Design and development of oil seals

Design and development of oil seals

The design of oil seal structures is primarily based on operating conditions, assembly conditions, and environmental conditions. Factors such as sealing performance, service life, materials, manufacturing process, and economic efficiency should be considered. When designing an oil seal, the first step is to select the appropriate seal material. The rubber compound formula used should provide a reasonable combination of properties that meet the requirements of heat resistance, oil resistance, wear resistance, and good process performance.

Oil seal usage parameters and design parameters
 

In structural design, the parameters used and the design parameters should be compatible. The relationship between the design parameters and the parameters used can be illustrated in Table 1.

When designing the oil seal structure, the structural parameters shown in the figure below should be considered.

(1) Lip Interference (d-d1)

If the interference is large, the lip will stretch excessively, causing aging and wear, shortening the service life. If the interference is small, the sealing performance will be poor. Because the interference is related to the radial force of the entire lip, it should be considered comprehensively. The interference values ​​shown in Table 2 are for reference only.

Table 2 Interference of different shaft diameters

(2)Spring position "R" value

This value is a theoretical contact width in design. A larger "R" value increases the contact width and friction. A smaller "R" value is not conducive to sealing. The "R" values ​​for spring positions in Table 3 are for reference only.

(3) Waist length

The radial force provided by the waist length is about 50% of the radial force of the oil seal lip. It is important to maintain a low radial force. One way to achieve this is to extend the length of the oil seal waist. However, the outer diameter of the oil seal is generally standardized. Even the non-standardized assembly space also limits this width. Therefore, the straight length of the waist is limited. This problem can be solved by deriving a curved section from the straight part of the waist.

(4) Waist section thickness

Experiments have shown that even under low pressure, deformation as shown in Figure (A) can easily occur. Simply thickening the waist is detrimental to the lip's ability to follow eccentricity. A thicker waist weakens the spring action, resulting in a less effective eccentricity-following capability than a thinner waist. To resolve the conflict between waist deformation and followability, it is recommended that the waist be reshaped as shown in Figure (B). This increases waist rigidity without compromising eccentricity-following capability.

(5) Head Top Length

Some oil seal cross-sectional diagrams design the head top length (t) to be equal to the spring groove radius (r). However, during use, the spring often falls off. To prevent the spring from falling off, the design should ensure that t is greater than r, at least satisfying the following relationship: t = 4/3 r.

(6) Spring groove shape

Many oil seals have made a mistake in the design of the spring groove, designing the spring groove radius (R) and the spring circle radius (r) to different values. Experimental verification has found that some oil seal lips have two contact zones. Therefore, when R=r, the stress distribution state of the lip is the best, with only one contact zone. However, due to mold processing, rubber shrinkage, etc., it is often difficult to make the two absolutely equal in manufacturing. The only way to maintain a small difference between the two is to maintain a small difference between the two.

(7) Metal frame design

The main function of the metal frame is to strengthen the structural rigidity of the oil seal. Its thickness and configuration method depend on the working conditions and assembly conditions of the oil seal.

(8) Spring Coil

There are two types of springs used in oil seals: garter springs and leaf springs. Garter springs are the most commonly used of the two. For calculations of spring diameter, extended length, and number of wire coils, refer to relevant standards and mechanical design manuals.

(9) Radial Force

Radial force is an extremely important parameter. Its effect on the performance of oil seals is summarized as follows:

1. If the radial force is too small, the sealing performance will be poor; 2. If the radial force is too large, wear will occur and the service life will be shortened; 3. The radial force directly affects the friction and temperature of the contact area. When the radial force is too large, friction generates large heat and accelerates the aging of the lip; 4. The wear of the shaft is also affected by the radial force; 5. When the shaft and the housing are eccentric, appropriate radial force must be applied to ensure that the lip has appropriate tracking ability; 6. The radial force limits the operating pressure of the medium. If the medium pressure is too high, further increase in the radial force will shorten the life of the oil seal.

Oil seal material

Currently, oil seals are primarily manufactured from synthetic rubber. Because its selection and structural design are key factors influencing the sealing performance and service life of oil seals, it is crucial to accurately understand the properties of rubber and select the appropriate material. The most suitable rubber material for oil seals should be determined based on the relevant parameters of the oil seal: the radial force on the shaft should be high enough to prevent leakage, yet low enough to maintain a certain oil film thickness to keep frictional heat low. The seal should have sufficient interference fit to overcome the effects of eccentricity during operation. The lip area in the contact zone is also a determining factor.

The oil seal material directly influences these three parameters. As the material changes with time and temperature, key parameters also change accordingly. For example, as temperature increases, the material's modulus decreases, causing changes in radial force. Thermal expansion, material swelling caused by the sealing medium, and the hardness of the rubber compound all affect radial force and interference fit.

For these reasons, the following properties should be considered when selecting oil seal materials: compatibility with the sealing medium, resistance to swelling or hardening due to the medium; good heat and wear resistance; and moderate elasticity to accommodate variations in shaft roughness and eccentricity.

Due to the constant evolution of rubber material formulations, with new materials emerging and existing materials constantly being improved, the following is a brief description of the most commonly used materials for oil seals: nitrile rubber (NBR), polyacrylate rubber (PAR), silicone rubber, fluororubber (FKM), and polytetrafluoroethylene (PTFE).

Nitrile Rubber

NBR may be used in greater quantities than all other elastomers combined in seal manufacturing. NBR is a copolymer of butadiene and propylene, with propylene content ranging from 18% to 40%. It is categorized as low, medium, and high propylene content. While NBR's oil resistance increases with propylene content, its low-temperature flexibility decreases. To achieve good low-temperature performance, some resistance to high-temperature fuels and oils is often sacrificed. Nitrile rubber has excellent physical properties, with better cold flow, tear, and abrasion resistance than most other rubbers. However, it is not resistant to ozone, weather, and sunlight, though these properties can be improved through formulation design. Nitrile rubber is suitable for use with petroleum-based oils, fuel oils, water, silicone oils and silicone esters, and mixtures of ethylene glycol. However, it is not suitable for contact with EP oils, halogenated hydrocarbons, nitrocarbons, phosphate ester fluids, ketones, strong acids, and certain automotive brake fluids.

Polyacrylate Rubber

Polyacrylate (ACM) rubber is an emulsion co-slurry of alkyl acrylates with other unsaturated monomers. Commonly used alkyl acrylates are ethylene ethyl acrylate and butyl acrylate. The performance of polyacrylate rubber lies between that of nitrile rubber and fluororubber. Because its main chain contains no double bonds, it exhibits high resistance to heat, ozone, and weathering. The presence of chlorine (Cl) or (CM) functional groups on its side chains further enhances its oil resistance, enabling use in hot oils at temperatures between 170°C and 180°C. A key feature of this rubber is its excellent resistance to mineral oil, hyperbolic oil, and butter at 178°C. It also exhibits excellent resistance to aging and flex cracking, making it suitable for oil seals. Its main disadvantages include poor processing, sticking to rollers during mixing, limited low-temperature performance, poor resistance to water and steam, poor resistance to ethylene glycol and highly aromatic oils, high compression set, and significant corrosion to metal molds and shafts. Its elasticity, wear resistance, and electrical insulation properties are also relatively poor. Furthermore, due to its high degree of saturation, it has a slow vulcanization rate. While its wear resistance can be significantly improved with proper formulation, it still falls short of nitrile rubber.

Silicone Rubber

Silicone rubber maintains its mechanical properties over a wide temperature range, remaining flexible at -65°C and capable of prolonged operation at 230°C. Although its mechanical properties can be enhanced through special compounding, its strength, tear resistance, and abrasion resistance are generally relatively poor. Its resistance to alkalis, weak acids, and ozone is generally good, but its oil resistance is moderate. Chemical properties can be improved with compounding agents, such as those for improving oil and fuel resistance. However, silicone rubber is generally not suitable for use in hydrocarbons such as gasoline, paraffin, and light mineral oil, as these media will cause it to swell and soften. Silicone rubber's primary advantage is its ability to maintain elasticity at very low temperatures. Furthermore, it can withstand high temperatures for extended periods without hardening, making it suitable for a wider range of high- and low-temperature seals than other rubbers. For rotary seals, its operating temperature is higher than that of standard rubber. However, silicone rubber is more expensive than most other rubbers.

Fluorosilicone rubber is a more expensive rubber. Its performance is essentially the same as silicone rubber, but its application range is narrower. Its main advantage is its oil resistance, which is comparable to or close to that of nitrile rubber. This allows it to be used outside the operating temperature limits of nitrile rubber while still offering the oil resistance that silicone rubber lacks.

Fluororubber

Fluororubber is a saturated polymer containing fluorine atoms on carbon atoms in the main or side chains. It possesses unique and excellent properties. It is characterized by resistance to high temperatures, oils, severe corrosion, solvents, weathering, ozone, low gas permeability, and excellent physical properties. It can operate continuously at temperatures between 200°C and 250°C. However, its disadvantages are poor low-temperature performance and high compression set. Considerable research has been conducted both domestically and internationally to improve the compression set of fluororubber.

Polytetrafluoroethylene

Plastics are generally semi-rigid and not generally used as seals. Polytetrafluoroethylene (PTFE) is an exception. It is a fluorocarbon compound with unique properties, most notably its resistance to chemical attack over a wide operating temperature range. It exhibits a low coefficient of friction against metals, but without filler reinforcement, its mechanical strength is low. PTFE is particularly useful in seals made from composite structures. For example, machined or molded PTFE can be used as both a low-friction surface and a chemical-resistant coating.

Oil Seal Material Properties

Operating Temperature of Oil Seal Materials

Operating temperature is a crucial factor affecting the service life of oil seals. The operating temperatures of several commonly used oil seal materials are shown in Table 4.

Table 4 Operating temperatures of commonly used oil seal materials

Performance changes at low temperatures differ significantly from those at high temperatures. As temperature decreases, nearly all elastomers gradually harden due to a loss of flexibility, slowing their recovery from deformation. Crystallization also occurs, albeit slowly. Before the material reaches brittleness, if there are no alternative elastomer materials, spring force can provide the necessary resilience. At high temperatures, all elastomers lose their elasticity and tend to soften. High temperatures also accelerate material aging, typically manifesting as a loss of elasticity and a gradual increase in hardness and modulus.

Wear resistance of oil seal materials

Material wear resistance is a crucial factor for oil seals. Rubber's wear resistance is related to its hardness and tear resistance. Generally, wear resistance improves with increasing hardness; better tear resistance also leads to better wear resistance. Furthermore, a material's wear resistance is also influenced by factors such as its coefficient of friction and the glossiness of the mating surface.

Compatibility with Sealing Media

As the material absorbs the liquid medium, its volume changes. Excessive expansion can degrade the material's physical and mechanical properties, rendering it unacceptable. Excessive expansion can also cause chemical reactions, such as dissolution, interactions between certain components within the material, or surface embrittlement, leading to cracking. In these cases, the sealing medium and material are incompatible. In some cases, the sealing medium may extract additives such as plasticizers and antioxidants from the rubber compound, altering the elastomer's composition and even causing shrinkage, leading to leakage. For information on the compatibility of oil seal materials with certain media, please refer to Table 5.

Table 5 Compatibility of oil seal materials

From the above, it can be seen that the structural design of the oil seal is very important. Even if the oil seal material is very good, if its structural design is unreasonable, effective sealing cannot be achieved.