InsaneHydraulics - Sergiy Sydorenko © 2009-2012 All Ridghts Reserved
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"
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
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
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!