Any discussion of sealing technology would not be complete without a brief review of "service problems". All the possible environmental and operating considerations are seldom available
when initial seal selection is undertaken and, as a result, it is sometimes necessary to reevaluate the seal selection in light of actual field performance. When seal problems do occur,
a visual examination of the worn or damaged seal is generally sufficient to allow the necessary corrective action to be initiated. Following are some of the more common types of problems
which may be the cause of seal failure:
SLOW, UNIFORM LEAKAGE
- Poor low-pressure sealability (esp. if a lip seal)
- Too little initial interference (if squeeze type seal)
- loss of interference or squeeze due to wear or compression set
- Seal shrinkage after installation (possibly chemically induced, or by leaching of plasticizers by solvent action)
- Possible omission or failure of static seal(s)
- Microscopic debris lodged under seal lip (lint, fiber, etc.)
- Scored lip due to passage of sharp particle under seal, leaving cut or nick
- Seal lip nicked or cut during installation (note whether leak starts immediately after seal installation)
- Non-repetitive overheating hardens compound (which loses its ability to conform to dynamic surface deviations)
- Off-center alignment puts all clearance on one side, all compression on the other (due to bearing wear, excessive side loads, etc.)
- Check static surfaces of dynamic seal (groove surfaces). They may have problems F, G or H hidden from view, and without self-cleaning tendency.
GRADUALLY INCREASING LEAKAGE
- Progressive wear
- Increasing compression set
- Progressive tear or erosion from initial nick
- Fine score mark on dynamic surface abrades seal lip progressively
SUDDEN, COPIOUS LEAKAGE
- Extruded seal
- Torn seal lip (see 1-D, ·E. -F, -G. -H, and -1; and 2-D)
- Twisted seal
- Dramatic bearing failure due to excessive side load, shock, etc.
- Spiral Failure
- Massive infusion of contamination (due to incorrect fluid added to system, or to upstream introduction of dirt or wear debris)
Slow rod seal leakage builds up behind tight wiper, then dumps ... giving appearance of catastrophic seal failure. If leak rate then diminishes, look for
slow leak or erratic leak causes. If high leak rate continues, look for true catastrophic leak origin.
- Reverse-pressure blowout of piston seal due to pressure trap or failure of opposed seal
ERRATIC (START-STOP) LEAKAGE
- Cold start-up shrinks seal; friction/fluid heating restores size
- Intermittent eccentric loading
- Fibrous contamination working Its way past seal lips
- Unstable seal (twists & returns. cocks, etc.) usually caused by shock loading
- Rod seal leaks slowly, tight Wiper periodically dumps accumulated leakage (see 3-G)
- Auld viscosity· changes as temperature cycles (e.g., fork lift truck alternately entering and leaving cold storage area)
- Worn-away low-friction surface treatment
- Breakdown of fluid lubricity due to contamination or deterioration of fluid
- Viscosity change due to temperature
- Excessive burnishing of dynamic surface to finer finish destroys ability of surface to maintain lube film (e.g., may go from 12 RMS to 4 RMS)
- Seal & bearing swell due to incompatible fluid and compound (possibly running hotter than temp. at which fluid is compatible)
- Thermal expansion of compound
- Pressure trap between dual squeeze seals or incorrectly installed lip seals
- Wedging of seal or back-up device into extrusion gap (if used, it is usually the back-up device that extrudes)
- In low-pressure systems. shock or other factors cock, can’t or misorient the seals in grooves. This may become more common as OSHA rulings force reduction of pneumatic pressures
- Bent rod, cocked head, etc., often due to unreported accident
SCORED ROD OR RAM
- Internally generated contamination, or
- Externally introduced rod dirt, dirty make-up, or disassembly/re-assembly dirt
- Misoriented exclusion devices (wiper/scraper); eccentric installation.
- Misaligned (eccentric) loads cock ram into metal-to-metal contact with head.
- Wiper in vertical ram forms catch-all pocket
- Inspect valve for leakage and full closure BEFORE disassembly. (Disconnect return line on valve & inspect visually for leakage)
- See problems 1 and 2, as applied to piston seals
- Misapplied cast iron rings in a "hold" cylinder (right ring in the wrong job)
- In "Retract-Mode" creep, check rod seal as well as piston seal
- Static internal seals may provide leakage path past piston
INCREASING CYLINDER DRAG
- Seal swell caused by improper (incompatible) installation lubricant (e.g., EPA seal lubed with petro-based grease or oil)
- Packing of contaminants into wiper groove of vertical ram
- Thermal expansion of bearings and/or seals
- APPARENT Drag increase due to undetected flow restriction in supply or return line ... or by-passing of pressure through improperly closing valve ... or obstructed check valve, etc.
- Cocked or twisted seal by-passing fluid and wedging into extrusion gap.
INCREASING CYLINDER/ROD TEMP
- See causes for problem 9. In their earlier stages, these problems may appear as hotter-running cylinders
- Internal leakage "throttling" past seals can cause rapid heating
Decreased lubricity of fluid can boost friction & heating (hotter fluid has lower viscosity = lower lubricity = higher friction = hotter fluid = lower viscosity, etc.).
Contaminated or deteriorated fluid can cause same cycle
- Diluted fluid can boost friction, etc
- Condensation in reservoirs can emulsify or hit cylinder as slugs of fluid with near-zero lubricity. Also, hot water can swell compounds such as urethanes, increasing friction.