PTFE Bush Guide
How to specify a PTFE plain bearing, and the three decisions that determine whether it lasts.
1. Is the duty inside the PV limit?
A plain bearing is governed by PV — contact pressure multiplied by surface speed. Every PTFE compound has a limit, and exceeding it does not cause immediate failure; it causes accelerating wear and heat build-up that eventually runs away. Calculate the projected bearing area (bore diameter × length), divide load by it for pressure, and multiply by surface speed.
If the PV is near the limit for unfilled PTFE, that is the signal to specify a filled grade rather than to increase the bearing size.
2. Which filler does the duty need?
- Bronze-filled — the highest PV capability, because it carries load and conducts frictional heat out through the wall. For dry, heavily loaded, faster-running bearings on benign media. Not for chemical or food service.
- Glass-filled — for sustained load at low speed, where creep closing the bore is the failure mode rather than wear.
- Carbon-filled — for wet and steam service, and where static must dissipate.
- Graphite-filled — where the shaft is soft, plated or refurbished and must be protected.
- Virgin PTFE — where chemical resistance leads and the load is light.
3. What is the counterface?
This is the decision most often skipped, and it governs service life as much as the compound does. A ground, hardened steel shaft will give roughly double the life of a soft turned one. Conversely, a glass-filled bush against aluminium or thin plating will wear the shaft rather than itself — the exact opposite of what a plain bearing is for.
Setting clearance
PTFE compounds expand far more than the steel around them. Set running clearance for the hot condition, not the bench measurement. A bush fitted to a cold clearance can nip the shaft once the bearing reaches running temperature, and the resulting heat accelerates the failure it caused.
Wall thickness matters too: a thicker wall insulates, trapping heat at the contact. Where heat is the constraint, a thinner wall in a conductive housing often outperforms a thicker one.
Fitting
Press-fit bushes are normally supplied slightly oversize and finish-machined after fitting, because the bore closes when the bush is pressed into the housing. State on the drawing whether the dimension given is the free or the fitted bore.
When to replace
Bore wear beyond the drawing clearance limit. Because filled PTFE wears steadily rather than seizing, measured bore checks at service intervals give reliable warning. A bearing that wears at an accelerating rate is telling you the duty has outgrown the grade.
Related pages
Grade-specific bush pages: bronze-filled, glass-filled, carbon-filled, graphite-filled and MoS₂-filled. See also PTFE bush and the PTFE glossary. Send bore, load, speed and media to HEMSUN.
