Home>> Let's Talk Hydraulics>> Displacement Limiters as a Diagnostic Tool>>
InsaneHydraulics - Sergiy Sydorenko 2009-2011 All Ridghts Reserved
The Simple Test
The Most Basic Basics
Let's Talk Hydraulics
Back-Engineer This!
News Archive
HomeIntroductionThe Simple TestBack-Engineering?The Most Basic BasicsLet's Talk HydraulicsBack-Engineer This!BattlefieldKaboomLibraryNews ArchiveBla-BlaimerContactsGuestbook |
     Today I would like to talk about bent-axis variable displacement hydraulic motors, more precisely about their displacement limiters, even more precisely about screw-type displacement limiters in a classical bent-axis design - like the Rexroth A6VM or A7VO series. Unfortunately not all bent-axis motor/pump manufacturers follow this design pattern, which saddens me a great deal, because those displacement limiters, aside from their natural function of limiting the travel of the valve plate, can also serve as a good alternative diagnostics and adjustment tool.

    From the drawing you can see that in "standard" bent-axis design the displacement limiting screws are "in direct contact" with the valve plate, allowing us to literally "touch and feel" it, which can be used to determine whether the motor is at maximum or minimum displacement, without reading servo-pressures and using any but most common tools - by simply loosening a limiter and screwing it in and out a little, just enough to "feel" if the valve plate is there.

   For example, when a hydrostatic transmission doesn't produce enough torque, one of the causes can be a variable displacement motor that is stuck at minimum displacement. Loosen the screw at maximum displacement side and check is the valve plate is there, if it is, then your problem is something else, like not enough pressure, high gear, etc....

   "Touching" the valve plate can also be a convenient way to find out exactly at what pilot pressure a displacement control starts. The short video below demonstrates how easily it can be done.

   In some motors it is possible to use these stops to lock the valve plate at a certain position and eliminate completely any influence from the part of the motor's displacement control  - again  for diagnostic purposes.

   Of course, this is an alternative method of "looking for the valve plate", and it can't replace direct servo-pressure reading, however it has certain advantages - it is very simple and fast to perform, it doesn't require installing additional hydraulic fittings, and it tells you exactly when the valve plate starts off. This ingenious technique has drawbacks too, of course,  - you must have good access to the adjustment screws, it can only give you information about end-of-travel plate positions, this method can only be used with bent-axis motors of classical design - those that have adjustable displacement limiters.

  By the way, in the video you'll see that the control starts at 13 bar, but re-seats at around 12 bar. This, my friends, is its majesty hysteresis - so clearly seen without any gages and meters.

   This little maneuver is a neat tool for you to carry along in your trick toolbox, but use it wisely - there's case pressure behind those screws (suction pressure in case of a A7VO pump) - turning one of these completely out on a working machine may end up not well!