For a Hydraulic system, Hydraulic Cylinders are vital part like Hydraulic Pumps & Valves. According to some studies about 25% Equipment failures are because of design failure. Same for hydraulic cylinders also. One out of every four-cylinder failure because of not proper design as per operating conditions. Most of Cylinder failures are because either of Seal Failure, Oil Contamination, Bent or Scored Piston Rod, Cylinder Misalignment, High Temperature, Corrosion, Chemical Attack, Dent on either Rod or Barrel etc.
Lost System Efficiency – Poor performing cylinder cause gradual loss of system efficiency. Such as increase internal leakage and friction between reciprocating parts. To counteract this inefficiency, sometimes operator increase system pressure, so rest of the system under more stress, lead to premature failure as well as increase electricity power bill. In a business the cumulative cost of inefficient hydraulic cylinder is much more. Below are some effects of inefficient cylinders –
➢ Increase heat loading to worn out seal and consequently failure.
➢ Excessive Pressure leading to fracturing of Rod Seals & Piston Seals.
➢ Erratic cylinder movement leading to bent Piston Rod, Score Piston Rod or Barrel.
➢ Excessive vibrations lead to fail mountings.
➢ Barrel deformation because of excessive internal pressure.
➢ Because of internal leak, system demand more flow, causing pump wear.
➢ Loss of efficiency leads to increase hydraulic motor wear & failure.
Cylinder Failure Risks – Hydraulic Cylinder failure often accompanied by fluid leak which can have negative consequences.
➢ Health Problems – Hydraulic flow exposure through inhalation, ingestion or accidental injection can have negative health consequences from skin irritation to gangrene or even death in extreme cases.
➢ Fire Hazard – It’s a Petroleum base fluid, means flammable specially when heated to high temperature. Fluid leak can potentially vaporize the fluid generating a flammable mist. With some circumstances can create an explosive environment.
➢ Environmental contamination – leakage from hydraulic pipe or hose run the risk of depositing fluid and chemicals on ground or on water bodies which become contaminated very easily. If hydraulic oil allows to mix with ground water it can have disastrous consequences for the local ecology.
➢ Injury Risk – Leaking hydraulic fluid creates slick surfaces & Hazard conditions having injury risk for person working there.
2 Symptom of Hydraulic Cylinder Wear –
➢ Abnormal Noise – It’s often cause by aeration or cavitation. Air present in system creates knocking noise. Often accompanied by erratic cylinder movement. Because of aeration, system components damage by overheating or seal damage. Cavitation occurs when there is insufficient oil supply to any system, part leads to pressure fall below vapor point of fluid. So, vapor cavities form which
implode when compressed. Cavitation causes metal erosion & damage components also causing contamination into the system.
➢ Sticking or Juddering – In a cylinder showing slip stick, cylinder movement is no longer smooth but having a jerky motion. Slip stick is usually a sign of increased friction possibly cause by worn seal & and/or oil contamination. It may also be a symptom of more extensive internal damage including bent piston rod.
➢ Decline System Efficiency – As a cylinder wears it increased friction and internal / external leakage require more energy to perform task.
➢ Slow operation – A slowing operation of cylinder shows something not proper, internal leak demand more fluid, increased friction due to rough / worn surface or worn seal result in drawdown of power to overcome this effect.
➢ Abnormal increase in cylinder temperature – An unusual rise in cylinder temperature may be a symptom of Cylinder abnormality. It’s a good idea to monitor cylinder operating temperature.
➢ Rod wear Asymmetrically – If rod shows sign of increased wear in one side, it’s a symptom of something abnormality may be misalignment, bent rod, excessive lateral loading or some type of internal wear.
Common Cause of Hydraulic Cylinder Failure: –
(i) Seal Failure – There are 2 types of seals, Dynamic seals, between parts having relative motion and Fixed seals, between fixed parts. Due to time of use or exposure to abuse, seals eventually fail & need to replace. There is specific design consideration for seal selection like, Fluid pressure range, Temperature Range, Fluid Type, Speed, Hardware Dimensions & Cylinder Application. The PV Value (Combined Pressure & Velocity) of sealing material is an important consideration for high pressure sealing. Every sealing material have a different PV value with respect to contacting material. A leaking hydraulic cylinder seal is a sign that either of seal has damaged or seal learance with its matching part exceed the limit.
3 Root Cause –
➢ Heat Degradation – Deterioration occurs When seal lost its elasticity and began to start break. The failure seal looks Hard, Brittle and/or a part of seal break away. It usually occurs because of high operating temperature or prolong exposure to sunlight and / or Ozone.
➢ Extrusion – It’s damage to the seal from the gap between the sliding surface and hosing. Excessive gap allows the seal to deform into these gaps during pressure causing material to creep/ Breakoff. Sometimes this is because of excessive internal pressure but can also be cause by mating surface worn out of spec. During seal assembly ensure tolerances are within acceptable limits.
➢ Grooving – It present as liner cut or groove worn into seal, usually cause by internal contamination when a hard or sharp foreign particle found its way under the seal.
➢ Wear – This happens as a result of friction between seal & running surface. It accelerates when there no sufficient lubrication, rough surface or excessive side load.
➢ Scarring – improper installation of seal can cause dents / cut on seal lip, which impact seal efficiency.
➢ Fracturing – This happens when seal crack or split, usually result of pressure spikes, high back pressure or low-grade seal material.
➢ Chemical Erosion – This happens when seal material is not compatible with the application fluid. Seal material break down when it encounters a corrosive fluid leading to swelling, shrinkage, softening or discoloration. Other modes of failure may be Dieseling – Damage cause by air bubbles, the fumes within bubble ignite when pressurize causing seal burning. Pressure trapping, Hydrolysis – Seal material
break down from exposure to water or water-based fluids at high temp.
(ii) Fluid Contamination – Contamination is a major cause of hydraulic cylinder failure as it leads to seal failure. It is essential to check fluid contamination, not only cleanness level but other parameters like water content, Air content, hard particles with no. of particles & sizes etc.
Root Cause –
➢ Ageing / Degradation – Fluid naturally degrades overtime and should be replace according to a define maintenance schedule. Ageing depends on many variables including oil quality, operating conditions and a potential contamination. Normally a recommended shelf life is 5 Yrs.
4 ➢ Using contaminated fluid – many times we top-up used oil which already contaminated, it ruins whole the system.
➢ Wear Out other part – Many times, contamination comes into cylinder from other damage parts like Pump or Piping seal.
➢ Worse Environment – From worse environment, abrasive particles come to cylinder through damage wiper seal.
(iii) Piston Rod Surface Finish, Scoring & Pitting: – A specific surface roughness is required neither too rough nor too smooth, optimal to reduce friction. Damage of Chrome plating in the form of gouges or scratches is a severe consequence of hydraulic seal leak which is inevitable and speed up quickly. Pitting appears as small holes on rod surface
Root Cause –
➢ External Impact from the operation environment – Hitting piston rod from external, can dent it, cause damage to chrome surface and with its movement, might damage rod seal.
➢ Fluid Contamination – Contaminated fluid can damage seal, Cylinder bore or piston rod.
➢ Misalignment – If the rod doesn’t move exactly parallel to barrel, it can damage rod, piston or barrel. Thrust force will put excessive load against rod & bearing and piston & barrel
➢ Poor Quality Chroming – Hard chrome plating is a specialized process involving Metallurgy, Chemistry, Skill, Knowledge & Experience. There are many things that can go wrong from improper pre-grinding to poor bath chemistry or even bad electrical connection.
➢ Corrosion – A highly corrosive environment like in marine application. For the cylinder rod to perform correctly, the surface finish must confirm very precise chrome
thickness & roughness. The surface chrome must be thick enough to ensure longevity and allow for wear. Chrome surface roughness must have a precise degree of roughness as it allows very fine striations to fill with fluid which provide a frictionless layer upon which the cylinder seal may run.
This improves seal life span. Unsuitable surface finish will not allow a proper fluid film under the seal. If the surface
roughness of Cylinder rod is too low (smooth), there will not be sufficient lubrication, causing reduce seal life. On contrary If surface roughness is too high, the seal to initially ride on surface finish peak and change seal surface finish, causing contamination ingression increase & unacceptable level of leakage can result. Piston Rod Chrome coating and hardening to be adjusted with surface finish. If it is too soft or thin coating, can be damage by seal, if surface too hard may damage the seal. To increase cylinder seal service life, consider cylinder rod surface as lubricated
5 wear surface and preserve it as such. For some cases installation of bellows or shroud also protect rod surface and seal from impact or contaminants and enhance similar life extension benefits.
(iv) Cylinder bore scoring – it appears as deep scratches on cylinder interior wall.
Root Cause –
➢ Misalignment – Happens for the cylinder not mounted properly, especially in long stroke cylinders.
➢ Side Loading – In certain application, cylinder exposed to increase side load forces which can cause the rod to become cocked in the barrel, leading to cylinder scoring.
(v) Bent Piston Rod – Hydraulic cylinder piston rod should be as straight as possible, however a run out of 0.5mm / Mtr. Is acceptable. Hydraulic cylinders are design for linear force & motion to a guided load. The path of the guided load MUST not impose any eccentric load on rod. Hydraulic cylinder bent rod occurs because of side load (Whether deliberately or inadvertently) and side load occurs when unequal pressure not centered on piston rod. Once the rod bent, the deforming load place on the rod seal, even a slide bend put excessive load on seal which increase leakage and ultimately premature seal failure.
➢ Insufficient rod diameter – Rod diameter should be calculated with considering Buckling, Sagging and bending. When theses parameters calculate properly, side loads will be minimized.
➢ Insufficient material strength – With higher yield strength, a reduced piston rod diameter can be use without compromising safety factor against bucking under axial compressive loads.
➢ Improper cylinder mounting arrangement – The effective way to make mounting proper is to use spherical bearing mount.
➢ Jam by some debris – Sometimes, because of some debris or mechanical obstruction, piston rod can bend.
➢ Excessive force – If applied force is excessive than design force, it can bend the piston rod. The allowable rod loading for a given cylinder arrangement can be check with Euler formula (F= nπ²EI /L², n – a factor as per load condition, E – Modulus of elasticity, I – Moment of Inertia, L – Mounting distance). If real load exceeds the design load, need to prepare new rod with higher tensile strength material or increase rod diameter to prevent bending. To check Rod straightness, place the rod on rollers and measure run out at middle of rollers with dial gauge. If the rod getting bent, check actual load against permissible load for the cylinder mounting arrangement and rod material tensile strength.
(vi) Ballooned Tubes – Ballooning is the expansion of the Hydraulic cylinder housing cause by either thermal or pressure situations. The Ballooning originated by hoop stress.
Root Cause –
➢ Cylinder wall thickness is not sufficient – Mostly Hydraulic cylinder wall thickness to be calculate by considering thin wall cylinder pressure vessel, if wall thickness is more than 10% of cylinder dia, more accurate to conder as thick wall cylinder.
➢ Excessive cylinder tube honing – Honing produces extremely tight tolerances in size, roundness, straightness and surface finish in cylinder bore. The honing purpose is to create cross hatch that leaves physical valleys which retain oil while allowing piston ring to slide over smooth surface.
➢ Material used for cylinder tube is not strong enough for given operation. Once the tube Ballooned, the precise level of tolerance between piston seal and Cylinder tube wall lost, High pressure fluid bypass the seal. This bypass oil generates localize heat because of pressure drop. This localizes heat and high-pressure fluid erode seal & reduce life. A ballooned tube is no serviceable, it requires to change.
Conventional way to test Hydraulic cylinders is full stroke bypass test (Cylinder pressurize at the end of stroke to measure seal leakage). But this method not reveal cylinder tube ballooning caused by hoop stresses. The ideal way to test a cylinder tube for ballooning is piston seal midstroke bypass test. The disadvantage of the procedure is, cylinder force has to resist mechanically which is not feasible in case of bigger dia high pressure cylinders. However, the midstroke test can be conducted hydrostatically using pressure intensification. Position the midstroke, close piston side Ball valve and direct fluid to the rod side. By adjusting Relief valve pressure, flow into rod side until rated pressure in rod side gauge. Now close rod side valve also and monitor both side pressure gauges. The pressure differential should be as per both side area ratio. If the differential pressure across piston seal not maintain, this indicates seal / tube problem. Never pressurize piston side with rod side valve closing, because of pressure intensification it may be harmful.
Major issues of cylinder failure can be reduced by preventive maintenance, as regular Oil quality testing (including air & water particle checking), Degradation of fluid viscosity, Corrosion or chemical attack, Temperature Checking. Some of the issues can be solve by visual inspection like misalignment, seal leak, dents on piston rod or barrel. Minor issues can be identified and repair it before becomes major issue.