Common Bearing Material Combination

The McMaster-Carr catalog gives PV values for some common bearing materials.

As the cheaper part, a sleeve bearing should generally be softer than the shaft. Your combo looks dubious on both counts, except for a prototype as mentioned.

Agreed, the sleeve should be softer. May be to use a lead sleeve? I just wondered what experience exists "out there" with AL as shafting. Probably none as I was afraid of. Perhaps the oil impregnated "oilite" material will be easier on such shafting and wear less.

From a torque point of view Al would do and I am looking for low weight and low hours of service, up to 100 only.

How about an aluminum shaft and a lubricated UHMW sleeve bearing? Not for long term, of course, but maybe good here.

I did say 2600 psi. Bearing sizes vary near between 1" and 2".

Your idea of a steel sleeve is a good one. Thanks.

The coupple steel-aluminum is used as any babbit in engines.Al is not so ordinary and the lub.is forced.Load could be big enough.

"Put a steel sleeve over it in the bearing area" is a good idea! Interference fit

For short term or proof of concept it would suffice, but it would eventually lead to dissimilar metal corrosion between the steel and aluminum diameters.

Maybe. But, this is a short term application and the environment will be oily and hot.

I missed that oily and hot part in the OP's post!!

OK,

I drew an inference here because OP said, "Its a little model aircraft engine. These usually last only 50 hours."

That's where I got the hot, oily environment.

Hi Lyn,

Your inference is quite correct, hot and oily.

The concern of dissimilar metal corrosion, although that is so, is likely not of great concern here due to the experimental nature and the oil present.

Thanks for the mention of Fluorosint® 500 Getting ready made bearings is a bit harder though.

Fixitorelse questioned the velocity 2200 fpm on bronze bushings. Is that too high? I know that the PV value is rather low but these values are likely given for long durations. I would think the PV can be exceeded by quite a bit on low hours of service, or not?

There is also General Magnaplate that have coatings for AL such as Tufram and Nedox and more. For a later stage that can be considered as well. How long would bronze last? After all it is a common bearing material. Wear takes time I figure.

and the point would be what? reduce rotating total weight???

2200 FPM? on a bronze bearing? Are you sure? What Size shaft? Not saying don't do it but I am not sure what your trying to do. Sounds pretty iffy at best.

The concern I would have using aluminum shafting is thermal expansion at the area of the aluminum bearing surface, regardless of the bearing being of bronze or sintered bronze [oilite]. (shaft diameter expanding & seizing inside of the bearing/bushing) My guess is that it would work fairly well at low RPM/Low load.

Phosphorus bronze is commonly used for valve guide material, but then again, is being used with steel valve stems and under constant lubrication

You might look up the thermal expansion coefficients for aluminum and steel and that may give you some guidance as to what to expect. Aluminum is something around .00001XXXXX lineal expansion/per unit length/per deg.F, while carbon steel is .000006XXXX. Considerable difference.

If you're proto-typing a small project for concept, try the aluminum (7075 has better tensile strength than 6061) and bronze bushing. (the last time I bought a 3/4 diameter x 6 inch length of oilite, it was $25 plus shipping)

You can always upgrade to a steel shafting material if problems occur. In that case you can buy a length of oil hardening ground rod and machine/heat treat to your requirements. Good luck with your project.

That is likely the main reason why Al shafting is not being used. Thanks.

Some quick look reveals that only ~ 84 deg C is enough paired with steel to eat up the usual bearing clearance of 0.001 * diameter or a bit more. Thermal expansion of Al per deg C is 0.000024. The diameter falls out, we have an allowable temperature rise = clearance / diff. in expansion rate = 0.001 / (0.000024 - 0.000012)=~ 84 deg C. This is a bit low, or apply extra large clearance. Yes, probably ok for low load/rpm as in a demonstrator model.

What 'ferquiza' said is also reversed. The Auto industry is using steel shafting in high silicone Al bushings that are surface treated to expose the silicon molecules, forming a hard surface layer.

There is also hard anodizing that only specialty shops do, or chrome plating where first Copper is needed for bonding. Again most chroming shops don't want to do that, especially for low volume or only one.

Having the housing also in Al neutralizes the expansion issue. Regarding the pairing I guess trying it out is the only way to answer if 6061 or 7075 work with bronze or sintered bronze.

Its a little model aircraft engine. These usually last only 50 hours. The 7075 is likely a better bet when untreated, I would think. Having higher strength will probably raise the point of galling. Any thoughts?

Just ran across this today.

It's a new product from Professional Plastics. (Not an endorsement)

Fluorosint® 500 has a nine times greater resistance to deformation under load than unfilled PTFE. Its coefficient of linear thermal expansion approaches the expansion rate of aluminum, and is 1/5 that of PTFE-often eliminating fit and clearance problems. It is 1/3 harder than PTFE, has better wear characteristics and maintains low frictional properties.

As noted elsewhere, the shaft should generally be harder than the bushing, so use of bronze or oilite bushings with Aluminum is not advisable. However, Pacific Bearing (pbclinear.com) has some products that use aluminum shafting with polymer bearings and specialty coatings that seem to be OK for light loads.

The surface hardness and wear resistance of aluminum can be augmented with coatings, which also can improve friction properties and increase the workable PV factors. Still wouldn't use bronze though. There are lots of polymer options available though, and often priced better than bronze.

Check Bunting bearings for potential pressure ratings. It varies with material.