Guide to Hydraulic Sealing
Original article date: June 1998
Modern seal scraper and guide elements are today developed as high performance machine elements. With proper choice and installation they should manage trouble-free operation with a long service life expectancy.
Oswald Seals’ so-called Really Useful Guide to Hydraulic Sealing contains four sections designed to help select the right type of seal and the correct seal profile as well as choosing seal combinations and undertaking problem diagnosis. Section 1 (see Table 1) provides illustrate information on making the right choice for a sealing application covering the variations of double and single-acting piston seals rod seals rod wipers bearing materials and O-rings.
To aid matching existing seal profiles over 40 seal profile illustrations are listed in Section 2. Each profile is accompanies by performance figures. A piston cutaway serves as a useful reminder showing all the seals which could need attention when a problem arises.
Hydraulic cylinders don’t just use one type of seal. Most types employ piston seals rod seals rod wipers and O-rings in their construction. When a breakdown occurs it doesn’t pay to replace only the seal that has failed. Good practice says replace all of them. Section 3 deals with selecting the right combination of seals.
Every conceivable seal failure scenario is examined in the final section. Everything from the initial reason for a failure to the physical cause and the resultant damage can be cross-referenced. Hydraulic systems work in extremely dirty environments such as foundries construction and rolling mills. Fig 1 indicates where contaminants can enter the hydraulic system and highlights other potential problem areas such as decomposition of the oil due to ageing high temperatures and shearing. Scraper seals can help to reduce the ingress of dirt.
| Damage through | Cause of damage/Seal failure | Consequences/Possible damage |
| Design | Excessively large gap | Gap extrusion |
| Pressure relief too fast with gases | Explosive decompression | |
| Incorrect installation dimensions tolerances | Excessive friction, Low pre-loading Blow-by effect | |
| Flow velocity too high | Erosive wear air bubble cavitation | |
| Contact pressure too high | Adhesive wear | |
| Surfaces too rough | Wear of seals | |
| Build-up of back pressure | Overloading of the seal elements | |
| Grooves too tight | No pressure activation Blow-by effect | |
| Grooves too large | Wear by blast jet | |
| Choice of the seals | Thermal overloading | Brittleness of elastomers,Melting or scarring of plastics |
| Chemical overloading | Swelling Shrinking Hardening | |
| Seals too soft | Gap extrusion Plastic deformation | |
| Unsuitable materials or type of seal | Stick-slip phenomena Loud noises | |
| Installation and start-up | O-ring twisted (due to installation) | Leakage due to spiral breakage |
| O-ring twisted (due to long stroke) | Shearing off the seal | |
| No lead-in chamfers | Dieseling effect Charring of the seal | |
| System not bled | Leakage during start-up | |
| Folding over of seal edges | ||
| Unsuitable installation tools | Premature failure | |
| Soiling in the system | Scoring of rods due to dirt particles or surface flaws | Leakage due to wear of the seals, Entry of dirt via wiper seals, Impaired functions due to guide wear |
| Scores in the cylinder barrel | Premature failure of the seals | |
| Water in the oil | Hydrolysis (eg of polyurethane) | |
| Solid particles in the system | Abrasive wear |
The Really Useful Guide to Hydraulic Sealing is Oswald Seals’ source of reference for hydrauylic designers and maintenance engineers. The 12 pages contain much information needed to achieve reliable and efficient sealing in hydraulic systems.
June 1998