Hydraulics 101

It's time to make sure your technicians know their hydraulics

"You've got cylinders and you've got two valves," he says. "When there is an issue, check your pressure, and if your pressure's good, the next thing is going to be your cylinders. With the auto cycling, you always want to make sure that the blade is retracting like it's supposed to and not staying in a position for a while where the guy will walk away from a truck and it still wants to extend rather than retract, and (pressure) sits there on the release valve, building up and then everything gets hot. There are times where they've got to walk to the back of the house so they pack the load but it doesn't retract, and if the parts aren't adjusted on the auto cycle valve, that tends to get everything too hot and stuff gets burnt up.

"Make sure it's going through the auto cycle correctly and make sure the pressure is set at 2,400 (psi) off the relief valve," Sullivan says. "If you have a problem, that's the first place to go. (Also) change the oil and you don't let it get hot--make sure they're not driving around with the PTO on, and that the valves aren't staying extended, like the relief valve. That's caused my most problems within the hydraulics system. Then the o-rings are getting hot, the seals are getting hot, and that's when everything else fails, because you're dealing with very high temperatures."


Barnes says perhaps the biggest part of a hydraulics system that gets overlooked is the "breather."

"People never change that, and that can cause the whole system to fail by not replacing it," he says. "If it's not breathing properly, all the air stays in the tank, goes back into the cylinders and pumps and causes premature failure throughout the system for a three dollar filter element that should have been changed. Everywhere I go, it's the same thing. We tell everybody, 'Every time you change the hydraulic return filter, always change the breather.' And they look at us and go, 'I've had that truck four years and I've never changed one.' But you go back to the truck and watch the system run, and you see the oil reservoir just swell in and out because it's not been able to breathe in air.

I've worked with dump trucks, small fleets, and (technicians) all go, 'Breather... where's that at?'"

Another common problem Barnes sees in the field is lack of knowledge about the relief valve, which controls the main pressure--without it, pressure does not build.

"A guy cranks the truck up, nothing works; he can't build any pressure," he says. "So the first thing he does is put a brand-new $3,000-$4,000 pump on it, cranks it back up and it still doesn't work. Then he finds out the $300 relief valve was not working. I always tell them, 'Go (to the valve) first and then back to the pump, because without that relief valve, nothing works. As a mechanic, it's hard to go back to your boss and say, "Well, if I spent $300 I wouldn't have needed that.' Most of them go back and say, 'Well, we needed 'em both.' You know how it is. They don't want to say 'I made a mistake and that pump was good.' We see that a lot."

Barnes says Class-3 trucks where hydraulics are under the hood--running off a belt-driven pump--can also be problematic.

"You just raise the hood and there's a belt running across the hydraulic pump, and they don't (realize) if those belts are slipping, the entire system doesn't work properly," he says.


Often overlooked, the condition of hydraulic oil is critical to the life of the system, Johnson says.

"The first step in making sure the hydraulic system is going to last is to make sure the reservoir is properly sized for the application," he says. "If it's too small, oil is going to go through the system too fast and generate excessive heat."

A general rule is keeping oil in a mobile hydraulic system under 160 degrees and checking that temperature regularly.

"If you suspect you have a problem--undersized components or anything--wait until it's been working good and hard and then check the temperature of the oil," Johnson says. "If it's about 160, you probably want to add a cooler to it, (but) a cooler is just another maintenance item."

Johnson says he has run hydraulic systems for years without changing out the oil if it was still in good condition. He says the main thing is to keep the oil clean and full, as low levels create problems.

"It creates heating condition (and) if it gets too low, you can get cavitation, which is going to tear up and damage your pumps," Johnson says. "It's when the pumps are sucking air or it may be trying to pump more fluid through than it has available, and it creates a vacuum and air bubbles form in a pump, and that will destroy a pump almost instantly. Never let your pump run dry."

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