Hollow Bar in PTFE BR55 GR MoS² 14/8x1000 mm natural

MoS² means added dry lubricant, so the surface slides without oil. A hollow bar in PTFE BR55 GR MoS2 is extruded PTFE filled with bronze and complemented with graphite and molybdenum disulfide. The 14/8x1000 mm format, that is 14 mm outer diameter and 8 mm inner diameter in a one meter length, is made to be turned into seals, guide rings and bushings with minimal waste.

The suffixes in the grade name tell the recipe: BR stands for bronze, GR for graphite and MoS2 for molybdenum disulfide. The bronze dominates the filler content while the two solid lubricants are added in smaller proportions. The combination has been in hydraulics for decades, for the simple reason that it works; the recipe is standardized and well documented.

Pure PTFE has very low friction but creeps under load, and that is exactly what the bronze filling addresses. The bronze gives compressive strength, dimensional stability and a wear resistance many times better than that of unfilled PTFE; it also raises the thermal conductivity, so the friction heat is led away from the sealing lip instead of staying in the plastic. The graphite and the molybdenum disulfide work the other way: they keep friction down even in dry running and compensate for the harder character of the bronze.

The combination is a classic in hydraulics. Piston seals, guide rings and slide rings in cylinders are typical parts, as are bushings, wear rings and seats in valves and pumps. Bronze filled PTFE works best against hardened and ground steel surfaces; against softer mating surfaces the choice should be checked, since the filler can act abrasively. We also see the material in mobile hydraulics and press tools, where pressure and temperature interact with long service intervals.

The dry running properties are worth pausing on. A hydraulic cylinder is in theory always lubricated by its oil, but reality serves up starts after long standstills, air in the system and pressure spikes; it is in those situations that the solid lubricants earn their place in the recipe, by protecting both the seal and the cylinder surface until the oil film is back in place. That is also why the grade is often specified for cylinder overhauls.

On temperature the material keeps the wide working range of the PTFE base, and the chemical resistance is still good with one important exception: bronze is attacked by acids and some aggressive media. In such environments carbon filled or glass filled PTFE, or pure PTFE, is a better choice; check the media resistance in the documentation from the manufacturer when the medium is something other than mineral oil. The same logic applies to food equipment, where bronze filled grades do not belong in product contact.

The hollow bar format saves both time and material. The inner hole is already there, so rings and bushings are turned straight out of the bar instead of being bored out of solid rod; for sealing shops with many small runs it is often the most rational raw material form. Count backwards from the finished part when you choose the blank: the outer diameter has to hold the finished dimension plus machining allowance, and the inner diameter must not be larger than the inside dimension of the part. The one meter length is easily divided into suitable blank lengths on a band saw.

Machining is like pure PTFE but with clearer tool wear, since the bronze wears on the edge. Choose carbide tools, keep the cutting speed moderate and take light cuts; thin rings spring away easily, so support the workpiece well. A tool geometry with a positive rake angle gives the finest surface. The chips are short and mixed with filler, so good extraction keeps the lathe clean.

Exact filler contents and mechanical values for the grade are given in the data sheet from the manufacturer.

Do you have a seal geometry on a drawing? Send it to us, and we will help you get to the right dimension and blank length.