Hydraulics 101

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

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.

We Recommend