July 7, 2015
The Importance of Seal Running Surfacenews
Several years ago while advertising for Air Jordan shoes, Spike Lee (a.k.a. Mars Blackmon) stated throughout the commercial, “It’s gotta be the shoes, It’s gotta be the shoes”. He was of course insinuating Jordan’s ability to leap, bound, and dunk like no other was because of the Air Jordans! Granted, there is no doubt that Michael Jordan’s amazing talent got him to where he was, we will never know if he could have done just as well wearing PF Flyers, Red Ball Jets, or Converse All Stars; however, having the right pair of shoes certainly didn’t hurt!
So what does this have to do with PTFE oil seals and running surface? Well, you can have a PTFE rotary lip seal made with the highest quality materials and superior design, but without the right (shaft) running surface it’s got a good chance of not lasting as long as you would like. (Insert: “It’s gotta be the surface, it’s gotta be the surface”!) I think Mars Blackmon would agree!
There are three extremely important running surface characteristics that need to be present to ensure the success of a seal:
- Proper shaft finish
- Proper shaft hardness
- No “lead” (spiral lines)
If any one of these characteristics is not present you are going to have a short lived PTFE rotary lip seal.
Now that you know what the three characteristics are let’s break down each one and talk about why they are important.
PTFE Rotary Shaft Seals are more abrasive than the run-of-the-mill elastomer seal. This isn’t the PTFE’s fault, but rather the fillers that are used in the PTFE resin that enhances seal mechanical properties to make the seal successful in your application. For example, the combination of glass fiber and molybdenum added to the PTFE resin allow the seal to have terrific wear properties.
On the flip side, it needs a hard surface to run on. We call out on our prints a recommended Rockwell C hardness of between 56 & 62 as the sweet spot. Most 300 series stainless steel shafts have a Rockwell C hardness from 10 to 22- well below the ideal range that a PTFE Lip Seal requires if you’re looking for longevity. Don’t fret…there is a solution!
Before we get to that let’s discuss the other two necessary surface requirements.
Just as important as surface hardness, a proper surface finish on the shaft is necessary for all seals. PTFE rotary lip seals run optimally if the Ra is between 8 and 16, a higher range than what an elastomer seal requires. The reason is that the friction caused from a semi-rough shaft surface allows the PTFE and its filler to go into the surface of the shaft, creating an additional sealing barrier. This is also why it isn’t recommended to lubricate the underside of the PTFE lip seal as is common with elastomer seals. If you do, this film of lubricant prevents the benefit of the PTFE lip seal from creating the seal barrier.
The last important characteristic that will give you the sealing success you require is a plunge ground shaft surface. This can be tricky to ensure. Why? Well, many surface grinders will tell you that they are giving you a plunge ground surface when in fact they are not. It may be “plunge ground” to their standards, but it won’t be to the seal. There cannot be any lead (spiral lines) on the surface or a leak path will be created, and nobody likes leak paths!
It goes as far as how the grinder dresses their grinding wheel. If not properly done when applying a “plunge” grind, it will create the dreaded spirals. Nearly 50% of all PTFE oil seal problems stem from an improperly ground running surface.
By now you’re probably dying to know the solution, right? Well it’s pretty simple: Seek out a seal producer who is not only knowledgeable, but will properly engineer and manufacture the wear sleeve. (That’s TRITEC, in case you were wondering!)
TRITEC has been providing the proper running surface for our customer’s oil seals for nearly 30 years through engineered solutions and carefully manufacturing. By having TRITEC provide you with the best running surface for your PTFE seal, you are assured that all three necessary characteristics of wear sleeves are being met! Additionally, there are several coatings and surface treatments that TRITEC can apply on the wear sleeve that will enhance its effectiveness in very severe applications. This could be ceramic, chrome, nickel, or even a surface treatment such as ion nitride, or TriArmor for hardening 300 series stainless to a 70 Rockwell C hardness! The experts at TRITEC would be happy to talk to you about your application and recommend a solution that is engineered and designed better than you ever thought possible!