Hidráulica sello cilíndrico de Análisis de Fallas

8/2/2019 Hidrulica sello cilndrico de Anlisis de Fallas

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Introduction

Careful inspection of the hydraulic cylinder seals after a failure can give indications of the rootcause. Document the complaint of the customer and determine if the problem is drift, leakage, ornoise. 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 incomparison to new seals. All scratches, nicks, cuts, foreign particles, dimensional changes, orcolor 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 mustbe properly cleaned. All parts that will be stored prior to assembly must be stored in a way toprotect them from contamination. Refer to Cleaning Guide Equipment and ChemicalRecommendations Reference Manual, SEBF8354 for complete cleaning instructions of all dealerareas. Refer to Spec Shop Cleaning Guideline, SEBF8360 for specifics on spec shop partscleaning.

Nomenclature

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

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Illustration 1




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(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-cupseal, or wiper seal ID lip.

Steel backed PTFE wear ring (used on HEX machines) is heavily scratched in one ormore 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. Anymetal part that comes in contact with a seal during assembly or operation must be free ofsharp edges.

Comments

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




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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 contactwith the rod.

The wiper seal is dislodged from the counterbore.

The contacting band of the wiper seal is not uniform in width around thecircumference 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. Ifnecessary, use a rubber mallet to tap around the edges of the seal. This will ensure properseating 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 properlyseated against the back of the counterbore, or that the rod was bent. Always use bearingadhesive on the OD of the metal shell to aid in retention and eliminate the seal OD as a




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possible leak path. However, adhesive on any other surface of the seal, rod, or head is notallowed.

Damage to the Head3.

Indications

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


The seals were removed in a manner that was too aggressive. Gouges were left in thegrooves by screwdrivers, chisels, or other hardened tools. The groove's surface finish wasdegraded 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 isfound on the rod in a location that will enter the seals during operation. Refer toIllustration 5.

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

Refer to Illustration 4.





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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 thatwill make contact with the seal.

Comments

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


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




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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 toIllustration 7 and 8.

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

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


Metal contaminants were not removed from the system before the contaminants migrated tothe 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 duringresealing because these components act as particle traps. The systems' cleanliness mayimprove after a cylinder reseal if the contamination was the result of manufacturingcleanliness 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 bandscan hold hard particles against the rod resulting in long axial scratches. These scratches maythen damage the U-cup seal. A single fine scratch on the ID of the U-cup seal can result in anoticeable leak.


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




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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 noticeableleakage.

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 thecylinder.

The head was not properly cleaned prior to resealing.




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Bent cylinder rods cause uneven wear of the wiper seal or a loss of lip to rod contactpressure 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 closeattention to the inside of the seal grooves prior to resealing. Straighten or replace any bentrod according to the guidelines in the Special Instruction, SEBF8072, "Inspection andSalvage of Hydraulic Cylinder Components" .

Comments

A small amount of very fine dirt or dust behind the wiper seal is to be expected after longservice. The area behind the wiper is dead space and system contamination will not resultunless the buildup is heavy. The U-cup or buffer seal may have black oil around the ODafter 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 wearband.

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 themachine 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 lowoil 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 elevatedtemperatures. 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.




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Damage to Mating Iron

Rod Damage1.

Indications

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

Nicks or scratches on the surface of the cylinder rod that are not completely straightand 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.




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

Remedy

Obtain a new head. The grooves cannot be machined to a larger diameter in order to removepitting 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




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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 itor 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 therod. Lower pressure relief settings. Replace damaged buffer seals.

Comments

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


A moderately extruded U-cup.




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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 arerated to 93 C (199.4 F) continuous operating temperature.

Remedy

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




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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 sealmaterial.

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 notlubricate the seals excessively. Any oil between the U-cup and wiper seals will leak. Theleak will occur during the first hours of operation.

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

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 mostcommon 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 theseal groove prior to the mating of the head with the cylinder tube. The ends of the seal outside thegroove were clipped.

Remedy




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Make sure that the head seal O-ring and the backup ring grip the head tightly before mating thehead with the tube. If the backup ring does not grip the head tightly, assembly compound can beused 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 frictionduring assembly.

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


Head seal Backup Ring Clipped during Assembly.


Head seal Backup Rings Clipped during Assembly.




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Head Seal and/or Backup Ring Extrusion

Indications

Ribbons of material that flowed into the clearance between the head and the cylinder tubeare 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 ofthe 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 sealmay need to be reduced. Always tighten threaded gland cylinder heads to the proper torque afterresealing.

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 areaaround the erosion is blackened and sometimes smells burnt.


When air or other entrained gases that are saturated with oil vapors form in the hydraulic oildieseling occurs. When these bubbles are compressed at a high rate the gases ignite due toadiabatic 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 thesystem after resealing a cylinder before using the machine. Extend and retract the cylinders severaltimes before heavy work is done in order to force air back to the tank.




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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 expanderring 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 therubber ring. If the damage accumulates the ring can no longer properly seal.

Remedy




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All efforts should be made in order to remove air or other entrained gases from the system afterresealing 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 andinstall 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 inthe hydraulic oil. Use protection piston seal backup rings. These rings reduce the area of therubber 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 hydraulicsystem.


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 tothe 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 theSpecial Instruction, SEBF8072, "Guideline for Reusable Parts and Salvage Operations". If




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contamination is the cause of the scratches all components must be thoroughly washed duringreseal 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 conductedinto the seals and the seals may be damaged.

Excessively high pressure spikes will damage the rod seals. PTFE step seals do not last aslong as a backed polyurethane buffer seal at pressures above 24 MPa (3500 psi). Anyincrease 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 thepiston in order to avoid damage to the parts when the head is assembled to the rod and whenthe rod assembly is mated with the cylinder assembly.




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

Note: Assembly Tips

After resealing a cylinder be sure to torque the piston nut properly. If the nut or the bolt isnot 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 withthe 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 eyewhile 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 acircumferential direction can help seat the ring to the cylinder bore.

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