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Coefficient of friction plastics

Obturate

Aluminum
Joined
Jun 17, 2010
Location
NH
Howdy all. I'm looking for a table that lists the coefficient of friction of numerous plastics. I'm designing a part that slides in an anodized aluminum bore. It has a plastic brake that expands in the bore slowing the part's rebound off a spring. The plastic would need good wear resistance as well as some friction against the aluminum.
Delrin has the right wear and hardness but is too slippery.
Thanks,
Dan
 
interesting problem. I had a similar situation that I considered various things for but the project was killed so I did not try the ideas. One is cast urethane, there a very wide range of properties available in urethane and it can be molded in silicone rubber molds. One approach I looked at was a HDPE shoe for sliding and a molded insert of high drag urethane for stopping. THe other is friction grease, it is what is used in some "slow" hinges.
 
From the Graingers product selection website, maybe it will help narrow your search. Also this is a chart for just straight friction properties ---> http://www.dotmar.com.au/co-efficient-of-friction.html

ABS: (Acrylonitrile Butadiene Styrene) Provides good thermal and dimensional stability, as well as good scratch resistance. Therrnoforrnable haircell sheets have a textured surface that is excellent for hiding cuts and scratches. Uses: Ideal for machined, flat, and vacuum-formed parts in the food and refrigeration industries. Used to fabricate furniture, workstations, displays, guarding, and partitions.

Acetal Copolymer: Stiff, high-strength material is easy to machine, with enhanced dimensional stability, low coefficient of friction, and excellent wear properties, especially in wet environments. Uses: For scraper blades, industrial bearings, electrical components, food processing applications, medical applications, and other applications that require close tolerances.

Cast Nylon 6: Unmodified cast type 6 nylon is the strongest and hardest of all the nylon 6 grades.

Cell Cast Acrylic: Provides the best optical quality, highest long-term design stress, and highest chemical resistance of any acrylic sheet, as well as superior weatherability and ease of fabrication.

CPVC: (Chlorinated Polyvinyl Chloride) Similar to PVC, with greater flexibility and increased chemical and heat resistance for use in more demanding environments. Uses: Pipe systems, lamp housings, and parts for the dental industry.

Extruded Acrylic: Sheets have a high molecular weight that lends itself well to thermoforming, bending, and flame polishing. Also provides excellent weather and impact resistance and low heat loss. Uses: Commonly used in fabrication, picture framing, skylights, displays, signage, and lighting applications.

Extruded Nylon 6/6: Food Contact Grade. Ultra-high performance, bearing-grade PA6 material provides superior wear resistance and load bearing capability. Uses: Commonly used for smaller machined parts, such as electrical insulators and food contact parts.

HDPE: (High-Density Polyethylene) Has good insulating properties, is easy to weld, and retains toughness at low temperatures. HDPE has low steam permeability, good electrical and dielectric properties. Uses: Ideal for use in pumps, valves, and in the medical and sewage industries.

LDPE: (Low-Density Polyethylene) Extremely flexible low-density polyethylene is easy to clean, lightweight, and doesn't absorb moisture. Uses: Commonly used for fabricated parts in which chemical and corrosion resistance is required. Also used for impact pads and thermoforming applications.

PAI: (Polyamide-imide) Offers excellent compressive strength and high impact strength. Available in Electrical Grade and Bearing Grade.

PEEK: (Polyetheretherketone) Used as an alternative to fluoropolymers in hostile environments, PEEK has good chemical, hydrolysis, and high-temperature resistance. It can also be used continuously in hot water or steam without permanent loss in physical properties and has very low smoke and toxic gas emission when exposed to flame. Uses: For structural parts, bushings, bearings, seals, and backup rings in aggressive chemical and high-temperature environments.

PEI: (Polyetherimide) Food Contact Grade Amorphous, high-performance copolymer has exceptional flame and heat resistance and maintains consistent dielectric properties over a wide frequency range. It is highly resistant to hydrolysis and acidic solutions, and capable of withstanding multiple autoclaving cycles. Uses: Structural components for high-performance applications such as analytical instrumentation and electrical/electronic insulators.

PET-P: (Polyester) Food Contact Grade Polyethylene terephthalate polyester has the best dimensional stability of any plastic, as well as excellent wear resistance and a low coefficient of friction. A melting point of nearly 150 degrees above that of acetals makes it suitable for use in precision mechanical parts for high-load and high-wear applications. Uses: For scraper blades, industrial bearings, electrical components, food processing, and other applications requiring close tolerances.

PETG: (Copolyester) Food Contact Grade Glycol-modified polyethylene terephthalate can be vacuum-formed to high definition and excellent cold temperature impact strength. Accepts surface printing with offset or screen printing techniques. Uses: Typical applications include videotape shelves, revolving merchandise racks, indoor signs, photo frames, and point-of-purchase, slat-wall inventory, greeting card, and menu displays.

Polycarbonate: General Purpose-Naturally transparent, crystal-clear sheets have excellent aesthetic properties, high impact strength, and good formability. Gray and bronze tinted sheets provide 50% less light transmission. Uses: Typical applications include glazing and security, as well as injection molding and sheet film applications, including lenses, compact discs, visors, and instrument panels. Machine Grade-Thicker sheets have a higher tensile strength and maximum temperature for improved formability.

Polypropylene: Food Contact Grade-Polypropylene is frequently machined into parts requiring good chemical resistance. It has excellent electrical properties and little or no moisture absorption. This plastic also has the ability to carry light loads for a long period in widely varying temperatures. Uses: For parts requiring good chemical or corrosion resistance, food packaging, laboratory equipment, automotive components, valves, and fittings.

PVC Type I: (Polyvinyl Chloride) Rated as self-extinguishing, PVC Type I has exceptional chemical resistance, good insulation properties, low moisture absorption, and high mechanical and tensile strengths. Can be easily cemented and welded. Uses: Ideal for use in structural building materials, plumbing applications, electrical insulation, signage, and modeling.

UHMW: (Ultra-High Molecular Weight Polyethylene) Ideally suited for wear applications, especially in contact with metals. Can be easily machined with ordinary woodworking tools. Material is moisture, odor, and stain resistant. Uses: Widely used in industrial applications such as chute linings, dock bumpers, paper machine parts, and conveyor wear strips.

UHMW-PE: (Ultra-High Molecular Weight Polyethylene) Ideally suited for wear applications, especially in contact with metals. Can be easily machined with ordinary woodworking tools. Material is moisture, odor, and stain resistant. Uses: Widely used in industrial applications such as chute linings, dock bumpers, paper machine parts, and conveyor wear strips.
 
Total braking force will involve coefficient of friction, and force normal to the internal tube surface. Coefficient of friction depends on both materials, so the anodized aluminum will affect the coefficient, and I couldn't find any tabulations of plastic friction on anodized aluminum. But if you have control of the part design, perhaps internal springs could increase the force against the tube surface?

The coeffiecient will change as the anodization wears off.

If you can figure a way to increase the normal force pressing your brake part against the tube, Delrin(TM) sounds ideal - it's an impressive material.
 
using friction to slow coasting things down is a stable system and easily tuned.
using friction to slow something which is being pushed is not.

I'm not aware of any "good" rebound damping systems that use plastic on bare aluminum sliding friction to slow down. if the bore can be closed off a sized air bleed hole can give really consistent tunable damping. you can use a check valve so it will only damp in one direction.
 
Dan, I don't have much experience with them (except as a consumer), but you should investigate damping greases. (Probably same thing as surplusjohn called friction greases.) These are selected primarily for their shear resistance properties and are frequently used to give a "silky smooth" feel, damped action and/or noise reduction to devices with rather loose tolerances. They also lubricate, so wear on the solid components is minimal.
 
Thanks for the help everyone.
This is for a hammer in an air rifle. I've made many and used abs plastic. I just made one for a new rifle model. The surface area of the brake is much reduced and I think it will work better if I could find a stickier material.
I can change the geometry of the parts to increase braking force as well.
Dan
 
TDegenhart, I agree. You cannot count on friction to control motion. Tolerances, wear, oil/grease build up, moisture, etc all affect the amount of friction.

Obturate, regarding the chart for coefficient of friction in plastics, here is the one I use: Plastic Selection.
While data sheets are fine, this chart makes it easy to compare multiple materials at a glace - in several different physical properties. Hope it helps.
 
what I was trying to do with my sliding braking part was to use a slippery shoe to allow it to slide but when it was suppose to stop, the sticky urethane member would be pushed out not only providing a high drag surface but to create a jamming situation. I tried a few different approaches but ultimately I wanted to use a machined nylon sliding body with a urethane shoe molded into it, the urethane would be flexible enough to allow the distortion needed to allow the jamming action.
 








 
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