Oil Filled Bushings

What are oil filled bushings?

Oil filled bushings, also called oil impregnated bushes or sintered bronze bushes, are self-lubricating plain bearings made from porous bronze that has been soaked in oil. The tiny pores in the metal act like a sponge, holding the oil inside the bush. As the shaft turns and the bush warms up, oil is drawn out to the bearing surface to lubricate it, and when the shaft stops, the oil is drawn back into the pores. This gives continuous lubrication with little or no maintenance.

A standard oil filled bushing is made up of:

• A sintered bronze body, made by pressing bronze, graphite, and other metal powders into shape and heating them
• A network of tiny interconnected pores running through the metal
• Oil held within those pores, which lubricates the shaft
• A plain cylindrical shape, or a cylinder with a flange at one end.

The sintering process is what makes the bush self-lubricating.

• Pressing and heating the metal powders leaves the bush porous, so it can absorb and hold oil.
• The oil supply lasts for a long time in normal use, so many oil filled bushes need no further lubrication for their working life.

Oil filled bushings are sized by their bore, outside diameter, and length, and by flange diameter and thickness on flanged bushes.

Why are oil filled bushings essential?

Oil filled bushings give a simple, low-cost bearing that lubricates itself. They are quiet, compact, and need little or no maintenance, which makes them ideal for parts that are hard to reach, for machines built in large numbers, and for applications where regular greasing is not practical.

Because they have no moving parts, they are less complex than ball or roller bearings, handle shock loads well, and run very quietly. They are well suited to electric motors, pumps, fans, and many other machines with rotating or sliding shafts.

Typical applications include:

• Small electric motors and fans
• Pumps and compressors
• Household appliances and power tools
• Office equipment and printers
• Agricultural and garden machinery
• Automotive components, such as wipers and seat mechanisms
• Hinges, pivots, and linkages
• Conveyors and light industrial machinery

The main types of oil filled bushings

• Plain oil filled bushes - Straight, cylindrical bushes with no flange. They are pressed into a housing and support the shaft radially.
• Flanged oil filled bushes - Bushes with a flange at one end. The flange locates the bush in the housing and can also take light axial loads against a shoulder or collar.
• Metric oil filled bushes - Made in metric sizes, with bore, outside diameter, and length in millimetres.
• Imperial oil filled bushes - Made in inch sizes, for older and imperial machinery.
• Oil filled thrust washers - Flat washers made from the same material, used to take axial loads alongside a bush.

How to choose the right oil filled bushings?

• 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.
• Plain or flanged - Choose a flanged bush where the bush needs locating in the housing or where there is a light axial load. Choose a plain bush where there is no axial load.
• Load and speed - Oil filled bushes suit light to moderate loads at a range of speeds. For very heavy loads or very high speeds, a different bearing type may be needed.
• Temperature - Check the bush and its lubricant suit the operating temperature, as very high temperatures can thin the oil and shorten its life.
• Shaft - Use a smooth, hard shaft with the correct running clearance, as a rough or soft shaft will wear the bush quickly.

Essential references and specialised ranges

How the numbering works:

Oil filled bushings are often referenced by a two-letter type code, followed by the bore, outside diameter, and length.

Metric bushes give the sizes in millimetres:

AM0812-16 = metric plain bush, 8mm bore, 12mm outside diameter, 16mm long

AL0812-08 = metric flanged bush, 8mm bore, 12mm outside diameter, 8mm long

Imperial bushes give the sizes in sixteenths of an inch:

AI0810-10 = imperial plain bush, 8/16 (1/2 inch) bore, 10/16 (5/8 inch) outside diameter, 10/16 (5/8 inch) long

AJ2024-12 = imperial flanged bush, 20/16 (1-1/4 inch) bore, 24/16 (1-1/2 inch) outside diameter, 12/16 (3/4 inch) long

Suffixes:

AM = metric plain bush

AL = metric flanged bush

AI = imperial plain bush

AJ = imperial flanged bush

PL = plain, FL = flanged

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

FAQ - All you need to know about oil filled bushings

How does an oil filled bushing lubricate itself?

The bush is porous and full of oil. When the shaft turns, friction warms the bush and draws oil to the bearing surface. When the shaft stops, the bush cools and the oil is drawn back into the pores, ready for next time.

Do oil filled bushings need extra lubrication?

In most applications, no. The oil inside lasts for a long time. On heavily used machines, a few drops of a compatible oil from time to time can extend the bush’s life, but avoid grease, which can block the pores.

What is the difference between a plain and a flanged bush?

A plain bush is a straight cylinder. A flanged bush has a lip at one end, which locates it in the housing and can take a light axial load against a shoulder or collar.

Can I ream or machine an oil filled bush?

It is best to avoid it. Reaming or machining can close the pores on the surface, which stops the oil reaching the shaft. If the bore needs sizing after fitting, use a burnishing tool rather than a cutting tool.

How do I fit an oil filled bush?

Press the bush squarely into the housing using a press and a fitting mandrel, never by hammering directly on it. The bore closes slightly when the bush is pressed in, so check the shaft runs freely afterwards. Keep the bush clean and avoid washing it 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. Note whether the sizes are metric or imperial.

How can I tell if an oil filled bush needs replacing?

Warning signs include:

• Squeaking, grinding, or noise from the shaft
• Play or wobble in the shaft
• The bush running hot or dry
• Visible wear, scoring, or an oval-shaped bore
• The bush moving or turning in its housing
• A shaft that has become stiff or hard to turn