I have always had a hard time deciding whether I am a DC or a Marvel guy. And I have always wanted to have superpowers... Wouldn't it be nice to come to a malfunctioning hydraulic system, put your hands over a couple of hoses and proclaim: "I see the cause!!!" all while rolling the eyes back as the thunder roars across the clear sky and people fall to their knees before you...
What if I told you that you could develop a set of skills, that would occasionally allow you to perform a trick similar to the one described above? All except the thunder and the kneeling part...
But before I start talking about how you can use your own human hands in the "art" of hydraulic troubleshooting, I want to leave one thing very clear - all the techniques that I am about to describe fall under the category of "extremely unsafe", and you should not use them. Period. Read the disclaimer, then read about the hazards of pressurized fluids, then re-read the disclaimer again. "Hands on" diagnostics of hydraulic machinery is like sky-diving and base jumping - it is a Russian roulette that kills even the most experienced practitioners.
Then - why bother writing about them? For information purposes. Do I use these unsafe methods - yes, do I recommend them - absolutely not. Do I think people should be aware that they exist? Yes, I do.
Another thing - none of these tricks will help you if you don't approach troubleshooting process systematically and fully understand the function of the hydraulic system before you. Remember - your best tool is the one that's between your ears!
Any troubleshooting challenge starts with you familiarizing yourself with the equipment, and then forming an action plan. And a sound troubleshooting plan always begins with checking simple things first. Although measuring absolute parameters, like pressure, flow, current, voltage, etc, is important, you should also be aware of the fact that often a lot of useful information about the function of a hydraulic system can be obtained from binary (i.e. true or false) checks - for example - flow in a certain line - present/not present, pilot pressure - present/not present, high pressure - present/not present, a solenoid valve is powered/not powered. And in certain circumstances you can perform these simple tests almost instantly by using nothing but your own hands, which can save you a lot of time by pointing you in the right direction right from the start.
Human Hands - such a wonderful tool! Perfectly designed closed loop control manipulators equipped with thousands of pressure and temperature sensors that provide instant multi-channel feedback directly to our brains. Delivered in pairs with a limited lifetime warranty. Free to obtain, tricky to repair, impossible to replace... While by no means this "private" sensory equipment can be used for obtaining absolute values, it's extremely good at sensing gradients - which means that while we can't give an exact value in grams or degrees Fahrenheit, we are very good at telling one value from another - just give us a baseline - and we are golden! Especially when the change is happening fast. And when you couple this natural ability to sense small gradients in force and temperature to the power of human brain - you get yourself an incredibly powerful tool which is always "on hand". (See what I just did here? On... hand?)
Anyhow - to the point now! These wonderful tools can provide you instant information about:
presence or absence of oil flow:
When hydraulic systems operate - the oil gets hot. If you see a line in a working system that is at ambient temperature, and the oil in the system is at 50 C, you know that there is no flow in this line, and the opposite is true as well.
A static load sensing line is very hot - there's oil flow where there shouldn't be any - look for a broken spool or a busted o'ring. A dynamic load sensing line is cold - that's not good, it should be nice and warm - look for a plugged orifice. Closed loop flushing line is cold - that's bad - it should instantly get hot when the loop is operating.
presence of a component with excessive leakage:
Parts of a hydraulic system where large amounts of oil are passing from a high pressure region to a low pressure region without producing any mechanical work can create "hot spots", or lines that are at higher temperature that the rest of the system. Sometimes you can find them simply by touching components. Especially in conditions when a system just begins operation and the oil is still at ambient temperature. Problematic spots will be the first ones to heat up. This principle directly applies to gear pumps - one of the few components that can easily be diagnosed "by touch".
ON/OFF solenoid coil condition:
An average valve coil will get very hot to the touch during operation, and you can use this fact to determine if a coil is powered or not. Sometimes you can get an indication that voltage is present if the coil was connected with a plug that has an LED indicator - but it will not guarantee you that the current is passing through the coil. If it's damaged or one of the pins is broken, for example, you will see the LED light up but the valve will not actuate. In this case the fact that the coil didn't heat up is a clear indicator that there's an electrical problem.
presence of pressure in a line:
Very often you can tell that a pressure in a hose rose by simply putting your hand over it, or bending it a little in your hand. Hydraulic hoses are elastic and become stiffer under pressure. You won't be able to measure the pressure this way, obviously, but you can definitely tell if the line went from no pressure to some pressure, which can be useful to know in some situations.
presence of pilot pressure in a proportional valve:
Often a person can feel the difference in the feedback force of a proportional hydraulic joystick when the P line is pressurized and when it's not. The control becomes somewhat "stiffer" - some operators will tell you that this is the case with hydraulic joysticks on excavators (especially older models) when they see that joysticks become "soft" when the cabin safety lever is down (pilot pressure OFF).
And now, as usual, an example from this week, not the best one - because it shows me using and abusing this technique. - But hey, a good example of a bad example is still a good example, right?
Anyhow - a guy calls to our office and says that his tipper truck stopped tipping, and, naturally, the problem has to be solved immediately, preferably yesterday, and if he doesn't have the truck repaired in an hour the world will end, to which I kindly reply that we have so much work right now already behind schedule that the world's ending would be doing us a huge favor, but if he could bring the truck to the shop I would "have a quick look" - no promises... He's at our doorstep half an hour later.
The troubleshooting took exactly five minutes and went like this:
See the problem:
- Would you turn on the PTO and set the control to lift, please? - Sure! The empty hopper's not moving - just as promised.
Familiarize with the system:
- a piece of cardboard on the ground - and I am underneath the truck inspecting the system - Simple stuff here: an oil tank, a gear pump, a common 3 position tipper valve and a single-acting cylinder connected with a fast coupling. Nothing new here...
Simple checks first:
Oil level? There's plenty of oil in the tank. Suction line? Open. Controls? The tipping valve is one of those that have an exposed spool end - how lucky! The driver operates the levers in the cabin - I see the spool move. The controls are fine.
Hand test Nº1 - check the pump:
I ask the driver to lift the hopper again - and bravely put my hand on the pump's body. 5 seconds pass, 15 seconds, 30 seconds, the pump is not heating up. No alarms so far, and I was hoping I'd be selling a new shiny pump today... moving on then
Hand test Nº2 - is the oil flowing somewhere?
I put my hand on the return line and I can feel that it's getting warmer. Clearly the oil is passing here. Now all I need is to discover why the oil is flowing to tank - is the relief valve opening at the set pressure or is there an internal leak in the valve's body or in the relief valve itself? In this situation any sane tech would get a pressure gauge, a test tee, and check the pressure at the pump's outlet - but hey, since I am on a groping spree I might as well do one last check, right?
Hand test Nº3 - is there pressure in the line?
I ask the driver to turn the tipping valve on and off intermittently and put my hand on the cylinder pressure hose. I can clearly feel the hose become stiffer. So now I know the pressure is reaching the hose - and by the stiffness I can tell that the pressure should probably be enough at least to move the empty hopper box - the cylinder is quite large. So then what could be the problem? There's only one part between the cylinder and the hose - the fast coupling. So we stop the engine, I disconnect the coupling and - sure thing - I see that the valve nose is busted and the fast coupling was simply not engaging, effectively blocking the line...
I go and get a new coupling set from the shelf and the truck is as good as new now..
Well - I won't lie... I did fetch a pressure gauge and did re-check the relief valve setting afterwards - and it was perfectly fine. And it didn't take me five minutes, more like fifteen. But the first five were marvelous... Get to the truck - no tools but heart and hand - touch here, touch there, - Boom! I know where the problem is! Thunder sound, anyone?...
To conclude - My personal point of view about the safety of such practices is as follows:
Being in an industrial environment next to a hydraulic machine is already dangerous as it is. Your experience should tell you what is relatively safe to do and what is not. I feel OK to put my hand on a pilot pressure hose to see if I can tell if it's pressurized or not, I feel OK to put my hand on a gear pump body to see if I can detect fast overheating, I would feel OK to check the temperature of a return line by hand, I definitely wouldn't feel OK to put my hand on a 4SH hose of a closed loop transmission that works at 450 bar, especially if the hoses looked "used". And I will never come near a hydraulic hose of a high pressure hydraulic tool. Was it OK for me to put my hand on a tipper cylinder line - the truth is - no, it wasn't OK - I rolled the dice on that one. So the hand test Nº3 was a no-no, but the first two were reasonalby safe.
Please, please, be safe!