Dry running of technopolymer gears: advantages, limits and the PV parameter
A gear that turns without oil, without grease, without scheduled maintenance. For those who design food, medical or automation machines, it's often the starting requirement. Technopolymers make it possible, but within precise limits: exceeding them means accelerated wear, overheating and early failure.
Dry running in technopolymers
Metal gears require oil or grease to limit friction and wear. Technopolymer ones can work without external lubrication, provided the design respects the material's tribological limits. The advantage isn't only operational: less lubricant means less contamination, less maintenance and cleaner work environments. In many industrial applications, replacing metal with technopolymer (metal replacement) also reduces weight, transport costs and environmental impact.
Dry running isn't a default condition that's always valid. If load, speed or temperature leave the specification, the risk is overheating and irreversible damage to the teeth.
Self-lubricating properties of technopolymers
Self-lubrication works by microscopic transfer: during contact between the teeth a thin film of material or of additives such as PTFE (polytetrafluoroethylene, commonly known as Teflon), graphite or MoS₂ (molybdenum disulfide) forms. This film reduces friction and wear without external lubricants.
The materials most used for dry running have different characteristics, suited to different contexts:
- POM-C (polyoxymethylene): low friction, good dimensional stability, ideal where precision is the priority.
- Glass-fiber PA6: high mechanical strength, excellent cost/performance ratio, suited to higher loads with careful management of the heat generated.
- PK (polyketone): good wear resistance, natural self-lubrication and fair thermal stability in moderately aggressive environments.
The surface quality and the compatibility between the materials of the paired gears directly affect durability. A poor surface finish increases wear in the run-in phase, even with suitable materials.
The PV parameter: pressure times velocity
The PV is the product of the contact pressure (P, in N/mm²) and the sliding velocity (V, in m/s). It indicates the combination of load and speed that a gear can withstand dry before the heat generated exceeds the material's capacity.
Technopolymers have low thermal conductivity: the heat produced by friction doesn't dissipate as quickly as in metal, but accumulates in the tooth. High PV values accelerate wear, deformation and loss of geometric precision.
For continuous-running applications, work at 50-70% of the limit PV value of the chosen material. This margin covers the load variations and thermal fluctuations that occur under the real operating conditions.
When dry running isn't enough
If load, speed or temperature exceed the PV limit, dry running is no longer sufficient. The signals to monitor are: temperature rise on the housing, abnormal noise, visible wear dust, shiny tooth surfaces or increased backlash relative to startup.
In these cases, evaluate hybrid solutions:
- Occasional lubrication with greases compatible with the material.
- Pairing different materials to reduce the overall friction coefficient.
- Improved heat dissipation with fins, forced ventilation or optimized housing geometries.
How to orient yourself in choosing the material
PA6, POM-C and PK answer to different application logics. Glass-fiber PA6 offers the highest mechanical strength among the three and an excellent cost/performance ratio: in applications with high loads it's often the more robust choice, with heat management addressed at the design stage. POM-C is preferable when dimensional stability is the priority and loads are moderate. PK is suited to chemically aggressive environments or when you want to minimize maintenance.
In any case, always verify the limit PV value of the material in the supplier's datasheets and design with a safety margin adequate to the real conditions, not to the nominal values.
If your application combines high loads, critical temperature and a dry-running requirement, write to the technical office. We assess material, geometry and PV margin together for your specific configuration.