Shafts in hot-gas fans must resist creep and oxidation; material and heat treatment decide survival above 200°C.
The High-Temperature Problem
Above about 200°C, ordinary carbon steel loses strength and begins to creep, so a shaft that is safe cold can slowly elongate and fail under the same load when hot, a change that is gradual and easy to miss until it is severe.
The fan shaft carries the impeller and the drive load, and its material and heat treatment decide how long it survives under bending, torsion, and, in hot duties, creep. A shaft that is right for a clean-air fan can fail early in a 300°C gas stream if the steel is not chosen for temperature.
For general duty a medium-carbon steel such as 45# is common, often quenched and tempered to raise strength and toughness so the shaft resists the fatigue from rotation and the occasional shock of a start or a balance shift. Alloy steel such as 40Cr adds hardenability for larger sections that must stay uniform through the heat.
High-temperature duty changes the rules. Above a few hundred degrees the same steel loses yield strength and can creep, so the shaft is specified in a grade rated for the operating temperature with margin, and the coupling and bearing on that end are chosen for the heat as well, because the whole train must live at the gas temperature, not just the wheel.
Surface finish and the fillet radius at a step matter as much as the alloy, because a rough shoulder is where a crack starts under cyclic load. The shaft is finished and radiused so stress concentrates nowhere, and it is inspected for straightness so the impeller runs true and the balance grade held at assembly is not lost in service.
The company's 80,000 m² production site builds fans under ISO 9001:2015 with CCC certification, and with more than 30 years of manufacturing experience its engineers respond to selection enquiries within 24 hours to specify shaft material and heat treatment matched to the actual gas temperature and load of the duty.
The shaft meets the impeller at a keyway and a coupling, and both are detail-sensitive: a poorly cut keyway is a stress raiser like a rough fillet, and a misaligned coupling loads the shaft sideways and shortens its life. The key is fitted to limit play, the coupling is aligned to the specified tolerance, and the assembly is turned over by hand or at low speed to confirm it runs true before the guard is closed and the unit is handed to the plant.
Oxidation scales the surface and reduces the effective section, while thermal cycling between hot and cold states induces fatigue cracks at stress concentrations such as keyways and shoulder radii where the geometry concentrates load.
The shaft also conducts heat toward the bearings, so the design must limit the temperature at the bearing seat, often with a cooling collar, finned spacer, or insulated span, because the bearing has a much lower temperature limit than the shaft.
Creep is time-dependent: a shaft may hold for weeks then deform, so the material choice is about long-term integrity, not just an instantaneous strength figure from a room-temperature table that no longer applies.
Material and Treatment
Alloy steels such as 35CrMo or 42CrMo are common for hot fans because they retain strength and resist creep better than plain carbon steel at elevated temperature, and they are weldable and machinable for the impeller fit.
Heat treatment, typically quenching and tempering, develops the toughness and strength needed, with the tempering temperature set below the service temperature so the shaft does not soften in use and lose the hardness the duty relies on.
For very high temperatures or corrosive gas, stainless or specialised alloys are specified, trading cost for the needed oxidation and creep resistance, and the choice is confirmed against the actual gas composition rather than a generic hot-air assumption.
Surface finish at the journals matters, because a rough seat under thermal stress initiates cracks; controlled machining and possibly nitriding improve life, and the fillet radii are opened up to lower the local stress that drives fatigue.
| Item | Specification | Note |
|---|---|---|
| Threshold | >200°C | Creep begins |
| Alloy | 35CrMo / 42CrMo | Creep resistant |
| Treatment | quench + temper | Below service T |
| Bearing seat | cooled | Limit heat rise |
| Finish | smooth journal | Crack control |
| Extreme | stainless alloy | Corrosive hot gas |
Selection Checklist
- Set the shaft material from the gas temperature, not just the duty.
- Use alloy steel above 200°C where creep is a risk.
- Temper below the service temperature so it will not soften.
- Cool the bearing seat to keep it within its range.
- Specify a smooth journal finish to resist cracking.
- Use stainless or special alloy for hot corrosive gas.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.