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How to Design the Perfect Rotary Shaft Seal

Cliff • September 1, 2020
PTFE Rotary Seal Shaft Considerations

When it comes to maintaining a high-functioning rotary shaft, you need to select the appropriate lip seal.

The shaft seal protects the rotary shaft from contaminants such as dust and dirt , and it keeps water out and lubricant in.

A rotary seal, also known as a 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 rotary seal’s outside surface is fixed to the housing, while the seal’s inner lip presses against the rotating shaft.

Common applications for shaft seals include motors, gear boxes, pumps and axles. They’re also increasingly used for food and chemical processing, as well in pressurized gas applications.

Three of the most important considerations when the choosing the best lip seal for a rotary shaft are:

  1. The material the seal is made of,
  2. the hardness of the shaft’s surface, and
  3. the roughness of the shaft’s surface.

Here’s our guide on how to achieve optimum performance and longevity for your seals and shafts, ultimately minimizing the risk of seal failure.

What Material Is Best for Rotary Seals?

The standout material of choice for a rotary shaft lip seal is polytetrafluoroethylene, also known as PTFE. This material is commonly used in seals because of its extremely low friction and resistance to wear and tear.

It also performs exceptionally well at high operating temperatures, requiring minimal lubrication. Plus, it is compatible with many chemicals, making it a good universal choice.

Rotary shaft lip seals made of PTFE are very tough and have a low coefficient of friction, which allows them to slip over the highest points of the mating surface (in this case, the rotary shaft) while resisting abrasion. And they can do it without lubrication.

Which Properties of Metal Rotary Shafts Most Affect Sealing Performance?

The roughness and hardness of a metal rotary shaft are the two most important characteristics to be considered when designing a lip seal for the shaft. Let’s look first at roughness.

What is Roughness in a Rotary Shaft?

Roughness, in the context of a rotary shaft, refers to the unevenness of the surface of the shaft. The way we measure shaft roughness is by measuring the high and the low points of the shaft’s surface, then calculating the difference. This is called the machined tolerance.

Roughness can be minimized by properly finishing the surface of the shaft. A well-finished surface with an appropriately machined tolerance will allow for excellent seal performance, as well as longevity.

A rotary shaft with a high surface roughness can allow paths for leakage through the low points on the shaft’s surface. The abrasiveness of the surface can also wear down the seal quickly, leading to failure.

Generally, the smoother the rotary shaft surface, the better the seal will perform. However, excessive smoothness beyond spec can actually decrease the effectiveness of a seal. This is because in extremely smooth surfaces, a film of lubricating fluid is not allowed to flow between the seal and the mating surface.

The passage of this film lubricates the seal and extends its longevity. Without it, the seal will wear out sooner. Essentially, if your rotary shaft’s surface is too smooth, a seal will perform too well — until it doesn’t.

What is Hardness in a Rotary Shaft?

The hardness of a rotary shaft is measured by looking at how deep an indenter can penetrate into the surface of the shaft when forced upon it at high pressure. Using a Rockwell scale, we can measure the penetration depth relative to that made by a reference pressure.

A seal should always be softer than the rotary shaft. This ensures it wears out instead of the shaft. So the harder the shaft, the more options you have for seal material.

In metal rotary shafts, the harder the surface, the better. Increased hardness allows for the use of highly reinforced seal materials, which will increase the lifespan of both the lip seal and the shaft itself.

The alternative, a softer metal shaft, is susceptible to abrasion and erosion from the best seal lip materials (PTFE). So when you’re working with a soft rotary shaft, you need to use a softer seal. This is a workable solution, but it does mean you’ll be stuck with a shorter seal life.

There is, though, one advantage to keeping rotary shaft surface hardness below 45 on the Rockwell C scale, and this is that most seals will actually polish the shaft surface during the initial “bedding in” period.

After that period, the wear upon the rotary shaft will ease off, depending on the PTFE material of the seal, the shaft’s surface finish, and Pressure*Velocity (PV rating) of the application.

When hardness exceeds 45 Rockwell C, the initial manufactured surface finish is absolutely crucial to seal life. Why? Because there is not much polishing allowed for in the “bedding in” period. When hardness is high, any roughness in the surface will cause wear on the seal.

So as you can see, the roughness and hardness of the surface of the rotary shaft are not independent properties when it comes to lip seal design — as the hardness goes up, roughness needs to go down. Alternatively, a less hard surface allows for a bit more roughness.

What is the Recommended Hardness for a Rotary Shaft?

The ideal hardness for a metal rotary shaft depends on shaft speed and environmental pressure.

  • At the low end of the spectrum, in rotary shaft speeds up to 150 sfpm (0.76 m/s), with 0 psi, hardness should be at least 35 Rc (with high lubrication) or 44 (if 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 engineering practice, the hardness used is normally a compromise between the expense of harder finish metals and seal life. As with any design task, the challenge is finding a solution that meets not only the technical requirements, but also falls within the logistical (i.e., budgetary) limits.

Achieving the Optimum Match Between Rotary Shaft and Lip Seal

Because of the wide range of PTFE fillers and material specifications, it’s not always easy to find exactly the right rotary seal for your shaft to ensure optimum seal effectiveness and longevity.

If you can’t find an existing shaft seal that meets your needs, you may need to design a custom seal. That happens to be exactly what our seal engineers specialize in here at Eclipse Engineering. We craft many types of seals for rotary shaft applications. including cased lip seals, O-ring OD lip seals, and MicroLip seals .

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Case Study: Dual Lip Crimped Case Seal in High Speed Rotary Vacuum Application

By Doug Montgomery • January 17, 2025
Eclipse deals regularly with challenging sealing applications from all industries. High pressures and speeds create unique sets of conditions where seal design and material properties are pushed to the limit. While reciprocating applications can certainly test seals to the edge of capability, often times rotary applications can present the greatest challenge to seal integrity and wear life. Unlike reciprocating configurations where the seal is acting on a different part of the shaft or bore throughout it’s operating range, rotary seals must operate on the same sealing area continuously. This makes things like heat rejection much more difficult, especially in unlubricated or dry running applications. Extreme localized heating can have negative affect on both seal and hardware life. Rotary applications also pose sealing difficulties due to the simple fact that surface speeds can be much higher than in reciprocating systems. A simple electric motor can operate at very high rpm, while long stroke, high speed reciprocating machinery is a major piece of equipment that is far less common (though Eclipse also has sealing solutions in a number of these situations). A customer approached Eclipse with an application that was beyond the scope and capability of any standard, off-the-shelf rotary seal. This sealing system would require a combination of both wear resistance in high-speed rotary, as well as excellent leakage control and sealability. Two factors that, more often than not, work in opposition to each other. The Customer Issue The customer was developing a test system that required an electric motor shaft passed through the wall of a large vacuum chamber. The testing apparatus needed a sizable motor to meet the speed and torque requirements. Adapting the motor to operate inside the chamber would not be practical due to contamination and motor cooling concerns. Therefore, the motor would have to be placed outside the chamber and a driveshaft would have to go through the chamber wall. Which, of course, would need a seal. Operating Conditions:
 Rotary Shaft Seal
 Shaft Diameter: 2.5”
 RPM: 7,500 RPM - unlubricated
 Pressure: Vacuum internal side / 1 ATM external side Temperature: 40° - 90°F The customer knew any kind of off-the-shelf rotary seal with a rubber element would not last any amount of time in the combination of speed and a dry running condition. They also knew a single lip PTFE seal would likely not meet their leakage requirements. Therefore, they turned Eclipse for a custom sealing solution.

Case Study: PEEK Spring Energized Seal in a High Temperature/High Radiation Application

By Doug Montgomery • November 25, 2024
Eclipse has engineered sealing solutions for applications all over the planet and in a plethora of environments. From the bottom of the ocean to orbiting the earth, Eclipse is challenged by the unique conditions in each application. Whether it be extreme temperature and pressure or severely caustic or abrasive media, Eclipse has a solution for most every sealing problem. One distinct environment presents a particularly challenging set of circumstances for seal design – high radiation. Eclipse’s primary seal material choice for many applications is PTFE and PTFE blends. With all the wonderful attributes PTFE possesses as a seal material, radiation resistance is not one. In high radiation environments PTFE’s properties can degrade to essentially rule it out as a suitable material. The options for effective sealing materials that are also radiation resistant becomes very limited. The seal designer is therefore confronted with creating a seal that is expected to perform in every way a typical PTFE seal operates, out of materials that are not as favorable to sealing. This is where Eclipse’s engineering experience and expertise in seal design come to the forefront. The Client's Issue Eclipse was approached by a customer that was looking for a seal solution for a sensor used in a nuclear application. It would be operating in an environment with both high temperature and high Gamma radiation. Operating Conditions:
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 Stroke: 1.5”
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By Doug Montgomery • November 14, 2024
Technological advancements in the area of robotics have led to more and more life-like creations existing only in works of science fiction a few decades ago. Development in autonomous logic processing and sensing allows bipedal robots to walk over uneven ground, up and down stairs, open doors and carry loads. Fast response to dynamic and unpredictable real-world environments is critical for the future use of robots in true-life service and practical employment in the years to come. While software and sensor development remain the primary focus of most research, the physical mechanics of next-gen robotics are also continually progressing. Physical components and control systems such as hydraulic pumps and cylinders, servo motors, and structural members are under pressure to continually be lighter, stronger, more efficient and less expensive. Increased demands on the physical components facilitate the need for innovative solutions in design and material usage. Advancements in construction and technology have spilled into all areas of robotic mechanisms and the many seals located throughout the system need to meet the challenges of tomorrow. Eclipse has been at the forefront of this research and has developed innovative solutions pushing the boundaries of conventional sealing devices. MicroLip™ by Eclipse is a prime example of most demanding applications forging new technologies in the sealing world. The Client's Issue Eclipse was approached by a leading robotics company looking for a sealing solution operating under a challenging set of conditions. While many components of tomorrow’s robotics are now controlled and actuated by servo/stepper motors and various electronic devices, the heaviest and most powerful movements are still driven by traditional hydraulics. The constant demand for more powerful hydraulic actuation in ever deceasing size and weight requirements has put tremendous strain on component design. But if robots are to progress to the point where they are usefully employed in the world, high power in a compact design is necessary. A robot, for example, used to survey and assist in a disaster zone too unstable for normal rescuers, must fit through doorways and over obstacles yet still be physically strong enough to render assistance. Large hydraulic systems are capable of moving extremely heavy loads but size and weight constraints of a humanoid size robot limit potential. The robot’s internal power supply to drive all components is also a limiting factor. Our client was developing a new hydraulic pump to drive all major motion aspects of their robotic systems. Their main objective was to minimize the pump’s physical size as much as possible while increasing output and improving power consumption efficiency. This means higher pressures and speeds on increasingly smaller and lighter components. Application Parameters: Shaft Diameter: Ø9.5mm Seal Housing Envelope: 5mm radial cross-section by 6mm axial width Rotational Speed: 3,500 RPM nominally; 6,000 RPM max Operating Pressure: 125 PSI min, 225 PSI nominal, 350 PSI max Surface Finish: 0.04µm Media: Hydraulic Oil While the above combination of pressure and speed might present difficulties for any conventional seal alone, the client’s extremely small physical envelope to house the seal further complicated the matter. If that wasn’t enough, the application presented the additional sealing challenge of up to 0.003” [0.08mm] of shaft runout. As part of the downsizing of all components in the pump, shaft support bearings were minimized leading to the possibility of runout. The wobbling effect of the shaft creates problems as the sealing lip has follow a moving, uneven mating surface, therefore potential leak-paths are created. Wear life can also be compromised due to higher concentrations of uneven loads. The combination of high pressure, high speed, high runout and minimal gland size present a worst-case scenario for a typical seal. Unsurprisingly, the client faced leakage of hydraulic fluid after only short periods of service with any conventional seal they had tested. Eclipse knew the had the perfect solution for this application. One developed to handle such extreme rotary sealing conditions: MicroLip™.
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