Hydraulic Leakage

Any hydraulic system will have a certain amount of leakage. Any leakage will reduce efficiency and cause power loss.
Some leakage is built in (planned), some is not. Leakage may be internal, external, or both.

Internal. This type of leakage (nonpositive) must be built into hydraulic components to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other moving parts. In some hydraulic valves and pump and motor compensator controls, leakage paths are built in to provide precise control and to avoid hunting (oscillation) of spools and pistons. Oil is not lost in internal leakage; it returns to a reservoir through return lines or specially provided drain passages.

Too much internal leakage will slow down actuators. The power loss is accompanied by the heat generated at a leakage path. In some instances, excess leakage in a valve could cause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case of flow or pressure-control valves, leakage can often reduce effective control or even cause control
to be lost.

Normal wear increases internal leakage, which provides larger flow paths for the leaking oil. An oil that is low in viscosity leaks more readily than a heavy oil. Therefore an oil’s viscosity and viscosity index are important considerations in providing or preventing internal leakage. Internal leakage also increases with pressure, just as higher pressure causes a greater flow through an orifice. Operating above the recommended pressures adds the danger of excessive internal leakage and heat generation to other possible harmful effects.

A blown or ruptured internal seal can open a large enough leakage path to divert all of a pump’s delivery. When this happens, everything except the oil flow and heat generation at a leakage point can stop.

External. External leakage can be hazardous, expensive, and unsightly. Faulty installation and poor maintenance are the prime causes of external leakage. Joints may leak because they were not put together properly or because shock and vibration in the lines shook them loose. Adding supports to the lines prevents this. If assembled and installed correctly, components seldom leak. However, failure to connect drain lines, excessive pressures, or contamination can cause seals to blow or be damaged, resulting in external leakage from the components.

Prevention. Proper installation, control of operating conditions, and proper maintenance help prevent leakage.

Installation. Installing piping and tubing according to a manufacturer’s recommendations will promote long life of external seals. Vibration or stresses that result from improper installation can shake loose connections and create puddles. Avoid pinching, cocking, or incorrectly installing seals when assembling the units. Use any special tools that the manufacturer recommends for installing the seals.

Operating Conditions. To ensure correct seal life, you must control the operating conditions of the equipment. A shaft seal or piston-rod seal exposed to moisture, salt, dirt, or any other abrasive contaminate will have a shortened life span. Also, operators should always try to keep their loads within the recommended limits to prevent leakage caused by excessive pressures.

Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could wear on a seal and interfere with desirable oil properties. Proper maintenance prevents impurity deposits and circulating ingredients that could wear on a dynamic seal.

Never use additives without approval from the equipment and oil suppliers. Lubrication can be critical to a seal’s life in dynamic applications. Synthetics do not absorb much oil and must be lubricated quickly or they will rub. Leather and fiber do absorb oil. Manufacturers recommend soaking a seal overnight in oil before installing it. Do not install a seal dry. Always coat it in clean hydraulic oil before installing it.

Hydraulic Cylinder Troubleshooting

Hydraulic cylinders are compact and relatively simple. The key points to watch are the seals and pivots. The following lists service tips in maintaining cylinders:

a. External Leakage. If a cylinder’s end caps are leaking, tighten them. If the leaks still do not stop, replace the gasket. If a cylinder leaks around a piston rod, replace the packing. Make sure that a seal lip faces toward the pressure oil. If a seal continues to leak,

b. Internal Leakage. Leakage past the piston seals inside a cylinder can cause sluggish movement or settling under load. Piston leakage can be caused by worn piston seals or rings or scored cylinder walls. The latter may be caused by dirt and grit in the oil.

c. Creeping Cylinder. If a cylinder creeps when stopped in midstroke, check for internal leakage. Another cause could be a worn control valve.

d. Sluggish Operation. Air in a cylinder is the most common cause of sluggish action. Internal leakage in a cylinder is another cause. If an action is sluggish when starting up a system, but speeds up when a system is warm, check for oil of too high a viscosity (see the machine’s operating manual). If a cylinder is still sluggish after these checks, test the whole circuit for worn components.

e. Loose Mounting. Pivot points and mounts may be loose. The bolts or pins may need to be tightened, or they may be worn out. Too much slop or float in a cylinder’s mountings damages the piston-rod seals. Periodically check all the cylinders for loose mountings.

f. Misalignment. Piston rods must work in-line at all times. If they are side-loaded, the piston rods will be galled and the packings will be damaged, causing leaks. Eventually, the piston rods may be bent or the welds broken.

g. Lack of Lubrication. If a piston rod has no lubrication, a rod packing could seize, which would result in an erratic stroke, especially on single-acting cylinders.

h. Abrasives on a Piston Rod. When a piston rod extends, it can pick up dirt and other material. When it retracts, it carries the grit into a cylinder, damaging a rod seal. For this reason, rod wipers are often used at the rod end of a cylinder to clean the rod as it retracts. Rubber boots are also used over the end of a cylinder in some cases. Piston rods rusting is another problem. When storing cylinders, always retract the piston rods to protect them. If you cannot retract them, coat them with grease.

i. Burrs on a Piston Rod. Exposed piston rods can be damaged by impact with hard objects. If a smooth surface of a rod is marred, a rod seal may be damaged. Clean the burrs on a rod immediately, using crocus cloth. Some rods are chrome-plated to resist wear. Replace the seals after restoring a rod surface.

j. Air Vents. Single-acting cylinders (except ram types) must have an air vent in the dry side of a cylinder. To prevent dirt from getting in, use different filter devices. Most are self cleaning, but inspect them periodically to ensure that they operate properly.