In the high-stakes worlds of automotive and aerospace engineering, precision isn’t just a goal—it’s a non-negotiable requirement. Every component, fluid, and material must perform flawlessly under extreme conditions. For lubricants and greases, this performance hinges on a single, critical property: viscosity.
However, a common point of confusion and potential error arises from the use of different viscosity units. While many engineers are familiar with the Reyn (a unit based on the pound-force-second per square inch), the lubricant industry often specifies products using a TP number (like TP 6.3). Understanding the conversion between Reyn and TP isn’t just academic; it’s essential for ensuring safety, performance, and longevity.
Language of Viscosity: Reyn vs. TP
Before diving into the “why,” let’s clarify the “what.”
- Reyn (lbf·s/in²): Named after the British physicist Osbourne Reynolds, this is an absolute unit of dynamic viscosity in the Imperial system. It defines the resistance to flow under specific force and area. It’s a fundamental unit used in engineering calculations, particularly in fluid dynamics and bearing design.
- TP (Thermoplastic) Number: This is a industrial classification based on kinematic viscosity, typically measured in centistokes (cSt) at 100°C (212°F). The TP number is not a direct unit of measurement but a code that corresponds to a specific viscosity range. For example:
- TP 6.3 corresponds to a kinematic viscosity range of 5.8 to 7.2 cSt at 100°C.
- TP 8.4 corresponds to 7.6 to 9.2 cSt.
- TP 10.5 corresponds to 9.3 to 11.7 cSt.
The key difference is that Reyn is a unit for dynamic viscosity, while the TP number is a code for kinematic viscosity. To convert between them, you need the fluid’s density.
The Conversion Process: From Reyn to TP
The conversion from Reyn to a TP number is a two-step process:
- Convert Reyn to Kinematic Viscosity (cSt):
Dynamic Viscosity (Reyn) = Absolute Viscosity (lbf·s/in²)
Kinematic Viscosity (cSt) = Dynamic Viscosity (cP) / Density (g/mL)First, convert Reyn to centipoise (cP), a more common metric unit for dynamic viscosity.
1 Reyn = 68,950 cPThen, use the fluid’s density (ρ) at the same temperature to find kinematic viscosity in centistokes (cSt):
ν (cSt) = η (cP) / ρ - Match the cSt Value to a TP Grade:
Once you have the kinematic viscosity in cSt at 100°C, you can compare it to the standardized TP classification chart to find the corresponding grade (e.g., TP 6.3, TP 8.4).
Example: A lubricant with a dynamic viscosity of 0.00009 Reyn at 100°C and a density of 0.86 g/mL would be converted as follows:
- η = 0.00009 Reyn * 68,950 cP/Reyn = 6.2055 cP
- ν = 6.2055 cP / 0.86 g/mL = 7.22 cSt
- A viscosity of 7.22 cSt at 100°C falls within the 5.8 – 7.2 cSt range, classifying it as a TP 6.3 grease.
Why Accurate Viscosity Conversion Matters in Critical Industries
Using the wrong viscosity lubricant can have catastrophic consequences. Here’s why getting the conversion right is paramount.
1. Optimal Bearing Performance and Lifespan
Bearings are the heart of rotating machinery in engines, transmissions, turbines, and landing gear. Their lubrication is a precise science:
- Too Low Viscosity (TP too low): A lubricant that is too thin cannot maintain a sufficient lubricating film. This leads to metal-to-metal contact, increased friction, wear, overheating, and premature bearing failure.
- Too High Viscosity (TP too high): A lubricant that is too thick creates excessive fluid friction (churning), leading to energy losses, higher operating temperatures, and increased power consumption. In cold aerospace start-ups, a grease that is too viscous can prevent proper circulation.
Selecting the correct TP grade ensures the film strength (Reyn value) is perfect for the application’s load and speed.
2. Sealing and Compatibility
Automotive and aerospace systems rely on complex seals to keep lubricants in and contaminants out. The viscosity of a grease directly affects its ability to:
- Seal effectively without being squeezed out.
- Be compatible with elastomeric seals and not cause them to swell, shrink, or degrade.
A misclassified viscosity can lead to leaks, seal failure, and contamination—a major risk in aerospace applications.
3. Temperature Stability and Performance
Both industries subject lubricants to wild temperature swings:
- Automotive: From cold engine start-ups in winter to blistering engine bay heat.
- Aerospace: From frigid high-altitude temperatures to the intense heat generated by jet engine bearings.
The TP classification at 100°C provides a standardized reference point for high-temperature performance. Converting a Reyn value specified at a certain temperature ensures the lubricant will maintain its protective properties across its entire operating range.
4. Standardization and Supply Chain Clarity
Engineers design components around specific performance requirements (e.g., “this bearing requires a lubricant with a viscosity of X Reyn”). However, procurement teams source materials based on industry-standard classifications like TP numbers. A clear and accurate conversion process bridges this gap, ensuring that the product ordered from the supplier (TP 6.3) perfectly matches the product specified by the engineer (~0.00009 Reyn). This eliminates costly mistakes and production delays.
Conclusion: Precision Drives Performance
In automotive and aerospace engineering, there is no room for approximation. The process of converting Reyn to TP is more than a mathematical exercise—it is a critical step in the language of reliability. It ensures that the fundamental property of a lubricant is correctly specified, sourced, and applied.
By mastering this conversion, engineers and procurement specialists guarantee that every bearing turns smoothly, every gear meshes perfectly, and every machine operates at peak efficiency and safety. In an industry where failure is not an option, getting the viscosity right isn’t just important; it’s everything.