Dynamic radial shaft seals are tasked with creating a barrier between surfaces in relative motion. They keep light contaminants away from a bearing and keep lubrication in. A seal can be the difference between optimal and worst-case system performance, which means that if the right seal is selected, installing it simpler and quicker, machinery will be less likely to fail prematurely, and overall performance is improved.
Weighing up your options
The problem is: there are hundreds of variants out there – all shapes, all sizes, and for all manner of applications. Simply matching a seal to a bearing isn’t enough. The seal should be selected in terms of the machine it will be fitted in and the operating conditions it will be subject to. Installation requirements (shaft assembly or spatial limitations), pressure, bore retention, resistance against chemicals, sealing lip protection, and lubrication requirements are the key operational considerations.
Some prime examples of what SKF terms “general industrial applications” are: pumps, fans, construction equipment, and especially drive line applications, such as industrial gearboxes.
Standard solutions, superior sealing
To support these applications, SKF has two sets of standard radial shaft seal solutions – measured in either millimeter or inch . HMS (5 and A10) or CRW (1, A1, H1, and HA1) seals are applicable in most general industrial cases and can replace the majority of existing seals, even if the design is slightly different. That said, shaft size does play a vital role when you are assessing your options. Fortunately, there is an extensive range of dimensions commonly used in the market and new designs are continually added. Contacting an SKF expert should shed light on your options.
The metric standard HMS5 and HMSA10 can withstand temperatures between -40 and 100°C and speeds of up to 18 m/s. The rubber-outside-diameter design optimizes sealing ability and lubrication retention. If your application involves dust or contaminants – such as gearboxes operated in environments with light contamination – the HMSA10 is fitted with an auxiliary lip for extra assurance to keep them away from the shaft.
CRW spring-loaded seals provide longer and more consistent service life than non-spring-loaded designs. What’s more, an SKF Wave lip which reduces surface pressure and results in 30 percent lower temperatures and 20 percent less friction – improving robustness. A metal diameter supports a smoother installation and an accurate fit in the housing bore.
The inch standard CR range extends to the general application pressure seals CRW5 and CRWA5, which have a metal casing to withstand pressure up to 50 psi. The CRWA5 also features an auxiliary lip to inhibit contamination exclusion.
Know your limits
It’s vital that you are aware of a seal’s limitations before you make a final decision. The following figures apply to the seals that we have discussed in this article. If your applications conditions are higher than the number stated, then you will have to look for a more suitable seal from SKF’s portfolio.
- Temperature: The permissible operating temperature for seals in NBR is –40 to +100 °C (–40 to +210 °F). Temperatures up to 120 °C (250 °F) can be tolerated for brief periods. If FKM material is used, seals can withstand temperatures up to +200 °C (390 °F).
- Pressure differentials: Standard seals are rated for no more than 0.07 MPa at 5 m/s, but CRW5 and CRWA5 can accommodate pressure of up to 0.34 MPa at 5 m/s.
- Speed: True permissible speeds are affected by a number of factors: surface, lubrication, pressure differentials, and running accuracy requirements. Please refer to pages 104 and 105 from our Industrial Shaft Seals catalogue.
Stay tuned for part four of our blog series on seal selection! We will conclude this series with selection criteria for shaft seals for heavy industrial applications. However, the selection process can be complex if you are unsure of what to look for. If you are ever in doubt, you can leave a comment under this article. An expert from the SKF team will be in touch!