Hydraulic Seal Replacement Prevents Leaky Cylinders
Sept. 28, 2010
Quick Tip: Need a Breather
Larry Stewart, Executive Editor
Maintenance programs that quickly repair seal leaks are integral to controlling hydraulic-system repairs and downtime. It's not a hard policy to justify. Wear is as easy to see as a drip of oil in the dust, and the vast majority of cylinders in construction equipment can be resealed in four hours for less than $250. On the flip side, working with a leaky cylinder starts a chain reaction of wear that cuts short the life of valves and pumps.
Once a seal begins to wear, it becomes less effective. Sharp edges designed to shear oil off the rod or cylinder barrel become rounded and let more oil pass. When the rod retracts, some oil is scraped off by the dust wiper and you get a slobbering or drippy hydraulic cylinder. Now's the time to replace those rod seals.
The reason repairing leaky rod seals is so critical is that as oil leaks out, dust gets in. Abrasive contaminants clinging to the oil film on the rod ride past the leaky seal and head downstream from the cylinder in the hydraulic lines. The first component downstream is typically a valve. Dirt particles wedge between the barrel and spool and gouge the surface. The valve's metering surfaces begin to erode.
The other significant source of contaminant in hydraulic systems is the reservoir breather. Many systems are sealed, but those that are not gulp in ambient air every time the fluid level drops in the reservoir. As cylinders extend, they draw oil from the reservoir. The fluid level drops, and air-loaded contaminant is sucked in through the breather. To protect the system's seals and tolerances, the breather filter should have the same beta ratio or efficiency rating as the system's fluid filters.
Contaminants slipping past the rod wiper wear down the piston seals — the critical interface for hard work. If piston seals don't contain oil on the high-pressure side, piston and cylinder efficiency suffers. Load-sensing hydraulic systems will compensate for some internal leakage by pumping more fluid, but when the cylinder is in neutral, it won't support a heavy load. It begins to drift.
In their service manuals, equipment manufacturers publish acceptable rates of drift and methods of measuring it. The process is simple — usually just raising the implement, placing the valve in neutral, and measuring how far it drifts during a fixed period of time. Measuring drift should be part of the regular inspections of any machine with hydraulic cylinders. When a cylinder drifts too much, the piston and head should be resealed.
Ignoring drift accelerates wear throughout the hydraulic system. Oil squeezing past piston seals creates a lot of heat. Oil degrades at higher temperatures, so lubrication breaks down. Metal-to-metal wear in valves, pumps and motors increases. Heat rapidly ages seals throughout the system.
Hydraulic systems are typically designed to operate at about 130 degrees Fahrenheit. Temperatures under the lip of a seal will be somewhat higher (as much as 50 F higher on rotating shafts) than system temperature because of friction at the seal surface. When temperatures climb above 180 F, seals begin to harden.
When seal leakage advances to the point where the rod begins to wear against its bearing or the piston scrapes the cylinder barrel, the whole hydraulic system is in trouble. Even after the cylinder is rebuilt or replaced, valves, pumps and motors often continue on their way to early failure unless the system is thoroughly flushed and the fluid cleaned.
In the ranking of most common cylinder killers, side loading is a distant third to contamination and heat. But it's worth noting because it's fairly easy to prevent.
When the pivot point where the rod connects to the machine is not greased regularly, the rod's eye binds on the pin that connects it to the implement. As the piston extends or retracts and sets the implement in motion, the piston rod wants to swing with the implement. There won't be enough force to bend or break the rod, but the lateral forces wear on the rod and piston seals.
Regular greasing will help keep the eye free to turn on the mounting pin. But it's a good idea to check the rod bearing for roundness when the head is torn down. Mark its orientation in the head before removing it. If the bearing is elongated, you can orient it in the head again to determine where the side loading occurs.
Chemical compatibility can also increase seal wear. If a foreign fluid — such as transmission fluid or brake fluid, glycol or diesel fuel — gets into the hydraulic system, it can age most hydraulic seals prematurely.
Nicks or dings in the cylinder rod that are big enough to catch a fingernail can ruin rod seals, too. A damaged rod should be repaired or replaced, and its seals replaced, immediately.
Preserving seals is the key to long cylinder life. They must be kept lubricated to prevent fast wear on their elastic surfaces. The goal is not a dry rod, but one on which there is no obvious quantity of fluid. It's pretty easy to see when they need attention. It's critical that they be serviced as soon as wear starts to show.
This article originally ran in the January 2001 issue.
Quick Tip: Need a Breather
Hydraulic systems that are not sealed gulp in dusty ambient air through the reservoir breather every time the fluid level drops. Dirt entering the reservoir circulates through the whole system before it can be removed by a return-line filter. To protect the seals and tolerances, the breather should be fit with a filter that's every bit as efficient in the same contaminant-size range as the main system filters.
Avoid Side Loading
Grease cylinder mounting points to preserve seal life. If the clevice that retains the eye of this stick cylinder is dry and bound up, when the cylinder retracts to extend the stick, as the stick pivots on its boom pin the cylinder rod wants to swing in a downward arc. The piston scrapes the barrel and the rod exerts force on one side of the head. The seals, and eventually the rod and piston, will be worn oval. None will last for a satisfying length of time.
The Front Line
Rod-seal failures are typically visible because they drip. Even before it affects performance, seal leakage is unacceptable because it ushers dirt into the hydraulics. Weeping oil also compromises safety, the environment and appearance.
Valve Protector
Most hydraulic systems use a return-line filter, so the cylinder is the first line of contamination protection for valves. Dirt that sneaks by rod seals into the system scours at least one valve before it reaches a filter.
Dull Dust Seal
Install the final dust seal with its dull edge outboard. This wiper lip is designed to repel all but the very finest dirt. The sharp edge is an oil seal. Special seals with a sharp wiper lip can be used to protect cylinders working in very wet conditions.
Oil-Assisted Seal
The open cup of a U-cup or V-cup seal faces the pressure side, so oil presses the lip against the sealing surface. They're frequently installed in sets as piston seals. In double-acting cylinders, two sets are used, each facing a pressure side of the piston.
Hot Oil Cooks Seals
An increase in operating temperature of 25 degrees Fahrenheit can cut seal life in half, according to Chicago Rawhide, a division of bearing manufacturer SKF that makes seals. It's the temperature under the lip of the seal that matters, and it can be as much as 50 F hotter than sump temperature.
If the oil is not suited for the application, lip temperature will climb and shorten seal life. Other factors that raise hydraulic temperature include extreme ambient temperatures, low hydraulic fluid level, dirty oil coolers, or problems with engine cooling.
An increase in operating temperature of 25 degrees Fahrenheit can cut seal life in half.
