Hydraulics 101

Like most problems in a shop, it starts small, but will your technicians be able to diagnose and fix it before it becomes a major problem?

A seal is leaking on a bucket truck's hydraulic pump--probably one of dozens of small jobs that could be done throughout the day. The question is, will your technicians handle the situation with some preventative maintenance, or will they let it slide until it ends up costing you much more?

Spotting something minor like this is what proactive PM scheduling is all about, and in this case, simply changing out the seal--keeping oil losses to a minimum--can save a fleet hundreds of dollars. Let it go, and soon the leaky seal could burn up from a lack of oil, forcing you to spend $600-plus on a new pump and more oil, not to mention associated costs of keeping a needed truck out of service.


Many fleets end up wasting precious dollars on their hydraulic systems because of a lack of proper education, says Joel Barnes, technical trainer manager for Heil Environmental.

"When you are charged with finding mechanics, the first person you look at is a highly qualified diesel mechanic, (but) that guy may not be well-schooled on the hydraulic side of it," he says. "Over the years, he's just seen what he's seen and done what he's done, he's never really been to any classes or schools on hydraulics. I find, everywhere I go, a guy who's a 20-year diesel technician, but knows nothing about hydraulics because he's never had to deal with it. And so it really gets left out a lot of the time."

And it all starts with the basics. As he travels across the country, training technicians on proper hydraulic maintenance, Barnes sees a startling lack of understanding about simple schematic diagrams.

"I'll go somewhere and the first thing I'll tell them is to get out a schematic and look at the entire system; that way you can see what's involved," he says. "Everywhere I go, they don't know how to read the schematic, and they sit there looking at it, (saying), 'I can look at that all day long and I don't know what these little symbols mean.' If you can't read a schematic, the first thing you ought to do is learn how, because that's the key."

Technicians who cannot understand schematics generally end up unnecessarily replacing parts, driving up shop costs.

"(They'll) replace the valve 20 times and not know why," Barnes says. "If they would look at the schematic, they would see it was another valve before that one that was causing the failure."

Effectively troubleshooting and maintaining hydraulic systems means identifying and correcting potential problem areas before they become major issues, says Bob Johnson, director of fleet relations for the National Truck Equipment Association (NTEA).

"It's pretty darn basic, but at the same time it can be complex," he says. "Inspect all the components; look at the hoses and seals--any locations where hoses can be subject to abrasion or they cross over, support or rub on each other--look for signs of hose damage or wear, re-position them or put a protective wrap on them if necessary. Look for signs of leakage in any fittings, and if any fittings are leaking, check them--if they're loose, tighten them.

"Then make a note where you found loose fittings," Johnson says. "If you find the hoses binding, try to relieve that. Look at your cylinder seals, look for leakage there; make sure that the wipers are in good condition. Make sure all your fittings are tight; all your mounting pins and bolts are snug, because if anything works loose it can cause problems down the road. Make sure the rod wipers are in good condition, make sure your motor seals aren't leaking."


Joe Sullivan, fleet supervisor for Long Island, NY-based Norsic and Sons refuse haulers has worked with Heil Retriever Class-5 trucks for more than a decade. He says setting the correct hydraulic pressure and making sure the system is going through its proper auto cycle correctly are two important factors in properly maintaining 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."

Barnes says most technicians are punctual about oil change intervals, but generally pay scant attention to hydraulic oil.

"They change the oil every 5,000 miles or 10,000 in some cases, and I tell them, 'You've got to look at hydraulic oil the same way,'" he says. "Most hydraulic oil has about 1,000 hours of use and will just break down and no longer be any good at all; all the additives disappear out of the oil. So we tell everybody at 1,000 hours, you've got to change hydraulic oil. And you can't go on 1,000 miles or 10,000 miles because the truck may run only a few miles, but the system may be on (longer). You have to put an hour meter on the hydraulic system and go by the meter, and change the filters every six months."

Sullivan has his technicians change out hydraulic oil and filters every three months, or about 250 hours.

"We don't do a lot of mileage, and none of these medium-duty trucks have clocks on them for hours, so usually three months is my key," he says. "We go from house-to-house, mostly in circles, so it's more time than anything else."


Technicians should always check oil for a burnt odor or changed color--an tell-tale sign of trouble.

"If it's overheated you can often tell by the odor or color, depending on the type of fluid you use," he says. "You want to check for moisture in the fluid--water will kill a system real quick. Again, it goes back to spec'ing and designing the vehicle--make sure you locate the hydraulic reservoir where it's not going to get water in it. If you suspect water, and it's cloudy, you can take a sample out in a glass jar and just set it on a shelf for a couple hours and the water will actually separate out."

If you want to test for water when the system is not running, test paste--a gray-colored paste that turns pink when it touches water--works, Johnson says.

"So you can put a little bit on a stick and push it to the bottom of the tank, (but) it's got to be after it's been sitting overnight," he says. "If there's any water in there the test paste will turn pink."

To better determine if hydraulic oil needs a change, it's a good idea to periodically take an oil sample for analysis to check the condition of additives, Johnson says.

"If it starts showing certain trace elements, you have a problem--copper, brass, zinc, steel, chrome--any wear metals that shouldn't be there," he says. "If you suspect you have a major problem, when you change the hydraulic filter, take the old filter and cut it apart and spread the paper out and sometimes you can actually see dirt or metal or things of that nature in the filter. Anytime you see that, you know you've got a problem."

Johnson says it is also important to make sure equipment is in the proper position when checking hydraulic oil.

"For instance a crane, (put) the boom at rest and the outriggers retracted," he says. "If you check it with the components in the wrong position, you may add too much oil, which can also cause problems."


Above all else, make sure your technicians respect the dangers of working on a hydraulic system--the higher the pressure of the system; the greater the potential for problems.

Johnson says most mobile hydraulic systems operate in the 1,800-2,300 psi range, but some high-performance systems operate at 3,000 psi or higher, and are more prone to seal failure and hose blowouts. He says any components that can create a pinch point or crush point should not be worked on unless proper safety provisions are installed.

And that 4x4 in the corner does not count.

"Typically, there should be a specifically designed component for a safety prop, which locks in place," he says. "If it's broken, or it's not working right, a guy just grabs the 4x4 and sticks it in there, and the weight of the bed hits it and the first thing that happens is the 4x4 kicks out and there you go."

Sadly, fatal accidents while working on hydraulic systems are not uncommon. A 22-year-old technician was killed recently when he was crushed by the lift bed of a refuse roll-off truck.

The model had no safety devices to hold the bed in a raised position, so an external safety jack stand was needed between the bed and frame of the truck when the bed was raised, but the technician--working alone--did not put the jack up. When he climbed between the bed and frame and disconnected the hose attached to the hydraulic ram that held up the container, it released the pressure in the system, causing the truck bed to fall on him.

Barnes, who devotes the first four hours of his week-long hydraulics classes on safety, says being crushed by a falling bed is just one of many potential dangers.

"I've seen guys have a small leak and they'll stick their hand behind the hose and tell someone to shift the valve to see where the pressure's coming from, and the oil coming out of there could penetrate your skin and shoot a hole through your hand," he says.


The hydraulics on medium-duty trucks might be one of the most dangerous, yet most-misunderstood systems on the vehicles. It is one area where some technicians seem to make repeated mistakes, misdiagnosing problems and unecessarily changing out parts, driving up both labor and parts costs.

The core problem is that many technicians do not receive proper training on hydraulics to begin with, and in other cases, years of working on the systems without incident can create a false sense of security, and when accidents happen, they can be sudden and fatal.

Make sure your technicians understand both the complexities and dangers of hydraulics systems, and your fleet will realize the benefits of fewer parts replaced and fewer hours lost to injuries, or worse.

Hydraulic System Safety Tips:

• Ensure lift beds of trucks are properly supported before performing maintenance or repairs.
• Ensure mechanics release pressure in hydraulic systems that are supporting loads or otherwise under pressure before beginning maintenance or repairs.
• Develop and implement a job hazard check sheet that all mechanics fill out and supervisors check and verify prior to staring each work assignment.
• Develop and implement a PM program which includes pre-shift inspections of the vehicle's operational systems and safety devices and the correction of identified defects prior to placing vehicle in service.
• Know the dump bed system owner's manual, including proper inspection and maintenance of the system and the safe work practices concerning system inspection and maintenance.
(Source - National Institute for Occupational Safety and Health)