Estimating wear of hard anodized aluminum

[DReimer]

I originally posted this in the Tribology forum, but it seems pretty quiet over there...

We are designing a cheap and cheerful linear guidance assembly using a formed sheet metal carriage supported using dirt-cheap skatewheel bearings within a hard anodized aluminum extrusion.

A vertical load of approximately 20 pounds will be supported by two 18mm (OD) bearings acting as wheels and rolling along the extrusion. There will be a moment load on the carriage that will be taken by the same type of bearing mounted within a delrin disk.

There is some concern about the wear of the aluminum with hardened steel bearings running on it. Our equipment often goes into food plants (not in the food processing area, though) and it is felt that visible metal wear particles would be offensive to many of our customers.

Is there a way to calculate an estimated material wear of the aluminum based on rolling-contact point loads? I've done some Googling without success. Any help is appreciated.

We will be doing physical tests of a prototype, but I want to move more quickly than that would allow.

Thanks,
Dean

 

[swall]

Have you considered running a Tabor abrasion test? It is a laboratory accelerated wear test. The apparatus consists of a (powered) rotating specimen (the hard anodized aluminum) and a rocker arm assembly with two rollers (your hardened steel items) which bear on the specimen. You can adjust the load on the rollers with deadweights to provide whatever downforce you need. However, you also need to consider that you may have more than a simple wear situation. If the aluminum flexes under load, the anodize could crack and pieces could spall off, leading to a very accelerated wear situation.

 

[MintJulep]

Don't use the the skate bearing, use the entire in-line skate wheel.

In that case, with only a 20 pound load the thing will last pretty much forever.

 

[alansimpson]

Try

http://www.igus.de

for materials. They have a range plastic bearing materials suitable for hard anodised surfaces.

 

[diamondjim]

If there is genuine concern
run a test. It really sounds
like your are doing everything
on the cheap. The military has
used steel pinions running against
aluminum that was anodized.
Do not know if they will make
their information available to
you. Check out the turret bearings
used in many of the tanks.

 

[rnd2]

The only way that chunks of Al are going to release is whether the point loadings exerted by the steel bearings are sufficient to displace the aluminium substrate. Very easy to determine byond any doubt. Run a test.

 

[DReimer]

Seems like testing is the way to go. We should have all the parts for our prototype by the end of next week. Our plan is to put air to it (carriage is moved by a 20mm rodless cylinder) and cycle it back and forth until we either break something or satisfy all our internal stakeholders that we aren't putting a warranty liability into the market. Given the nature of the application we can simulate 10 years of use in less than a week.

While we aren't looking to design something "on the cheap" we are certainly looking to make it as inexpensive as possible. Given the low loading, low speed (it moves about 3/8" every 2-4 seconds) and low precision requirements of the guideway, we felt that we could make our own linear guide much more cost effectively than using the linear bearings and shaft than we use now.

Mint: due to space constraints we can't fit anything that large, but we do have a source for a similar bearing with a urethane "wheel" that we can consider if this doesn't work. The extrusion is a 4" x 2" aluminum C-channel with additional flanges, as below:
----
| '
|
|
| ,
----
Within this channel we need to fit a cylinder, wheels to support vertical, horizontal and moment loads. Plus the carriage will be nearly flush with the outer flanges. It will be a very tidy, clean and easy-to-assemble component, vastly superior in every respect to what it replaces.

diamond and rnd2: Personally, I'm not concerned. The people in the company that are concerned are non-engineers who don't appreciate how hard the anodized aluminum is, and how lightly loaded the application really is. The biggest naysayer just happens to have his own pet solution, so there is some not invented here going on.

Thanks for the ideas, guys.

Dean

 

[Tofflemire]

Hi,

The question is will the steel bearings stand up to rolling over the hard anodized aluminum? I read a report that hard anodized aluminum is the second strongest material next to Diamonds.

Tofflemire

 

[swall]

To clarify what I wrote in my previous post above--hardcoat anodize is indeed extremely hard and wear resistant. BUT, it is very thin, usually no more than .002".IF the substrate is flexed due to high contact loads, the substrate will deflect, but the hardcoat, having nil ductililty, can't flex with it and will crack. Then under repeated loading, the cracked hardcoat can spall, leaving patches of bare substrate and free particles of hardcoat, which can then act as an abrasive.

 

[GregLocock]

I like the tire idea.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cooperjer]

If you haven’t already looked at rolling contact stress I would start there. If you have a copy of Fundamentals of Machine Component Design by Juvinall and Marshek check out chapter 9.13, or net search for “rolling contact stress.” If you do not have a copy of Fundamentals of Machine Component Design I highly recommend it.

cooperjer


Cooperjer
Mechanical Engineer