Thrust washers, strips, maintenance-free
Features
Maintenance-free thrust washers are used for rotary and oscillating motion. Strips are used predominantly for linear motion or as the basic material for other designs, such as half-cups or special geometries.
These plain bearings are bearings for very small design envelopes.
The plain bearings are supplied with either steel backing or bronze backing. Bearings with a bronze backing have high corrosion resistance and very good thermal conductivity and are antimagnetic.
ACHTUNG
If the plain bushes are to be used in the aerospace sector or in the food or pharmaceuticals industry, please contact the Schaeffler engineering service.
Maintenance-free plain bearing material
For maintenance-free metal/polymer composite plain bearings from Schaeffler, the sliding material E40 and E40-B is used. The basis of the dry lubricant is polytetrafluoroethylene PTFE with embedded chemically non-reactive additives.
In the three-layered material, the steel or bronze backing has a sintered porous tin/bronze sliding layer whose pores are filled with the superimposed running-in layer, see table, ➤ Figure and ➤ Figure. The running-in layer is a plastic composite comprising PTFE and additives.
Sliding and running-in layer
E40, E40-B
Chemical element | Proportion of mass | Layer thickness | ||
|---|---|---|---|---|
w | ||||
% | mm | |||
Sliding layer | Running-in layer | Sliding layer | Running-in layer | |
Molybdenum disulphide MoS2 | ‒ | max. 8 | 0,2 – 0,4 | 0,01 – 0,05 |
Polytetrafluoroethylene PTFE | ‒ | 80 – 86 | ||
Fillers | max. 5,5 | max. 19 | ||
Tin Sn | 7 – 12 | ‒ | ||
Copper Cu | Balance | ‒ | ||
Maintenance-free plain bearing material E40
Maintenance-free plain bearing material E40-B
Resistance of the plain bearing material
The resistance of the material E40 depends on the chemical characteristics of the individual layers:
- The material E40 is resistant to water, alcohols, glycols and numerous mineral and synthetic oils.
- The tin-plated steel surface gives adequate protection against corrosion in most cases.
- In the case of the material E40-B, the bronze backing is additionally resistant to water vapour and seawater.
ACHTUNG
The material E40 is not resistant to acidic media (pH ＜ 5) and alkaline media (pH ＞ 9). The bronze backing of E40-B is not resistant to oxidising acids and gases such as free halides, ammonia or hydrogen sulphide, especially if these gases have a high moisture content.
Technical data for E40
The sliding layer E40 is maintenance-free. It can be used for rotary and oscillating motion as well as for short stroke linear motion.
The low-wear material has good sliding characteristics (no stick-slip), a low coefficient of friction and high resistance to chemicals. It does not absorb water (it is highly resistant to swelling), does not tend to weld to metal and is also suitable for hydrodynamic operation.
The maintenance-free plain bearing materials E40 and E40-B have the following mechanical and physical characteristics, see table.
Characteristics of E40 and E40-B
Characteristic | Loading | ||
|---|---|---|---|
Maximum pv value | Continuous operation | pv | 1,8 N/mm2 · m/s |
For short periods | 3,6 N/mm2 · m/s | ||
Permissible | Static | pmax | 250 N/mm2 |
Rotary, oscillating | 140 N/mm2 | ||
Permissible | Dry running | vmax | 2,5 m/s |
Hydrodynamic operation | ＞2,5 m/s | ||
Permissible operating temperature | ϑ | –200 °C to +280 °C | |
Coefficient | Steel backing | αSt | 11 · 10–6 K–1 |
Bronze backing | αBz | 17 · 10–6 K–1 | |
Coefficient | Steel backing | λSt | ＞42 Wm–1K–1 |
Bronze backing | λBz | ＞70 Wm–1K–1 | |
Relative electrical resistance after running-in | Rrel min | ＞1 Ω · cm2 | |
Lubrication
Plain bearings with the sliding layer E40 contain dry lubricants and do not therefore require lubrication.
Lubrication can be used to protect the mating surface against corrosion or simply to provide sealing against contamination. It should be checked in advance, however, whether it is more advantageous in such cases to use a corrosion-resistant material for the mating surface or a different means of sealing the bearing position.
In certain applications, the sliding layer E40 can be used in fluid media. In this case, the improved heat dissipation may considerably increase the operating life.
ACHTUNG
The compatibility of the media with the sliding layer E40 must be checked. Further advice should therefore be sought from the Schaeffler engineering service.
Lubricants
Oil and grease lubrication, even in very small quantities, impairs the transfer of material during the running-in phase.
Over time, grease and small quantities of oil mix with wear debris to form a paste that promotes wear. Solid lubricants such as zinc sulphide, molybdenum disulphide or similar grease additives are not permitted, since they promote this paste formation to an increased extent.
Relubrication
If it is not possible in exceptional cases to avoid the use of grease lubrication, the bearings must be relubricated periodically. During relubrication, old grease is replaced by fresh grease. At the same time, the grease flushes wear debris and contaminants out of the bearing.
ACHTUNG
If periodic relubrication is carried out, the formation of paste comprising debris and contaminants must be avoided.
Operating temperature
The permissible operating temperature for maintenance-free metal/polymer composite plain bearings is between –200 °C and +280 °C.
ACHTUNG
The running-in layer and sliding layer may undergo swelling in the presence of some mineral oils at temperatures above +100 °C. This could lead to jamming of the bearing.
This can be remedied by increasing the bearing clearance, since other characteristics of the sliding layer E40 are unaffected.
Suffixes
Suffixes for available designs: see table.
Available designs
Suffix | Description | Design |
|---|---|---|
E40 | Maintenance-free sliding layer, with steel backing | Standard |
E40-B | Maintenance-free sliding layer, with bronze backing |
Design and safety guidelines
The guidelines on the design of bearing arrangements and on mounting and dismounting must be observed.
Friction
At high specific bearing load and low sliding velocity, the coefficient of friction is more favourable.
Running-in process
During the running-in process, part of the running-in layer is transferred to the mating surface:
- This compensates uneven areas.
- A mating surface with a low coefficient of friction is formed, which acts favourably on the operating behaviour.
- After running-in, some of the porous bronze layer is visible on the sliding layer as individual areas of differing sizes, ➤ Figure. This shows that the bearing is running correctly.
Typical wear pattern of sliding layer E40
Operating behaviour
After running-in, the wear of maintenance-free plain bearings proceeds in a linear manner, ➤ Figure.
Typical operating behaviour during operating life
Dimensioning and rating life
The dimensioning of thrust washers and strips is summarised in the chapter Technical principles.
Depending on whether the bearing is subjected to dynamic or static load, the following must be checked:
- static load safety factor S0
- maximum permissible specific bearing load p
- maximum permissible sliding velocity v
- maximum specific frictional energy pv.
ACHTUNG
The rating life of thrust washers can be calculated if the range of validity is observed.
For a rating life estimate in the case of strips EGS, the Schaeffler engineering service should be contacted.
Design of mating surfaces
The shaft and mating surface of the bearing arrangement must be produced in accordance with the following specifications.
The shafts or parts of the mating surfaces should be chamfered and all sharp edges should be rounded. This allows easier mounting and prevents damage to the sliding layer.
Measures
The mating surface should always be wider than the bearing to prevent the formation of steps in the sliding layer.
The optimum operating life will be achieved with a roughness depth of the mating surface of Rz 2 to Rz 3:
- with dry running of the sliding layer E40.
ACHTUNG
Very small roughness values do not have a beneficial effect on the operating life, but larger roughness values reduce it considerably.
Surface quality
Ground or drawn surfaces are preferable as a mating surface. Surfaces that have been precision turned or rolled by precision turning, even with Rz 2 to Rz 3, can cause greater wear since helical manufacturing grooves are created by precision turning.
Spheroidal graphite cast iron GGG has an open surface structure and should therefore be ground to Rz 2 or better.
Heat dissipation
Correct and sufficient heat dissipation must be ensured:
- If hydrodynamic operation is present, the heat is dissipated predominantly via the fluid lubricant.
- In the case of maintenance-free plain bearings, the heat is dissipated via the housing and shaft.
Protection against corrosion
Corrosion of the mating surface is prevented by sealing or the use of corrosion-resistant steel. Alternatively, suitable surface treatments may be carried out.
Fretting corrosion
Due to the tin coating applied as standard, fretting corrosion between the steel backing of the plain bearing material and the housing occurs only rarely. In such cases, electroplated protective coatings can be used to achieve a delaying effect.
Electrochemical contact corrosion
In unfavourable conditions, electrical cells (local elements) can be formed that reduce the operating life through corrosion of the steel. This should be checked at the design stage and clarified by means of tests. In case of doubt, please consult the Schaeffler engineering service.
Machining of plain bearings
Metal/polymer composite plain bearings can be machined by either cutting or non-cutting methods, such as shortening, drilling or bending.
The procedure is as follows:
- Separate the plain bearings starting from the sliding layer side, since the burr formed in cutting will impair the running surface
- Clean the bearing elements thoroughly.
- Protect any bright steel surfaces such as cut edges against corrosion by means of oil or electroplated protective coatings.
ACHTUNG
In electroplating with high current densities or or long coating times, the sliding layers should be masked to prevent deposits.
The machining temperature must not exceed +280 °C in the case of the sliding layer E40, otherwise there may be a health risk.
Alternative joining methods
If it is uneconomical to use dowel pins or screws, there are economical alternative methods for location:
- laser welding
- soft soldering
- adhesive bonding.
ACHTUNG
The machining temperature must not exceed +280 °C in the case of the sliding layer E40, otherwise there may be a health risk.
The running-in or sliding surface must always be kept free of adhesive.
If adhesive is used, the adhesive manufacturer must always be consulted, particularly on the selection of adhesives, preparation of the surface, hardening, strength, temperature range and elongation behaviour.
Electrical conductivity
The electrical conductivity of new bearings may be lower because the running-in layer is still present. The bronze layer is partially exposed after the running-in process, thus improving the electrical conductivity, ➤ Figure.
The electrical resistance is dependent on the size of the contact surface.