised it was still in one piece with all the radial fissures running across it. I am sure I could break it in half with my bare hands if I wanted.
The case pressure spike was caused by the quick coupling in the motor drain line that was connected incorrectly (one of those situations that you hear about all the time and think it'll never happen to you). This particular motor came from the drilling head of a raise-boring rig. These heads need to be disconnected from the HPU for transport, so it's very common for these rigs to use quick couplings in all lines. One day the quick coupling wasn't inserted all the way and - Boom! My condolences.
Whenever I come across a system that uses a quick coupling in a drain line, and there is no way to "engineer it out", I always recommend installing a 5 bar check valve teed to the drain line as an "emergency atmospheric vent" to protect the motor from exploding when the drain coupling is disconnected (or misconnected).
The direction in which the "business end" of the emergency relief will be pointing depends on the relationship existing between the maintenance crew and the operators of the machine in question. I've seen solutions ranging from a carefully placed transparent plastic tank to a hose pointing directly at the control booth.
I have also heard an opinion that in such cases the motor shaft seal can "save the motor" and blow out before something breaks. Well, sometimes it can, but sometimes it can't - have another look at the pictures, they are educational. In systems with large drain flows or forced flushing, even if the seal does blow out, the pressure can still be high enough to crack the case. If you repaired enough Danfoss series 90 pumps - I am sure that you've come across one with the case cracked just above the round side cover - they all have shaft seals, you know, and the cast body always "saves" the shaft seal when a case pressure spike is big enough.
There is another place where such predicaments can happen - hydraulic test benches, which almost universally use quick couplings for all connections, and therefore have the same vulnerability. Even experienced techs make mistakes.
I am not ashamed to admit that I did it (actually - I am, kind of, ashamed, but I want to report my blooper anyway). And it happened recently - about a year ago. I was testing this huge multi-section Parker directional valve with proportional solenoid control. This model used a dedicated drain line for the pilot pressure reducing valve. The test was going OK, but then something urgent came up - and I needed to test another thing on the same bench and disconnect the main lines from the Parker valve.
The next thing that I did was pure genius - since the drain line of the pilot section was causing the oil to fill and overflow the now open ports of the valve (because it was still connected to the slightly pressurized collector of the test bench), I disconnected the female quick coupling of the drain line, but left it "sitting in place" over the male so that I could quickly re-connect it back as soon as I'm done with the second test - something I never remembered to do...
So I finish the second test, go back to the first one, and re-connect everything BUT the drain line, because the female coupling is fit over the male coupling and although it's not engaged, it looks like it kind of is... The pump starts, I turn on the pressure supply, try to control the valve - and - immediately - I see (and hear) oil mist coming from several proportional solenoid caps...
Three of them had to be replaced, and these don't come cheap, by the way, because the solenoid and the proportional pressure reducing valve underneath it make a single assembly...
Two lessons here:
For machine owners and operators - respect the drain lines of your hydraulic motors! And if you have to use a quick coupling in a drain line - never leave it unprotected.
For mechanics - pay attention when you test stuff on test benches and double-check all the quick couplings before initiating a test, your years of experience don't mean squat when you aren't paying attention.