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    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.
Sauer Danfoss M46PV
Sauer Danfoss M46PV
Sauer series 20
Sauer series 20
Sauer Danfoss series 90
Sauer Danfoss series 90
Rexroth A4VG90
Rexroth A4VG90
Rexroth A4VG125
Rexroth A4VG40
Rexroth A4VG40
Rexroth A4VG40
Oilcontrol A VSL-R valve
Oilcontrol A VSL-R valve