What would you say if I asked you: "What is the first pressure-regulating hydraulic valve a person learns when she is introduced to the oil hydraulics?" I bet most of you will answer "the relief valve, of course!" And you would be absolutely right. But would you believe me if I told you that a good half of people who work professionally with hydraulic systems (and this includes both general mechanics, who happen to change a hydraulic setting on rare occasions, and specialized hydraulic techs, who do this kind of stuff daily) don't know what they are doing when they are adjusting relief valves?
This statement may sound contradictory, offensive even - but the goal of this post is not to label my hard-working peers as stupid or unprofessional but to a) call attention to the fact that a tech can be perfectly capable of setting a relief valve in a particular system correctly, while still not knowing why he is supposed to be setting the valve in this or that manner, and b) promote a more methodical (or scientific, if you will) approach to setting relief valves, as a great way of achieving the correct setting while also expanding your "hydraulic erudition".
Allow me to explain what I mean in a different way:
Imagine a mechanic, who's given a test bench and a hydraulic relief valve and asked to set it at 100 bar.
A rookie will say "yes Sir", and proceed with connecting the valve to the test bench.
An intermediate tech may ask at what flow he should be setting the pressure.
An advanced tech will inquire what function the vale performs in the target hydraulic system.
A very advanced tech will ask all of the above and then will note the valve's make and model and look it up before doing the adjustment.
And a very very advanced technician, aside from making sure that he knows the valve's function, type, and the target system parameters, will also inspect the test bench he's given to make sure the pressure gauge that's sitting on the front panel is connected in the "right" place.
I guess what I am trying to say here is that while there's absolutely nothing wrong in following the instructions like "connect a pressure gauge to the point A and then turn the adjusting screw clockwise till the indicated pressure reaches ... bar", it can only teach you that much. Looking into the valve's function and discovering what it does in a system, how it functions, and why this particular type of valve was chosen may take more time, but this time is not wasted!
Here's the list of what I like to know when I am setting a relief valve:
It gives me information about the expected pressure setting and alerts me if the requested value is in the right/safe ballpark or not.
It gives me information about the expected flow. For example, a relief valve that is regulating the charge pressure of a closed loop is constantly venting the charge flow, and therefore should be set with the nominal charge pump flow, while a thermal relief valve that is protecting a hydraulic cylinder should be set at its cracking pressure, and confirmed to be leak-free within a certain pressure range.
It gives me information about the expected oil viscosity (some relief valves "toil" with lubrication oils, you know, sometimes very "syrupy" stuff...)
And last, but definitely not least - it gives me the information about how precise the adjustment really needs to be. I probably should have placed this one at the top, because having a good idea about the margin of error that is acceptable for the valve you're setting is very important. If you need to adjust the main relief of a log splitter at 250 bar, setting it in the 240-270 bar ballpark most likely won't make much of a difference both for the end-user and the whole "system", and so there's no real need to worry about the correct oil temperature as you set it, or make sure that the flow of your test stand matches the tractor's (or whatever else they used to feed the splitter), on the other hand setting the charge pressure of a closed-loop pump with automotive control and a considerable pressure carry-over angle of the valve plate a couple of bar lower can render the transmission "weak" for no apparent reason.
One more thing - knowing a relief valve's function also allows you to make a decision whether any wear, if detected, is acceptable or not. In the case of relief valves "wear" pretty much equals "excessive leakage". Again - if it is a valve that is used in a "constantly open mode", a small amount of leakage before it cracks can be absolutely acceptable, while in the case of a valve that is working as a "fuse" in an accumulator circuit, or the thermal relief example from above - even minor leakage means the valve is faulty and should be replaced.
This is very important because knowing the relief valve's type (e.g. direct-acting seated, balanced piston, balanced poppet, etc..) gives me the information about
This is something that gets overlooked often, especially in the field, where the choice of accessible measuring points is limited, but it can happen in a shop as well. For example - if you have your pressure gauge connected at the pump's outlet (a place where you will often find a conveniently placed test coupling in most systems), and it's a classic closed center load sensing system and you are setting anti-chock relief valves mounted on the actuator (or in the outlets of the DCV) - there's a good chance that your setting will be delta-P bar lower than the desired.
Now, of course, there's more. I like double and triple-checking the "adjustability" of a valve when I am setting it - i.e. seeing how well the pressure change corresponds to the turns of the adjusting screw, I like placing my oval gear flow meter in the T line to check leakage and catch the "cracking" pressure (if I can), there can be more things to do and check, we are doctors, machines are our patients, and every case is different, so don't consider this post as the ultimate guide to adjusting relief valves. Look at it as an encouragement to learn more about the screw you were told/taught to turn. Do that and you'll be a top-tier tech in no time!