Hollow Bearing Blanks BP25

Sintered bronze is produced by pressing and sintering bronze powder, which leaves a network of interconnected pores — typically 20-25% of the volume — that is vacuum-impregnated with lubricating oil. In service, shaft rotation forms a hydrodynamic wedge and generates slight frictional heat; because the oil expands more than the surrounding bronze, it is drawn out of the pores to build a thin lubricating film between shaft and bore. When motion stops and the bearing cools, capillary action reabsorbs the oil back into the pore structure, so the reservoir is continuously recycled rather than consumed.

This self-replenishing film is why the finished bearing runs maintenance-free, needs no external greasing and tolerates dry starts. For a machined blank the critical point is that the porosity must stay open at the running surface: cutting or forming operations that smear or burnish the bore will close the pores and starve the film, so light, sharp finishing cuts are essential to preserve the oil-release mechanism right through the machined wall.

Because these are semi-finished blanks, they are first machined to your target dimensions and then installed like any sintered bush. Preserving the oil-bearing porosity is the priority at every stage.

• Machine dry, or wetted with the impregnated oil only — use sharp carbide tooling, high cutting speed and light feed; avoid flood coolant that washes oil out of the pores.
• Do not burnish, roll or ball-size the running bore, as these operations smear and close the surface porosity; finish-turn or lightly ream instead.
• Consider re-impregnating the machined part in warm oil to top up the reservoir before it enters service.
• Press-fit the finished bush into a housing machined to an H7 bore; the interference closes the outside diameter and typically reduces the bore by a similar amount, so size or check the bore after pressing, or allow for that close-in.
• Press squarely with a piloted arbor — never hammer — and lead in with a small chamfer on the housing rather than shaving the bush edge.
• Run against a hardened, ground steel shaft with a surface finish of about Ra 0.4 µm or better, allowing appropriate running clearance for the finished bore.
• Keep blanks in their oiled packaging until machining and handle with clean gloves to avoid contaminating the open pores.

Choose BP25 hollow blanks when no finished bush in the standard range matches your bore, outside diameter or length, or when you need a stepped, flanged or extra-long bearing that has to be produced by machining. Their thick 10-16 mm walls give the stock to cut both diameters to a custom print from a single part. If your dimensions already exist as a ready-made cylindrical bush, a finished BP25 bushing is more economical than machining a blank. Where higher loads, shock or elevated temperatures push beyond what bronze suits, consider FP20 sintered iron blanks; for very low-friction or chemically aggressive duties look at SO16 alloy or Metagliss composite plates. Within the machinable hollow-blank family, BP25 is the default when you want bronze's thermal conductivity, corrosion resistance and smooth self-lubricated running.