radial bearings at 15000 rpm?

[Talcite]

Hey guys, I'm designing a slot car racer and it goes 20m in 1 second.

That works out to roughly 15000rpm, so I'm wondering what kind of bearings would give me the least friction? The car weighs 55 grams and the axles are pretty thin, only 5mm in diameter.

With such a low load, what kind of radial bearing would be best for this application? The axle diameter can be changed, but not significantly. I was considering fluid dynamic bearings, but are the ones from computer fans going to work well at 4 times their normal rpm? 0.o

 

[randay]

Could try some bearings from those 2 stroke nitro engines for RC cars, those motors go upwards of 40,000 rpm and are quite small.

 

[bobsmith1492]

Tear apart a 15000 RPM SCSI drive and steal its bearings - may be somewhat expensive...

 

[Talcite]

harddrive bearings are not only expensive and too large, I'm also wondering which design would ensure the least friction. Does anyone know if fluid dynamic bearings are lower friction than ball bearings? Oh btw, harddrive bearings are also designed to take axial loads, not only radial, which wouldn't be needed for this application.

 

[dkozloski]

Try bearings out of a dentists drill. These operate in the 400,000+ RPM range.

 

[jagec]

For something that small, you'll just want a bit of graphite powder in a thrust bearing, I'd think.

Dentist's drill bits basically float on a cushion of air while operating.

 

[Talcite]

graphite powder in a thrust bearing? I'll look into it =D.

Btw, what unit is FPM? I came across it while looking at different types of bearings and their typical stats.

Hmm... I looked into thrust bearings and they're not able to support alot of radial load =/. They're actually primarily designed for axial loads. The car just needs to be separated from the axles, which puts radial loads onto the bearings. Anyone have an idea?

 

[Howard]

Foil air bearing. Probably isn't feasible though.

 

[Talcite]

foil air? 0.o sounds exotic. Now can they make that in 3-5mm inner diameter? =P

 

[Never Knows Best]

Fluid Dynamic bearings or the simpler version Sleeve bearings will probably be what you want. They are small and depending on the fluid or oil they can be tailored for a specific RPM shaft.
http://en.wikipedia.org/wiki/Bearing_%28mechanical%29 for a little info.
I love that site has just about everything you can think of.

 

[cheesehead]

For 15,000RPM, you can get some really, really tiny ball bearings. They're used by modellers to build tiny gearboxes for R/C airplanes.

 

[GalvanizedYankee]

Ceramic ball bearings :thumbsup:

 

[Talcite]

ahh yeah, i was looking at ceramic ones too. Too bad full ceramics set me back about $200 each >.>'

Are hybrid ceramics comparable? They're still $30 each 0.o

 

[GalvanizedYankee]

I think in the hybrid ceramic ball bearings, just the balls are replaced.
The higher the RPM rating the looser the ball fit. In larger bearings the shaft and outer race's press fit is taken into account as this further reduces running clearence once installed.
Ceramic balls hold lubricant very well and do not grow with heat gain, in fact the friction is so low that there is much less heat gain than with steel balls. F1 and Moto GP are crazy about ceramic bearings in gear boxes.

Have you considered ceramic bushings to hold the axels. About 3/8" long and super glue gel them inplace. http://www.zippy.com.tw/ is big in ceramics

 

[cheesehead]

http://indoorflyer.co.uk/index.asp?function=DISPLAYPRODUCT&productid=134

Use a teensy bit of oil, or maybe white lithium grease, for best results.

Yes, these work at 15,000RPM.

EDIT:

5mm? That's 1/2 a centimeter - my bicycle has axles only a little thicker than that!

 

[jagec]

Originally posted by: Cheesehead

5mm? That's 1/2 a centimeter - my bicycle has axles only a little thicker than that!

He probably means .5mm.
Don't most of those really small-scale applications just drill a couple holes in the plastic base and slide the metal axle through them, making sure it doesn't bind? At that scale, it makes more sense to use a more powerful motor than four tiny, super-special bearings, if friction is seeming to be a problem. At least, in terms of the cost+complexity:benefit ratio.

 

[Talcite]

well, this slot car racer runs on CO2 =/. There's no engine or anything, just a CO2 cartridge and a puncture. Its not .5mm, its 5mm.

I was looking into those ceramic bearings galvanized, and it seems most of them I've come across are ceramic raceway, metal balls. The full ceramic bearings set me back $100 each =o so they're kind of out of the picture, they also don't come in the right size. I'm going to look into them though, the guy at the company still hasn't gotten back to me with any technical specs.

I was thinking of going to 0.125 inch axles since it would be easier to find bearings in that size. Not much of a change, just modify the CAD drawings and let the CNC machine cut them out again =P.

Btw, all I could find at zippy was ceramic fans. Is there somewhere else where they have them?

 

[cheesehead]

Trust me, the inexpensive ball bearings I linked you to will work great. They're a huge improvement over sleeve bearings for high-RPM applications like tiny R/C helicopters. You can get them at other vendors, too, in sizes ranging from .5mm to 3mm, and for a buck or two each.

Also, assuming your wheels are 1" in diameter, they will only be rotating at about 6725RPM tops. Yes, I did the math.

 

[Talcite]

I don't know how you got 6725 rpm... They're 1 inch wheels, which means 25.4mm diameter, which means 79.756mm circumference. 20 meters is 20 000mm. Make it 80 mm circumference to be simple, 2000/8 = 250. Since its 20m in 1 second, 250 x 60 = 15 000RPM.... is there something I'm doing wrong in my calculations?

 

[dfuze]

I happen to work in an aerospace bearing company (no, our bearings would be waaay too much!). I talked to one of the engineers who is also into RC racing, and he suggested that you go to Boca Bearing if you haven't already.

 

[Talcite]

Yup, I was looking at boca's stuff. Seriously considering the hybrid ceramic bearings. Now if only the salesman would get back to me about the damn technical specs...