POM (Polyoxymethylene) has established itself as a go-to material for bearing applications in modern engineering. Whether you’re designing new components or looking to upgrade existing ones, understanding these essential properties will help you make informed decisions about using POM in your bearing applications.
POM excels in bearing applications due to four essential properties: excellent self-lubrication characteristics, superior wear resistance with specific PV value ratings, broad chemical resistance, and outstanding mechanical properties under load conditions. These properties make it an ideal choice for both dry-running and lubricated bearing systems.
Let’s explore why these four properties make POM the preferred choice for many bearing applications, from simple bushings to complex bearing systems. Whether you’re an engineer, designer, or manufacturer, understanding these properties will help you optimize your bearing designs.
Table of Contents
1. Self-Lubrication Properties
POM’s ability to operate with minimal or no external lubrication sets it apart in bearing applications. This property comes from its specific molecular structure, which provides a low coefficient of friction – typically ranging from 0.15 to 0.35 depending on operating conditions. Think of it as having a built-in lubrication system that works continuously throughout the bearing’s life.
As the bearing surface experiences friction, POM forms a transfer film that acts as a solid lubricant. This molecular-level process is reflected in its measurable performance:
– Static friction: 0.20-0.25 against steel
– Dynamic friction: 0.15-0.20 in dry running conditions
– Speed limits: Up to 3 m/s in continuous operation
– PV value: Maximum 0.32 MPa × m/s for continuous operation
The practical impact of these properties makes POM particularly effective in:
– Food processing equipment where external lubricants are restricted
– Medical devices requiring clean operation
– Hard-to-reach mechanisms where maintenance is difficult
– High-precision equipment requiring consistent performance
– Conveyor components
– Gear systems
– Sliding elements
– Cam mechanisms
Operating Condition Friction Coefficient Maximum Speed Benefits
Dry Running 0.15-0.20 3 m/s Reduced maintenance
Light Lubrication 0.10-0.15 5 m/s Extended life
Water Lubrication 0.05-0.10 10 m/s Maximum performance
Pro Tip: For optimal performance, maintain operating temperatures below 90°C to preserve POM’s friction properties. While POM performs well without lubrication, in high-load applications, a small amount of initial lubrication can significantly extend bearing life.
2. Wear Resistance and PV Value
Think of wear resistance like the durability of your car’s tires. Just as tires gradually wear down from constant contact with the road, bearing materials wear down from continuous operation. Wear resistance measures how well a material maintains its original shape and dimensions under this constant mechanical stress and friction. In bearing applications, better wear resistance means longer service life and less frequent replacements.
Evidence of POM's Superior Wear Resistance
Performance Data:
– Maximum wear rate: 20 μm per 100 km sliding distance under dry running
– Continuous service temperature: -50°C to 90°C (-58°F to 194°F)
– Shaft hardness requirement: Minimum 45 HRC
– Optimal surface roughness: Ra 0.2-0.8 μm
Wear Performance Under Different Conditions
Operating Condition Wear Rate Maximum Speed
Dry Running 20 μm/100km 3 m/s
Water Lubricated 5 μm/100km 10 m/s
Oil Lubricated 2 μm/100km 5 m/s
Comparison with Other Materials:
POM shows superior wear resistance compared to:
– 3x better than standard thermoplastics
– Comparable to bronze in lubricated conditions
– Excellent performance against hardened steel
What is PV Value?
Think of PV value like the speed limit for your car under different road conditions. Just as you need to adjust your driving speed based on road conditions and vehicle load, PV value tells us the safe operating limits for bearings. It combines two critical factors: Pressure (how much load the bearing carries) and Velocity (how fast it moves). This value helps engineers determine if POM can handle specific operating conditions safely and effectively.
Evidence of POM's PV Value Performance
Performance Limits:
– PV value for continuous operation: 0.32 MPa × m/s
– Maximum static load: 80 MPa
– Typical operating pressure: 20-35 MPa
– Maximum sliding velocity: 3.0 m/s (dry)
Operating Conditions Comparison
Condition PV Value (MPa × m/s) Operating Pressure (MPa) Max Velocity (m/s)
Dry Running 0.32 20-35 3.0
Light Oil 0.45 35-45 5.0
Water Lubricated 0.90 45-55 10.0
These values demonstrate POM’s versatility in bearing applications, particularly when:
– Continuous operation is required
– Variable loads are present
– Speed fluctuations occur
– Different lubrication conditions exist
3. Chemical Resistance
Think of chemical resistance like your kitchen countertop’s ability to withstand various cleaning products and food spills. In bearing applications, chemical resistance determines how well a material maintains its properties when exposed to different substances, from lubricants to harsh cleaning agents. This property is crucial because bearings often operate in environments where they contact various chemicals.
Evidence of POM's Chemical Resistance
Performance Data:
– Operating temperature range in chemicals: -50°C to 90°C
– Water absorption: < 0.5% after 24 hours
– Resistant to most solvents at room temperature
– Excellent resistance to hydrocarbons and cleaning agents
Resistance Performance Table:
Chemical Environment Resistance Level Max Temperature (°C) Notes
Hydrocarbons Excellent 90 No significant effect
Weak Acids Good 60 Minor surface effect
Weak Alkalis Good 60 Minor surface effect
Strong Acids Poor 20 Not recommended
Industry Applications and Benefits:
Industry Environment POM’s Advantages
Food Processing Cleaning agents, oils FDA compliant, resists sanitizers
Automotive Fuels, lubricants Maintains properties, no swelling
Medical Sterilization processes Biocompatible, chemical stable
Industrial Coolants, oils Long-term stability, low absorption
Chemical Processing Various chemicals Predictable performance, good stability
4. Mechanical Properties
In bearing applications, mechanical properties are the fundamental characteristics that determine a material’s ability to perform under real-world conditions. These properties encompass strength, stiffness, and durability – all crucial factors that influence how well a bearing handles loads, impacts, and repeated stress cycles throughout its service life.
Evidence of POM's Mechanical Performance:
Key Strength Values:
– Tensile strength: 67 MPa
– Compressive strength: 80 MPa
– Flexural modulus: 2,800 MPa
– Impact strength (Charpy): 8 kJ/m²
Performance Under Different Conditions:
Property At 23°C At 60°C After Moisture
Tensile Strength (MPa) 67 45 65
Elastic Modulus (MPa) 2,800 1,900 2,700
Impact Strength (kJ/m²) 8 12 7.5
Long-Term Performance:
– Creep resistance at 1000 hours: <2% under 20 MPa load
– Fatigue strength: 30 MPa at 10⁶ cycles
– Dimensional stability: ±0.2% over temperature range
– Stress relaxation: <15% after 1000 hours
Application-Specific Benefits:
Application Critical Property Performance Benefit
High Load Bearings Compressive Strength Maintains shape under pressure
Continuous Operation Fatigue Resistance Extended service life
Precision Components Dimensional Stability Consistent performance
Dynamic Loads Impact Strength Reliable under stress
Conclusion
POM’s success in bearing applications stems from the synergy of these four essential properties. Each property contributes uniquely to make POM an exceptional bearing material:
Key Property Benefits:
– Self-lubrication provides reliable performance with minimal maintenance
– Wear resistance and PV values ensure long service life
– Chemical resistance enables use in demanding environments
– Mechanical properties deliver consistent performance under load
Whether you’re designing bearings for food processing equipment, medical devices, or industrial machinery, POM offers a unique combination of properties that make it an ideal choice for demanding applications.
Frequently Asked Questions
POM offers comparable performance to bronze in many applications while providing additional benefits like self-lubrication and chemical resistance. It particularly excels in clean environments and where maintenance access is limited.
POM’s low moisture absorption (< 0.5% after 24 hours) ensures dimensional stability and consistent performance even in wet environments. Water can act as a lubricant, potentially improving PV values.
Hardened steel shafts with minimum 45 HRC hardness and surface roughness Ra 0.2-0.8 μm provide optimal performance. Proper shaft specifications help maximize wear resistance and bearing life.
While POM performs well without lubrication, additional lubrication can enhance performance in high-load applications or when maximum PV values exceed 0.32 MPa × m/s in continuous operation.
Proper operating temperature, appropriate PV values, and correct shaft specifications are crucial. Regular monitoring of wear patterns and maintaining clean operating conditions also contribute to longer service life.
POM bearings perform optimally between -50°C to 90°C. Above 90°C, mechanical properties begin to degrade, potentially affecting bearing performance and longevity.
