Matching Rotary Lip Seals to Shaft Characteristics for Optimum Performance
Posted on 2017/06/27 | Cliff | Seals
Rotary shaft hardness and roughness are important considerations when selecting the right PTFE lip seal.
Here’s a guide to avoiding seal failure by getting optimum performance and longevity for your seals and shafts.
A lip seal’s job
A rotary lip seal, or radial shaft seal, typically sits between a rotary shaft and a fixed housing—such as a cylinder wall—to stop fluid leaking along the shaft. The seal’s outside surface is fixed to the housing, and the seal’s inner lip presses against the rotating shaft.
Common applications are motors, gear boxes, pumps and axles. They’re also increasingly used for food and chemical processing, and with pressurized gas purposes.
PTFE: the standout material of choice
Polytetrafluoroethylenes, or PTFEs, are used for seals because of their extremely low friction, minimum wear, good performance at high temperatures with little lubrication and chemical compatibility.
Due to their toughness and low coefficient of friction, PTFE lip seals can slip over the high points of the mating surface (shaft) and resist abrasion—even without lubrication.
The two key characteristics of shafts
The properties of metal shafts that most affect sealing performance are:
Roughness is essentially surface unevenness. To measure roughness, the difference between high and low points of the shaft—or machined tolerance—is calculated.
Surface finish is crucial for seal performance and longevity. Rotating surfaces that are too rough can allow leak paths (through the low points on the shaft) and can also be very abrasive, causing seal wear and failure.
Although sealing generally works better with smoother shafts, excessive smoothness beyond spec can decrease the seal’s effectiveness by not allowing a film of fluid to flow between the seal and mating surface. This film lubricates and extends the life of the seal.
Hardness is measured by the depth of penetration of an ‘indenter’ which is forced on to the shaft at high pressure. The Rockwell scale measures the penetration depth relative to that made by a reference pressure.
A hard surface is usually required for metal shafts, allowing the use of highly reinforced seal material to increase seal and shaft life.
Some seal lip materials are abrasive and will erode softer metal shafts, so softer seals are required, which means shorter seal life. The seal should always be softer than the shaft to ensure it sacrificially wears out instead of the shaft.
One advantage of shaft surface hardness being below 45 on the Rockwell C scale is that most seals will polish the shaft surface during the initial ‘bedding-in’ period. After that, wear will ease off, depending on the PTFE material, surface finish, and Pressure*Velocity (PV rating) of the application.
When hardness exceeds 45 Rockwell C, the initial manufactured surface finish is crucial to seal life, as there is not much ‘bedding-in polishing,’ and any roughness will cause wear.
Recommended hardness varies depending on shaft speed and environmental pressure:
- At the low end: up to 150 sfpm (0.76 m/s) with 0 psi, hardness should be at least 35 Rc (well lubricated) or 44 (no lubrication).
- 70+ Rc may be required for 1000 psi at the same speed (up to 150 sfpm).
- 60+ Rc may be required for 2500 sfpm at the same pressure (0 psi).
In typical practice, hardness used is normally a compromise between the expense of harder finish metals and seal life.
Getting the optimum match between shaft and seal
Because of the wide range of PTFE fillers and material specifications, it’s not always easy to find exactly the right lip seal for your rotary shaft to ensure optimum seal effectiveness and longevity.