Designing Plain Bearings and Wear Components Is Actually a Material Selection Process
When early man rubbed sticks together to create friction and heat to make a fire, they didn’t know that the science behind this would one day be called tribology. The same is true of the ancient Egyptians, who used lignum vitae as ore locks as one of the first self-lubricating plain bearings. Advancements in tribology, although not yet named, continued to be made throughout history, from Leonardo da Vinci’s studies in friction to the rise of gears and roller bearings during the first industrial revolution. The evolution of this scientific discipline continued, but it wasn’t until 1967 that tribology was formally defined as “the science and technology of interacting surfaces in relative motion.” It includes friction, corrosion, and lubrication. Today, engineering plastics play a significant role in tribological applications that require self-lubricating plain (or plane) bearing and wear components.
Plain-bearing designs must integrate load, speed, temperature, and various environmental conditions such as UV exposure, impact, vibration, and chemical contact. High-Performance Engineering plastics are ideal for meeting these requirements.
What is a Plain Bearing?
Plain Bearings come in all shapes and sizes. Known by many names, such as bushings, bearings, slide plates, flanged bearings, thrust washers, etc., they are used to control the rotation or sliding motion of a shaft or flat surface with minimal friction and to simultaneously “bear” the dynamic load. Most are designed to reduce friction at the point where two materials come in contact. They are used in a multitude of products, ranging from everyday items, such as automobiles, washing machines, and air conditioning units, to industrial applications, such as chemical pumps, railroad safety appliances, and food processing equipment.
Most bearings are either rolling element bearings or plain bearings. Rolling element bearings employ either metal ball bearings or cylindrical rolling elements to maintain dynamic separation between the inner and outer raceways. Plain bearings are typically designed from solid material containing no moving parts. They tend to be simple in design, less expensive, and carry higher loads than their rolling element counterparts.
What is Self-Lubrication?
Most plain bearings are self-lubricating and do so in one of two ways. The first method of self-lubrication is called “debris lubrication.” Where the two materials rub together, tiny particles exfoliate from the bearing itself and become micro ball bearings between the two dynamic surfaces. Due to its inefficient nature, materials that employ debris lubrication are reserved for low PV applications where the pressure is light and velocity low to near static.
The second method is called “material transfer lubrication” and is by far the more efficient of the two. This technology typically employs a filler material, such as PTFE (polytetrafluoroethylene), MoS2 (Molybdenum Disulfide), or graphite. One or more of these solid lubricants are added to the resin during the manufacturing process and is uniformly dispersed throughout the material. As in the case of a typical shaft and bushing application, as the shaft makes its first few rotations, the solid lubricant is transferred or ‘smeared‘ onto the mating surface and fills the microscopic recesses of the machined surface. Thus, becoming a self-lubricating, low-friction surface.
Benefits of Plain Bearings
When designers are faced with bearing design, Ensinger High Performance Plain Bearings often win out over rolling element bearings due to several reasons.
- They tend to be less expensive than rolling element bearings.
- They are self-lubricating, requiring little to no maintenance.
- They’re simple in design.
- Size for size, they have a higher load capacity than ball bearings.
- They can handle heavy, vibratory loads.
- They can survive under water and in aggressive chemical environments.
- They have predictable life cycles, which simplifies maintenance planning.
- They have a predictable wear pattern and a non-catastrophic failure mode.
The last point described above is critical, especially for food preparation and pharmaceutical equipment manufacturers. During use, a self-lubricating plain bearing will wear at an even, predictable rate. Unlike the catastrophic failure mode of a ball bearing, where the bearing seizes, comes apart, and the ball bearings end up in food and medicine products and/or containers. Not finding all the ball bearings could result in a costly event.
The Material Selection Process
The material selection process is one of the most critical elements in plain bearing design. The designer must consider many factors, the five most basic of which are the following:
- Bearing Pressure (P)– What is the load on the bearing?
- Relative Velocity (V) – How fast are the parts rubbing against each other?
- Service Temperature – What is the ambient temperature in use?
- Temperature Variations – Will the ambient temperature fluctuate?
- Environmental Conditions – UV, chemicals, water, debris, food contact?
Pressure (P)
It’s essential to know the load each bearing must handle in pounds per square inch (lb./square in.) since every material has a minimum static pressure rating (P). Pressure (P) is calculated by dividing the load in pounds by the projected area of the bearing in square inches. The projected area is calculated by multiplying the inner diameter (ID) in inches by the length of the bearing in inches.
Velocity (V)
Equally essential is knowing the velocity at which the two mating surfaces are traveling against each other since every material has a maximum no-load velocity rating (V). Velocity (V) is calculated by taking the shaft circumference in feet (ft) and multiplying it by the rotations per minute (RPM). The product is the distance (in feet) that a point on the shaft has traveled in one minute.
PV (Pressure X Velocity)
We then multiply pressure (P) by the velocity (V), which gives us P x V or PV. These combined factors create frictional heat and must be considered in tandem since every material has a maximum PV limit. Think of the example of rubbing two sticks together. With enough pressure and velocity, you can generate enough heat to start a fire. Frictional heat is the enemy of plain bearings because excess heat causes premature wear. Therefore, choosing a material that can withstand the temperature in operation is critical for a successful application.
P, V, and PV Summary
It is important to consider each of these factors individually since every material has a maximum pressure (P) limit, every material has a maximum velocity (V) limit, and every material has a maximum combined pressure x velocity (PV) limit.
Temperature Considerations
Continuous service temperature is also critical since every material has a maximum continuous service temperature. For example, if a bearing is going to be used in a high-temperature application (such as an oven conveyor bearing), the material must withstand not only the oven temperature but the additional frictional heat as well (P x V). Designers must also consider fluctuating temperatures as materials will expand and contract with changes in temperature. Not all materials, however, have the same rate of thermal expansion (coefficient of linear thermal expansion or CTLE). Which means that they do not expand and shrink at the same rate. The changes in size could result in excessive clearance between the shaft and the bearing, or the opposite could occur where the bearing closes on the shaft, causing bearing seizure.
Cost Considerations
It is important to note that some ultra-high-performance materials can be expensive. That is why they are only used when the application demands their capability. Conversely, materials with excess capability (higher P, V, and PV) typically result in a factor of safety and/or longer bearing life. For example, if use is intermittent, the system has time to cool down between cycles, and therefore a lower-rated material (lower PV rating) could then be used. In that case, it might then be possible to focus on one factor, such as pressure (P).
P, V, and PV Comparison
Clearly, considering as many factors as possible is essential during the material selection process. Listed below are just some high-performance and engineering plastics used as plain bearings. These are listed in descending order of Max PV. Please note that they are not in descending order of pressure (P) or velocity (V) capability.
| Material | MAX PV | MAX P | MAX V | Cont. Service Temp (F) |
|---|---|---|---|---|
| TECASINT XP-130 /Graphite Polyimide | 300,000 | 6,000 | 1,000 | 650 |
| TECASINT XP-132 /Graphite & PTFE filled Polyimide | 250,000 | 6,000 | 500 | 550 |
| TECAPEEK XP-99 / Bearing Grade filled PEEK | 100,000 | 4,500 | 750 | 480 |
| TECAPEEK PVX / Bearing Grade filled PEEK | 100,000 | 4,500 | 750 | 480 |
| TECAPEEK XP-92 / 15% CF filled PEEK | 50,000 | 1,500 | 400 | 450 |
| TECATRON PVX / Bearing Grade filled PPS | 50,000 | 20,000 | 500 | 425 |
| TORLON 4301 / Graphite Filled Polyamide-imide | 50,000 | 1,000 | 900 | 500 |
| POLY-TEXX HPVT / Bearing Grade Composite | 40,000 | 34,000 | 50 | 266 |
| HYDEX 4101L / PTFE filled PBT / FDA / USDA | 7,500 | 1,000 | 200 | 245 |
| DELRIN AF / PTFE filled Delrin | 7,500 | 1,000 | 100 | 180 |
| HYDLAR Z / Aramid Fiber Filled Nylon 66 | 7,500 | 1,500 | 100 | 230 |
| HYDEX 4101 / PBT / Chemical Resistance | 6,000 | 1,000 | 100 | 245 |
| NYLON / TECAMID 6/6 | 2,700 | 300 | 60 | 210 |
| UHMW | 1,000 | 800 | 50 | 180 |
Need Help? – Trust the Experts
Our engineering staff stands ready to help you through the material selection process. We have the application knowledge and experience to design and manufacture cost-competitive plain bearings that will meet your needs. We will take the time to understand your application to ensure we can provide you with a material that will withstand the rigors of use and deliver the longest life possible.
Contact us today to start the conversation!