Fitting Self-Lubricating Bushings: Housing Preparation, Press-Fit and Running-In

Why installation decides bearing life

An oil-impregnated sintered bushing is only as good as the way it is fitted. The self-lubricating action depends on an open, oil-charged surface and on a controlled running clearance, and both are easily destroyed by careless installation. The guidance below applies to BP25 bronze, FP20 iron and SO16 bushes and machined blanks alike, and following it is the difference between a bearing that runs maintenance-free for its design life and one that seizes on the first shift.

Handling and storage

Keep the bushes in their packaging until fitting. The pores are already charged with oil, so avoid prolonged exposure to dust, swarf and abrasive grit, which can embed in the surface. Do not degrease the bearing in aggressive solvents before fitting: doing so strips the impregnated oil out of the pores. If a bush must be cleaned, wipe it and re-impregnate it with the matching oil (TURBO T100 for BP25 and FP20, METADOP for SO16) before assembly. Handle with clean hands or gloves to keep the running surface uncontaminated.

Preparing the housing

The housing bore carries the interference that holds the bush and, just as importantly, sets the finished running bore. Prepare it deliberately:

Tolerance. Machine the housing bore to a clean, round, parallel finish at the recommended fit, commonly an H7 bore, so the bush is retained by interference without being over-compressed.
Lead-in. Provide a small chamfer, in the order of 15 to 30 degrees, at the mouth of the bore so the bush enters square and does not shave material from its outside diameter.
Cleanliness. Remove all burrs, chips and high spots. A single burr under the bush will distort the bore and create a hard spot against the shaft.
Blind versus through bores. For a blind housing, allow for the displaced air and confirm the seating depth so the bush is not driven onto the bottom of the bore.

The press-fit

Sintered bushes are installed by interference (press) fit, not adhesive or set screw. Press them in with steady, axial force:

Use an arbor press or a controlled hydraulic press, never hammer blows, which fracture the porous wall.
Press through a piloted mandrel or insertion tool that locates in the bore and bears squarely on the bush face, so load is applied evenly and the bore is supported during entry.
Keep the bush square to the bore throughout; cocking it will gall the outside diameter and skew the seated bearing.
Press to the correct depth in one continuous movement and support the housing so it cannot flex.

Bore close-in: the point most often missed

When a sintered bush is pressed into an interference housing, the wall is compressed and the bore closes in: the finished internal diameter ends up smaller than the free bore measured before fitting. This close-in is normal and expected, and standard bushes are made so that, after fitting into the recommended housing, the bore arrives at the intended running clearance with the shaft. The critical rule follows directly:

Do not ream, drill or turn the bore to size after fitting. Machining the running surface smears the metal, closes off the surface pores and cuts the bearing off from its own oil supply, which is the fastest way to ruin a self-lubricating bush. If the bore must be corrected, use ball sizing or a burnishing pin (a sizing mandrel pushed through the fitted bush) rather than a cutting tool. Sizing re-establishes the diameter by burnishing without smearing the pores closed.

Shaft requirements

The shaft is the counterface and shares responsibility for the oil film. As typical guidance for this bearing class:

Surface finish. A smooth, ground shaft, in the region of Ra 0.4 to 0.8 micrometres, lets the released oil form a continuous film; a rough shaft scrapes the surface and pumps oil away.
Hardness. A hardened, ground shaft resists wear and preserves the fit over the life of the bearing, particularly against the harder FP20 iron grade.
Clearance. Maintain the recommended diametral running clearance for the bore size after fitting. Too little clearance risks seizure as the assembly warms and the oil expands; too much spoils the film and allows knock. Confirm the target clearance for the specific bore rather than assuming a single value across all sizes.

Running-in

Give the bearing a gentle start. During the first period of operation, run it in at reduced load and speed so the surfaces bed in and a stable oil film is established across the full bearing area. A short running-in phase lowers the initial friction, sets up the self-lubricating cycle described in the technical guide, and markedly improves long-term life. Expect a small amount of oil to appear at the surface as the bearing warms for the first time; this is the reservoir doing its job, not a fault.

Machining from blanks

When a bespoke bearing is turned from a solid or hollow blank, treat the porous material with the same care as the fitted bore. Use sharp tools, light finishing cuts and no cutting fluid that would flush the impregnated oil out of the pores, or re-impregnate afterwards with the matching oil. Finish the bore by a light cut followed by sizing rather than by aggressive reaming, so the working surface keeps its open porosity.

Installation checklist

Housing bore machined to the recommended fit (commonly H7), round, parallel, chamfered and de-burred.
Bush kept oil-charged and clean; not degreased, or re-impregnated if it was.
Pressed in squarely with a piloted mandrel on an arbor press, to the correct depth, in one movement.
Bore close-in allowed for; the bore not machined after fitting, only ball-sized or burnished if correction is needed.
Shaft ground and hardened to a suitable finish, with the correct running clearance confirmed for the bore.
Bearing run in at reduced load and speed before full duty.

Follow this sequence and a correctly selected BP25, FP20 or SO16 bush will deliver the quiet, dry-running, maintenance-free service the technology is designed for.