Bearings facilitate movement while reducing friction. When designing bearings, engineers often face the question Bearing or Bushing? But they have two types of bearings at their disposal: Rolling bearings and plain bearings. Most know what a rolling bearing (or ball bearing) is, but have difficulty with the different plain bearing varieties.
Most plain bearings are simple, metal or plastic in the desired shape. Some plain bearings are a bit more sophisticated and consist of a carrier material (metal) and a polymer layer.
Plain Bearings are used in a wide range of applications, and can broadly be divided into radial bearings, thrust bearings and linear bearings, depending on whether they’re working with rotational or linear movements. Its alternative name of “sleeve bearing” gives you the best idea of a plain bearing use and appearance. The plain bearing has a sliding layer and because they are usually self-lubricating, they provide a smooth and reliable operation. The simplest type of bushing is a cylindrical, sleeve-shaped design, and it’s used in radial loads. For additional axial loads, a bushing with a flange is the best option. Plain Bearings are widely used across many industries, particularly if there are heavy load requirements. They’re found in machinery with rotating or sliding shafts, and depending on the material, can be a good option when quiet operation is needed.
The benefits of the versatile NORGLIDE® Plain Bearing are unique properties which result in some key advantages;
• Wear & Friction
• Tolerance compensation
• Corrosion resistance
• Noise & Vibration
By changing the metal backing or interlayer the NORGLIDE® Bearing can be tuned to accommodate different levels of loading. Depending on speed, load, mating surface or lubrication the different PTFE-Compounds fit to your system, resulting in low wear and friction. The bearings, with their unique thick PTFE layer, can allow clearance to interference fits, with no change in lifetime performance. For a precise fit; calibration is possible with interlayered materials ensuring the plain bearing operates in optimum conditions. The right coating or metal backing can be chosen to avoid corrosion. The thick PTFE Layer has damping properties and avoids excessive torque with higher wear rates.
Materials such as NORGLIDE® PRO & NORGLIDE® SMTL are designed with a higher load capability than other plain PTFE-Bearings or plastic bushings. This feature enables smaller bearings to be created allowing smaller mating components to be used. The result is an application that takes up less space, weighs less and has reduced overall cost. NORGLIDE® Bearings can also be used as ball bearing replacements, providing a reduction in component weight and space.
The coefficient of friction of a composite material is not a constant. It is determined by the materials of the mating contact surfaces and by the roughness.
With combinations that have very different strength values (such as polymer with steel) the coefficient of friction also depends on the load. In addition, due to the polymer’s strong tendency to change all mechanical properties under temperature, the coefficient of friction is also affected by speed and ambient temperature.
Graphs A-C show the influence in the coefficient of friction (COF) of PTFE on steel (100 Cr6 1.3505). Graph A demonstrates the influence of temperature in the COF. The COF drops as temperature increases. Graph B demonstrates the impact of speed in the COF. The COF increases as speed increases. Graph C demonstrates the influence of load in the COF. The COF drops as load increases.
During the wear-in period, a polymer transfer layer forms on the metal contact surface. After this, a consistent performance range is found which is controlled by the PTFE compound layer.
Generally speaking, the coefficient of friction of NORGLIDE® materials is outstandingly low and consistent in comparison with other plain bearings, due to the use of PTFE as the main component in the bearing surface.
A mark of quality in any product is the ability to perform consistently from part to part. This is how a company can build a brand by having a consistent and smooth feel when parts are actuated. Often, this can lead to a difficult decision: consistent performance can be guaranteed through precision manufacturing but this comes at a significantly increased cost.
With NORGLIDE® Bearings, Saint-Gobain offers a solution to this dilemma due to the forgiving configuration of the low friction PTFE layer. This layer is able to absorb tolerance stack ups and perform consistently for a wide range of customer part tolerances, resulting in less total cost for our customers without the need to compromise on performance.
Two major considerations of performance throughout a tolerance range are assembly force and torque. If the parts are made with too much interference due to a tolerance stack-up, then both the assembly force of the shaft into the housing-mounted bearing and the torque to turn the shaft within the bearing can become too high. Not only this but the force and torque will be quite different compared to that of the same system but in the opposite tolerance extreme, i.e. very low interference or even clearance.
Figure 1 shows the assembly, or press in, forces of a shaft into a housing-mounted bearing for five different sizes of pin, simulating a tolerance range of 200 µm. As can be seen the NORGLIDE® solutions are significantly superior in maintaining a consistent assembly force for the different pin sizes. Two of the competing PTFE-based solutions failed as the pin (shaft) diameter was increased as the assembly forces became too high.
The torque was also tested for different pin sizes. The torque is generated purely from the radial surface of the bearing. Torque can also be generated by applying an axial force to the flange. The results can be seen in Figure 2. A similar result as the assembly test is obtained, as expected. This shows a consistent performance of the NORGLIDE® Bearings throughout the 200 µm range whilst the alternative PTFE-based solutions increased significantly. Obviously if the pin couldn’t be assembled in the press-in force test for high interference, then they could not be tested for torque either.
failed after 0.10 mm and 0.15 mm interference
failed after 0.10 mm and 0.15 mm interference
Some systems are controlled by electric motors which require software to run. In these applications, consistent performance will decrease the complexity of the software. One input parameter for the software will be the speed of actuation. If the bearing’s torque changes too much for different speeds, then calculating the right amount of input power to the motor can become a difficult task. Therefore, it gives confidence to our customers to know that we test our bearings with such detail that sliding speed is considered, especially as PTFE performance is sensitive to speed and pressure. Figure 3 presents the change in torque for three different angular speeds, 45°/s, 90°/s and 135°/s. The stability of torque over the three speeds is significantly better than all but one of the competing solutions. This allows confidence for our customers that not only will their systems have a consistent feel but also that the software doesn’t have to become needlessly complex.
Figure 3 – left: torque generated by the plain bearings for three different speeds (45°/s, 90°/s and 135°/s). This test highlights the choice in torque level our customers have by choice of bearing. Right: the same test as the left graph but plotted as a change in torque between the 3 data points per material (Δ torque) to highlight the consistency of the bearings when the speed changes. NORGLIDE® Bearings perform consistently when compared with all but one of the alternative PTFE-based bearings. Test conditions: housing and pin are dummy components made out of hardened steel; pin size = 10.05 mm; axial load = 0 N; test stroke = 90°; cycles = 5.
The impressive tolerance compensation qualities exhibited by NORGLIDE® Bearings are achieved due to the unique composition of the products. The PTFE layer seen in Figure 4 has the ability to absorb tolerance stack-ups of mating components. The low friction layer also has the benefit of reducing the impact of wear on the performance throughout the lifetime of the system.
Figure 4– NORGLIDE® T (left) and a sintered bronze plain bearing (right). The PTFE layer on the NORGLIDE® solutions has a unique structure which allows greater compensation of tolerance and more consistent performance over time than sintered bronze plain bearings.
A large number of aspects have to be taken into account when protecting assemblies against corrosion. A wide variety of environmental influences and diverse material combinations require specific measures. NORGLIDE® plain bearings with their modular design allow the metal backing and the PTFE sliding layer to be matched exactly to requirements. As a result, NORGLIDE® Bearings offer premium corrosion protection with over 1000 hours without red rust in the salt spray test.
The NORGLIDE® Bearing steel shell can be provided with an anti-corrosive coating which can be a combination of zinc plating, Cr6 free passivation and sealing, depending on what kind of protection is needed. In a few special cases, other corrosion-protection systems can be used to avoid contact corrosion. All systems comply with the European Union End-of-Life directive for automotive vehicles 2000/53/EG.
In comparison with components of solid refined metal, specially structured NORGLIDE® Bearings have a shorter corrosion resistance when exposed in the salt spray chamber. The corrosion resistance of the individual components is not necessarily identical to the life expectancy of the assembly. For choosing the proper anti-corrosive coating, the composition and operating conditions should also be taken into account. Actual corrosion resistance should always be verified in the assembled condition.
The resistance to red and white corrosion of individual NORGLIDE® Bearings is tested to DIN EN ISO 9227 in our own salt spray chamber. In special cases the salt spray test is conducted on parts assembled in mating components.
Some NORGLIDE® materials are reinforced by corrosion resistant metal structures of the materials shown in Table 1 and do not need additional corrosion protection. Also in this case the electro-chemical potential must be taken into account to prevent contact corrosion.
Some NORGLIDE® types use a stainless steel backing and do not need additional corrosion protection. Some NORGLIDE® types use a backing of aluminium which also does not require further protection because it passivates itself. In both cases however, the electro-chemical potential must be taken into account to prevent contact corrosion in the assembled condition.
To avoid contact corrosion, attention must be given to the electrochemical series. For instance, a plain bearing containing bronze will have a high electrode potential (or electrochemical corrosion potential) when paired with zinc which is often used to coat steel housing bores. For this reason, it is not advised to use bronze containing bearings with zinc plated steel housing bores.
In this case it is good to use a metal that has a high affinity with zinc, such as aluminium. The aluminium clad bearings, such as NORGLIDE® SMALC, NORGLIDE® TALC or NORGLIDE® SALC, are good to use in this instance as they contain no bronze and the steel has been replaced with aluminium clad steel.
Learn more about Corrosion Resistant Bearings in this Insights article.
Our engineers minimize noise by developing solutions that reduce the noise source strength, interrupt the noise path and absorb any noise or vibrating energy. NORGLIDE® Bearings are lined with a PTFE compound. PTFE has viscoelastic properties which absorb energy that would otherwise radiate as noise. Combined with the unique structures in NORGLIDE® Bearings noise is kept to a minimum. It is because NORGLIDE® Bearings can be designed so that the shaft can be press fit into the bearing that rattle noise can be eliminated completely by eliminating clearance.
Using our state-of-the-art anechoic chamber, our engineers are able to perform benchmark testing to fully understand the current problems of the customer. A comparison test with NORGLIDE® Bearings then means that our customers can have confidence that our parts will give superior performance.
The chamber has a 7x7 meter space with a 20 dB noise floor so can be used to test most automotive assemblies, no matter how big or how quiet.
Traditional metal-to-metal joints can create vibration. Our solutions have a vibration damping effect due to the viscoelastic nature of PTFE, eliminating excessive vibration creating a smoother performance.
4 x Reduction – up to 15 db(A) – in noise compared to competing solutions under rattle test conditions.
Misalignment of an assembly can cause issues for manufacturers such as, increased assembly forces, excessive torque, and higher wear rates. These issues can diminish the perceived quality of a system.
An example of misalignment is found in the seat track mounting in the automotive industry. The tier 1 seat manufacturer could design a perfectly aligned seat, however the mounting holes in the car body in white can be misaligned which can distort the shape of the seat. Figure 5 shows a mechanism in which the distortion of the seat can produce misalignment between two housings that hold the seat cross-tube. Misalignment in this case can cause excessive torque and wear when the seat is adjusted.
Figure 5: left - the diagram above shows a mechanism of misalignment where two housings with a shaft running between them are misaligned. Right - when the shaft is rotated within the bearing, torque is produced. Misalignment between the two housings can result in higher torque values. Saint-Gobain engineers replicated the test as shown.
Saint-Gobain engineers replicated the test as shown in Figure 5. One of the housings was misaligned by varying amounts and the torque to turn the shaft was measured. This was performed on different plain bearing solutions. The resulting torques are shown in Figure 6. The plastic POM bushing showed a significant increase in torque, even at low misalignment values. The NORGLIDE® Bearings show a great improvement in torque consistency throughout the misalignment range. Particularly, the NORGLIDE® MP material shows excellent misalignment compensation properties.
Where plastic bushings have a hard and unforgiving surface, our unique NORGLIDE® Bearings maintain their low friction properties of the PTFE, as shown previously, which helps to minimise issues caused by misalignment in assemblies.
Figure 6: The graphs above show the resulting torque under different values of misalignment between two housings. POM plastic bushings show a significant increase in torque, even at low value of misalignment. NORGLIDE® Bearings show far greater compensation for misalignment with the NORGLIDE® MP material performing best.
The NORGLIDE® Bearings brochure will give you further insights.
Find out more about NORGLIDE® PTFE-Bearing Compound.