September 15th, 2020
Does your application need plain bushings? When it comes to specifying the material for your bushings (aka bearings and sleeve bearings), think twice before plumping for plastic. Plain bearings are a simple-looking component, and unlike ball bearings, they contain no moving parts. However, the material has a huge impact on the bearing’s performance. Plastic bushings seem like the obvious solution: they are low-cost and off-the-shelf, and plastic has many operational advantages. Plastic bushings are self-lubricating and maintenance free, with a low coefficient of friction.
However, when you’re looking for the right bushings for high loads or high temperatures, quality, reliability and performance are everything. Don’t waste the time and skill that’s gone into the design of the mating components, by sticking the easiest and quickest option in your shopping cart.
We’ll take you through the 5 main reasons why plastic bushings may not be the best solution for your application – and suggest a superior alternative.
In the automotive industry, there’s been a huge shift towards quieter engines. A smooth, silent glide to work in the mornings is seen as a badge of quality. There’s a downside to a low-noise engine: every rattle, every tiny little tick or click, can be heard by the driver and passengers.
So, for automotive engineers, there’s now a drive to source the quietest components. A plastic sleeve bearing either has to be a precise fit to have the right clearance, or it can create a rattle that’s audible in the cab.
Precise dimensional tolerance is not a key feature of plastic bushings – and requires precision manufacturing. Where we all appreciate precision, it’s not always the most practicable solution. When it comes to clearance, there is the risk of movement between the metal shaft and the bushing, which creates an annoying rattle.
We’d always recommend PTFE bushings if you need silent operation, such as NORGLIDE®. The softer PTFE has noise-dampening properties, unlike harder plastic, reducing sounds considerably, even in a clearance situation with a metal shaft. 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.
Another advantage of PTFE as a bushing material is its ability to maintain consistent torque. It creates a more compliant sleeve bearing that is consistent throughout the range of tolerances that our customers’ mating components have. Because of this superior tolerance, you won’t have to go down the costly precision manufacturing route.
Compared with other bushings, NORGLIDE® bushings perform consistently over a wide range of dimensions. The layer of low-friction compound can compensate tolerances from the mating component, while ensuring consistent, smooth adjustments. In Figure 2 below, you can see how a plastic material bushing failed during the lab test after 0.06 mm interference.
The location tolerance of automotive components can cause misalignment. This is clearly illustrated by the mechanics of automotive seats, where something as simple as a plastic bushing can have a genuine and negative impact on the driver’s experience.
The mounting holes on the seat frame are made using a stamping process. This method is prone to variations in the holes’ positions. The cross tube aligns between the two sides of the seat (Figures 3 and 4), and so if these aren’t perfectly aligned, the torque increases. If the car manufacturer has used a plastic bushing, it will push against one side of the shaft causing an increase in pressure and higher friction. Trust us, the driver notices.
Figure 3: Mechanism of misalignment - side view
Figure 4: Mechanism of misalignment – top view
Our engineers at Saint-Gobain simulated this situation and tested different types of bushing material. Figure 5 shows the differences between a plastic bushing and various NORGLIDE® bearings. Higher misalignment shows a significant increase in torque with the plastic material, while the NORGLIDE® bearings show far greater compensation for misalignment. Their MP material performed the best.
Figure 5: Torque at different misalignments
Figure 6 shows a seat frame with the NORGLIDE® bearings highlighted in red. The magnification on the right shows the pivot points on the seat height adjuster and cross tube. NORGLIDE®’s consistent torque, reduced noise and reduced vibration provide superior comfort – and a better driving experience.
If you have a high load application, plastic materials have a limited function – unless they’re supported by a metal backing layer. Again, we’ll take a look at how this affects the seat height adjustment pivot points. Under heavy loads, plastic can crack and transmit pressures onto the mating components. As well as causing damage, this can result in an increase in friction.
NORGLIDE® bearings are supported by a metal backing layer. This spreads out the load, which protects the mating components as well as the bushing itself. Figure 7 demonstrates how the load on the bushings propagates through to the mating components, using finite element analysis results.
To get new products out at high speeds, complete assemblies of bushing and mating components are often painted together on assembly lines. This is done using e-painting (electrophoretic deposition) at high temperature. Efficient yes, effective no! The high temperatures can cause the metal mating components to expand and the bushing material to soften.
The result of this is a permanent thinning of the plastic, which leads to increased clearance and decreased torque levels. The predictable end result is the rattle we spoke of in point 1.
This can be solved by opting for a metal-backed bushing. A steel backing reduces the risk of plastic deformation as well as offering higher load capabilities.
When you’re specifying or selecting components, don’t compromise on service life and superior performance based on short-term benefits.
PTFE bearings are vastly superior to plastic across all applications. Abrasion resistant PTFE is a great all-round material for a range of components. It’s a solid lubricant that can be used in wet environments, has excellent chemical resistance, and is an insulator with a high tolerance to heat.
Saint Gobain’s engineering expertise at the design stage ensures your application will have the highest level of performance. All your efforts could be undone by cheap bushings, so build in quality bearings and mating components from the start.
Our engineers will work in close collaboration with you, making sure that every single specification is robust and reliable, down to the last piston ring. Contact us today to discuss your custom-made NORGLIDE® Bearings.