September 15th, 2020
When engineers are specifying bearings, there are broadly two main types to choose from: roller bearings and plain bearings. Most people understand what a roller (or ball) bearing is, but struggle by the different kind of plain bearings. Most plain bearings are metal or plastic in the required shape. Some plain bearings are more sophisticated and made from a backing material (metal) and a polymer compound layer. Plastic bushings are a tempting choice as a low cost, off-the-shelf product. Plastic has the same advantages, as other materials used for plain bearings; It is self-lubricating, maintenance-free, wear resistant and has a low coefficient of friction. But injection molded plastic has limitations in key performance areas that many applications require. This is critical when so much time and effort is given to the design of the mating components. The nature of plastic material can be unforgiving. In some cases, it requires very precise dimensional tolerances of mating components. This article lists 5 of the main reasons that plastic bushings might not be the best solution for your application.
In the automotive industry there has been a huge shift towards quieter engines which have made the journey to work in the morning so much more pleasant. As engines have become quieter, noises that were once hidden by the engine can now be heard by the occupants. Because of this, detailed design work is needed even for the smallest of components.
Plastic bushings are less compliant than PTFE bearings and cannot be used in interference conditions. This means that plastic bearings have to be used in either clearance or with an exact fit, which requires precision manufacturing of mating components. In a clearance condition, there is a risk of relative movement between the metal shaft and the bearing. This can create an annoying rattle noise. The more compliant PTFE bearings (for example NORGLIDE®), can be used in interference conditions preventing rattles from occurring. Even if the NORGLIDE® bushing is used in clearance conditions, the rattle noise is far less as the metal shaft hits against the soft PTFE compound rather than against the harder plastic. Due to the superior damping properties of PTFE, the noise is reduced and the quality of noise is increased. Figure 1 shows the rattle noise generated based on laboratory testing at 2 mm clearance, with a 20 Hz excitation frequency at 40 m/s2 acceleration.
A more compliant sleeve bearing brings further advantages over plastic bearings. One advantage is the ability to maintain a consistent torque throughout the range of tolerances that our customers’ mating components have. Figure 2 shows the change in torque over a range of interference conditions. There is a clear advantage of choosing a PTFE bearing such as NORGLIDE®, as it can compensate a greater tolerance range. This gives the ability to reduce or eliminate costly precision manufacturing of components.
NORGLIDE® Bearings perform consistently over a wide range of dimensions, compared to other bushings. The layer of a low-friction compound can compensate tolerances from mating components and it ensures a consistent, smooth adjustment feeling. In the Figure 2 below, you can see how a plastic bushing failed during the lab test after 0.06 mm interference.
Location or position tolerances of automotive components can cause misalignment. One example is the automotive seat. A stamping process is used for the mounting holes on the frame structure. The variations during the stamping process lead to a variation in the position of the holes. The cross tube rotates between the two sides of the seat, as shown in Figures 3 and 4. If the two sides are not aligned, the torque increases. A plastic bushing, having less compliance, will push against one side of the shaft and this causes an increase in pressure and higher friction. That increase in torque is felt by the end-user during an adjustment of the seat. Saint-Gobain engineers simulated this situation and tested different bearings.
Figure 3: Mechanism of misalignment - side view
Figure 4: Mechanism of misalignment – top view
Figure 5 shows a mechanism of misalignment, with the shaft running between two misaligned housings. The shaft rotates within the bearings and creates a torque. Higher misalignment between the two housings results in higher torque values. Figure 5 shows the torque results under different values of misalignment. A POM bushing shows 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.
Figure 5: Torque at different misalignments
Figure 6 shows an automotive seat frame with the location of NORGLIDE® Bearings in red. The magnification on the right shows the pivot points on the seat height adjuster and cross-tube. NORGLIDE® provides superior comfort due to consistent torque and reduced noise and vibration. And can make a big difference to the overall driving experience.
Without the support from a metal backing layer, plastic materials have limited function in high load applications. Examples are bearings in seating applications, like the seat height adjustment pivot points. If the load becomes too high, then the plastic can crack and will transmit similar pressures onto mating components. This causes damage or an undesirable increase in friction. Conversely, a high load on a NORGLIDE® sleeve bearing can be supported by the metal backing layer and that load can be spread out to protect mating components as well. Figure 6 shows how the load on the sleeve bearings propagate through to the mating components, using finite element analysis results.
E-painting (electrophoretic deposition) is used to paint complete assemblies faster, containing the assembled bushing and mating components. This process is done at high temperature. The high temperature causes the metal mating components to expand and the bushing material to soften. This results in a permanent thinning of the plastic and increased levels of clearance and it causes a reduction of torque and an increase in rattle noise after painting. With a metal-backed plain bearing, the level of plastic deformation is reduced, and will result in more consistent performance overall.
Achieving a high quality perception for your applications involves detailed design work to ensure your components can achieve the desired performance. This effort can be undone by bushings that have not gone through the same detailed design process. Therefore, consider the mating components from the start.
Saint-Gobain engineers work in close collaboration with you, to get the highest levels of performance. This way your custom-made NORGLIDE® Bearings make a big difference with your applications in mind.