Oil leakage in a hydraulic system reduces efficiency and leads to increased power losses. Internal leakage does not cause loss of fluid in the system as the fluid returns to the reservoir. External leakage represents a loss of fluid in the system. An improperly assembled pipe fitting is the most common cause of external leakage. Shaft seals on pumps and cylinders can get damaged due to misalignment, leading to leakages in the system.
Seals are used in hydraulic equipment to prevent excessive internal and external leakages and to keep out contamination. Seals can be of a positive or non-positive type and are generally designed for static or dynamic applications. Positive seals do not allow any leakage whatsoever. Non-positive seals permit a small amount of internal leakage.
Static seals are used between mating parts, which do not move relative to each other. Figure 11.1 shows some of the static seals including flanged gaskets and seals. The static seals are compressed between two rigidly connected parts. They represent a simple and non-wearing joint, which would be trouble free if properly assembled.
Dynamic seals are assembled between mating parts, which move relative to each other. Dynamic seals are subject to wear and tear as one of the mating parts rubs against the seal. The most widely used seals of this type are:
• Compression packings
• Piston cup packings
• Piston rings
• Wiper rings.
V-ring packings are compressible type seals, which are used in virtually all reciprocating motion applications. These include rod and piston cylinders in hydraulic cylinder applications, press rams and jacks. Here, proper adjustment is essential since excessive tightening will accelerate wear and tear.
Piston cup packings
Piston cup packings are designed specifically for pistons in reciprocating pumps and hydraulic cylinders. They offer the best service Ufe for this type of appUcation. Figure 11.2 shows a typical installation of piston cup rings for double acting and single acting operations.
Sealing is accomplished when the pressure pushes the cup lip outwards against the cylinder barrel. The backing plate and the retainers clamp the cup packing tightly in its place, allowing it to handle very high pressures.
Non-metallic piston ring packings
These packings are made out of tetrafluoro ethylene (TFE), a chemically inert, tough waxy solid. Their extremely low coefficient of friction permits them to run completely dry and at the same time prevent scoring of the cylinder walls. This type of piston ring is very ideal for applications where the presence of lubrication can be detrimental or even dangerous.
The following are the most common types of materials used for seals:
• Leather. This is rugged and inexpensive. However it tends to ‘squeal’ when dry and cannot operate above 93 °C (200 °F). Leather also does not operate well at cold temperatures of around -50 °C (-58 °F).
• Buna-N: This material is rugged, inexpensive and wears well. It has a wide operating range between -45 °C (-50 °F) and 121 °C (250 °F) and also maintains good sealing characteristics in this range.
• Silicone: This elastomer has an extremely wide temperature range between -68 °C (-90 °F) and 232 °C (450 °F). Hence it is widely used in rotating shaft seals and static seals. Silicone is not used for reciprocating seal application as it has a low resistance to tear.
• Neoprene: This material has a temperature range of -54 °C (-65 °F) to 120 °C (250 “^F). It has a tendency to vulcanize beyond this temperature.
• Viton: This material contains about 65% fluorine. It has become a standard material for elastomer type seals for use at elevated temperatures up to 260 °C (500 °F). The minimum temperature at which these seals operate is about -29 °C (-20 °F).
• Tetrafluoroethylene: It is a form of plastic and is a very widely used seal material. It is quite tough and chemically inert in nature and has excellent resistance up to temperatures of 370 °C (around 700 °F). It additionally possesses an extremely low coefficient of friction. One major disadvantage with this material is its tendency to flow under pressure forming thin films. This can be neutralized to a large extent by using filler materials such as graphite, asbestos and glass fibers.