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:
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.
