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!
