InsaneHydraulics - Sergiy Sydorenko © 2009-2010 All Ridghts Reserved
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 a setting 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...