Simply put, the function of seals is to minimize or prevent leakage between two parts. While seals provide this very basic role, choosing the right seal for the job takes real consideration and knowledge. Selection also assumes that the seal purchaser grasps the potential effects of the environment on the seal in addition to understanding the machine or equipment where the seal will reside.
Choosing the right seal for virtually any application depends on operating conditions and the physical dimensions of the seal itself. While there are many considerations, here is a breakdown of the top four things you need to know when selecting a seal and why each is important.
The temperature within the seal’s environment is the main factor that dictates the seal materials. There is generally a stated temperature range that is optimal for that particular seal. For example, there may be a stated range of 4 to 49° C. When the operating environment is too cold a seal may become brittle and at high temperatures the seal material may show greater elasticity. Increased temperature also accelerates the aging of the rubber. When axial cracks at the sealing edge are visible, the seal has been exposed to excessively high temperature, clearly out of its recommended range. An ambient air temperature increase of 10° C can halve the theoretical useful life of the rubber.
It isn’t just air temperature that can affect seals. Temperatures can fluctuate from:
- High-speed rotation that generates excess heat
- Insufficient lubricating and thermal management capabilities of the lubricant
- The circumferential velocity
- Applied pressure
Position or Size
Measurement is a key element for proper seal selection. The groove or the position into which the seal will be housed must be measured correctly so that there is as perfect fit as possible. This is critical because if your seal does not fit, it will not be viable and leakage or contamination is a certainty.
Seal size is determined by the seal bore, the diameter of the hole in the housing where the seal will be fitted, seal outer and inner diameters (OD and ID) and seal width, which is the total width of the seal including inner and outer shells.
- Seal bore represents the diameter of the hole into which the seal will be seated.
- Seal OD: Measurements should be taken in at least three places equally spaced around the seal. The average of these readings can then be used as the OD.
- Seal width is a measurement of the seal height when laid on a level surface, which includes inner and outer shells.
- Shaft ID: A seal’s inner diameter can vary, so the shaft diameter is used as the inside dimension. If you don’t know the actual shaft diameter, you can estimate it by measuring the seal’s inside dimensions.
As the amount of pressure grows, the radial load and friction of the sealing lip increases as it contacts the shaft. Like temperature, recommended pressure for optimum performance is spelled out for each seal. Excess pressure causes seals to wear faster and shortens their life.
When a seal is under too much pressure, specified values of peripheral speed cannot be maintained but must be lowered relative to the magnitude of the pressure. At high pressures leakage between the seal’s periphery and the housing bore can be avoided by using seals with rubber-covered cases. When a seal is under pressure there is also a risk of axial movement in the housing bore, which is prevented by locating the seal against a shoulder with a spacer ring.
The speed at which a seal can effectively operate depends on such conditions as pressure, temperature, the lubricant or fluid involved, shaft finish and seal design. Lubrication between the seal lip and a moving surface reduces friction, an important seal degradant. The thickness of the lubricating film establishes the level of friction. Initially, when velocity increases, friction decreases as the lubricant is dispersed. Even at increased velocity, frictional forces will begin to climb, causing wear.
Various seal designs allow for a variety of maximum peripheral speeds. Higher peripheral speeds are appropriate for larger shaft diameters more than for smaller diameters, as cross-sectional area increases in proportion to the square of the diameter, thus increasing the heat dissipation capacity.
While higher speeds affect performance and seal life, speeds under and over the recommended range cause increased friction, impacting the seal material.
Common Causes of Failure
While all seals eventually wear out, the most common causes of rapid failure include:
1) Installation—damaged by handling, incorrect measurement, etc.
2) Seal face open during operation, enabling particles to penetrate and affect seal fastening
3) Improper seal selection
4) Poor design
5) Inadequate environmental controls
Not often considered is the chemical composition of the seal components, corrosion resistance, effects of compression, chemical attack, material choice and changing ambient conditions during use.
Today, there are interactive tools such as a Fits & Tolerances Calculator that allows you to determine lower and upper limit deviations and maximum and minimum interferences based on the selected tolerance classes for seal bore and shaft.
The information in this article should provide the basics of selection. In addition, should a seal fail, look at the evidence, which should lead to the reason for the failure. Is the seal brittle or too elastic? The issue is temperature. Are particles getting through the seal? Check the seal face opening.
Finding the best seal for your application does not need to be a larger-than-life exercise. For that best seal, begin here.