Comprehensive Introduction to Skeleton Oil Seals

Oil seals are mechanical components used for sealing, also known as rotary shaft lip seals. Since oil can enter the friction parts of machinery during operation, oil seals are used to prevent these oils from leaking out of the machinery. The skeleton oil seal is a typical representative of oil seals, and generally, the term "oil seal" refers to the skeleton oil seal.

Common representation method

Oil Seal Type-Inner Diameter-Outer Diameter-Height-Material

For example

TC40*62*12-NBR

Indicates

Double-lip inner skeleton oil seal, inner diameter 40, outer diameter 62, thickness 12, material is nitrile rubber.

Nitrile Rubber (NBR)

Wear-resistant, oil-resistant (cannot be used in polar media), temperature resistant: -40~120℃.

Hydrogenated Nitrile Rubber (HNBR)

Wear-resistant, oil-resistant, aging-resistant, temperature resistant: -40~200℃ (better temperature resistance than NBR).

Fluoroelastomer (FKM)

Acid and alkali resistant, oil-resistant (resistant to all oils), temperature resistant: -20~300℃ (better oil resistance than the first two).

Polyurethane Rubber (TPU)

Wear-resistant, aging-resistant, temperature resistant: -20~250℃ (excellent aging resistance).

Silicone Rubber (PMQ)

Heat-resistant, cold-resistant, low permanent compression deformation, low mechanical strength, temperature resistant: -60~250℃ (excellent temperature resistance).

Polytetrafluoroethylene (PTFE)

Good chemical stability, resistant to acids, alkalis, oils and various media, wear-resistant, high temperature resistant, high mechanical strength, good self-lubricating properties.

Generally speaking, materials commonly used for skeleton oil seals are nitrile rubber, fluoroelastomer, silicone rubber, and polytetrafluoroethylene due to its self-lubricating properties. Especially after adding bronze, the effect is even better, used for making retaining rings, glyd rings, step seals, etc.

C-type skeleton oil seals are inner skeleton oil seals and can be divided into SC-type, TC-type, VC-type, KC-type, DC-type: single-lip inner skeleton oil seal, double-lip inner skeleton oil seal, single-lip inner skeleton oil seal without spring, double-lip inner skeleton oil seal without spring, double-lip double-spring inner skeleton oil seal respectively.

G-type skeleton oil seals have external threads, similar in type to the C-type, but are modified on the outside to have threads to enhance sealing and fix the oil seal to prevent loosening.

B-type skeleton oil seals have rubber material inside or no rubber material inside or outside. No rubber material allows better heat dissipation.

A-type skeleton oil seals are assembled oil seals with relatively complex structures and better pressure resistance.

Skeleton oil seals are composed of three parts

self-tightening spring, sealing body, reinforcing skeleton

Usually, in its free state, the inner diameter of a skeleton oil seal is smaller than the shaft diameter, meaning it has a certain "interference fit." Therefore, when the oil seal is installed into the oil seal housing and on the shaft, the pressure of the oil seal lip and the contraction force of the self-tightening spring exert a certain radial tight force on the shaft. After a period of operation, this pressure will quickly decrease or even disappear, so the spring can compensate the self-tightening force of the oil seal at any time.

The sealing principle of the skeleton oil seal

An oil film controlled by the oil seal lip exists between the oil seal and the shaft, and this oil film has fluid lubrication properties. Under the influence of surface tension within the skeleton oil seal, the film stiffness is just right for forming a meniscus at the air–oil interface, preventing the working medium from leaking and achieving the rotary shaft's sealing.

The sealing ability of an oil seal depends on the thickness of the oil film on the sealing surface. If too thick, the oil seal will leak; if too thin, it may cause dry friction, leading to wear on both the oil seal and the shaft. If there's no oil film between the sealing lip and the shaft, it may cause heating and wear.

Therefore, during installation, apply some oil on the sealing ring and ensure the oil seal is perpendicular to the shaft centerline. If not perpendicular, the sealing lip of the oil seal will drain the lubricant, leading to excessive wear on the sealing lip. During operation, a small amount of lubricant should seep out to form a film at the sealing face.

The function of oil seals is generally to isolate lubricated components in the transmission parts from the output parts, preventing the leakage of lubricants, and are usually used for rotating shafts as a type of rotary shaft lip seal. The skeleton acts like the steel bars in concrete components, reinforcing the oil seal and helping it maintain shape and tension.

According to the type of skeleton, oil seals can be divided into inner skeleton oil seals, outer skeleton oil seals, and inner & outer exposed skeleton oil seals.

Skeleton oil seals are made using high-quality nitrile rubber and steel plates, providing stable quality and long service life.

• Prevents mud, dust, water vapor, etc., from intruding into the bearing;

  
  

• Prevents the leakage of lubricating oil from the bearing. Requirements for oil seals include conformity to specified dimensions (inner diameter, outer diameter, and thickness), appropriate elasticity to clamp the shaft properly to achieve the sealing function, heat resistance, wear resistance, good strength, medium resistance (oil or water), and long service life.

  
  

When choosing skeleton oil seals, it is important to consider the sealing medium and operating conditions. The sealing medium influences the material choice for the oil seal, and the operating conditions mainly consider the pressure, operating speed, and temperature range.

When selecting the oil seal material, consider the material's compatibility with the working medium, its adaptability to the operating temperature range, and the ability of the lip edge to follow the rotary shaft at high speeds.

The temperature at the lip edge of an oil seal during operation is usually 20~50℃ higher than the working medium temperature, which should be considered when selecting materials.

For standard oil seals, the operating pressure should not exceed 0.05MPa. If the operating pressure exceeds this value, pressure-resistant oil seals should be chosen.

Under the same diameter conditions, oil seals made of different materials have varying capabilities to adapt to the rotary velocity of the shaft surface. Generally, the rotary velocity range for oil seals is less than 15m/s.

The working temperature range for oil seals varies with the material used: -40~120℃ for Nitrile Rubber (NBR), -30~180℃ for Acrylic Rubber (ACM), and -25~300℃ for Fluoroelastomer (FPM).

Before installation, the oil seal lips should be coated with lithium grease containing molybdenum disulfide to prevent dry friction at the lips during instant startup, which could affect the interference fit. Installation should be performed promptly, and if the oil seal is not installed immediately, cover it with a cloth to prevent foreign matter from adhering. Ensure that hands and tools used to apply lithium grease are clean.

The oil seal should be installed flat without inclination, preferably using hydraulic equipment or sleeve tools. The pressure should not be too high, and the speed should be uniform and slow.

During installation, lubricate the oil seal and the shaft end and shoulder edges.

Ensure the lip edge faces the sealed oil side during installation and avoid reverse installation.

Use specialized tools to push the oil seal into the seating hole to prevent misalignment.

Take precautionary measures to protect the lip edge from damage when passing threads, keyways, splines, etc., and use specialized installation tools.

Avoid damaging the inner surface of the chamber or the shaft surface during oil seal removal.

Used oil seals should not be reused, even if they appear to be in good condition, and new oil seals must be installed.

Repair any nicks or rust on the shaft surface before installing a new oil seal.

Install the new oil seal to a contact area different from the original seal to avoid prior friction marks by using spacers.