Introduction to Seals :
Oil under pressure is moving in every hydraulic circuit. Since oil is a liquid it has the tendency to ‘leak’ through every gas/slot it finds during movement. This leakage of oil results in (bad effects of leakage)
- Loss of efficiency of the hydraulic circuit.
- During leakage, oil is coming out without doing any work hence is the loss of power.
- Sometimes there is a temperature rise due to leakage.
- Hydraulic oil is costly and hence when it is leaked there is a loss of money (monitory loss )
- If leaked oil is dropped on hot surfaces then there is a possibility of fire. Hence there are fire hazards due to leakage.
- When leaked oil falls on the ground, it becomes slippery and there is the possibility of an accident.
The seal is an agent or element which prevents leakage of oil from hydraulic elements and protects the system from dust and dirt. A mechanical seal is a device that helps join systems or mechanisms together by preventing leakage.
Classification Of Seals :
According to the nature of the application:
a. The types of seals used in hydraulic circuits are static seals and dynamic seals
The seals used between the mating parts that do not move relative to each other are termed as static seals. These seals are compressed between two rigidly connected parts. These seals make leak-proof joints because of pressure applied in tightening the bolts. Under pressure, the seal material flows and fills the irregularities in the surface making the joint leak-proof. A static seal may often termed as a gasket and is usually cut from compressible flat sheet material like paper, cork, rubber, or asbestos. The thickness is ranging from 0.25 mm to 3 mm. The figure shows a static flange joint and rubber seal molded in a metal ring. An O-ring static seal is the simple and most versatile seal used for static applications. The O-ring can be made circular, rectangular, or U-ring in cross-section.
The seal between the mating parts that move relative to each other is called as dynamic seals. These seals are subjected to wear as one of the mating part rubs against the seal. These seals prevent leakage around a moving component. Ex. Piston rings, O- ring on rotating and reciprocating shafts.
Types of dynamic seals
2) Lipped seals
3) Piston cup packing
4) Piston rings
5) Wiper rings
According to Degree of Sealing ( Extent of Sealing ):
1. Positive seal: when sealing is required for 100% leak-proof and no oil is allowed to leak.
2. Non-positive seal: when sealing is allowed minute amount of oil leakage for lubrication of spools and moving parts of valves.
Classification of seals based on shape:-
a) ‘O’ Ring seal
b) ‘V’ Ring seal
c) U-packing seal
d) T- ring seal
e) Cup seal
O’ Ring Seal:
The figure shows an ‘O’ ring seal. These are the most common and simple seal with a circular cross-sections like ‘O’. Hence called O-ring is used as a static as well as dynamic seal. The material used for O-Ring is a synthetic rubber and is specified by its ID/OD. The round cross-section is non-positive seals. O-rings are fitted with back ring the following figure depicts sealing of cylinder and piston by using ‘O’-ring with the backup ring.
Fig. depicts the sealing of the cylinder and piston by using o- ring with the backup ring.
‘V’ Ring Seal :
V-ring seal is also called ‘ V ‘ packaging. It is commonly used in hydraulic systems. These seals are available in pieces to facilitate assembly and specific size. It has a cross-section resembling to letter ‘V’. There is male and female adapter at the extreme ends as shown. In between V- seal rings can be assembled.
These are made up of fluorocarbon reinforced with cotton, asbestos, or are made up of neoprene reinforced with asbestos.
U- Packing Seal :
U packing seal is most versatile in various applications and used ad ID or OD piston seals. U packings are available in rectangular cross-section shape to suit piston seals.
- U-packing with long lips: Suitable for eccentric operations.
- U-packing with short lips: Suitable for a pressure system.
U-packaging is usually made up of leather or fabric reinforced rubber. These seals are balanced seals and no staking like a V-ring seal is required.
T-ring Seals :
These are very modern seals. These seals can be installed as ‘o’- ring. But these seals are equally effective as static as well as dynamic seals. T- ring seals can be used on road as well as a piston.
Cup seals :
These seals are most commonly used with the piston as shown in fig. These seals can take higher pressures up to 700 bar. The material used for these seals is polyurethane or leather. For low-pressure applications, neoprene is used.
The function of Each Type of Seal :
The function of each type of seal:
1) Positive seals do not allow any leakage whereas non-positive seal permits a small amount of internal leakage.
2) ‘O’ ring is a static seal that gives very effective sealing at high pressure.
3) Quad ring seal is quite versatile and can be used as a static seal as well as for rotary and reciprocating motion.
4) V- Packing seal is capable of holding almost all pressure.
5) U-Packing seal is used in various dynamic sealing.
6) Cup Packing seal is exclusively used to seal pistons in both low and high pressure hydraulic and pneumatic applications.
7) Composite seals is a combination of various types and are available in the form of sets used for special application.
Factors for seal selection:
1) Type of fluid used in the system
2) Maximum temperature of the system in working condition
3) Functional reliability expected
4) Cost of seal
5) Working pressure of the system
6) Environmental condition
Purpose of Oil Seal.
1) To stop leakage of oil
2) To maintain the pressure
3) To keep out contamination in the system
4) To enhance the working life of the system
5) To enhance the functional reliability of components over a longer period
General functions of seals
1) To stop leakage of oil.
2) To maintain the pressure
3) To keep out contamination in the system.
4) To enhance the functional reliability of the components over a longer period.
5) To enhance the working life of the system.
Reasons for seal failure :
1) Incompatibility of seal material with oil.
2) Low speed of actuators
3) If seals are not correctly installed then there is a possibility of seal failure.
4) High temperature of the oil can burn the seals.
5) If the seal is excessively squeezed then it can fail.
Factors to be considered for selection of seal
1. Shaft Speed
The maximum allowable shaft speed is a function of the shaft finish, runout, housing bore and shaft concentricity, type of fluid being sealed, and the type of oil seal material.
The temperature range of the mechanism in which the seal is installed must not exceed the temperature range of the seal elastomer.
Most conventional oil seals are designed only to withstand very low-pressure applications (about 8 psi or less). If additional internal pressure is present or anticipated, pressure relief is necessary.
4. Shaft Hardness
Longer seal life can be expected with shafts having a Rockwell (RC) hardness of 30 or more. When exposed to abrasive contamination, the hardness should be increased to RC 60.
5. Shaft Surface Finish
Most effective sealing is obtained with optimum shaft surface finishes. The sealing efficiency is affected by the direction of the finish tool marks and the spiral lead. Best sealing results are obtained with polished or ground shafts with concentric (no spiral lead) finish marks. If you must use shafts with spiral finish leads, they should lead toward the fluid when the shaft rotates.
When the bore and shaft centers are misaligned, seal life will be shortened because the wear will be concentrated on one side of the sealing lip.
7. Shaft and Bore Tolerances
The best seal performance is achieved when the close shaft and bore tolerances are present. Other factors include shaft eccentricity, end play, and vibration.
Runout must be kept to a minimum. Movement of the center of rotation is usually caused by bearing wobble or shaft whip. When coupled with misalignment, this problem is compounded. Contrary to popular belief and common practice, the installation of flexible couplings cannot correct or compensate for misalignment.
Seals perform much better and longer when they are continuously lubricated with oil that has the correct viscosity for the application and that is compatible with the seal lip elastomer material. The consideration of seal incompatibility, particularly with certain additives and some synthetic lubricants, should not be ignored.
Causes of failure
1. Improper installation is a major cause of hydraulic seal failure. The important things to watch during seal installation are (a) cleanliness, (b) protecting the seal from nicks and cuts, and (c) proper lubrication
2. Hydraulic system contamination is another major factor in hydraulic seal failure
3. Chemical breakdown of the seal material.
4. heat degradation problems may involve reducing sea life.
Failure reasons for seals:
(a) Excessive clearance: Excessive clearance between moving parts causes extrusion of the O-ring and other rings and reduces the performance of the seal.
(b) Fluid temperature: High working temperature of the fluid may cause seal abrasion.
(c) Damaged or worn out parts: Causes improper fitting of the seal.
(d) Incompatibility of the seal material and oil: When the oil is improperly selected with respect to the type of oil.
(e) Excessive side loads/overloading: It causes wobbling of the piston rod and dislocates the seals.
(f) Vibrations: Caused due to bent shaft, broken vane, misaligned or damaged bearing, defective coupling, etc.
Application of seals:
- Static Seals: These seals are used in reservoirs, gearboxes, body and casing/cover assembling of storage tanks, etc.
- Dynamic seals: These seals are used in applications where mating parts are having relative motion. Hence the applications like piston and cylinder, rotating shaft and body, oscillating or limited rotary hydraulic motors.
- Cup seals are used where seals have to withstand higher operating pressure up to 700 bar. v- packing and u packing seals are used as ID rod or OD piston seal.
- O ring is used as static as well as dynamic seal.
- General Applications where these all seals for different purposes are: Hydraulic pump, hydraulic motors, hydraulic actuators, valves, filter, reservoir
Material Used For Seals :
Two materials of seals used in hydraulic systems.
1) Metallic seal-like Aluminum alloy.
2) Non-metallic seal-like Synthetic rubber.
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