Does your application need plain bushings? When it comes to specifying the material for your bushings (aka plain bearings or sleeve bearings), think twice before plumping for plastic. 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.
Plain bearings are simple-looking components and, unlike ball bearings , they contain no moving parts. However, the material chosen has a huge impact on the bearing's performance. Plastic bushings seem like the obvious solution. For a start, they're low in cost and easily available as off-the-shelf products. Plastic also has many operational advantages. In particular, 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. Here's why…
1. Plastic bushings can be noisy
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. But there's a downside to a low-noise engine: every rattle, and 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. For a plastic sleeve bearing, a precise fit is needed if the right clearance is to be right – otherwise it can create a rattle that's audible in the cab.
Precise dimensional tolerance is not a key feature of plastic bushings. It's something that requires precision manufacturing. And while we all appreciate precision, it's not always the most practicable solution. When it comes to clearance, there's always a risk of movement between the metal shaft and the bushing – which creates that annoying rattle.
We'd always recommend PTFE bushings, such as the ones made from our Welfine material, if you need silent operation. The softer PTFE has noise-dampening properties, unlike harder plastic. This reduces sounds considerably, even in a clearance situation with a metal shaft. Figure 1 shows rattle noise generated in some laboratory tests.
Figure 1: Rattle noise generated at 2 mm clearance, with a 20 Hz excitation frequency at 40 m/s2 acceleration
Figure 2: Torque at different clearance
Figure 3: Mechanism of misalignment - side view
2. PTFE bearings offer superior tolerance compensation
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 applying to our customers' mating components. Thanks to this superior tolerance, you won't have to go down the costly precision manufacturing route.
Compared with other bushings, our plain bearings perform consistently over a wide range of dimensions. The layer of low-friction compound can compensate tolerances in the mating component, while ensuring consistent, smooth adjustments. In Figure 2, you can see how a plastic material bushing failed during the lab test after 0.06 mm interference.
3. Plastic bushings can misalign
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 significant 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 (Figure 3). If these aren't perfectly aligned, the torque increases. When the car manufacturer uses a plastic bushing, it pushes against one side of the shaft – causing an increase in pressure and higher friction. Trust us, the driver notices.
Our engineers have simulated this situation and tested different types of bushing material. Figure 4 shows the differences in torque results between a plastic bushing and bearings made from various Welfine materials. Higher misalignment produces a significant increase in torque with the plastic material, while our plain bearings show far greater compensation for misalignment. The Welfine material performed best of all.
4. Plastic bushings have a lower load capacity
If you have a high load application, plastic materials have limited functionality – unless they're supported by a metal backing layer. Again, let's look at how this affects seat height adjustment pivot points. Under heavy loads, plastic can crack and transmit pressures into the mating components. As well as causing damage, this may result in increased friction.
Our plain bearings made from Jiande Welfine are supported by a metal backing layer. This spreads out the load, which protects the mating components as well as the bushing itself. Figure 6 demonstrates, using finite element analysis (FEA) results, how the load on the bushings propagates through to the mating components.
Misalignment Plastic bushing
Figure 4: Torque at different misalignments
Figure 5: Automotive seat frame, showing (in red) the location of our plain bearings
5. Plastic bushings can fail after e-painting
To get new products out at high speeds, complete assemblies of bushings 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. This leads to a permanent thinning of the plastic, which in turn increases clearance and decreases torque levels. The predictable result, ultimately, is the rattle we spoke of in point 1.
The problem 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.
Conclusion: don't compromise on your bushing material
When you're specifying or selecting components, don't compromise on service life and performance for the sake of 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.
Our engineering expertise at the design stage ensures your application will enjoy 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 plain bearings.