Insane Hydraulics
Bold and audacious blog about fluid power
When a person starts getting acquainted with industrial hydraulics, the first "real" component he or she 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 curious and looking for new knowledge is never a bad thing, and when learning about valves the first two "enlightenments" to occur (given the above-mentioned component types) are:
a) understanding that 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) realization that a restrictor valve pretty much resembles a common kitchen faucet (again, based on the right-hand screw design, in which turning the faucet clockwise closes the water flow).
However, it is exactly this basic knowledge that gives origin to one of the most common "hydraulic adjustment myths", that 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 truth is - this statement is valid for about 95 percent of "adjusting scenarios", leaving the remaining five to potentially hazardous situations if the same "adjusting logic" is applied. Allow me to illustrate with a couple of examples:
The DA valve (automotive control) on Rexroth A4VG closed loop pumps. Due to the design of the valve, turning the adjusting screw in (clockwise) decreases the transmission response (makes it start at a higher rpm 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 in a shop, making the owner of the shop happier and the owner of the machine sadder (and poorer). Not a good thing.
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 turning the adjusting screw clockwise actually increased the priority flow. Unfortunately for the machine, the priority flow was feeding precisely the crane-like arm hydraulics, including the rotation.
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 swang with what could be described as "satirical speed" and before the astonished operator could react hit the travel limiter with a loud bang, projecting it (along with a part of the pretty robust frame) 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 design examples where turning an adjustment screw in will decrease a setting value or reduce restriction to flow.
There is a notion around hydraulics related folks (especially those with little experience, yet brave enough to fiddle with adjustments) that turning an adjusting screw in (clockwise) either increases a 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 the 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" often don't facilitate this option, therefore being ready for "left adjustment surprises" is an extremely useful quality.
Forewarned is forearmed!