Insane Hydraulics

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Using Displacement Limiters in Bent Axis Units as a Diagnostic Tool

Today I would like to talk about bent-axis variable displacement hydraulic motors, more precisely about their displacement limiters, even more precisely about the set screw type displacement limiters used in the classical bent-axis design - like the Rexroth A6VMs. Unfortunately not all bent-axis motor/pump manufacturers follow this pattern, which saddens me a great deal, because these displacement limiters, aside from their natural function of limiting the travel of the valve plate, can also serve as a good diagnostics and adjustment tool.

From the cutaway drawing, you can see that the displacement limiting screws come in direct contact with the valve plate, allowing you 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 wiggling 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 shifting. The short video below demonstrates how easily it can be done.

In some motors, it is possible to use these screws to lock the valve plate in a certain position and eliminate 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 the 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 the end-of-travel positions, this method can only be used with bent-axis motors that follow the classical design and have adjustable displacement limiters.

By the way, in the video below you'll see that the control starts at 13 bar, but re-seats at around 12 bar. This, my friends, is its majesty hysteresis.

This little maneuver is a neat tool for you to carry along in your trick box, but remember - there's case pressure behind those screws, so don't get "too enthusiastic" screwing them out on a working machine, or this may end badly!