Plastic bearings are integral components in various applications, offering advantages such as reduced weight, corrosion resistance, and the potential for self-lubrication. Designing plastic bearings requires careful consideration of multiple factors to ensure optimal performance and longevity.
1. Bearing Load (Pressure, P)
The load applied to a bearing is expressed as pressure (P), measured in pounds per square inch (psi). It's calculated by dividing the load (in pounds) by the projected area of the bearing (in square inches). The projected area is determined by multiplying the bearing's inner diameter by its length. Understanding the bearing pressure is crucial, as it influences the bearing's capacity to withstand applied loads without deforming or failing.
2. Shaft Speed (Velocity, V)
Shaft speed, or surface velocity (V), is the speed at which the bearing's surface moves relative to the shaft. It's expressed in feet per minute (FPM) and calculated using the formula:
V=Shaft RPM×Shaft Diameter (in inches)×0.262V
This calculation helps in assessing the bearing's ability to handle dynamic loads and dissipate heat generated from motion.
3. PV Value (Pressure-velocity Product)
The PV value represents the combined effect of pressure and velocity on bearing performance. It's calculated by multiplying pressure (P) by surface velocity (V):
PV=P×V
Operating a bearing beyond its specified PV limit can lead to premature failure due to excessive heat generation and material degradation. Therefore, it's essential to select materials and design parameters that keep the PV value within recommended limits.
4. Running Clearance
Plastic bearings typically require greater running clearances than metal bearings, primarily due to their higher coefficient of thermal expansion. This means they expand more with temperature changes, necessitating additional clearance to prevent binding or seizing. As a guideline, the diametral clearance between an assembled shaft and bearing should be between 0.3% and 0.5% of the shaft diameter, ensuring smooth operation across varying temperatures.
5. Shaft Material and Hardness
The material and hardness of the shaft significantly influence bearing performance and wear rates. Shafts made from soft metals like mild steel or non-ferrous materials such as aluminum or brass may not pair well with plastic bearings, even those with friction-reducing additives. Harder shafts, particularly those case-hardened to Rockwell C60 with a fine surface finish, are recommended to minimize wear and extend bearing life.
6. Operating Temperature
Operating temperature affects both the bearing material and the lubricant (if used). Plastic bearings have specific temperature limits, and exceeding these can lead to material degradation, dimensional changes, or loss of mechanical properties. It's crucial to select bearing materials that maintain their integrity under the expected temperature ranges of the application.
7. Lubrication
While some plastic bearings are designed to operate without external lubrication, others benefit from it. Lubrication reduces friction, minimizes wear, and can enhance the bearing's load-carrying capacity. However, the choice between dry operation and lubricated operation should be based on the specific application requirements, environmental considerations, and maintenance capabilities.
8. Housing Material
The material of the bearing housing plays a role in heat dissipation and overall bearing performance. Housings with good thermal conductivity help dissipate heat generated during bearing operation, preventing overheating and maintaining optimal operating conditions.
9. Duty Cycle and Environmental Factors
Understanding the bearing's duty cycle—how frequently and for how long it operates under load—is essential for selecting appropriate materials and design parameters. Environmental factors such as exposure to chemicals, moisture, UV radiation, or food-grade requirements also influence material selection and design. For instance, certain polymers offer excellent chemical resistance, making them suitable for harsh chemical environments.
10. Bearing Geometry and Design Features
Incorporating design features such as axial grooves or through-holes in the bearing can enhance performance by providing pathways for wear debris to escape, reducing the risk of abrasive wear. Grooves should be deep enough to accommodate wear particles and wide enough to prevent clogging. Typically, the groove width is about 10% of the shaft diameter, and at least three grooves are recommended for effective debris removal.
Conclusion
Designing plastic bearings necessitates a comprehensive understanding of material properties, operational conditions, and application-specific requirements. By carefully considering factors such as load, speed, clearance, material compatibility, temperature, lubrication, housing, duty cycle, environmental conditions, and bearing geometry, engineers can optimize bearing performance, enhance service life, and ensure reliability in diverse applications.
