Rotary Seals for Dummies: Four Questions about Shaft Surfaces for PTFE Rotary Seals
Polymeric Rotary Shaft Seals:
When it come to polymeric rotary shaft seals, the mating surface of the shaft is of vital importance. In this article we look at four questions that you need to know the answers to when it comes to the shaft surface for a PTFE rotary shaft seal. Let’s get started!
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Question #1 – Surface Finish: Can It Be Too Smooth?
If a rotating surface is too rough, two things can happen: a leak path can be created, and the rough surface can be abrasive to the PTFE seal and reduce its life. However, too smooth of a surface can actually decrease the effectiveness of a dynamic seal. So what is the solution?
The actual surface roughness, Ra, is dependent on the type of media you are sealing in. For example, the smoothest surface would be for cryogenic applications, with a recommended surface finish of 4 µin. Gasses like helium and freon would require a recommended surface of no more than 6 µin. Aircraft and automotive fuel, natural gas, air, or nitroge would require a surface roughness of no more than approximately 8 µin. Fluids like water, crude oil, and hydraulic oil allow you to have the roughest surface of these, with a recommended finish of 12 µin.
A surface needs to be of sufficient roughness to allow a thin film of fluid to flow between the PTFE seal lip and the mating surface of the shaft. This helps to lubricate the mating surfaces and therefore extand the life of the seal. If it is too smooth, then you will not get optimum performance from the seal.
The problem with achieving a certain surface finish is cost: the smoother the surface, the higher the cost of manufacturing.
Question # 2 – Surface Hardness: Can It Be Too Hard?
Another issue with the shaft surface is its hardness. The rule of thumb is that when a harder shaft surface is chosen, then a harder, reinforced seal material can be used which will increase the effectiveness of the seal and extend its life. However, there are some other things that must be taken into consideration. With extremely hard surfaces, you actually run the risk of reducing the seal effectiveness and life.
When the Rockwell hardness is below 45 RC, typical seal materials will end up polishing the the running surface. This results in a break-in period after which the seal wear will taper off. However, if the Rockwell hardness if above 45 RC, the break-in time, and associated wear, will be much longer because the surrface will be harder to polish. If the hardness exceeds 65 RC then the surface won’t polish. In short, the higher the hardness of the shaft, the more critical it is that the initial surface finish be appropriate. The plus side is that you can expect better overall seal performance with a harder surface.
Most engineers aim for a hardness between 50 RC and 60 RC. Again, cost comes into play: the harder the surface, the more expensive it will be to achieve a smooth surface finish for the initial break-in period. If you don’t, then you will greatly reduce the effective life of the seal.
Questions #3 – Surface Hardness: What Happens if it is too Soft?
Remember that the seal has to remain the sacrificial component, as opposed to the shaft or other dynamic hardware. If you are using a softer running surface, then you will need to select a seal material with less wear resistance that will not damage the shaft. If you have a softer shaft and an abrasive seal lip material, there is a chance that the seal lip will actually cause wear on the shaft. It is necessary to achieve a balance between the surface finish and hardness of the shaft, and the properties of the seal lip material.
Question #4 – Shaft Lead: It Does What!?!
One final issue with surface finish involves the shaft lead. When a shaft is turned to size on a lathe, the cutting edge of the tool will leave behind a spiral groove on the shaft. If this groove is not removed, then, depending on which direction the shaft is rotating, it can act as an auger to pump contaminants into the bearing housing or pump oil out past the seal lip. This defeats the whole purpose of the seal. Even if the surface of the shaft is going to be plated, this lead must be removed prior to installation.
When specifying a shaft for PTFE seal applications, the surface finish is of utmost importance. If it is too rough, then a leak path can develop and the life of the seal can be compromised. On the other hand, if it is too smooth then you won’t get the best performance out of the seal. If a shaft is too hard, then it can damage the seal; on the other hand, if it is too soft, then the seal will no longer be the sacrificial component. Finally, the shaft lead can actually help pump lubricant out or contaminants in, defeating the purpose of the seal. In short, don’t forget the needs of the rotary seal when specifying the surface finish and hardness of a shaft.
NOTE: Advanced EMC Technologies can supply the mating hardware for the seal assembly which eliminates surface finish and machining lead issues for customers that chose not to manufacture these critical components internally.
Find out more about advanced polymer sealing solutions in the Advanced EMC Sealing Solutions resource guide.