External gear motors are the simplest and the cheapest hydraulic answer to many "simple rotating demands". And so, after deciding to "join the hydraulic brotherhood", every rookie learns what parts these motors have inside, how they generate torque, that there are unidirectional and bidirectional gear motors with their peculiarities, how to service these units (read - replace seals) and the rest of the gear unit related bla-bla you find in any beginner industrial hydraulics course.
However, these motors possess one very interesting peculiarity that's never mentioned in catalogs or technical manuals, but which is extremely useful to be aware of, because from my personal experience and other tech's stories I can assure you that when this peculiarity "pops up" and one's not ready for it - lots of head-scratching is guaranteed. By saying "peculiarity" I am referring to the genetic predisposition of some external gear motors to start-up stalling.
Allow me to illustrate with an example:
A common gear motor (most likely driving a fan) breaks down/wears out and needs to be replaced. Naturally, you go to the hydraulic store, pick up a new motor and install it on the machine. Then you flip the switch and...
95% of cases - you flip the switch and everything works as it should.
5% of cases - you flip the switch and... what the deuce? The brand new motor is not turning! It's "stuck" so bad that you can't even turn the fan by the blades! Now what?... And the "gear motor stall" troubleshooting begins...
Lack of pressure? Warn-out pump? - No, the pressure in the line is at its maximum level and is only being limited by the relief valve at, say, 200 bar. You spend a couple of hours connecting your flow tester, the pump is tested and is OK.
Backpressure in the return line? - Nope! In fact, in a very unsafe attempt to "figure it out" you leave the return line disconnected, and all you can see is a tiny trickle of oil coming out of the return port (as well as the drain), yet the motor is not turning...
Damaged parts inside the motor, bad mount, jammed seals? - You disassemble the motor and all's well, then you assemble it, then you disassemble it again just to make sure that this time you didn't pinch the freaking back-up seal... Nope, not crazy...
You put it all back together - test it again - still nothing. Turn the machine on - the pressure rises to the maximum level and the motor is stalled - you can't even move the blades. Turn the engine off - and the motor is easily turned by hand...
At this point, most "not-enlightened" techs pass to the above mentioned "head-scratching" phase and think "how the hell something so simple can be so difficult?!!" Further actions may vary, but you get the point - a brand new gear motor has been just mounted on a very simple hydraulic system and it's not turning for, apparently, no reason.
The explanation of the "mystery", as always, is quite simple - this phenomenon is caused by the fact that not all new gear motors are "run in", especially when they are assembled locally (read in the workshop) with new parts or parts from other motors/pumps to fit the client's specific demands. By design gear motors are not balanced - so during the start-up the gear teeth are pushed against the not run-in body by the system pressure and they literally dig into it. When the body is not sufficiently run-in, the stopping force created by the gears can be greater than the torque produced by them - and the motor stalls! To make things seem even more complicated, it can even happen that you may have no apparent problems starting and running the motor with an empty shaft, and it will still stall when a load (e.g. the fan) is connected!
What can be done to set such a motor "straight"? You need to run it in under load. In an ideal workshop environment, you can attach the motor to a test stand, start it without load, and then increase the load/speed (by applying braking torque to the shaft - not by counter pressure) to the nominal value and let it run for a couple of minutes, which should be enough to finish the body.
Alternatively - you could connect and run the motor as a pump, and gradually raise the test bench pressure to its nominal value, and again - a couple of minutes under load should be more than enough.
But what if you are in the field and don't feel like taking the motor all the way back to your workshop?
Well, the solution is easy - you pressurize the system and then award the fan with a mighty blow - in other words - kick-start it, (in case of fans - don't even think of using your hands for the purpose, unless you want to see your fingers fly away from you - use an adequate object, like a wooden mallet or any other "next best thing"). If you are feeling like a gentleman - you can lower the relief valve setting, and kick-start the motor at a lower pressure and then raise it to the normal level. Normally, after working for several minutes under load the motor starts to start by itself, without kick-starting - which is exactly what we want!
Please note that the mallet technique is an "in-field only" fix that is last-resortish in nature and should be applied with caution.
Anyway - the point of this article is - not run-in gear motors can start-stall - which is not a malfunction but rather an indicator that the motor "needs some help".
So - next time you see someone hitting a fan with a mallet, know that he is no mad man but an experienced hydraulic technician and quite probably a reader of this blog!