Designs and variants
Solid bronze bushings can be lubricated with oil or grease, whereas the lubricant not only improves the sliding properties, but also reduces wear and prevents corrosion. Those bushings with a bore diameter d > 14 mm have an an axial lubrication groove in its sliding surface and are indicated by the designation suffix G1.
All surfaces of a solid bronze bushing are machined.
SKF does not recommend machining or grinding of the sliding surface of a porous sintered bushing due to the risk of closing the bushing pores.
The diamond-shaped lubrication pockets in the sliding surface (fig. 7) need to be filled initially with grease. A good quality lubricant reduces friction and wear by separating a bronze bushing from its shaft. To protect the bushing and lubricant in highly contaminated environments, SKF recommends using seals.
SKF PTFE composite bushings are available as straight bushings (fig. 8) in metric and inch sizes, as flanged bushings (fig. 9) in metric sizes. These bushings are self-lubricating as well as maintenance-free and due to their low friction provide a long service life. Despite of their compact design, they can accommodate heavy radial loads and are suitable for relatively slow rotational or oscillating movements. They also have good dimensional stability and thermal conductivity.
PTFE composite bushings consist of a sheet steel backing on which a 0,2 to 0,4 mm thick porous layer of tin/bronze is sintered (fig. 10). Furthermore, by a rolling process the pores of the sintered layer are filled with a mixture of PTFE (polytetrafluoroethylene) and molybdenum disulphide additives. This additional 5 to 50 μm thick running-in layer provides an optimum combination of the mechanical properties of the sintered tin/bronze and the good sliding and lubricating properties of the PTFE mixture.
During a short running-in period, minute amounts of the PTFE material from the running-in layer are transferred to the counter surfaces. After this transfer has taken place, the characteristics low friction and wear properties of these bushings are reached.
POM composite bushings consists of a sheet steel backing on which a 0,2 to 0,4 mm thick layer of tin/bronze is sintered. The principal characteristic of these bushings is their relatively thick (approximately 0,25 to 0,45 mm) covering layer of acetal resin (POM – polyoxymethylene) with additives. This covering layer has pockets to retain grease and is firmly attached to the sintered tin/bronze layer (fig. 12).
The thickness of the covering layer makes these bushings less sensitive to misalignment and the edge loading associated with that misalignment.
However, filament wound bushings have only a limited capacity to accommodate solid particles embedded into the filament wound material. Therefore, SKF recommends protecting the sliding surface against the ingress of contaminants (Design of bushing arrangements) when the bushing is to be used in highly contaminated environments.
SKF filament wound bushings, which are often dimensionally interchangeable with solid bronze or steel bushings, can be machined using normal methods on all surfaces, except for the sliding surface. To facilitate mounting, the bushing may be split lengthwise into two halves by using a diamond coated grinding wheel. Sufficient cooling fluid should be applied as well to avoid excessive temperatures, as they can destroy the bushing.
SKF filament wound bushings are manufactured by a winding technique from a self-lubricating composite. The unique sliding surface layer consists of high-strength polyester and PTFE (polytetrafluoroethylene). The backing consists of wound high-strength tensioned glass fibres. Both layers are manufactured by winding endless strands in a criss-cross pattern (fig. 16) and are embedded in an epoxy resin matrix so that they are firmly attached to each other.