Sintered Bronze Bushings

What are sintered bronze bushings?

Sintered bronze bushings are self-lubricating plain bearings made by compressing fine bronze powder into shape and heating it until the particles bond together, a process called sintering. This leaves a strong but porous metal structure, which is then filled with mineral oil. The oil-filled pores act as a built-in lubricant supply, so the bushing runs smoothly without any added oil or grease. Sintered bronze bushings are especially suited to rotating shafts running at high speeds.

A standard sintered bronze bushing is made up of:

• A porous bronze matrix, formed from compressed and sintered bronze powder
• Mineral oil filling the pores, which provides lubrication in service
• A precision-sized bore and outside diameter
• A straight cylindrical shape, or a cylinder with a flange at one end.

The porous structure allows high sliding speeds.

• As the shaft turns, the oil is drawn to the bearing surface to form a lubricating film between the shaft and the bush.
• This film keeps friction and heat low, so sintered bronze bushings can run at very high sliding speeds compared with most other plain bearings.

Sintered bronze bushings are self-lubricating and maintenance-free, and are sized by their bore, outside diameter, and length.

Why are sintered bronze bushings essential?

Sintered bronze bushings give an accurate, quiet, and maintenance-free bearing for rotating shafts. Because the lubricant is held inside the bush, there is no need for grease nipples, oil supplies, or regular servicing, which keeps designs simple and running costs low. They are ideal for compact, mass-produced machines and for places that are difficult to reach once assembled.

The sintering process also allows bushings to be made accurately and consistently in large numbers, so they give dependable performance at a low cost.

Typical applications include:

• Electric motors, generators, and electrical equipment
• Household appliances and power tools
• Printing machinery
• Machine tools and small gearboxes
• Fans, blowers, and pumps
• Office equipment and business machines
• Automotive auxiliaries, such as wiper and window motors
• Rotating, oscillating, and light linear movements

The main types of sintered bronze bushings

• Straight sintered bronze bushes - Plain cylindrical bushes, pressed into a housing to support a rotating shaft. They are the most common type.
• Flanged sintered bronze bushes - Bushes with a flange at one end, which locates the bush in the housing and takes light axial loads.
• Spherical sintered bronze bushes - Bushes with a rounded outside, fitted in a matching seat so they can self-align with the shaft, often used in small motors.
• Sintered bronze thrust washers - Flat washers made from the same porous material, for light axial loads.
• Metric and imperial sizes - Sintered bronze bushings are made in a wide range of standard metric sizes, as well as imperial sizes for older machinery.

How to choose the right sintered bronze bushings?

• Movement - Choose sintered bronze for rotating shafts, especially at higher speeds. For slow, heavily loaded, or oscillating pivots, a solid bronze or wrapped bronze bush may be better.
• Size - Choose a bush with a bore to suit the shaft, an outside diameter to suit the housing, and a length to suit the space available.
• Straight or flanged - Choose a flanged bush where the bush needs locating in the housing or where there is a light axial load.
• Load - Sintered bronze bushings suit light to moderate radial loads. Because the material is porous, very heavy or shock loads can damage them.
• Shaft and housing - Use a smooth, hardened shaft and a correctly sized housing, as the bore closes slightly when the bush is pressed in.

Essential references and specialised ranges

How the numbering works:

Sintered bronze bushings are usually referenced by a type code, the bore, outside diameter, and length in millimetres, and a material code. In the widely used SKF range:

PSM = straight sintered bronze bush

PSMF = flanged sintered bronze bush

Examples:

PSM202525A51 = straight bush, 20mm bore, 25mm outside diameter, 25mm long

PSM455565A51 = straight bush, 45mm bore, 55mm outside diameter, 65mm long

PSMF253525A51 = flanged bush, 25mm bore, 35mm outside diameter, 25mm long

Suffixes:

A51 = the sintered bronze material grade used for the bush

F = flanged bush

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about sintered bronze bushings

Are sintered bronze and oil filled bushes the same?

They are very closely related. Sintered bronze is the porous material, and oil filled describes the oil held within it. Most sintered bronze bushes are supplied filled with oil, so the terms are often used for the same type of bush.

Why are sintered bronze bushes good for high speeds?

The oil in the pores forms a thin lubricating film between the shaft and the bush, which keeps friction and heat low. This allows higher sliding speeds than bronze bushes that rely on grease.

Do sintered bronze bushes need extra lubrication?

Not usually. They are self-lubricating and maintenance-free. In heavy use, a few drops of a compatible oil can top up the supply, but grease should be avoided, as it can block the pores.

Can I ream a sintered bronze bush?

It is best not to. Cutting the bore can smear and close the surface pores, which stops the oil reaching the shaft. If the bore needs sizing after fitting, use a burnishing tool instead.

How do I fit a sintered bronze bush?

Press the bush squarely into the housing using a press and a fitting mandrel, never by hammering directly on it. Check the shaft runs freely after fitting, as the bore closes slightly. Avoid washing the bush in solvents, which can remove the oil.

How do I identify the bush I need?

Check the reference on the old bush or in the machine’s parts list. If it cannot be read, measure the bore, outside diameter, and length with callipers, and on a flanged bush, the flange diameter and thickness.

How can I tell if a sintered bronze bush needs replacing?

Warning signs include:

• Squeaking, whining, or noise from a rotating shaft
• The bush running hot or dry
• Play or wobble in the shaft
• An oval-shaped or worn bore
• Cracks or crumbling in the bush from overload
• The bush moving or turning in its housing