Hydraulic cylinders

Through hundreds and thousands of cycles a day, hydraulic cylinders bear the brunt of the heavy lifting that makes you money in the construction business. These simple, rugged components may seem bulletproof, but eventually they wear out. And it is possible to damage them before that, either through careless operation or by neglecting maintenance.

Reading the signs
Most external damage to hydraulic cylinders is obvious. Dents in the tube or oil running down the chrome rod can be spotted during the daily walk around. But internal leaks, normal wear and unseen damage require more rigorous attention.

Bucket or attachment drift is a good indicator of internal leaks. A standard test for bucket drift, says Rudy Urbano, senior training consultant for Caterpillar, is to put the cylinder in the extended position, turn off the engine and wait. If the bucket or attachment moves more than a 1/2 inch in three minutes then you have an internal leak across the piston seals.

Field tests where you use a stopwatch to time cycles are another way to gauge cylinder health. Sometimes the operator will notice a sluggish feel or poor performance, but Urbano cautions not to rely on this exclusively. Cylinder performance usually degrades slowly over time and may have deteriorated as much as 20 percent before an operator realizes it.

If you do notice bucket drift or prolonged cycle times, you’ll need to do additional tests, usually with your dealer service technician, to determine if the problem is just the cylinder or if you have a faulty hydraulic pump or motor.

Damage control
Clawing deep into trenches, brushing up close to walls and truck beds, most heavy equipment hydraulic cylinders work in a rough and tumble environment. The chrome gets scratched, tubes get dented or the seals get nicked or cut.

Oil leaking through the seals onto the cylinder rod demands a quick response. If oil is leaking out, dirt is being sucked back in and dirt that gets under the seal contaminates the hydraulic fluid and scratches the surfaces of the internal seals.

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Contamination from dirt ingestion through the cylinder seals also accelerates wear on expensive hydraulic pumps. “When I started 38 years ago, 2,500 psi was a high-pressure system,” Urbano says. Today’s hydraulic systems frequently operate in the range of 5,000 to 6,000 psi. “With those kinds of pressures, the clearances have to be small and any contamination can be forced between two moving metal surfaces and scratch and nick them and become a leak path,” he says.

Piston-style pumps provide greater control for the operator, compatibility with electro-hydraulic and pilot control systems and are replacing many of the gear-and-vane-style pumps. But the gear-and-vane style pumps were designed to cope with more contamination than is healthy for piston pumps. In addition to causing accelerated wear, contamination also leads to overheated hydraulic fluid, which degrades cylinder seals, which in turn invites more contamination and becomes a self-perpetuating problem.

To keep contamination at bay during the cylinder rebuild process Caterpillar has a certification method for its dealer shops. Hydraulic oil must meet the ISO 18/15 standard for cleanliness. “We used to be able to just do a rebearing and reseal and install it on a machine, but that’s not acceptable anymore,” says Bill Svoboda, exchange reman manager for
Cashman Equipment in Nevada. One of the strategies Cat dealers use to meet that standard is to filter the hydraulic fluid as it’s being pumped into the shop’s storage tanks and again as it’s going into the machine’s reservoir. After repairs the hydraulic fluid is filtered through a high-efficiency, kidney-loop filtration machine to pull out any debris or contamination that may have inadvertently entered during the rebuild or repair process.

Cylinder lifecycles and repair
In a relatively clean environment with no accidental damage, a cylinder seal will normally last 4,000 to 5,000 hours, Urbano says. Reseal kits are widely available and can be installed by dealers or contractors with their own shops. At that point in the cylinder’s lifecycle you also want to inspect any bearings at the eye or rod end and replace those if they are worn.

If you’re doing this yourself, cleanliness is crucial. (For more on maintaining a clean shop, see the article “Contamination control,” on page 31 of the August issue of Equipment World.) And even if you don’t have a kidney loop machine to super-clean your hydraulic fluid after the repairs, you can upgrade to a high-efficiency synthetic hydraulic oil filter that will do much the same.

As a cylinder ages, resealing alone may not suffice to restore the cylinder to its peak performance. The chrome rods get scratched, worn or bent a few thousandths of an inch. And over time the inside of the tube becomes worn by friction, typically in one spot, where most of the motion takes place. At the rebuild stage, rechroming or replacing rods is standard at Cashman Equipment, Svoboda says. And rehoning the barrel needs to be done every time the seals are replaced.

Need for precision
Checking these components for this kind of wear, particularly on large cylinders, is beyond the technical abilities of many contractor shops. Full repair or rebuild requires a disassembly bench, hone, cylinder washer and tester. Most contractor shops don’t have the equipment, Urbano says.

Measuring the cylinder barrel for wear requires sophisticated instruments. “We have a long tube with a computer electronic microscope on it to view the inside of the barrel to see how the cross hatch looks along with any bad areas,” Svoboda says. “Another tool, an inside dial indicator, can run the complete length of the barrel and see how much it fluctuates inside.” The final stage of a rebuild involves testing the rebuilt cylinder for signs of leakage, again using specialized equipment.

Some cylinders, usually the more expensive ones, are built with thicker wall tubing to allow for additional repair, Urbano says. After the several reseal and honing repairs have increased the tube’s inside diameter 10,000ths-of-an-inch oversize from the nominal dimension, other options are available. Instead of completely replacing the tube, new seals are available that will compensate for up to 30,000ths-of-an-inch oversized tube inside diameter.