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    My discussion of closed loop charge pressure related stuff (started in part1 and continued in part2) would be incomplete without addressing the loop flushing subject, which is , therefore, the prime topic of the following article.

     The main purpose of a closed loop flushing system is loop oil renewal during transmission operation. You will find many closed loops out there that aren't equipped with loop flushing and apparently work fine and problem free, so naturally a question pops up - why bother?  - Well, the main reason loop flushing exists and is widely used is very simple - transmissions equipped with loop flushing last longer, because the oil inside the loop runs cooler and cleaner.

    Any loop flushing system can be divided in two parts - the directional part, which serves to always pick low pressure side of the loop to work with, and the purging part, which serves to divert   oil from the loop. As a rule, in vehicle propulsion closed circuits the flushing system is incorporated into the drive motor. In industrial applications, however, it is not uncommon to find external loop flushing manifolds.

    The classic and time proven layout of the directional part consists of a dumbbell shaped spool (picture, and a cut-view), which functions in the following manner. It is possible to come up with a different technical solution, like for example two touching spools instead of a single one, but the final purpose will always be the same - choosing the low side of the loop to purge. In some cases system designers adopt the use of solenoid valve/valves instead of a shuttle spool and in this case the loop flushing is operated electrically (for the record - I don't like this solution). And in some motors the complete flushing system is integrated into a special valve plate (like Rexroth AAFM, A2FE motors, etc..). I don't have pictures of such a valve plate at the moment, but as soon as one turns up I'll update.

   The purging part of the loop flushing arrangement is the part that should interest us the most, because it is directly connected to the loop charge system, and therefore can influence the charge pressure. Let us take a look now at different ways the purging can be performed:

   1) Classic loop purging, done by means of a relief valve. In this arrangement it is very important to adjust the purging valve below the setting of the pump's charge pressure relief valve (2-4 bars lower), as well as to understand that it is this valve that defines the charge pressure level during transmission operation. In this system the flushing flow equals charge pump flow minus the leakage, and it is very easy to confirm its correct operation by monitoring the charge pressure drop when the pump is on stroke.

    2)  Orificed relief valve purging, done by means of a relief valve combined with an orifice (schematics). A relief valve is set to a relatively low threshold, and the purge flow is then determined by the orifice size. Most of the times the spring setting is fixed, although some designs will allow you to exchange orifices. Depending on the size of the orifice, the charge pressure level may and may not drop during the operation. Very often the charge pressure will exhibit momentary drop during the initial phase of the pump stroke, and then will return to its normal value - a behavior which can be easily misinterpreted as a malfunction or incorrect adjustment if the technician isn't familiar with peculiarities of this system. The best way to confirm its correct function is through flushing flow (motor case flow) measurement - an important verification - because like any orifice, this one can get blocked and cause flushing malfunction (example). In this system flushing flow is a percentage of the charge pump flow, and is defined by the orifice size and the charge pressure setting.

   3) Simple orifice as a purge valve - by far the most compact and the simplest system you could possibly imagine, often used for flushing solutions when small size is mandatory. Its main drawback is the fact that it doesn't have an opening threshold like the above system (defining minimum charge pressure level), which can become an issue during low speed and low charge flow operation, causing the charge pressure to drop beyond acceptable level. The integrated flushing valve built into motor valve plate is one example of such a system - there's no place for a relief valve inside the valve plate, therefore the purge flow limitation is done by restriction, performed either by an orifice, or by a slotted shim mounted in the center of the valve plate.

    4) Flow limiter as a purge valve - a solution which is rarely used in closed loops (no need to go that complicated) but can be occasionally found in open loop transmissions. When such a purge valve is used in a closed loop, there also will be no noticeable charge pressure drop during the operation.

     As you can see, it is the type of the flushing system that defines if and how much the charge pressure drops when the pump is on stroke, and consequently it may and may not influence the charge pressure setting. For that reason it is important to identify which type of flushing system is used in a closed loop transmission before adjusting the charge pressure.

     Serving the same purpose, closed loop flushing systems elaborated by different brands have their design and performance peculiarities, therefore the most perfect loop flushing learning material is technical catalogues, which will also provide you with characteristic curves and technical data. That is why I am concluding this article with these files, which are a good place to start for those who like going into detail.

Sauer Danfoss Flushing Block
Rexroth Flushing Block
Poclain Flushing Block

   P.S.

    Another important loop flushing issue to consider is the fact that, like any high pressure exposed component, the directional part of a loop flushing system (the shuttle spool) can become the point of high pressure leakage and cause the transmission to malfunction when damaged or worn out, that is why it is important to check it for excessive clearances every time such a motor is overhauled . A good example of this type of wear is in this short video, which shows a loop flushing block from a Sauer Danfoss series 20 motor worn beyond repair.
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