Self-Lubricating Sintered Bearing Technology: A Technical Overview

What an oil-impregnated sintered bearing is

An oil-impregnated sintered bearing is a plain (journal) bearing whose wall is a porous metal skeleton saturated with lubricating oil. Unlike a solid machined bush turned from bar, the bearing wall is not fully dense: a network of interconnected pores runs through the material and acts as an integral oil reservoir. Because the lubricant is stored inside the bearing itself, the component supplies its own oil film during service and, in a great many applications, runs for its whole design life without a single top-up. This is the origin of the terms self-lubricating, maintenance-free and oilless bearing.

Bushingo supplies this bearing class in the classic Metafram-type grades: BP25 sintered bronze, FP20 sintered iron and the SO16 alloy grade, in cylindrical and flanged forms as well as solid and hollow blanks for machining into bespoke geometries.

How the porous matrix is produced

These parts are made by powder metallurgy rather than by casting or bar turning. Fine, bronze- or iron-based metal powder is blended and then compacted in a die under high pressure to form a fragile green compact, held together at that stage only by mechanical interlocking of the particles. The compact is then sintered: heated in a controlled protective atmosphere to a temperature below the melting point of the base metal, so that the particles metallurgically bond at their contact points without the mass melting and slumping. The outcome is a coherent, dimensionally stable solid that deliberately retains an open pore structure.

For sintered bronze of this type the interconnected porosity is typically of the order of 20 to 25% of the volume, which gives a large internal reservoir; structural iron grades are usually run at somewhat lower porosity to favour strength. After sintering the bushes are sized in a further pressing operation, which brings the bore and outside diameter to their finished tolerances and burnishes a smooth, closed-pore running surface.

Vacuum impregnation with oil

The porous bush is charged with lubricant by vacuum impregnation. Parts are submerged in oil under vacuum so that trapped air is drawn out of the pore network; when atmospheric pressure is restored, oil is forced into the evacuated pores. Because the open pores fill with lubricant, a fully impregnated sintered bronze bush holds an oil charge of roughly a fifth of its volume, following directly from the porosity. The oil is matched to the grade and the duty: Bushingo supplies TURBO T100 for BP25 bronze and FP20 iron, METADOP for the SO16 alloy grade, a food-grade oil (PE1152) for hygienic applications, and PE1116 where a wide service window from minus 40 to plus 150 degrees Celsius is required.

The self-lubricating cycle

The mechanism that makes these bearings work is thermal and capillary, not mechanical pumping:

• At rest, capillary force holds the oil inside the pores, so the bearing does not drip and the surface remains only lightly wetted.
• On start-up, relative sliding between shaft and bore generates frictional heat. Oil has a much higher coefficient of thermal expansion than the surrounding metal, so as the interface warms the oil expands and is drawn out of the pores onto the running surface.
• In steady running, the released oil forms a film that separates the surfaces. At adequate speed and load a partial hydrodynamic film develops; at low speed the bearing operates in the boundary or mixed regime, still protected by the oil it releases.
• On stopping and cooling, the oil contracts and capillary action reabsorbs it into the pore network, recharging the reservoir for the next cycle.

Because the reservoir is replenished every time the bearing cools, a correctly loaded self-lubricating bush can complete a very large number of start-stop cycles without any external lubrication, which is the principal reason the type is specified for sealed, inaccessible or low-maintenance assemblies.

The PV limit

The single most important design parameter for any plain self-lubricating bearing is the PV value: the product of the bearing pressure P (load divided by the projected area, bore multiplied by length) and the sliding velocity V at the bore surface. Frictional heat generation rises with PV, so every material has a limiting PV above which the oil film breaks down and the bearing overheats. A bearing may therefore carry a high load at low speed, or run fast under a light load, but not both at once. As a class, sintered bronze grades tolerate higher speeds and a higher PV than sintered iron, which is generally chosen for slower, more heavily loaded or intermittent motion. Permissible load is always higher for oscillating or intermittent duty than for continuous rotation, because the interface has time to cool and reabsorb oil between movements. Bushingo confirms the running limits for a specific grade and size against the intended duty rather than publishing blanket figures, because the safe PV also depends on shaft finish, clearance, housing conductivity, ambient temperature and duty cycle.

Typical characteristics and honest limits

Framed as typical properties of the material class rather than certified ratings for an individual size:

• Sintered bronze bushings of this type typically operate to around 100 degrees Celsius continuous on standard mineral impregnation oil; a special oil such as PE1116 widens the usable window at both ends.
• They are best suited to low-to-moderate speed rotary and oscillating motion where a clean, quiet, dry-running and maintenance-free bearing is wanted.
• They are not intended for heavy shock loading, heavily contaminated environments without shielding, or as a direct substitute for a rolling-element bearing where very high speed and very high load occur together.

Standards and the product family

Dimensionally, sintered bushes of this family follow the recognised standards for the type, principally ISO 2795 for sintered bushes and DIN 1850 for bush dimensions, which is why they interchange readily with equivalent parts from other manufacturers. Bushingo's cylindrical bronze range covers bores from 2 mm up to 125 mm, cylindrical iron from 3 mm to 100 mm, and flanged grades from 3 mm bore upward with an integral collar for axial location. Where no catalogue size fits, solid blanks with outside diameters from 20 mm to 145 mm and hollow blanks up to 175 mm outside diameter are supplied for machining into custom bearings. For flat sliding duties rather than rotary journals, self-lubricating composite friction plates are offered as 375 by 375 mm plates in thicknesses from 4 mm to 20 mm, cut to shape as wear pads and slide ways.

Why engineers specify them

The attraction of the type is the combination of low cost per part, precise powder-metallurgy tolerances straight from the sizing press, quiet running, and freedom from a lubrication schedule. Within their PV envelope they are among the most economical and reliable plain bearings available, and the same porous technology scales from a 2 mm instrument bush to a machined blank over 100 mm in diameter.