PTFE composite bearings operate through a self-generated transfer film, not through classical lubrication[reference:0]. Shaft surface roughness (Ra) directly governs whether that film can anchor, form continuously, and remain stable over the bearing's service life.

For many MG‑1 and MG‑1i dry‑running applications, Ra 0.3–0.6 µm is a practical starting window for stable transfer‑film formation. The final shaft finish should be verified against PV level, shaft hardness, motion type, load stability, and contamination exposure.

In MG-1 and MG-1i PTFE bearing systems, shaft roughness directly governs transfer film formation, wear rate evolution, and lifecycle stability.

Why Transfer Film Is the Real Wear Control Mechanism

PTFE bearings do not rely on classical lubrication. Instead, they operate through a self-generated tribological layer: the transfer film.

Formation Process

During operation:

• PTFE micro-particles are sheared from bearing surface — a mechanism clearly illustrated in MG-1B bronze-based Pb-free self-lubricating bushings, where the PTFE and oleophilic fibre mixture forms a protective transfer film on the mating shaft during motion[reference:1].

• Particles adhere to the shaft surface

• A thin, continuous transfer layer forms

• System transitions to PTFE-on-PTFE sliding

This process is well documented in real-world applications such as port machinery, where PTFE composite bearings consistently form a protective film on the mating shaft.

Engineering Impact

Once stabilized:

• Friction coefficient drops significantly

• Direct metal contact is eliminated

• Wear rate becomes predictable and stable

For a broader comparative perspective on why PTFE-based systems outperform traditional metallurgies in dry or boundary-lubricated conditions: Polymer Composite Bearings vs Bronze Bearings: Engineering Trade-Offs Explained.

Procurement insight:
If transfer film does not stabilise, material upgrade alone will not improve bearing life.

Transfer Film Lifecycle and Wear Evolution

PTFE bearing performance follows a three‑stage lifecycle:

1. Running‑in Stage (High Initial Wear)
   – Surface asperity interaction
   – Controlled PTFE material transfer
   – Temporary high wear rate

2. Film Formation Stage (Transition Instability)
   – Partial coverage of transfer layer
   – Mixed contact conditions
   – Wear rate begins to decline

3. Steady‑State Stage (Design Life Phase)
   – Continuous transfer film coverage
   – Stable low friction behaviour
   – Predictable long service life

For field verification of how this lifecycle plays out across different industrial sectors — including mining, construction, agricultural and port machinery: Applications overview, which details real-world transfer film behaviour under varying duty cycles.

Shaft Ra determines how fast—and whether—the system reaches Stage 3.

Surface Roughness: Too Smooth vs Too Rough

1. Shaft Too Smooth (Ra < 0.2 µm)

Although visually ideal, ultra‑smooth surfaces reduce tribological anchoring:

• Weak transfer film adhesion

• Delayed stabilisation phase

• Increased stick‑slip in low‑speed motion

Failure Outcome:
Unstable lubrication → early wear escalation → shortened service life

2. Shaft Too Rough (Ra > 0.8–1.0 µm)

Excessive roughness creates abrasive interaction:

• PTFE layer continuously cut and removed

• Increased debris generation

• Third‑body wear acceleration

Failure Outcome:
Abrasive wear → clearance growth → premature functional failure

Recommended Shaft Roughness Specification

Standard Engineering Range (MG‑1 / MG‑1i systems)
Ra = 0.3 – 0.6 µm (ground surface recommended)

Acceptable Extended Range
Ra = 0.2 – 0.8 µm

As a direct engineering reference, the MG-800 bimetal self-lubricating bushings product page explicitly lists surface roughness parameters, providing a quantifiable benchmark for how this specification is managed across our product range.

PV Condition Effect (Critical Design Factor)

PTFE performance is strongly affected by PV value (Pressure × Velocity).

• High PV Systems
  – Require stronger and more stable transfer film
  – Excessive Ra increases thermal disruption — for a deeper understanding of the thermal mechanisms involved: A Preliminary Analysis of the Heating Mechanism of Sliding Bearings
  – Ultra‑low Ra delays film anchoring

Design Conclusion
Higher PV conditions require tighter Ra control, not looser tolerance.

For applications with demanding PV parameters, the MG‑CR long‑fibre wound bearing series provides explicit PV ratings that can serve as a useful reference when defining shaft finish requirements.

Shaft Material Interaction (Engineering Constraint)

Best performance is achieved when Ra is controlled in combination with:

• Hardened carbon steel shafts

• Stainless steel (controlled finishing) — for corrosive environments, the MG‑1S stainless-steel-based self-lubricating bearing offers the same transfer-film mechanism on a corrosion-resistant substrate[reference:2].

• Surface‑coated shafts (requires tighter Ra window)

When coatings or special surface profiles are involved, the micro‑texture of the mating surface becomes even more critical. Our technical article on surface profile optimisation for PTFE liners discusses how liner texture interacts with shaft roughness to promote stable film transfer.

For practical guidance on achieving proper shaft-to-bearing fit during assembly: How to properly install an oil-free bearing?

Wear Mechanism Comparison

Engineering vs Real‑World Cost Impact

From a procurement perspective, shaft roughness directly affects lifecycle cost:

Incorrect Ra Specification Leads To:

• 2–5× faster bearing replacement cycles

• Increased maintenance downtime

• Unexpected field failures

• Higher spare parts inventory pressure

Correct Ra Control Enables:

• Stable long‑term operation

• Predictable maintenance intervals

• Reduced total cost of ownership (TCO)

For maintenance strategies addressing unexpected damage, the Repair process for partial damage and welding of sliding bearings offers practical recovery methods that can extend service life when surface conditions deviate from specification.

In many OEM cases, shaft machining control provides more life improvement than upgrading bearing grade.

MG‑1 vs MG‑1i Selection Logic (Critical OEM Decision Tree)

MG‑1 Series (Standard PTFE Composite Bearing)

The MG‑1 uses a low-carbon steel backing, sintered porous bronze interlayer, and PTFE-based surface layer[reference:3]. It is suited for applications with consistent shaft machining and stable operating conditions.

For a detailed breakdown of MG‑1 construction and its pre-lubrication benefits: What benefits can you get from pre-lubricated sliding bearings?

Best for: General machinery, light industrial systems, high‑volume OEM assemblies

MG‑1i Series (PTFE Bearing with Improved Stability)

For projects involving load fluctuation, shock, varying PV conditions, or extended maintenance interval requirements, the suitability of MG‑1i or higher-grade series should be evaluated against actual PV ratings, load capacity data, and application-specific testing[reference:4].

Best for: Heavy‑duty equipment, agricultural machinery, variable‑load industrial systems

Engineering Selection Decision Tree

• Step 1: Shaft surface control level
  Consistent Ra control → MG‑1  |  Variable Ra conditions → Evaluate MG‑1i or higher-grade series

• Step 2: Load condition
  Stable load → MG‑1  |  Shock / dynamic load → Evaluate MG‑1i or higher-grade series

• Step 3: PV severity
  Low–medium PV → MG‑1  |  Medium–high PV → Evaluate MG‑1i or higher-grade series

Key Engineering Insight for OEM Procurement

PTFE bearing life is not a material selection problem—it is a system surface engineering problem.

Most premature failures are caused by:

• Incorrect Ra specification

• Poor shaft machining consistency 

• Ignoring transfer film stability window

Conclusion

Shaft surface roughness is a primary life‑determining parameter in PTFE bearing systems.

By controlling Ra and aligning it with PV conditions and load requirements, OEMs can directly control:

• Transfer film formation speed

• Wear rate stability

• Maintenance cycle length

• Total lifecycle cost

For MG‑1 and MG‑1i PTFE bearing systems, the design principle is clear:

No stable transfer film = no predictable bearing life

In modern OEM engineering, performance is not only designed into the bearing—it is manufactured into the shaft surface specification.

Submit Your Application Parameters for Engineering Review

Every PTFE bearing application has unique operating conditions. To determine the optimal shaft finish, bearing grade, and design configuration for your specific requirements, submit your application parameters for technical evaluation.

Submit Your Inquiry with Application Parameters →