- tags: CNC MTM #Making Prototypes
- General Design principles for Engineering Polymers -Dupont Principles for plain bearings Delrin
- Wear resistant plastic for bearings
- https://www.professionalplastics.com/BearingWearResistantPlastics Huge library of wear resistant plastic.
- Linear motion tip guide to plastic bearing
- 2:1 rule for defining fixed and floating bearing plain barings for cantilever For a cantilevered load in a 2 rail 4 bering system the force applied should not be at more than 2 times the length of the bearing. The rail closes to the cantilever load should be the fixed and the other side should be floating.
- Guide to plain bearing materials
- Plastic bearing materials
- Plastic bearing properties and selection
- Delrin design guide Delrin #Acetal (Delrin)
- Delrin deign guide dupont Mod3
- **Surface hardness and finish of the metal shaft - ** Harder the shaft the better the wear characteristics. Dlerinc can be used on aluminium and hardened brass shafts with initial lubrication, but with more wear.
- Geometric accuracy of bearing bore - to prevent imperfect contact.
- Correct clearance- Clearance should be at least 2-4%of diameter.
- Grooves or holes in the sliding surface- to prevent contamination of the rubbing surface and clearance for collecting debris.
- Initial lubrication- provides the best conditions for the run-ins and set the bearing for dry running later.
- Means to protect the bearing against dirt penetration and to retain the lubricant.
Types of plastics
- The most commonly used are phenolics, acetals, Teflon (PTFE), ultra high molecular weight polyethylene (UHMWPE), and nylon. The major limitations involved in the use of plastics have to do with high temperatures and possible cold flow under heavy loads.
- Phenolic Laminate The compatibility of the phenolics makes them easily lubricated by various fluids. They have replaced metal bearings in applications as propeller and rubber-shaft bearings in ships and electrical switch-gear, rolling-mill, and water-turbine bearings. In small instruments and clock motors, laminated phenolics serve as structural members as well as a bearing material. They have excellent strength and shock resistance coupled with resistance to water, acid, and alkali solutions. Phenolic plastic bearings work well in heavily loaded systems provided sufficient clearance and cooling is provided.
- Nylon Although the phenolics have predominated in heavy-duty applications, they are frequently replaced by nylon, which has the widest use in plastic bearings. Nylon bushings exhibit** low friction and require no lubrication. Nylon is quiet in operation, resists abrasion, wears at a low rate, and is easily molded, cast, or machined to close tolerances. Improvement in mechanical properties, rigidity, and wear resistance is obtained by adding **fillers such as graphite and molybdenum disulfide to nylon. While the maximum recommended continuous service temperature for ordinary nylon is 170°F, and 250°F for heat-stabilized compositions, filled-nylon parts resist distortion at temperatures up to 300°F. As mentioned in some of our other posts, nylon is also very inexpensive.
- Teflon (PTFE): It has an exceptionally low coefficient of friction and high self-lubricating characteristics, immunity to almost all types of chemical attack, and ability to operate over an extremely wide temperature range (-330 to 360°F continuous, to 550°F short-term). Normally very soft and usable directly for light loads, PTFE’s load capacity depends on construction and reinforcing material. PTFE is a great choice for applications that include exposure to weather, chemicals, or vapors which can attack metals, lubricants, and some plastics. Teflon bearings are also ideal for applications like sluice gates that involve the need to operate smoothly, reliably, and without sticking after prolonged idle periods. Other applications for PTFE include those with low rpm, oscillatory or intermittent service, or where reliable service without lubrication is vital. The major drawback to using Teflon is that the cost of PTFE is high relative to plain metal or other resins.
Acetal (Delrin): Delrin is a popular material for inexpensive bearings in a wide variety of automotive, appliance, and industrial applications. Delrin is particularly useful in wet environments because of its stability and resistance to wet abrasion.
- Bearings of DELRIN® acetal resin offer the unique characteristic of no ‘‘Slip-stick’’, or the static friction coefficient being equal or lower than dynamic
- Ultrahigh-molecular-weight polyethylene (UHMWPE): UHMWPE bearings resist abrasion and have a smooth, low-friction surface. UHMWPE is often an ideal replacement material for parts typically made from acetal, nylon, or PTFE materials.
- Polyimide, polysulfone, polyphenylene sulfide: These are high-temperature materials with excellent resistance to both chemical attack and burning. With suitable fillers, these moldable plastics are useful for PV factors to 20,000 to 30,000. Polyimide molding compounds employing graphite as a self-lubricating filler show promise in bearing, seal, and piston ring applications at temperatures to 500°F.
Stick Slip #Plastic bearing
- When two objects slide against each other, friction works against their motion. But there are two types of friction: static (also referred to as breakaway) and kinetic (also referred to as dynamic). At a fundamental level,** static friction is caused by the molecular bonding that occurs when two surfaces are in contact. Kinetic friction is primarily caused by surface roughness, which impedes the motion of the two bodies relative to each other.**
Static friction increases with time of contact. In other words, the longer two surfaces have been in contact at rest (not moving), the higher their static friction will be. Kinetic friction has been found to be mostly constant regardless of velocity, although some variation can occur at very slow speeds. (The velocity of the surfaces determines how long any two areas on the surfaces are in contact with each other.)
- The relationship between friction and velocity for lubricated surfaces (as with recirculating bearings) is shown on a Stribeck curve. The thickness of the lubrication film is an important parameter in friction, and there are three regions of lubrication in which bearings can operate:
- Boundary Lubrication—where friction is dominated by surface properties
- Mixed Lubrication—where friction is affected by both surface properties and the properties of the lubricant, and is also dependent on speed
- Hydrodynamic Lubrication—where friction is dominated by the viscosity of the lubrication film
- Stick slip is caused by the transition from higher static friction to a lower dynamic friction which can cause the motion system to overshoot its target and create position errors.