InsaneHydraulics - Sergiy Sydorenko © 2009-2011 All Ridghts Reserved
I've troubleshot enough hydraulic systems to know that maybe ten
percent of equipment malfunctions and the consecutive
operation/production stops is caused by what I like to call "bogus"
failures. By bogus I mean failures that are caused not by broken or warn
out parts, but rather by simple and instantly "fixable" reasons, which
despite being the main reason the equipment stopped normal operation,
still do not justify major interventions, like dismounting components or
replacing the malfunctioning machines.
In this article I would like to address a component,
which can easily cause a "bogus" failure of a closed loop circuit -
namely the bypass valve. Most closed loop transmissions are
equipped with bypass valves (or the bypass function integrated into a
multifunction valve) to allow towing of the vehicle or movement of the
actuator in emergency situations. This modest feature, when activated
accidentally, can easily render a transmission inoperable, and cause a
lengthy downtime and even an expensive overhaul of the loop components -
not because it breaks or damages components, but because operators and
often even mechanics simply forget to check the bypass position and draw
wrong troubleshooting conclusions. I've seen this happen more than
once. Last time was a couple of weeks ago, when I witnessed a plant
production stop for several hours to replace the malfunctioning closed
loop power-pack with a spare one due to a partially open by-pass valve.
The mechanics followed the standard failure protocol - so no one
remembered to check the bypass. In the end it turned out that all that
was needed to fix the failure was tightening the small socked head
screw. Time and effort were wasted to replace the 500 kg power-pack, then
money, time and effort were wasted to send the power-pack to our
workshop to "repair" the problem. Quite a bite for tightening a screw,
don't you think?
The bypass system can be built into a closed loop
pump, it can also be built into an external loop flushing manifold, or
it can be a simple ball valve mounted somewhere on the frame of the
vehicle. To the left you can find some examples of bypass designs:
Pic. 1, Pic. 2 - Sauer Danfoss M46PV closed loop pump.
Pic. 3, Pic. 4 - Sauer Danfoss series 20 closed loop pump (the model with gerotor type charge pump).
Pic. 5, Pic. 6 - Sauer Danfoss multifunction valve for series 90. The last image shows the bypass function activated.
Pic. 7, Pic. 8 - Rexroth multifunction valve for A4VG90, last image shows the bypass function activated.
Pic. 9 - Rexroth multifunction valve for A4VG125, the bypass is activated by screwing the whole valve out a couple of turns
Pic. 10, Pic. 11, Pic. 12
- Fixed relief valve for Rexroth A4VG40 pump. This is a somewhat
treacherous design, as one may look at the bypass actuator, and consider
it a pressure adjustment screw. It does look like one, doesn't it?
Pic. 13, Pic. 14 - Oilcontrol AVSL-R closed loop purge valve, with built in bypass.
Bypassing of a closed loop in emergency situations can be
also achieved by lowering the setting of loop relief valves
(alternatively - removing the springs) or removing the spool of the loop
flushing block. And, of course, towing can only be performed at low
speeds and for short distances.
The most fundamental principle of hydraulic system
troubleshooting is - check simple things first! And checking if a closed
loop transmission is equipped with a bypass valve and if it is
activated for some reason is the simple one minute check a hydraulic
technician should do whenever he faces a closed loop with "not enough
force" symptom. I confess I have made this mistake once - and was the
one responsible for bringing over a closed loop transmission to the
workshop to discover that the bypass valve was open! From then on I have
developed a reflex - whenever a phrase "closed loop troubleshooting"
comes up - my head gives an instant subconscious response thought -
don't forget to check the bypass! I know I'll never make this mistake
again, and I hope that after reading this post, you too never will.