InsaneHydraulics - Sergiy Sydorenko © 2009-2011 All Ridghts Reserved
A series of nasty repeated breakdowns of an industrial street sweeper
closed loop transmission lies in the origin of this short article. That
damned mechanized mop wouldn't stop breaking synchronizing shafts of one
of the two Sauer Danfoss series 51 motors, which were driving rear
wheels through planetary gearboxes (example).
The shafts wouldn't last for more than a couple of months, and since
the insistent breakdowns felt like monotonous kicks to the groin for the
company behind the overhauls, a lot of heated discussions concerning
probable causes of the failures were born during the investigation.
Having heard a couple of these debates made me realize that not
all "hydraulic professionals" fully understand one very important
difference between the bent axis and swashplate motor designs -
something I like calling "what drives what", which means that I am being
genetically forced by the IH code to make my own attempt to fill this knowledge gap.
Allow me to explain what exactly I mean by saying
"what drives what": in a swashplate type hydraulic motor pistons slide
on an angled surface and drive the barrel which in turn drives the output shaft via the spline in the center of the barrel, while in a bent axis type hydraulic motor, the pistons drive the shaft, and the shaft drives the barrel via a mechanical linkage, which can be a) a synchronizing shaft, or b) a timing gear, or c) pistons themselves - tapered or cylindrical with rods.
I am not going to address the peculiarities of different linkage
designs in this article - all I want is to underline the point - in
swashplate type motors the barrel drives the shaft, while in bent axis motors the shaft drives the barrel.
Period. By the way, this is why the above mentioned shaft is called
"synchronizing" - because its purpose is to synchronize the rotation of
the barrel and the shaft.
Now, since I already started to talk about broken
synchronizing shafts of Sauer Danfoss series 51 motors - I would like to
theorize a little on the topic and to toy around a couple of ideas that
have been playing in my head. I have no doubts that in the above
mentioned cases the shafts failed due to fatigue caused by cyclic
torsion stress - this is my opinion based on my very limited knowledge
of how fatigue failures should look like. If it were my call - I
would love to show all of these broken shafts to metallurgical
consultants to get their professional opinion, but root cause analysis
is still traditionally regarded as something eccentric and unnecessary
by most Portuguese companies, so the broken shafts ended up in
scrap and all I was left with were a couple of pictures I was fast
enough to take.
In any case - when I think about what can be done to
create a cyclic torsion load stressful enough to cause the shaft to
start cracking and eventually break - the only thing that comes to my
mind is - intense and repetitive accelerations and decelerations of the
main shaft (or torsional vibrations, if you will) caused by the load.
Even in the 80 cc model the barrel is quite heavy, so accelerating or
decelerating it at high rate can subject the synchronizing shaft to
significant stress - make this situation repeat itself long enough - and
you have a fatigue failure on your hands. Anyway - this is just me
For that particular type of vehicle, if I were asked where
to concentrate the troubleshooting efforts, I would advise to
investigate (aside from the obvious general closed loop parameter
checking, like speed/pressure/temperature/etc...):
- the terrain the vehicle was operating on - the presence of the speed
bumps or curbs the operators would like to pass over at full speed on an
- the presence of an unfavorable mechanic condition in the planetary
gearbox - like excessive play, caused by spline or gear wear;
- the motor displacement control function.
It would be really interesting to take one of these
problematic sweepers and study it in detail to determine with one
hundred percent certainty the real root cause of the failures, but this
is virtually impossible in real life, not in this economy anyway, so all
I can do now is to hypothesize about it and ... well... blog about
If any of you ever came across a similar problem with an alike
bent axis motor - shoot me a letter - I would really like to hear about
your experiences and your opinions on this subject.