Adrian Carrera
Surface finish is one of the most critical variables in bearing design. It has a direct and measurable impact on friction, wear rate, noise, and overall bearing service life.
At TriStar Plastics, surface finish is evaluated alongside material selection, load, speed, and environment, not as an afterthought. Achieving the right finish is a balance between plastic form and plastic function, and getting it wrong can prevent even the best self-lubricating bearing from reaching its full performance potential.
Is Surface Finish Really That Important?
Yes, and surface finish refers to far more than appearance. It directly determines how well and how long a bearing will perform in a real application.
Surface finish affects:
- Transfer film formation
- Coefficient of friction
- Wear rate of both shaft and bearing
- Noise and vibration
- Overall service life
In plastic and composite bearings, surface finish is a functional design parameter, not a cosmetic one.
When the Surface Finish Is Too Rough
If a shaft surface is excessively rough:
- The bearing experiences abrasive wear
- The polymer surface is rapidly removed
- Heat and friction increase
- Bearing life drops dramatically
This condition is often compared to running sandpaper against the bearing surface. Even highly wear-resistant materials will fail prematurely if paired with an overly rough shaft.
When the Surface Finish Is Too Smooth
Surprisingly, a surface that is too smooth can also be problematic. Mirror-polished surfaces may prevent self-lubricating polymers from functioning properly.
Why?
- Self-lubricating bearings rely on transfer film formation
- Migratory lubricants (PTFE, graphite, MoS₂) need micro-texture to embed
- An overly smooth surface prevents proper film adhesion
Without a stable transfer film:
- Friction can increase rather than decrease
- Wear accelerates
- The bearing fails to deliver its intended self-lubricating benefit
Why Surface Finish Must Be Defined Early in Design
Surface finish should be specified early in the bearing design process, because it directly influences:
- Bearing material selection
- Shaft material compatibility
- Lubrication mechanism effectiveness
- Noise and efficiency targets>
The right finish enables the formation of a stable lubrication layer, reducing friction, minimizing wear, and supporting quieter, more efficient operation.
For deeper technical guidance, explore our Bearing Design Technical Paper.
TriStar Test Data: Surface Finish vs. Wear Performance
TriStar recently conducted rotary wear tests using Rulon® LR bearings, measuring wear performance across multiple shaft finishes.
Results showed a clear relationship between surface finish and wear rate:
- 8 RMS: Wear rating 1.0 (best performance)
- 16 RMS: Wear rating 1.4
- 32 RMS: Wear rating 2.2
- 63 RMS: Wear rating 5.3 (significantly higher wear)
Surface preparation method also mattered:
- Roller-burnished surfaces performed best
- Ground and polished finishes followed closely
- Turned or milled finishes showed higher wear, even at 16 RMS
These results reinforce that RMS alone does not tell the full story: surface topology and finishing method matter.
Expert Note from TriStar: Surface finish is a tribological interface, not a tolerance detail. Our engineers routinely see bearing life double, or fail prematurely, based solely on shaft finish. Matching surface texture to the bearing’s lubrication mechanism is one of the most effective ways to improve performance without changing materials.
Why Surface Finish Matters More Than Ever
As modern equipment:
- Operates at higher loads
- Runs at higher speeds
- Requires quieter and cleaner operation
Surface finish has become a primary performance driver in bearing design. Engineers who account for surface finish early gain better wear control, longer service life, and more predictable performance.
To help engineers in the field, TriStar also provides practical inspection guidance, including:
- Recommended RMS ranges by material
- Preferred finishing techniques
- Simple validation methods (including the fingernail test)
Learn More
Watch the video below to learn:
- How to match surface finish to bearing material
- Which finishes perform best for self-lubricating bearings
- How to check surface finish quickly during inspection
Need help selecting the right shaft finish for your bearing application? Ask a TriStar Expert for material and surface finish recommendations tailored to your design.
