Applied Failure Analysis Hydraulic Cylinder Seal Failure Analysis

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Applied Failure Analysis Hydraulic Cylinder Seal Failure Analysis{7562} Media Number -REHS2811-00 Publication Date -01/11/2005 Date Updated -29/11/2005

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Hydraulic Cylinder Seal Failure Analysis{7562}SMCS - 7562

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IntroductionCareful inspection of the hydraulic cylinder seals after a failure can give indications of the root cause. Document the complaint of the customer and determine if the problem is drift, leakage, or noise. Leakage can occur at the rod seals between the cylinder head and rod or at the head seals between the head and the tube. Leakage can also occur internally at the piston seals, between the piston and the tube. At disassembly, all seals should be inspected for any change in appearance in comparison to new seals. All scratches, nicks, cuts, foreign particles, dimensional changes, or color changes should be noted. The following sections describe most common problems, indications of the root causes, and remedies.

Cleanliness of hydraulic components during assembly is critical. Before assembly, all parts must be properly cleaned. All parts that will be stored prior to assembly must be stored in a way to protect them from contamination. Refer to Cleaning Guide Equipment and Chemical Recommendations Reference Manual, SEBF8354 for complete cleaning instructions of all dealer areas. Refer to Spec Shop Cleaning Guideline, SEBF8360 for specifics on spec shop parts cleaning.

NomenclatureThe following illustration shows the position of all the seals, rings and bands that will be discussed in this document. A bolted head cylinder is shown, but threaded gland and threaded crown cylinders have the same basic seal positioning.




Illustration 1




(1) head o-ring seal

(2) head seal backup ring

(3) wear band

(4) buffer seal

(5) u-cup seal

(6) wiper seal

(7) nut

(8) piston

(9) head

(10) rod

(11) piston seal expander ring

(12) piston seal ring

(13) wear band

Rod Seal Leakage

Assembly Damage

Assembly Cone Not Used1.

Indications

A small section of the material is found to be clipped out of the buffer seal, U-cup seal, or wiper seal ID lip.

◦

Steel backed PTFE wear ring (used on HEX machines) is heavily scratched in one or more discrete areas.

◦

Possible Root Causes

A sharp edge, usually at the end of the cylinder rod, has clipped or scratched the seals as the head is pushed over the rod.

Remedy

It is recommended that a seal guide be used when reassembling the head to the rod. Any metal part that comes in contact with a seal during assembly or operation must be free of sharp edges.

Comments

The wiper seal is most commonly clipped when a guide is not used. The head lands pilot the head after the wiper goes over the rod. An attempt to assemble the head over the rod in one motion should be made, because it is possible for the wiper lip to be caught between the edge of the sealguide and the rod chamfer if a jerking motion is used. All burrs and sharp edges should be removed from the end of the rod prior to reassembly.




Illustration 2 g01202321

Steel backed PTFE wear band heavily scratched by the edge of the rod.

Wiper Seal Not Installed Properly2.

Indications

The wiper seals' metal shell is dented, or deformed.◦

Dirt enters the cylinder through the wiper seal because the lip is not in close contact with the rod.

◦

The wiper seal is dislodged from the counterbore.◦

The contacting band of the wiper seal is not uniform in width around the circumference of the seal.

◦


The wiper seal was not installed properly or the seal was damaged during installation.

Remedy

A piloting, wiper seal driver should be used to install wiper seals that have a metal shell. If necessary, use a rubber mallet to tap around the edges of the seal. This will ensure proper seating of the seal against the counterbore. Do not dent the metal shell.

Comments

An uneven contact band on the ID of the wiper indicates that the seal was not properly seated against the back of the counterbore, or that the rod was bent. Always use bearing adhesive on the OD of the metal shell to aid in retention and eliminate the seal OD as a




possible leak path. However, adhesive on any other surface of the seal, rod, or head is not allowed.

Damage to the Head3.

Indications

Upon removal of the seals, scratches or gouges are found in the seal grooves of the head.

◦


The seals were removed in a manner that was too aggressive. Gouges were left in the grooves by screwdrivers, chisels, or other hardened tools. The groove's surface finish was degraded beyond the ability of the seals to properly function. Refer to Illustration 3.

Remedy

Care should be taken in order to prevent damage to the groove when removing rod seals. Use a seal pick or other tools of low hardness to remove the seals.


Buffer groove damage by removing a seal with a screwdriver.

Contamination

Paint Contamination1.

Indications

The front face of the wiper seal is painted to the wiper lip edge. Paint overspray is found on the rod in a location that will enter the seals during operation. Refer to Illustration 5.

◦

Other indications are pieces of paint adhering to the inner diameter of the rod seals. Refer to Illustration 4.

◦





The wiper seal and/or rod was not properly masked during painting of the cylinder.

Remedy

Before painting, protect the entire front face of the wiper seal and the section of the rod that will make contact with the seal.

Comments

Small paint chips cause leakage by getting between the sealing lip of the U-cup seal and the rod. The U-cup lip edge must be clean of any contaminant to function at maximum efficiency.


Paint of front face of wiper seals. Paint on the lip edge indicates that the rod was probably painted.


Strut in which the wiper seal and the rod just outboard of the seal was not masked prior to painting.

Metallic Contamination2.

Indications




Small, medium sized axial scratches are found across the sealing lip of the U-cup. Scratches that match are often found on the buffer and wiper seals. Refer to Illustration 7 and 8.

◦

Shiny metal flakes and chips are embedded in the wear bands, and in the sides of the plastic piston seal ring. Refer to Illustration 6.

◦

The rod is scratched in one or more places. The scratches are straight running the length of the rod stroke.

◦


Metal contaminants were not removed from the system before the contaminants migrated to the cylinders. The contaminants may have been the by-product of the manufacturing process, wear or the result of repairs. The contaminants damage the rod seals.

Remedy

Replace wearing components. Always replace the wear bands and piston seal during resealing because these components act as particle traps. The systems' cleanliness may improve after a cylinder reseal if the contamination was the result of manufacturing cleanliness instead of system wear. Replace any rod with scoring or a nick deep enough to be felt with a fingernail.

Comments

There is nothing in the rod seals that can scratch a rod. However, rod seals and wear bands can hold hard particles against the rod resulting in long axial scratches. These scratches may then damage the U-cup seal. A single fine scratch on the ID of the U-cup seal can result in a noticeable leak.


Metallic particles are shown embedded in the piston seal ring. These particles cause leakage by scratching the U-cup sealing lip.





The scratches in the U-cup lip were caused by metallic contamination.


Fine scratches are shown on the sealing lip of the U-cup. Very small scratches will result in noticeable leakage.

Dirt Contamination3.

Indications

Heavy scratches or wear is found on the ID of the rod seals.◦

Large amounts of dirt are found between the seals and around the wear bands.◦

The rod is scored to varying degree.◦


The wiper seal was damaged or improperly assembled, and dirt is entering the cylinder.

◦

The head was not properly cleaned prior to resealing.◦




Bent cylinder rods cause uneven wear of the wiper seal or a loss of lip to rod contact pressure on one side of the seal.

◦

Nonmetallic contaminants were built into the cylinder or other hydraulic components.◦

Remedy

Ensure proper assembly of the wiper seal. Thoroughly clean the head and pay close attention to the inside of the seal grooves prior to resealing. Straighten or replace any bent rod according to the guidelines in the Special Instruction, SEBF8072, "Inspection and Salvage of Hydraulic Cylinder Components" .

Comments

A small amount of very fine dirt or dust behind the wiper seal is to be expected after long service. The area behind the wiper is dead space and system contamination will not result unless the buildup is heavy. The U-cup or buffer seal may have black oil around the OD after a period of service. This material is a combination of extremely small wear particles from the head wear band and chrome from the rod. This does not contribute to seal failures.


The cylinder rod was lightly scored by aluminum oxide contaminants that were trapped by the head wear band.

Water Contamination (Hydrolysis)4.

Indications

Any of the rod seals, but especially the buffer, is cracked and brittle. ◦

The seal has large pieces missing from the sealing edge.◦

Material can be scratched off the seal with a fingernail.◦


Water or coolant has contaminated the hydraulic oil. Cooler failure is possible if the machine is water to oil cooled. Water can condense on tank walls. Water may also reach

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higher concentrations around the rod seals. The higher concentration of water is due to low oil flow around the seals.

Remedy

If the oil sample indicates water in the oil, change the oil. Use hydrolysis resistant rod seals. Always use special carboxylated nitrile (black) U-cup and wiper seals, and a PTFE step buffer seal when using water/glycol hydraulic fluids.

Comments

A very small amount of water or coolant can quickly damage rod seals at elevated temperatures. At moderate temperatures, the breakdown occurs more slowly.


Buffer seal destroyed by hydrolysis. Note the leathery "skin" on the outside of the part.


Head oval seal destroyed by hydrolysis.




Damage to Mating Iron

Rod Damage1.

Indications

There may be scratches on the ID of the rod seals and uneven wear patterns on the rod seals and on the wear band.

◦

Nicks or scratches on the surface of the cylinder rod that are not completely straight and axial are other indications.

◦

Pits or cracks can be found on the chrome surface of the rod.◦


The rod is bent.◦

The rod has been scratched or abraded the working environment.◦

The chrome surface on the rod is of low quality or was improperly polished.◦

Remedy

Straighten or replace any bent rod according to the guidelines in Special Instruction, SEBF8072, "Inspection and Salvage of Hydraulic Cylinder Components". Re-chrome, polish, or re-rod any damaged rod according to the guidelines in Special Instruction, SEBF8072, "Inspection and Salvage of Hydraulic Cylinder Components".


Rod scuffed by poor handling.

Head Groove Damaged or not Made to Standards2.

Indications

Seal grooves are corroded and pitted.◦

Tool drag marks are evident, surface finish does not meet standards. ◦





Water has entered the head, or the head was left exposed to the environment. The head grooves were not properly machined.

Remedy

Obtain a new head. The grooves cannot be machined to a larger diameter in order to remove pitting or tool marks without reducing the efficiency of the rod seals.


Step incorrectly machined into the wall of the buffer seal groove.


U-cup and buffer seal grooves with poor surface finish.

U-cup Extrusion

U-cup Extrusion1.

Indications




The buffer seal is worn flat or ribbons of material are coming off the back of the seal.◦

The ID of the U-cup seal closest to the wiper seal has small chunks removed from it or it has a melted appearance.

◦

Ribbons of material have flowed in between the rod and the head land behind the U-cup.

◦


High pressure spikes have occurred in the cylinder.◦

The relief pressure setting may be set too high.◦

The buffer seal has been damaged. This exposes the U-cup to full system pressure.◦

If extrusion is seen on truck struts, overloading or overcharging may have occurred.◦

Remedy

Use a backup ring behind the U-cup seal that closes the clearance between the land and the rod. Lower pressure relief settings. Replace damaged buffer seals.

Comments

Extrusion is not related to melting. The melting temperature of polyurethane is much higher than the temperatures that can be generated in a normal operating hydraulic system.


A moderately extruded U-cup.





A heavily extruded U-cup seal.

Heat Damage

Heat Damage1.

Indications

The buffer and U-cup seals may appear very dark in color. ◦

The U-cup has taken a heavy compression set.◦

The seals are cracked and brittle.◦


The hydraulic system temperature is too high for standard rod seals. Standard rod seals are rated to 93 °C (199.4 °F) continuous operating temperature.

Remedy

Check cooling components for efficiency and fix any problems. Modify machine operation in order to avoid overheating. Use high temperature rod seals. Use appropriate hydraulic oil for the your specific operating conditions for good lubricity.





A buffer seal destroyed by oil temperatures higher than the rated operating temperature of the seal material.


U-cup seal destroyed by hydraulic system temperatures higher than the rated operating temperature of the seal material.

Note: Rod Seal Assembly Tips

Push the U-cup to the back of the groove toward the wiper. The U-cup will remain in that position during operation after the first pressure cycle.

•

Lubricate the rod and rod guide lightly before pushing the head over the rod. Do not lubricate the seals excessively. Any oil between the U-cup and wiper seals will leak. The leak will occur during the first hours of operation.

•

For optimum rod seal performance, the rod surface finish should be 0.40 micrometers (16 microinches).

•

Head Seal Leakage

O-ring seal or Backup Ring clipped during Assembly

Indications

A small section of the O-ring or backup ring appears "bitten" out of the part. This is most common of the ends of split backup rings.

•


The backup ring was in a "sprung out" condition, and the ends of the ring did not remain in the seal groove prior to the mating of the head with the cylinder tube. The ends of the seal outside the groove were clipped.

Remedy




Make sure that the head seal O-ring and the backup ring grip the head tightly before mating the head with the tube. If the backup ring does not grip the head tightly, assembly compound can be used to prevent the ends from extending outside the seal groove in some cases.

Comments

Screw in the threaded gland head slowly in order to avoid pinching and stretching the O-ring head seal.

•

Lubricate the OD of the seals lightly before assembly.•

Many threaded gland head seal O-rings have a blue PTFE coating in order to reduce friction during assembly.

•

The head seal O-ring on the threaded gland cylinder is often cut during disassembly by the threads.

•


Head seal Backup Ring Clipped during Assembly.


Head seal Backup Rings Clipped during Assembly.




Head Seal and/or Backup Ring Extrusion

Indications

Ribbons of material that flowed into the clearance between the head and the cylinder tube are present at disassembly.

•

Large amounts of material are missing on the low pressure side of the head seal O-ring.•


Head seal extrusion is usually caused by oil pressures that are too high.•

Head seal extrusion failures can also occur if the tube bore is too large or if the diameter of the land behind the seal is too small (extrusion gap is too high).

•

Some failures have been attributed to the stretch of head bolts on the bolted cylinders. Threaded gland cylinders have had failures because of the expansion of the tube at very high pressures.

•

Remedy

If the backup ring is extruding use one of a harder material. The pressure around the head seal may need to be reduced. Always tighten threaded gland cylinder heads to the proper torque after resealing.

Piston Seal Drift

Dieseling

Indications

The cylinder drifts at a high rate.•

Oil temperatures are elevated due to flow of oil through small clearances at high pressure.•

The piston seal ring or rubber expander ring is eroded in a finger-like pattern. The area around the erosion is blackened and sometimes smells burnt.

•


When air or other entrained gases that are saturated with oil vapors form in the hydraulic oil dieseling occurs. When these bubbles are compressed at a high rate the gases ignite due to adiabatic heating. The resulting explosion erodes adjacent components, especially nonmetallic parts.

Remedy

All efforts should be made in order to remove air or other entrained combustible gases from the system after resealing a cylinder before using the machine. Extend and retract the cylinders several times before heavy work is done in order to force air back to the tank.





Piston Seal Ring damaged by Dieseling.


Piston Seal Ring Damaged by Dieseling at the Step Cut.

Explosive Decompression

Indications

The cylinder drifts at a high rate.•

The rubber expander ring has large chunks of material missing.•

There is no smell of burning or damage to the seal ring.•


Explosive decompression (ED) occurs when gases that have penetrated into a rubber expander ring suddenly expand due to a very rapid drop in hydraulic oil pressure, or when voiding occurs in the cylinder. The rapid expansion of the entrained gas bubble pops a chunk of material out of the rubber ring. If the damage accumulates the ring can no longer properly seal.

Remedy




All efforts should be made in order to remove air or other entrained gases from the system after resealing a cylinder before using the machine. Extend and retract the cylinders several times before heavy work is done in order to force air back to the tank. Refer to the cylinder removal and install procedure for the specific model for more specific instructions. Use dieseling/ED resistant piston seals. These seals are made of a rubber material designed to be less permeable to gases in the hydraulic oil. Use protection piston seal backup rings. These rings reduce the area of the rubber expander ring that is exposed to the oil and therefore reduce the amount of gases that penetrate the rubber.

Comments

ED and dieseling are very similar. The root cause of ED and dieseling is air in the hydraulic system.


Piston seal expander ring heavily damaged by explosive decompression.

Scratched Piston Seal Ring

Indications

The cylinder drifts slowly.•

The split plastic seal ring has heavy axial scratches on it that run from one side of the seal to the other.

•


Bore damage •

Contamination•

Remedy

If a damaged bore is the root cause of a scratched seal ring the bore will need to be lightly honed or oversized in order to remove the damaged areas. Follow the recommendations found in the Special Instruction, SEBF8072, "Guideline for Reusable Parts and Salvage Operations". If




contamination is the cause of the scratches all components must be thoroughly washed during reseal of the cylinder.


Cylinder heavily scored by metallic contamination.


Piston seal ring scored by a cylinder bore with a poor surface finish.

Note: Other Information

Do not apply excessive heat to the cylinder with the seals in place. Heat can be conducted into the seals and the seals may be damaged.

•

Excessively high pressure spikes will damage the rod seals. PTFE step seals do not last as long as a backed polyurethane buffer seal at pressures above 24 MPa (3500 psi). Any increase in the line relief pressure settings will reduce the life of the rod seals.

•

A head wear band should spring out when in the groove. A piston wear band should grip the piston in order to avoid damage to the parts when the head is assembled to the rod and when the rod assembly is mated with the cylinder assembly.

•




Oversized piston seals and wear bands should be used if the bore is honed 0.25 mm (0.010 inch)past nominal.

•

Note: Assembly Tips

After resealing a cylinder be sure to torque the piston nut properly. If the nut or the bolt is not torqued properly the cylinder may drift due to leakage between the piston and the rod mating surface.

•

The step cut on the piston seal can be slightly open before the rod assembly is mated with the cylinder assembly. The gap will close completely when the assemblies are mated.

•

If the cylinder pressure test shows cylinder drift it may be necessary to rotate the rod eye while slowly stroking the cylinder to help seat the piston seal in the groove.

•

Slight scuffing of the piston seal ring with a fine grit sandpaper or emery cloth in a circumferential direction can help seat the ring to the cylinder bore.

•

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Applied Failure Analysis Hydraulic Cylinder Seal Failure Analysis

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