Wrapped Steel Bushings

What are wrapped steel bushings?

Wrapped steel bushings, also called composite bushings or dry bushes, are thin-walled plain bearings made from a flat strip of layered material that is rolled, or wrapped, into a cylinder. The strip has a steel backing for strength, a porous sintered bronze layer, and a PTFE-based lining that forms the bearing surface. This construction makes them light, compact, and self-lubricating, so they can run dry without oil or grease, giving an economical plain bearing for lighter-duty machinery.

A standard wrapped steel bushing is made up of these layers, from the outside in:

• A steel backing, which gives the bush its strength and load capacity and carries heat away from the bearing surface
• A thin copper or tin plating on the outside, for corrosion resistance
• A porous sintered bronze layer, which conducts heat and holds a reserve of the PTFE mixture
• A PTFE lining, which forms the low-friction bearing surface.

The bush has a joint, or split, along its length, where the ends of the wrapped strip meet.

• When the bush is pressed into its housing, the joint closes up and the bush is held firmly in place.
• Because the wall is thin, wrapped bushes take up very little space, which lets designs use smaller housings or larger shafts.

As the shaft moves, some of the PTFE transfers onto the shaft to form a thin solid lubricant film, which keeps friction and wear low without oil or grease.

Why are wrapped steel bushings essential?

Wrapped steel bushings give a self-lubricating, maintenance-free bearing in a very small space and at low cost. They are ideal for parts that are hard to reach, for machines built in large numbers, and for applications where oil or grease would be messy, would attract dirt, or could contaminate a product.

Their PTFE lining gives very low friction, prevents stick-slip, and copes with rotating, oscillating, and sliding movements. They work across a wide temperature range, from around -200°C up to around +280°C, and resist many chemicals, making them suitable for a broad range of light and medium-duty uses.

Typical applications include:

• Automotive components, such as hinges, pedals, and seat mechanisms
• Pumps, compressors, and hydraulic gear pumps
• Agricultural and garden machinery pivots
• Office equipment, printers, and appliances
• Packaging and textile machinery
• Hydraulic cylinder guides and shock absorbers
• Pneumatic equipment and valves
• Light industrial machinery and conveyors

The main types of wrapped steel bushings

• Plain PTFE-lined wrapped bushes - Straight, cylindrical bushes with a PTFE lining. They are the most common type and are designed to run dry.
• Flanged PTFE-lined wrapped bushes - Bushes with a flange at one end, which locates the bush and takes light axial loads.
• Wrapped thrust washers - Flat washers made from the same layered material, for axial loads alongside a bush.
• Metric wrapped bushes - Made in metric sizes, with bore, outside diameter, and length in millimetres.
• Inch wrapped bushes - Made in imperial sizes, for older and imperial machinery.

How to choose the right wrapped steel 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.
• Housing and shaft tolerances - Wrapped bushes rely on the housing to close them to the correct size. Check the housing bore and shaft diameter are machined to the recommended tolerances, such as an H7 housing.
• Shaft finish and hardness - For long life, use a smooth, hardened shaft, as a rough or soft shaft will wear the PTFE lining quickly.
• Load and speed - Wrapped bushes suit light to medium loads at moderate sliding speeds. Check the load and speed are within the bush’s PV rating, which combines the two.
• Dry or lubricated - Wrapped PTFE bushes are designed to run dry, but can also be used with oil, which raises their speed and load limits.

Essential references and specialised ranges

How the numbering works:

Wrapped steel bushes are usually referenced by a type code and their bore, outside diameter, and length.

Metric bushes give the sizes in millimetres, for example:

WMU060808 = metric plain PTFE-lined bush, 6mm bore, 8mm outside diameter, 8mm long

MB1015D = metric plain bush, 10mm bore and 15mm long, with a 12mm outside diameter

Inch bushes give the bore and length in sixteenths of an inch, for example:

03D06 = inch bush, 3/16 inch bore and 6/16 (3/8) inch long, with a 1/4 inch outside diameter

Suffixes:

D = PTFE-lined dry bearing material

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 wrapped steel bushings

Do wrapped steel bushings need lubrication?

No. The PTFE lining is self-lubricating, so the bush is designed to run dry. It can also run in oil, which allows higher speeds and loads, but grease should be avoided as it can carry dirt into the bearing.

Why does a wrapped bush have a split?

The bush is made by rolling a flat strip into a cylinder, so there is a joint where the ends meet. When the bush is pressed into its housing, the joint closes and the bush is held firmly in place at the correct size.

Can I ream or machine a wrapped bush?

No. The PTFE lining is very thin, and reaming or machining would remove it. The bush is designed to reach its correct bore size when pressed into a correctly sized housing.

What is the difference between wrapped steel and solid bronze bushes?

Wrapped steel bushes are thin-walled, self-lubricating, and lower in cost, and suit light to medium loads. Solid bronze bushes are thick-walled, need grease or oil, and carry heavier and shock loads.

How do I fit a wrapped steel bush?

Press the bush squarely into a clean, correctly sized housing using a press and a stepped fitting mandrel, never by hammering directly on it. Position the joint away from the main load if possible. Check the shaft runs freely once fitted, and do not ream the bore.

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, and on a flanged bush, the flange diameter and thickness. Note whether the sizes are metric or inch.

How can I tell if a wrapped steel bush needs replacing?

Warning signs include:

• The grey PTFE lining worn through to the bronze layer underneath
• Squeaking, noise, or increased friction
• Play or wobble in the shaft
• The bush turning or moving in its housing
• Scoring on the shaft or bush
• A bush that has cracked or opened at the joint