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    Although I have already written before about these multi-function valves in my short back-engineering article on Sauer Danfoss series 90 closed loop pumps, I feel it is necessary to address the subject again, because every once in a while I have to confront an opinion that these valves are "all the same". There is a big chance that this myth is a local Portuguese phenomenon which emerged due to the lack of technical curiosity from the part of local Sauer Danfoss dealers, who traditionally give more attention to commercial side than to the technical details of the product they sell and promote. In this article I will explain, yet again, the function and principle of operation of these multi-function valves, as well as the difference in dynamic behavior of different types of these valves.

    First, let us review the basic operation. The multi-function valve of a Sauer Danfoss series 90 closed loop pump performs four functions:

- check valve
- by-pass valve
- pressure limiter
- pressure relief

    The check valve function is for the loop filling purpose - it allows the charge oil to enter the low pressure side of the closed loop. The function is performed by the two-piece poppet (cut-view, you can see the actual parts here ). The larger poppet lifts the smaller one from its seat and lets the charge flow to enter the circuit (picture). This assembly functions as a logic element check valve configured for annulus flow (free flow from side to bottom).

    The by-pass function is for connecting the work ports of the pump when necessary (for towing, for example), and is activated by loosening the hexagonal actuator (cut-view), which lifts the poppet from the seat.

    The pressure limiter function acts to reduce the pump's displacement when the high pressure setting is reached. The function is preformed by the tiny poppet, which is held in place by the adjustable spring (cut-view). When the system pressure lifts the poppet from its seat, the oil flows directly to the servo-cylinder opposite to the one that is being used to tilt the swashplate, and thus reduces the effective delta P of the servo system and destrokes the pump.  You can see the circuit here, and also the basic series 90 closed loop schematics here

   And finally - the pressure relief function, which cuts through the unusually fast pressure surges by venting the high pressure to the low pressure side of the loop when the limiter function isn't fast enough. The inner part of the two-piece poppet (the outer part ensures the check valve function, the assembly would work with a single-piece poppet, but the two-piece design makes the relief poppet much lighter and therefore increases its opening speed) acts as a logic element piloted by the pressure limiter poppet above.

   It is this particular function - cross-port relief -  that behaves in a different way according to the type of a multifunction valve.  The main difference between the three types that you see on the picture is the size of the poppet orifice. And it is the size of the orifice that defines the dynamic behavior of a logic element. Let's look into it. The relief poppet is lifted when the force, created by pressure differential between the external face of the poppet and the spring chamber overcomes the force of the spring. When the small pilot poppet lifts from its seat and the oil is directed to the servo-cylinder, flow is building through the orifice. As the flow increases, the pressure differential acting on the poppet also increases, however with a smaller orifice it takes less flow to reach the delta P sufficient to lift the poppet than it is with a bigger orifice. At the same time, the orifice is a flow limiting factor, which conditions the flow to the servo-piston and therefore the destroking speed. Thus, by reducing the size of the logic element orifice (relief poppet orifice) we make the relief function faster and the limiter function (desroking) slower.

    Official Sauer Danfoss technical literature (readily available from their site) states that the relief function of a multifunction valve is sequenced to operate at approximately 35 bar above the pressure limiter setting, and that the relief response time is approximately 20 ms. The key word is approximately. The actual values will depend on how fast a pressure surge is, which will be determined by the system design and operation conditions. However, if we tested the three types (type 1, 5 and 2) of the multi-function valves under the same surge rate, we would obtain different results (see the diagram). Under repeating conditions the type 5 multi-function valve would spend more time cross-porting, thus contributing to oil heating but also maintaining the maximum surge at a lower level than type 1, for example, which in its place would provide a much faster limiter function.

     The correct choice of a multi-function valve is determined by the system it is used in, and that is why  special care should be taken when replacing damaged valves. By saying "special care" I do not mean that it is imperative to replace a broken multi-function valve with the same type. What I mean is - common sense and knowing of what about a system's operation can alter if the valve type is changed should be present.

    The reason I am putting only the type 1, 2 and 5 valve on the picture is very simple - these are  the most common three types to pass through our shop, however according to the Sauer Danfoss parts list for series 90 there are more.

    Sauer Danfoss multi-function valves for series 90 have simple and compact design, and as a rule last as long as the pump itself. I have seen them shatter and cause a catastrophic pump failure, but it's not an everyday malfunction. If I could change one thing about these valves that would be the  hexagonal adjusters - I would make them taller and therefore easier to clamp a standard wrench on, the ones they have now are too flat.
Sauer Danfoss Multi-function Valve
Sauer Danfoss Multi-function Valve
Sauer Danfoss Multi-function Valve
Sauer Danfoss Multi-function Valve
Dynamic response