Hollow Bearing Blanks FP20

Sintered iron is pressed and sintered from iron powder so that roughly 20-25% of its volume remains as a network of interconnected pores. Under vacuum this pore network is flooded with lubricating oil, turning the bushing wall into an integral oil reservoir. In service, frictional heat and the slight pressure of the rotating shaft draw oil out of the pores by capillary action, building a hydrodynamic film between shaft and bore. When motion stops and the part cools, capillary forces pull the oil back into the porosity, so it is retained rather than lost. This closed cycle is what lets an FP20 bush run dry — without grease fittings, oil lines or periodic re-lubrication — for much of its working life.

Because the oil supply lives inside the wall, the mechanism only works if machining preserves the surface porosity. Smeared or glazed bore surfaces seal the pores shut and starve the interface, so blanks must be finished with sharp tooling and light cuts (detailed under Installation) to keep the capillary paths open on the finished running surface.

These are semi-finished blanks: the running surface is created by the customer, so machining technique decides whether the finished bush self-lubricates. Preserve the surface porosity throughout.

• Cut dry. Do not flood with cutting fluid or soluble oil — coolant is drawn into the pores and displaces the impregnated lubricant. Machine dry, with sharp carbide tooling, high surface speed and light feed to shear rather than smear the surface.
• Avoid reaming the bore. Reaming and heavy burnishing glaze the pores shut. Bore or turn to size, then finish to fit with a sizing pin that opens rather than seals the porosity.
• Part off from length. The 120 mm billet parts cleanly into several shorter bushes or thrust washers; deburr edges lightly without rolling material over the bore mouth.
• Press-fit into the housing. Finished bushes are retained by interference in a rigid H7 housing bore, using an arbor and a lead-in chamfer — never hammered directly. No adhesive or grub screw is needed.
• Allow for bore close-in. Press-fitting contracts the bore slightly; machine the finished ID with an allowance so it closes onto the target running clearance, and verify clearance after assembly.
• Re-impregnate if heavily worked. After extensive machining, blanks can be re-soaked in bearing oil to top up the reservoir before assembly.

Choose FP20 hollow blanks when no standard bushing matches the geometry — an unusual bore, a long or flanged sleeve, a thrust collar — and you want to machine it in-house from self-lubricating stock rather than from solid steel that would then need separate lubrication. Within the sintered range, specify FP20 sintered iron over BP25 sintered bronze where the duty is heavier and slower and cost matters: iron offers higher hardness and load capacity at the expense of corrosion resistance and high-speed capability. If the environment is wet or corrosive, or the speed is high, favour a bronze grade; if you need a ready-to-fit standard size rather than a billet, choose a finished bushing instead of a blank. The 120 mm length and thick wall make these blanks particularly economical when several bushes are parted from a single billet.