When a person starts getting acquainted with industrial hydraulics,
after learning the most basic basics like the Pascal law and stuff, the
first component he (she on a rare occasion) learns is the direct acting
pressure relief valve, and the second one is the needle type
restrictor. The same can be said about those, who aren't purposefully
learning industrial hydraulics, but are somehow involved with hydraulic
machinery, and are curious enough to peek at whatever the
technician-called-to-solve-the-hydraulic-problem is doing, and since
relief valves and restrictors (flow controls) fall under the category
of "routinely adjusted components", there's a good chance that such an
"industrial voyeur" will get a general idea of how such valves work and
how their adjustment is performed.
Being curios and looking for new knowledge is
never a bad thing, and when learning about valves it is indeed an
enlightenment to understand that
a) most pressure control valves function is based on
the principle of balancing two forces - one created by a spring,
pre-tensioned by means of a right handed screw, and the other one
created by hydraulic pressure acting on an area, and
b) that a restrictor valve pretty much resembles a common kitchen faucet.
However it is exactly this basic knowledge
that gives origin to one of the most common "hydraulic adjustment
myths", which can be resumed to the assumption that turning an
adjusting screw of any hydraulic valve in (clockwise) either increases a pressure setting or strangulates oil flow.
The real life truth about it lies in the fact that this
statement is valid for about 95 percent of "adjusting scenarios",
leaving the remaining five to potentially hazardous situations if the
same adjusting logics is applied, which I would like to illustrate with
a couple of examples.
Example one (not uncommon, must I add) - the
adjustment of a DA valve (automotive control) on a Rexroth closed loop
transmissions. Due to the design of the valve, turning the adjusting
screw in (clockwise) decreases the transmission response (makes it
start at a higher rpms and therefore turns the transmission less
responsive). If a mechanic/operator expects an increase in
performance/speed after turning the adjustment screw in,
chances are that he will label the pump as malfunctioning, and the
complete transmission will end up at a shop nearby, making the owner of
the shop happier and the owner of the machine poorer. Not a good thing.
Example two (witnessed by myself a couple of weeks
ago). The piece of equipment in question was a second hand cattle feed
transporting trailer, equipped with a hydraulically driven dispenser,
which consisted of a long rotary jack-moved crane-like arm and a
feeding screw. Due to the fact that there were more independent
circuits than pumps available, a heavy duty priority valve (hammer
valve) was adapted to provide the necessary flow division. However, the
restrictor part of the priority valve was not of a needle type,
but of a spool type, and was designed in a way that screwing the
adjusting screw in (clockwise) actually increased priority flow.
Unfortunately for the machine, the priority flow was feeding precisely
the crane-like arm hydraulics, rotation included. When the machine
started, the crane movements were dangerously fast, and the attending
mechanic screwed the flow adjusting screw all the way in, and then
turned the adjusting screw back just a tiny bit before doing anything
else - the perfectly logical idea was to supply the arm with a very low
flow, and then to adjust its speed to the desired level. What happened
was exactly the opposite - screwing the adjusting screw all way in
directed all of the pump flow to the priority outlet... The first "test
movement" happened to be the arm rotation - the arm turned with what
could be described as "satirical speed" and before the astonished
operator could react hit the mechanical stop with a loud bang,
projecting the stop (along with holding screws and part of the pretty
robust structure) several meters away... It was then that the valve was
disassembled and the fact that the flow restricting part was the other
way around was discovered...
When you deal with different brand hydraulics long
enough, you will find a decent share of valve design examples where
turning an adjustment screw in will decrease asetting value or increase restriction.
The point of this post is plain - there is a notion
around hydraulics related folks (especially those with little
experience, but brave enough to fiddle with hydraulic
adjustments) that turning an adjusting screw in (clockwise) either
increases pressure somewhere (which is generally considered by them as
something positive), or restricts (closes) flow. This is not
true. NOT ALL valve
designs follow this pattern, and it is very important to bear this fact
in mind every time you are about to adjust a component that you aren't
entirely familiar with. Even if you know and understand its function as
a part of hydraulic circuit, there's still a chance that this
particular valve manufacturer opted for a different design solution,
making the adjustment direction opposite to the one you are used to.
The best way to go is, of course, getting technical
data before adjusting a component, but, unfortunately, "field
conditions" rarely offer this option, therefore being ready for "left
adjustment surprises" is an extremely useful quality. Forewarned is
forearmed...
