Key Characteristics of Plain Bearings:

• Simplicity & Cost-Effectiveness: Generally less complex and cheaper to manufacture than rolling element bearings.

• High Load Capacity: Can often handle very high static and dynamic loads, especially radially.

• Quiet Operation: No rolling elements means less noise generation.

• Compactness: Often have a smaller radial dimension compared to rolling bearings for the same shaft size.

• Shock Resistance: Generally handle shock loads well.

• Friction: Can have higher starting friction than rolling bearings, but running friction can be very low with proper lubrication.

• Material Variety: Can be made from a vast range of materials (metals, polymers, composites).

What is a Bushing (or Bush)?

The term “bushing” or “bush” is often used interchangeably with “plain bearing,” particularly when referring to the sleeve-like component that lines a hole to provide a bearing surface for a shaft. So, a bush is essentially a type of plain bearing.

Introducing the Flanged Bush

A flanged bush (or flanged bearing, flanged sleeve bearing) is a specific type of plain bearing (bush) that incorporates an integral flange or lip at one end.

• Construction: It’s a standard cylindrical bush with an added radial extension (the flange) perpendicular to the axis of the bore.

The Purpose of the Flange:

The flange serves several critical functions that a simple cylindrical bush cannot:

1. Axial Location/Positioning: The primary role of the flange is to positively locate the bush axially within its housing. It prevents the bush from sliding through the hole it’s pressed into.

2. Thrust Load Resistance: The flange provides a surface to bear axial loads (loads parallel to the shaft’s axis), also known as thrust loads. A plain cylindrical bush primarily handles radial loads (loads perpendicular to the shaft). The flange acts as a simple thrust washer integrated into the bush.

3. Simplified Assembly: It can make assembly easier by providing a clear stop point when pressing the bush into a housing.

4. Increased Mounting Surface: In some designs, the flange might offer a larger surface for mounting or securing purposes.

Comparison: Plain Bush vs. Flanged Bush

Materials for Flanged Bushes (and Plain Bearings)

Like all plain bearings, flanged bushes can be made from various materials, depending on the application requirements (load, speed, temperature, environment, lubrication):

• Bronze Alloys (e.g., Tin Bronze, Bearing Bronze C93200/SAE660, Aluminum Bronze): Excellent wear resistance, good load capacity, good machinability. Often oil-impregnated (sintered bronze) for self-lubrication. Connects back to the previous article – specific bronze types offer different strength/wear trade-offs.

• Steel (often with plating or overlay): High strength, but often requires good lubrication or surface treatments (like chrome plating) or bearing overlays (like Bimetal) to act as the bearing surface.

• Polymers (e.g., Nylon, PTFE, PEEK, Acetal): Low friction (especially PTFE), self-lubricating properties, corrosion resistance, lightweight, good for lower loads/speeds.

• Composite Materials: Metal-backed polymers (e.g., PTFE-lined steel), fiber-wound composites. Offer combinations of strength, low friction, and self-lubrication.

• Cast Iron: Good wear resistance, often used in lower-speed machinery.

1. Flanged: As discussed before, this means the bush has an integral flange (a lip or rim) at one end. This flange provides positive axial location and acts as a surface to bear thrust loads.

2. Wrapped: This refers to the manufacturing method. Instead of being machined from solid bar stock or cast, these bushes are formed by wrapping a flat strip of composite material into a cylindrical shape. The seam where the edges meet is typically a butt joint. This method allows for:

   • Thin Walls: Wrapped bushes can be made with very thin walls compared to solid bushes, saving space and weight.

   • Cost-Effectiveness: Often more economical for high-volume production than machining solid bushes.

   • Composite Structures: Easily allows for layered material construction.

3. Steel: This specifies the material used for the backing layer. A strong steel backing provides:

   • Structural Rigidity: Gives the bush its overall strength and shape.

   • Press-Fit Capability: Allows the bush to be securely pressed into a housing bore with an interference fit, holding it firmly in place.

   • Heat Dissipation: Steel helps conduct heat away from the bearing surface.

4. Plain Backed: This is slightly redundant with “Steel” but emphasizes that the steel is the structural backing material. It implies there’s another layer on top of the steel that forms the actual bearing surface. The bearing surface itself is plain (meaning smooth, without rolling elements).

5. Bush: The fundamental component – a sleeve bearing designed for sliding motion.

Putting it all Together: The Structure

A Flanged Wrapped Steel Plain Backed Bush is typically a multi-layer composite bearing:

• Layer 1 (Outer Backing): Steel Strip. Provides strength, rigidity, and the press-fit capability. The flange is formed from this steel backing.

• Layer 2 (Intermediate – Optional but Common): Porous Sintered Bronze. This layer is sintered onto the steel backing. Its porosity serves two main purposes:

  • It creates a strong mechanical bond between the steel backing and the sliding layer.

  • It acts as a reservoir to hold lubricant (often impregnated during manufacturing).

• Layer 3 (Inner Sliding Surface): Bearing Material. This is the crucial layer that the shaft runs against. Common materials include:

  • PTFE (Polytetrafluoroethylene) based compounds: Often mixed with fillers like lead (historically), molybdenum disulfide (MoS₂), graphite, or other polymers. This provides extremely low friction and self-lubricating properties (often referred to as DU-type bushings, a trademark now widely used generically). Requires no external lubrication for many applications.

  • Acetal (POM – Polyoxymethylene) based compounds: Often has indents or pockets on the surface to retain grease. Designed for marginally lubricated or grease-lubricated applications (often referred to as DX-type bushings, also a widely used generic term). Requires initial greasing and potentially periodic re-lubrication.

  • Other polymers or composites.