Productivity Pointers: Seals – Part 2

This is the second tip in a two-part series on seals. Last week, we covered basic seal types and design. This week, we’ll take an even closer look at design and also discussed seal wear.

Seal design: An in-depth look

When designing a seal, choosing the correct material is essential to proper operation. Seal materials include:

  •  Butyl Rubber: Suitable for phosphate ester (PE) and silicon oils. Often used as grease seals.
  • Nitrile or Buna-N: Used with most oils but has poor resistance to EP additives.
  • Neoprene: Synthetic elastomer used in temperature applications between -54 to 149 C. Found in all industrial fluid systems with the exception of PE fluids.
  • Perfluoroelastomers: Includes Teflon and Viton. Suitable for most oils and may be used in applications with a large temperature range: 56 to 316 C.
  • Ethylene Propylene: Used is phosphate ester systems with a temperature range of -54 to 149 C.
  • Fluorocarbon: Found in all industrial fluid systems with the exception of PE fluids.
  • Fluorosilicones: Seal found in fuel systems.

The choice of seal type and design is influenced by the lubricant chosen, specifically grease or oil and the additives they contain:

  • Grease is a semi-fluid and as such, given typical operating condition, stays in position. Any good seal, with or without a spring, will generally retain grease.
  • Because oil is fluid, it is more difficult to retain than grease. The seal used to retain oil most often contains a spring and a lip molded in such a manner to direct oil back into the sump. Because the lip is molded, the contact area is larger, facilitating load support and oil retention.

Proper seal design and choice is critical to achieving maximum component life when lubricating rotating equipment.

Seal Wear

The most common cause of seal wear is improper selection, particularly failure to consider the operating or environmental temperatures. With static seals, the polymers used may dry out with age and lose their memory. When that happens, leakage occurs and it is time to replace the gasket or o-ring. Mechanical seals, being more complex, can suffer from many different types of wear. These include:

  • Dry running: Occurs when there is no liquid around the seal, either due to the absence of pumped medium or poor venting.
  • Poor Lubrication: Causes frictional heat to be generated on the seal face, resulting in the seal cracking and drying out.
  • Contamination: Small amounts of hard particles on the seal face which results in abrasive wear.
  • Chemical degradation: Incompatibility with a chemical media and/or thermal environment may cause degradation. Signs include blisters, cracks, or discoloration. In some cases, the degradation may only be determined by measuring the physical properties of the seal.
  • Wear: Results from the thickness of the lubricating film being at or near surface roughness of the rotating shaft.  
  • Improper Installation: Some mechanical seal failures come from wrong mounting and handling. Examples can be shaft misalignment, seals not mounted perpendicular to the shaft, axially moving shaft and wrong assembly length, etc.
  • Corrosion: Grey or black streaks across the bearing raceway indicate that water has penetrated the bearing during static conditions. General rust indicates the presence of water or other corrosive substances.

It is difficult to state causes of shaft seal failures exactly without performing a root cause failure analysis. This would include a look at the shaft seal, rings, elastomers and other parts, in addition to knowing the operating and environmental conditions.

If it’s not already obvious, seals are super important. They are designed to not only keep the lubricant in but also to keep environmental factors out. Just 0.002% water entering a bearing will cut the rated life by 50%, so clearly proper seal selection impacts productivity.

I hope you enjoyed reading this tip as much as I did creating it. If you have any questions, leave them in the comments section below. Thanks for reading!