Rotary seals are essential to maintaining the life of the equipment. While choosing the appropriate rotary seal materials, the rotary shaft mating surfaces are equally important.
Below, we will discuss rotary seals, materials used for seals, and rotary shaft properties.
Rotary seals work to help keep the system lubricated while excluding contaminates. A properly fitting seal can positively impact the life of the lubricant. Oil’s life span at 86°F (30°C) is 30 years. However, as the oil heats up, the life span diminishes rapidly to no more than a 30-day life span.
The addition of contaminants and water also limits the life of the oil and the ball bearings. For example, adding .002% water into the oil lubricant will reduce the ball bearing’s life by 50%. The cause of the ball-bearing integrity loss is called hydrogen embrittlement.
Common Materials Used for Rotary Seals
Rotary seals come in several different materials. The optimal choice is dependent on the environment in which it is used. The most common materials include nitrile rubber, polyacrylate rubber, fluoroelastomers (FKM), and PTFE.
While these materials are a good choice for rotary seals, conventional rubber seals are common in static applications where temperature and chemical compatibility are not a concern. PTFE is the solution in high-speed dynamic service requiring low friction seals or where exposure to severe temperature or chemicals exists. There are different types of PTFE used for rotary shaft seals. Users can choose from
- Virgin PTFE
- Molybdenum Disulfide Filled PTFE (MoS2)
- Carbon Filled PTFE
- Carbon and Graphite Filled PTFE
- Carbon and MoS2 Filled PTFE
- Glass Filled PTFE
- Glass and MoS2 Filled PTFE
- Polymide Filled PTFE
Properties of Rotary Shafts Mating Surfaces That Affect Sealing Performance
Most often, rotary shafts are metal. However, the rotary shaft mating surface could be made from plastics. No matter what material the shaft is made from, some properties will affect the sealing performance of the shaft.
The properties of the rotary shaft that affect sealing performance are the shaft harness and the shaft roughness. The sections below explain in greater detail how the hardness and roughness of the rotary shaft can affect the rotary shaft’s sealing performance.
Rotary Shaft Hardness
The hardness of the rotary shaft is how deep an indenter can penetrate the surface of a shaft. The shaft’s hardness is measured in the Rockwell C scale. The higher the number, the more complex the surface.
As a general rule, the rotary shaft should always be harder than the seal to ensure the seal wears out before the shaft. Additionally, if you choose a harder surface, there are more options for seal materials.
With a rotary shaft with a hardness exceeding 45 Rockwell C, the seal doesn’t have time to polish and “bed in.” That means that any roughness on the surface will cause issues with the seal, wearing it down quicker than average. A shaft with a hardness under 45 Rc requires a softer seal which doesn’t have as long of a life.
The choice of hardness depends on the environmental pressure and shaft speed. For example:
- In environments of 1000 psi with rates up to 150 sfpm, a shaft with 70Rc or greater is necessary
- Settings using shaft speed 2500 sfpm and 0 psi need a hardness of 60Rc or greater.
- Rotary shaft speeds of up to 150 sfpm and 0 psi need a hardness of at least 35Rc, with lubrication, or 44, with no lubrication.
Rotary Shaft Mating Surface Roughness
The rotary shaft’s roughness refers to the shaft surface’s unevenness. To measure the roughness, measurements of high and low points of the shaft and taking the difference to determine the machined tolerance.
Ideally, a smoother surface will increase the seal life and offer outstanding performance. But on the other hand, when the surface is exceptionally smooth, there is no way for the oil to flow between the mating surface and the seal. As a result, the seal wears out quicker without lubricant between the seal and the mating surface.
Of course, a high roughness level can allow leaks through low points on the shaft. Therefore, the rotary shaft roughness needs to be relatively smooth but not so smooth that the seal cannot be lubricated.
Rotary shafts work with the rotary seals to keep lubrication from dirt and water. Understanding the properties of the rotary shaft mating surfaces helps determine the type of seal material chosen. Contact us today for your rotary seal and shaft mating needs.