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    External gear motors are the simplest and the cheapest answer to many "rotating demands" in oil hydraulics. And so, after taking the decision 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 bla-bla you find in any beginner's industrial hydraulics course. However -  these motors possess one very interesting peculiarity that's never mentioned in catalogues 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 out" (it is rare to happen but it does happen) and one's not ready for it - lots of head-scratching is guaranteed. By saying "peculiarity" I am referring to the genetic predisposition of new external gear motors to start-up stalling.

   Allow me to explain 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 warehouse, pick 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. The pump is tested and is OK.

    Counter pressure in the return line? - No, in fact you leave the line disconnected, and all you can see is very little 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? - No, you dismount the motor and all's well, then you mount it, then you dismount it again just to make sure...  Nope, not crazy...

    You mount it all back - test it again - still nothing. Turn the machine on - the pressure rises to 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 parts from other motors/pumps to fit the client's specific demands, or the bodies are machined (to accommodate a different type of flange, for example). 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 the body. When the body is not sufficiently run-in the stopping force created by the gears can be greater than the torque produced by the gears - 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 no load, and it will still stall when a fan is attached!

    What can be done to set such a motor "straight"? You need to run it in under load. Ideally - in a 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. But what if you are in the filed 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 generous punch - 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 - use an adequate object, like a wooden mallet or any next best thing). If you are feeling like a gentleman - you can lower the relief valve setting, and kick start the motor at lower pressures an then raise them to the normal level. Normally, after working for several minutes under normal load the motor starts to start by itself, without kicking - which is exactly what we want!

   Note that the latter is an in-field solution, therefore the mallet technique described is last-resortish in nature and to be applied with caution.

    Anyway - the point of this article is - new (not run-in) gear motors can start-stall - which is not a malfunction but rather an indicator that the motor "needs a little help".

   So - next time you see someone hitting a hydraulic motor driven fan with a mallet, know that he is no mad man but an experienced technician and quite probably a reader of this blog!   
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