Do oil-impregnated bronze bushings ever need re-lubricating?

In normal service they do not - the oil is stored inside the porous wall and re-circulated as the bearing heats and cools. Adding a compatible oil at assembly does no harm and can top up the reservoir, but there is no relubrication interval to maintain. Life ends when the oil charge is eventually consumed or contaminated, at which point the bush is replaced or re-impregnated.

Can I ream the bore of a sintered bush to fit my shaft?

No. Reaming or machining the running surface smears metal across the pore openings and cuts the bearing off from its own oil supply. If the fitted bore needs adjusting, use a ball- or plug-sizing operation, which burnishes to size without closing the porosity. Remember that press-fitting into the housing already reduces the bore, so check the bore after installation.

What determines how long the oil charge lasts?

Chiefly temperature, cleanliness and how hard the bearing works relative to its load and speed limits. Cool, clean, moderately loaded duty preserves the oil for years; high temperature, abrasive contamination or operation near the material's PV ceiling consumes it far faster. Selecting the correct impregnation oil - standard TURBO T100, high-temperature PE1116 or food-grade PE1152 - matches the reservoir to the environment.

How Oil-Impregnated Sintered Bronze Bushings Work

Oil-impregnated sintered bronze bushings are among the most widely used maintenance-free plain bearings in industrial machinery. Bushingo supplies them as the BP25 grade, in cylindrical and flanged forms, with bores ranging from 2 mm through to 125 mm. Their appeal is straightforward: they carry a load, run without any external lubricant supply, and need no re-greasing schedule. The reason lies entirely in how the porous bronze structure stores and releases oil.

A porous skeleton, not a solid bush

Unlike a bush turned from solid bar, a sintered bearing is produced by powder metallurgy. A blended copper-tin bronze powder is compacted in a precision die under high pressure, then sintered below its melting point. Sintering bonds the particles metallurgically while deliberately leaving an interconnected network of fine voids throughout the wall. An oil-impregnated sintered bronze of this class typically retains around 20-25% interconnected porosity by volume - effectively a rigid bronze sponge. After sintering and sizing, the component is vacuum-impregnated so that the pore network fills with lubricating oil. Dimensional and tolerance standards such as ISO 2795 and DIN 1850 govern this class of bush, which is why a sintered bearing can be supplied finished and pressed straight into a housing.

Capillary retention and thermal release

The interconnected pores act as a distributed oil reservoir. At rest, surface tension and capillary forces hold the oil within the matrix, so a correctly stored bush does not simply drain. When the shaft begins to turn, two effects bring lubricant to the sliding interface. First, frictional contact generates heat, and because oil expands more than bronze as temperature rises, the warming oil is driven out of the pores toward the bearing surface. Second, the moving shaft develops a local pressure field that draws oil into the loaded zone. Together these establish a thin lubricating film - partly boundary, partly hydrodynamic depending on speed and load - that separates shaft and bore.

When the machine stops and cools, the process reverses. Capillary action and the contraction of the cooling oil pull lubricant back into the pore network, recharging the reservoir for the next start. This self-regulating cycle is what allows the bearing to lubricate itself for long periods without any external feed.

Why it runs dry and maintenance-free

Because the lubricant is stored inside the bearing wall rather than delivered from outside, an oil-impregnated sintered bush is genuinely a dry-running, maintenance-free component in normal service. There is no grease nipple, no oil line and no relubrication interval. This suits the technology to sealed assemblies, inaccessible pivots and high-volume mechanisms where servicing individual bearings would be impractical - small motors, actuators, linkages, domestic and garden equipment, and general machine building. Service life is governed chiefly by the rate at which the oil charge is gradually consumed or degraded: running cool, clean and within the material's load and speed limits preserves the oil and extends life, while running hot, contaminated or overloaded consumes it faster.

Fitting without closing the pores

The porous structure that makes these bearings work can also be spoiled by poor handling. A few practical rules apply:

Sintered bushes are press-fits. Fitting into a correctly sized housing elastically closes the bore by a predictable amount, so the finished bore should be verified after installation rather than before.
Do not ream, hone or machine the running surface. Cutting smears the surface metal and closes off the pores, cutting the bearing off from its own oil supply. Where a finished bore is critical, use a ball- or plug-sizing operation instead of a cutting tool.
Keep bushes away from aggressive degreasers and solvents, which wash the oil out of the matrix. If parts must be cleaned, they should be re-impregnated before service.
Provide a clean, chamfered shaft with an appropriate surface finish. A rough shaft abrades the bore, while an over-polished shaft can starve the boundary film.

The Bushingo BP25 range

Bushingo's BP25 sintered bronze bushings are offered across a broad dimensional envelope. Cylindrical references span three size bands - bores from 2-15 mm, 16-32 mm and 35-125 mm - while flanged references, designated C03 through C60, add an integral collar for locating the bearing axially and taking light thrust; each flanged size specifies bore, outside diameter, length, collar diameter and flange thickness. The bushes are impregnated with Bushingo's TURBO T100 oil, the standard charge for BP25 and FP20 bearings. Where the application demands it, the same porous bronze can be impregnated with food-grade PE1152 oil for hygienic equipment, or with high-temperature PE1116, rated from -40 degC to +150 degC, for hot or cold-service duties. Sintered bronze bearings of this type typically operate to around 100 degC continuously on standard oil, so the impregnation oil is often the deciding factor at the temperature extremes.