High PV value friction material?

[RichLeimbach]

We currently make a product using a sintered bronze clutch material in a clutch-flywheel arrangement. The material is subjected to a maximum of 3800 fpm relative movement and a maximum pressure of 1500 psi. It runs against a 4140 flywheel machined to a 32 mi finish. Coefficient is around .19. The bronze material is holding up well in this application, however...

We are making a new product that needs to have a maximum relative speed of 8500 fpm and a max pressure of around 3000 psi. Test results are all preliminary so far, but I have some concerns that the large PV jump will be too much for this material to handle. Stability of the friction coefficient is important (needs to be held within +/- 20%).

My question is this... Does anyone have any input on a material that would be good for this application? We have tried most standard organic & semi-metallic materials previously and have found them lacking in frictional stability. I would prefer to use a wrought material (steel, bronze, aluminum, etc.) for their ease of manufacture and mechanical properties, but I am unsure how they would react after a number of cycles (I have some concerns about galling, etc.). If anyone has good ideas, let me know. Thanks. (I'm also posting this to the tribology forum).

- Rich

 

[ccw]

Hi Rich,

While you are experimenting, try precision machined then hard coated or martin coated aluminum for the surfaces. We made some fail safe brakes(electromagnet off, spring set) a few years back and had good success with holding a constant friction factor after wear-in process (running partially engaged for several seconds). We had to be able to overdrive the brake after it was set with 120% normal motor current. The clutching or braking process will shed some aluminum oxide which can be abrasive. This was a dry process and the application obviously did not have to absorb as much energy as an active clutch -- just park and hold; or grab, decel, and hold on power outage. They were used on one horsepower dc motors and outlasted the brushes during torture tests.

 

[RichLeimbach]

ccw,
Thanks for the tip. I'm not familiar with martin coating or aluminum, can you give some more info on this or the name of a company that deals with it?

- Rich

 

[ccw]

Martin coat was an old process trade name for hard coat anodizing. It is basically an electrolysis process of forming a hard skin of aluminum oxide on aluminum parent material of considerable depth and servicable hardness properties. It is not the more familiar color anodizing. Look at

http://www.fortwayneanodizing.com

for a better explanation.

 

[RichLeimbach]

CCW,
Thanks for the reply. We do a lot of hard anodizing on other products, so this should be a pretty easy way to go if it works.

- Rich

 

[kenvlach]

Hard-anodized aluminum is not suitable for this application. Results would be good initially until the hard anodize layer (about 0.004” max) wore through, very erratic for a brief transition period, and finally erratic behavior (fade, then‘grabby’ when hot) during a fast wear through the softer aluminum. You need a material with uniform wear properties throughout its thickness. [My company anodizes aluminum brake calipers, etc., but not parts with wear surfaces.]

Composite Materials, vol. II, Chapter 3, Applications of Composite Materials, Mel Schwartz, Prentice–Hall (1997) discusses a number of composites used in aircraft and automotive applications that would be worth considering.

An MMC (metal matrix composite) of aluminum filled with SiC or Al₂O₃ particulate should work well. Both wear-resistant and good thermal conductivity to avoid overheating. Al/SiC_p has been used for disc brake rotors, and I believe Waupaca Foundry has some experience with Al/ Al₂O₃ brake parts. A 1994 reference reports that both of these materials were being field tested for brake disks for the Japanese bullet trains. The weight savings are 50% vs. the original Ni-Cr-Mo cast iron. And of course, the lower mass is beneficial at high rotational speeds. Toyota uses an Al/alumina silicate fiber MMC for a crankshaft pulley dampener on 3L inline 6-cylinder engines.

Carbon-carbon composites (CCC) brakes (I believe first used on either the Concorde or the space shuttle) are used on all modern large aircraft, and were being tested in the French TGV train (as of 1994). Pretty expensive, though.

There are also a number of graphite fiber (e.g., Kevlar)-reinforced, 300^oC-resistant, PMCs [polymer (some epoxies, phenolics, polyimides) matrix composites] in use in aircraft engines.

I hope this helps, or at least gives you more ideas.
Ken

 

[RichLeimbach]

Ken,
Thanks for the info. We had previously tested the carbon-carbon and have found it to be a pretty good material for our application, which had some firctional variation, but generally stayed within the range we needed. It was definitely expensive, but less so than a failed product. We may end up retesting some of this material.
Thanks for the lead on the MMC aluminum. I'll check into that.

- Rich

 

[EdDanzer]

We used aluminum bronze in a hydraulic apply spring release break the holds 20,000 ft lbs torque in a 10” diameter X 8” wide package. This material has worked well at slower speeds with hydraulic oil lubrication. One thing to consider is the PV value of any material considered, the disks are bearings seeing increased load and heat as the speed differences of the parts decrease. The lubrication and cooling will have a major effect on success or failure in the design. The primary advantages we have seen with aluminum bronze are higher yield strength, and it tends to be non galling at low speeds when dry. In our design the higher yield strength was critical to hold the OD as small as possible.

 

[RichLeimbach]

Ed,
We're also trying to hold the OD as small as possible and the increased yiled strength is something we are looking for. Unfortunately, we are not able to lubricate this surface, but with our very small time period of slip (3 milliseconds) heat has not proven to be a big problem so far.
For my reference, can you give some more info on the aluminum bronze you used? Was it sintered, or was it a wrought material? Do you have any material designation for it or is is a proprietary material?
Any info is appreciated. Thanks.

- Rich

 

[EdDanzer]

Rich,
We used commercial CA954 plate ¼” thick, about 40,000 psi yield. We should have had it water jet or laser cut, but got circles and milled the shape of the outside. The housing was just filled with oil, and this lubricated the bushings on the shaft as well as the disks. We purchased the plate from Copper and Brass Sales. We used 4150 for the disks on the shaft, with a 45° spline interface, again to reduce diameter. This material broaches better than 4140 HT and will be about 110,000 psi yield. Just for your information we made push broaches for the steel disks and the shaft anchor to reduce package size.

Ed Danzer,
Danzco Inc.
360-264-2141