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    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 speculating...

   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 daily basis;

- 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 it...

  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.
Broken Syncronizing Shaft
bent-axis motor with syncronized shaft
bent-axis motor with timing gear
bent-axis motor with tapered pistons
bent-axis motor with cylindrical pistons