About Stage 2
At Stage two we need to record the leakage pattern and volume. Make a note of the actuation performance (i.e. whether smooth or erratic movement) and watch for drift, or loss of force.
Check the cylinder temperature for unexplained heat. Any such observable condition that can be recorded will provide critical information to establish accurate conclusions.
Ultimatley, this stage can be divided into two detailed categories:
- Leakage Pattern
- Actuator Performance
The following is a list of different leakage patterns - together with typical causes.
Slight, steady leakage
- Diminished pre-load of the Rod Seal will cause a slow leak during the low pressure phase of the stroke.
- Compression setting or loss of elastic memory will reduce interference or squeeze between the seal and the sealing surfaces.
- Insufficient initial interference or squeeze of seal due to seal choice or groove design. This situation usually will cause leakage beginning from start up.
- Side loading of rod or ram will cause excessive compression and wear on one side of seal and excessive clearance on the other side.
- Poor dynamic surface finish will score dynamic seal lip causing a leak path for fluid.
Example of poor static surface finish
- Groove or box bore will inhibit the ability to seal tightly.
- Compression setting, extrusion or omission of static seals (i.e. Gaskets or O-Rings).
- Seal lips cut during installation. This situation will cause leakage as described beginning from start up.
- Hardening of the seal material due to heat. Hardened material is less conforming to sealing surfaces.
- Dirt or grit under static or dynamic lip.
- Seal shrinkage due to excessively cold temperature.
Leakage increases gradually
- Compression setting of seal material increases over time causing gradually increasing leakage.
- Continued wear of the dynamic sealing lip.
- Tear on seal lip continues to leak causing erosion of seal and gradually increasing leakage.
- Fine score mark on dynamic surface abrades seal lip progressively
- Lack of back-up ring.
Heavy, sudden leakage
- Rolled or twisted seal.
- Extruded seal causing bursting or total loss of sealing integrity.
- Severely torn seal lip.
- Seal split due to excessive radial space relative to seal cross section. Improper seal fit or excessive bearing wear can cause this failure.
- Sudden failure of bearing causing a crushing of the seal.
- Dieseling due to air bubbles in fluid causing burning or bursting of seal.
- Changing of hydraulic fluid causing sudden seal attack and failure.
Inconsistent, erratic leakage
- Gradual rod seal leakage builds up behind wiper which eventually releases pressure and dumps fluid.
- Side loading occurring intermittently causes erratic leakage.
- Dirt or grit interfering with seal lip eventually works its way past.
- Cold leakage at start-up which gradually decreases as fluid temperature rises.
- Seal is rolling or twisting in seal groove causing erratic leakage.
Chatter or stick-slip
- Insufficient fluid pressure to smoothly overcome the resistance.
- Excessive drag or friction for the available fluid pressure.
- Use of high friction seals.
- Over compressed stacked V-Rings.
- Insufficient fluid lubrication.
- Dynamic surface finish is too smooth to maintain film. Minimum surface finish. should be 8 RMS
- Piston bypass.
- Valve problems.
Creep or Drift
- Piston seal bypass
- Piston bypass through static piston-to-rod O-Ring.
- Valve bypass. Inspect return line on valve for leakage while in the closed position.
- Cast iron or steel rings utilized as piston seals in a holding application
- Rod leakage on retraction stroke.
Excessive drag or seizing
- Over compressed or improperly packed stacked V-Rings.
- Incompatibility of seal with fluid or assembly lubricant causing swelling of seal and binding and/or seizing
- Thermal expansion of seals.
- Thermal expanding of bearing
- Extrusion of seal or back-up ring into clearance causing a binding and/or seizing of the actuator.
- Bent rod, dented tube, or other damage.
- Misaligned head or follower, or unevenly torqued tie-rods
- Pressure trap between piston seals. May be squeeze type seals or lip seals or lip seals installed with sealing lips facing each other.
- May be a problem with system causing apparent drag or seizing.
- Extreme side loading.
- Improper fits.
Increasing actuator temperature without external heat source
- See causes of excessive drag or seizing These conditions also create heat
- Piston bypass raises fluid temperature
- Valve bypass
- Fluid break down will reduce lubricity and increase friction and heat which further breaks down fluid.
- Lack of lubrication in compressed air or insufficient lubrication in a water mixed fluid.
- Improper dynamic surface finish. May be too smooth or too rough.
When analyzing cylinder performance, it is important to consider the conditions in which a cylinder is operating.
To assure proper seal design and compound selection and to utilize the best rebuild design, carefully consider the following:
- Temperature of the fluid or operating environment
- Fluid Type
- Fluid contamination
- Reciprocating speed
- Shock loading
- System pressure
- Surface finish
The following is a more detailed description of each of these areas, together with suggested causes and possible solutions.
Built in interference or preload is necessary for a seal to maintain proper sealing forces throughout the pressure range. Excessive temperatures can cause seals to lose memory or embrittle and crack, resulting in fluid leakage. Proper selection of seal compound and maintenance of nominal fluid temperature will prevent premature seal failure.
Shield cylinder, insulate, utilize and maintain cooling system. Also consider choosing a seal compound capable of higher temperature
Piston seal bypass
Avoid metallic piston rings, select piston seal materials and designs least likely to bypass without sacrificing seal efficiency.
Friction related to bearing and/or compressed packings.
Select non metallic bearings; avoid compression packing. Use lubricated seal material.
Improperly set or functioning control or relief valves.
Check valves for proper functioning. Reset, repair, or replace
Check pump temperature relative to reservoir temperature. If pump temperature is more than 20F (11C) hotter than reservoir temperature, check pump for wear.
Hydraulic fluids or pneumatic lubricants which are not compatible with the seal material can result in chemical attack. Proper seal compound selection can prevent failures.
|Cause of Failure||Solution|
Incompatibility of seal and fluid causes seal to swell, lose memory due to excessive friction while swollen, or become gummy and sticky.
Change seal compound or change fluid
Foreign matter in hydraulic or pneumatic systems can cause abrasive attack on seals, resulting in weepage and premature seal failure, along with associated equipment damage. Effective rod wipers, proper filtration and preventative maintenance of system solves this condition. Sharp, positive rake seal lips operate better with contaminated media.
|Cause of Contamination||Solution|
Pump deterioration, i.e. gears, housing, plates.
Avoid air ingestion; cavitation wears parts. Check fittings on suction side, change filters, clean strainer. Use magnet in reservoir.
Metal to metal contact; piston to bore, rod to gland, etc.
Use non metallic bearing bands when possible.
Braising of primary wear surface.
Braise only lands, not primary wear surfaces.
Deterioration of packings, usually fabric reinforced rubber.
Avoid fabric reinforced rubber packing whenever possible.
Clogged filter elements, bypassing filters.
Replace clogged filter elements.
Dirty work areas and poor cleanliness procedures.
Use clean work areas, tools and rags. Clean all mating surfaces. Use care when replacing hoses and filters and when rebuilding cylinders.
Inadequate wiper material or design.
Use sharp lip, positive rake polymer wipers. Avoid wipers made of felt, rubber or any rigid, inflexible material. Replace existing fluid if overly contaminated.
High surface speeds may cause elevated heat due to friction at the seal lip. This elevated temperature can cause conditions as described under Excessive Temperature. In addition, hydroplaning of the seal lip can also cause leakage. Speed limits vary according to such factors as ambient temperature, surface finish, viscosity of fluid, pressure and seal compound.
|Cause of Failure||Solution|
Speed creates frictional heat. Heat embrittles or softens seal compound, causing failure.
Choose lubricated seal compound. Choose seal design with proper preload. Choose seal compound capable of higher temperature.
Some applications have shock loading, resulting in intense pressure spikes. Seals can burst, roll or extrude, depending on other factors. Consider that these pressure spikes can exceed the normal operating pressure by as much as ten times.
|Cause of Shock Load||Solution|
Operator abuse – slamming forces, i.e. driving dump truck with bed up, manual valve abuse, driving blade into immovable force.
Train and supervise operators. Weigh production gains, if any, against cylinder equipment repair or replacement costs. Select seal designed to prevent bursting or rolling.
Improperly functioning relief valves or metering valves.
Check valves for proper functioning. Reset, repair or replace.
Using under designed equipment.
Match equipment to the task it is to perform.
Cylinders in holding position can swell.
Use proper back up rings.
Zeroing in on Leakage
The principal target area when working on reducing leakage is that of fluid sealing.
Fluid sealing could include gaskets, packing, mechanical seals, static seal, reciprocating, elastomeric seals and wipers used in hydraulic and pneumatic actuators.
Fluid leakage can be split into 2 areas:
- Rod seal leakage
- Piston seal bypass
Rod seal leakage
Rod seal leakage is also known as External Leakage. The effects of rod seal leakage are obvious - fluid leaking out of the cylinder.
Piston seal bypass
Piston seal bypass is also known as Internal Leakage. The effects of piston seal bypass, or internal leakage are less obvious.
In hydraulic applications, piston bypass creates heat. Heat in turn, breaks down hydraulic fluids and seal materials. Additionally, piston bypass can cause conditions such as 'drift' or slow, erratic actuation.
In a pneumatic application, piston bypass is direct air loss. As compressed air leaks past the piston seal, it goes directly to the exhaust phase of the cycle. As compressed air is an energy intensive medium, air loss is energy loss. Like hydraulic fluid bypass, internal air leakage can result in slow, inaccurate or erratic actuation.