Cookies on the SKF website

We use cookies to ensure that we give you the best experience on our websites and web applications. By continuing without changing your browser settings, we assume you give your consent to receive cookies. However, you can change your browser cookie setting at any time.

Design of bushing arrangements

The factors that are most important to consider when selecting the material and surface finish of the counter surface on which the bushing slides are the load conditions such as load, angle of oscillation, type of movement, and environmental influences. 

Where there is a risk of corrosion, the counter face should be sufficiently resistant. Corrosion scars in the counter surface and for example abrasive particulate contamination as a by-product of corrosion increase the surface roughness accelerate wear. In this cases, the use of stainless steel or a surface treatment such as hard chromium, nickel plating or electrolytic oxidation should be considered.

This page provides bushing type specific recommendations on the following design considerations, as far as applicable:

  • shaft and housing tolerances
  • shaft roughness and hardness
  • material and surface finish of counter surfaces
  • design of associated components
  • sealing solutions
Solid bronze bushings
Shaft tolerance
e7 to e8

Housing toleranceH7

Shaft roughness
Ra ≤ 1,0 µm

Shaft hardness
165 to 400 HB


Sintered bronze bushings
Shaft tolerance
f7 to f8

Housing toleranceH7

Shaft roughness
Ra: 0,2 to 0,8 µm

Shaft hardness
200 to 300 HB


Wrapped bronze bushings
Shaft tolerance
e7 to f8

Housing toleranceH7

Shaft roughness
Ra: 0,4 to 0,8 µm

Shaft hardness
150 to 400 HB


PTFE and POM composite bushings
Shaft tolerances

Metric PTFE composite bushings (table 1): 

  • d ≤ 75 mm: f7
  • d > 75 mm: h8

Metric POM composite bushings: h8 (table 2)

Inch PTFE composite bushings (table 3)

Inch POM composite bushings (table 4)

Housing tolerances1)

Metric PTFE composite bushings (table 1):

  • D ≤ 4 mm: H6
  • D > 4 mm: H7

Metric POM composite bushings: H7 (table 2)

Inch PTFE composite bushings (table 3)

Inch POM composite bushings (table 4)

Shaft roughness

PTFE composite bushings: Ra ≤ 0,4 µm; Rz ≤ 3,0 µm

POM composite bushings: Ra ≤ 0,8 µm; Rz ≤ 6,0 µm

Shaft hardness

PTFE composite bushings: 300 to 600 HB

POM composite bushings: 150 to 600 HB

1)The recommended tolerances and guideline limits quoted in the tables are valid for steel and cast iron housings. Where light alloy housings are used, a greater degree of interference may be required because of the different thermal expansion characteristics of the housing and the bushing. There is otherwise a risk that the greater thermal expansion of the housing would no longer provide radial location for the bushing and that the operating clearance would be too large. If it is not possible to adopt a greater interference fit, it might be possible to use an adhesive to retain the bushing. In special cases it may be necessary, by selecting a suitable tolerance for the shaft, to prevent an inadmissible increase in operating clearance.



Material and surface finish of counter surfaces
For PTFE composite and POM composite plain bearing arrangements, soft carbon steels with a ground surface are usually adequate for the counter surface.
For more demanding applications, SKF recommends hardened counter surfaces. These should have a surface hardness of at least 500 HB (50 HRC). Alternatively, hard chromium, nickel plating or some other form of surface treatment can be considered. In all cases Ra should not be greater than 0,3 μm (Rz ≤ 2 μm). The better the surface finish, the better the running and wear properties.


Design of associated components
To prevent the sliding surface from becoming stepped, the counter surface should always be wider than the actual bushing width – particularly where axial displacement of the shaft relative to the housing may occur as a result of axial expansion.
To facilitate mounting, shaft ends and housing bores should have a lead-in chamfer with an angle of 10 to 15° (fig. 1). These chamfers make it easier to press the bushings into the housing bore and to insert the shaft into the bushing bore, without the risk of damaging the sliding surface.
The housing shoulders intended to locate the bushing axially should have a bore diameter that is equal to or greater than d + 0,8 mm.
When PTFE composite bushings operate without a lubricant it is especially important to accurately align all bearing positions. If misalignment between the positions cannot be avoided, take steps during the design stage to prevent inadmissibly high edge stresses from occurring. For example, the housing bore seating can be relieved on both sides, or a wider bushing can be used so that it extends beyond the housing bore seating on both sides (fig. 2).
In case of misalignment have to be compensated for and the operating conditions permit the use of POM composite, then the bushings of this material should be chosen. The covering layer of this material can be machined to a minimum degree after the bushing has been mounted in a housing bore.
For shafts that need radial support and axial location, flanged bushings or a combination of a bushing and a thrust washer (Design of thrust washer arrangements) can be applied (fig. 3) depending on the magnitude of the axial load. Flanged bushings or thrust washers are advantageous even if axial loads are light and a surface is not adequate to accommodate the thrust, either because the material or its finish is unsuitable. Note that the counter surface should completely cover the sliding surface of the flange of a flanged bushing. For bearing arrangements where flanged bushings are used, the transition from housing bore to abutment should be chamfered so that it does not contact the bushing where it meets the flange (fig. 4).


Sealing solutions
Seals can significantly influence the service life of composite bushings. When selecting seals, consider especially the design and the available space. The justifiable expense should also be taken into consideration since, for example, composite plain bushings can tolerate embedded contaminant particles especially in their POM composite layer and are thus relatively insensitive to contamination. They generally require no special protection against normal airborne dirt.
If, however, the bushing position is subjected to heavier contamination it should be sealed from the outside and protected against the ingress of contaminants. Simple and efficient sealing can be obtained if adjacent components can also serve as seals (fig. 5).
Radial shaft seals with a low cross sectional height, for example G design radial shaft seals as used for needle roller bearings, can provide adequate protection for composite plain bearings in most cases (fig. 6). If the demands placed on the sealing arrangement are high, it may be necessary to resort to special seals of rubber, plastic or similar materials, for example SKF wiper seals (fig. 7).
In highly contaminated environments, particularly where sand or clay is present, rubber or plastic seals tend to have a very short service life. Under these conditions, a good sealing solution can be achieved by regular relubrication.

PTFE polyamide bushings
Shaft tolerance
h8 to h9

Housing toleranceH7

Shaft roughness
Ra ≤ 0,8 µm

Shaft hardness
100 to 300 HB


Filament wound bushings
Shaft tolerances
h8

Housing tolerance
H7

Shaft roughness

Ra: 0,2 to 0,4 µm
Rz: 1,0 to 2,0 µm

Shaft hardness
≥ 490 HB (≥ 50 HRC)



Material and surface finish of counter surfaces
SKF generally recommends using hardened counter surfaces for filament wound bushings.
Profiles having sharp edges, for example high Rr values, result in heavy wear even if the Ra values are within the recommended range. Although greater surface roughness might be acceptable for oscillating movement, heavy wear occurs when the bushings rotate or perform linear movements.
To fully exploit the potential service life of the bushing it may be necessary to optimise the counter surface. The best results have been obtained with polished, hard chromium plated as well as with nitrided surfaces.
Whether an adequate service life can be obtained if the condition of the counter surface is outside the recommendations has to be checked from case to case.
For additional information, contact the SKF application engineering service.


Sealing solutions
Filament wound bushings have only a limited capacity to accommodate solid particles embedded into the filament wound material. Therefore, seals can significantly increase their service life when the bushings are to be used in highly contaminated environments. Contaminants otherwise damage the strands of the sliding surface layer and the continuing destruction of the structure of the sliding layer would result in rapid increase in wear. SKF recommends using SKF wiper seals, which allow the same sliding velocities as the filament wound bushings, to protect the sliding surface against the ingress of abrasive contaminants (fig. 7). Radial shaft seals or Radial shaft seals with a low cross sectional height can also provide suitable sealing solutions. 


SKF logo