Bronze-filled PTFE is the workhorse for heavily loaded bearings and piston rings in hydraulic and compressor service where the fluid is benign.
Typical content
Forty to sixty percent by weight. Because bronze is about four times denser than PTFE, 60 percent by weight is only about 25 to 30 percent by volume. Density is roughly 3.7 to 4.0 g/cm3.
What it improves
- Load and PV capacity. The highest of the common fillers. It can carry loads that glass-filled PTFE cannot.
- Thermal conductivity. High, so heat is removed from the contact.
- Creep and compressive strength. Excellent.
- Wear. Very good, with a smooth film.
- Dimensional stability. Thermal expansion much lower than virgin.
What it costs you
- Chemistry. Bronze corrodes in many acids and alkalis, in ammonia, in steam and in seawater. Where the fluid is aggressive, switch to carbon or graphite.
- Electrical conduction. The compound conducts. Not suited to insulating parts.
- Counterface hardness. Needs a hard, smooth shaft. Bronze particles are fairly gentle on hardened steel, rougher on soft surfaces.
- Weight and cost. Heavy and costly per part.
- Food and drinking water. Usually not approved because of copper and tin content, unless specifically certified.
Typical applications
Hydraulic piston and rod bearings, guide rings, compressor piston rings in oil-free service, linear bearings, bridge and structural slide pads, and heavy-load valve seats.
Design pointers
- Check the fluid first. If it attacks copper alloys, do not use it.
- Keep surface finish about Ra 0.2 to 0.4 micrometres and hardness 50 HRC or more.
- Allow adequate clearance. Dense, stiff fillers reduce the compliance of the ring, so tolerances matter more.
- For dry running, a blend with MoS2 or graphite can reduce break-in wear.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
