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Post by Peter Seager-Thomas on Oct 9, 2012 11:41:24 GMT -5
A question directed at the more serious racers.
Has anyone done proper comparisons between plastic and metal axle bushes and ball bearings? Are there any reasons for going for a particular type?
The plastic bushes are of course cheap and easily fitted. On several cars I have machined off one flange and used them as top hat bushes, where due to the nature of the material, a good push fit can be achieved, and replacement is simple.
I would guess brass bushes would permit the axle to run far more freely.
For me, the poser is ball bearings. There must be a fair amount of drag in a sealed/shielded bearing running in grease, though I would guess if the bearings are cleaned out and run in light oil they would be fine, though ingress of foreign matter could cause problems.
Opinions and preferences please.
Peter.
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Post by alfetta on Oct 9, 2012 12:22:41 GMT -5
I have in the past tried all of the above. As for Ball races `i can find no useful advantage particularly on a cost benefit analysis. Plastic OK but wears out fairly quickly however they are cheap and easy to replace. On balance I prefer Brass/Oilite bearings. Peter.
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Post by Andrew Rowland on Oct 9, 2012 23:52:29 GMT -5
Peter As you might expect i'm a ball race man.
I have done comparative testing with plastic and brass/bronze.
I have found that due to the manufacturing process plastic are just never perfect. They are two part moulded not 'turned' so of course they are never true and the hole is never perfect. On my old Scaley cars you could get a serious upgrade in power and lap time just by changing to brass.
Ball races are great for reducing friction. I buy from SMB in the UK. They sell Chinese and Japenese parts. I buy Japanese as the Chinese don't have the metalurgy apparently!!
Depending on how limited the space is there are shielded and non shielded so in normal situations buy shielded and crud wont get in. Using thick oil seems to do the trick. Getting grease in is the hard part on the shielded and attracts dirt.
Now the only reason I can find for NOT using bearings is that they reduce breaking effect. That can be crucial for handling ESPECIALLY with slim line motors which already don't have good breaking due to low torque....
To get an idea of how much friction your bushings are giving you don't try to compare acceleration but do a braking test. Run your car at max to a certain point and see how far it rolls when you release the trigger. You'll be amazed at how much further the ball races get you. Now if you can convert that to lap times depends on the track and your driving style!!!!
Personally i've gained several meters at the end of the race when the power is switched off while we're running just through running bearings.....
Then, ..... of course ....... they're more, ...... well ...... ENGINEERING aren't they? SO I just LIKE them!
Hope this ramble helps in your choice Peter, Andi
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Post by nuvolari on Oct 10, 2012 3:52:36 GMT -5
Sorry guys, I don't have any experience with plastic-, brass- or ball bearings. Up until now, all my rear axles run in a brass tube that is soldered to the chassis. For me this works fine, since I don't build for competition. But I was also thinking to experiment with ball bearings, since it's lifetime will be longer and you won't get any cavity over the long time. Can anybody tell me if there are, and where to find, ball bearings with a bore for a 3/32 standard axle? A Internet link(s) would be fantastic.
Cheers, Danny
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Post by alfetta on Oct 10, 2012 5:08:49 GMT -5
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Post by Andrew Rowland on Oct 10, 2012 9:15:35 GMT -5
Be very careful when ordering bearings (and any other engeneering parts).
Always check tollerances and whether they are plus or minus tollerances.
Then 3/32 axles aren't all 3/32. For example a slot.it axle is 3/32 so it wont go inside a 3/32 SMB bearing.....
You'll need a 2.35 or number #47 (i think) axle to fit snugly inside......
Cheers Andi
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Post by slo on Oct 10, 2012 14:32:03 GMT -5
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Post by Peter Seager-Thomas on Oct 10, 2012 16:35:07 GMT -5
Oh good, a topic of interest! Thanks for all the contributions.
I'm taking a greater interest in racing than in times past, hence the thread. I will probably go with brass bearings for all cars with chassis' which take the standard type bearing. I'll countersink each side to reduce the contact area, accepting the inevitability of worn bushes
'Special' cars are different, and as mentioned on another thread as well as Andi's comment a few posts back, it is the engineering content accepted (and enjoyed) by the scratch-builder which means ball bearings must be considered, indeed some are already fitted.
The braking issue is one I must look into. It probably explains why my favourite race cars both have SCX motors, they brake well.
Going slightly off thread, do the proposed 'Hurricane ' motors brake? Thoughts are that the magnets used would be 'effective'.
Peter.
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Post by Mark Huber on Oct 10, 2012 16:55:40 GMT -5
Peter,
Do the Hurricane motors have brakes? Is the Pope Catholic? Do ducks have wings?
The magnets on those motors will pull fillings out of teeth at 25 meters. I hope they are replaced with something a bit less potent when/if the motor goes into production.
I would not recommend that motor for racing it it's current configuration.
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Post by EM on Oct 10, 2012 22:17:24 GMT -5
The basic parameters that have to be considered in bearing choice are rotational speed and load. In the applications for our little cars both of these are modest. Using, as an example, a motor speed of 25,000 RPM and a gear ratio of 3 to 1, the axle will be turning at a little over 8000 RPM. This corresponds to a surface speed about 5.5 ft./s. This is a very modest rate. The loads imposed by these cars are insignificant. If we assume that a properly lubricated axle is actually turning on a thin film of lubricant, a principal source of drag in a bronze bushing will be the viscous drag of the lubricant. It is my opinion that the typical 3 mm width of a bearing insert is overkill for this load. One of the things that I typically do in preparing car is to chuck the bearing in my lathe and counter bore the bearing to reduce the bearing surface to perhaps 1.5 mm. I think this makes a difference but I don't really have any data to prove it.
Ball bearings are another matter entirely. Ideally there is no sliding action involved. The balls are rolling so the only source of resistance should be the viscous drag of the lubricant on the ball. To my mind this would argue for a low viscosity lubricant with just sufficient body to maintain a thin film between the ball and the race.
While there may very well be applications for ball bearings in the very high-speed motors used in wing cars, I'm not convinced that they offer measurable advantage as axle bearings in scale racers. They look nice and I have used them but only as shaft bearings in front motored cars.
As always, this opinion is worth exactly as much as was paid for it.
EM
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Post by nuvolari on Oct 11, 2012 3:36:59 GMT -5
Hi,
You're all are speaking a little Chinese when it comes to ball bearings. Sorry that I don't have the technical view you all have on this matter, but if I don't ask I will never know, as probably a lot of others who are to scared to loose face to. But as a teacher ones sad to me, there is no such thing as a stupid question, only a stupid answer.
I was brought up with the metric system, so I always have difficulties with the Imperial system. Up until now, I order 3/32 axles from "slotting plus", I buy bras tubes with a 3/32 bore and I've got drills to bore 3/32. This is the little technical know-how I've worked with.
With all the information you guys have given on ball bearings, going to the SMB website, opening the products list, choosing under Imperial, Flanged ext. inner miniature brgs 0.0469 - 0.3125" bore, open it, and than I'm lost ;D
So my question is, I've got a 3/32 Slotting plus axle, I want Shielded Bearings, can you please give me the SMB-ref Nr. that suits my course?
What I also want to know is, what size of drill do I buy, to drill out the O.D. so the bearing fits nicely into a brass plate or what ever? A model number on the Maritool site would be a perfect start?
Thanks for your understanding and help.
Cheers, Danny
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Post by Andrew Rowland on Oct 11, 2012 9:10:34 GMT -5
Hey Danny Yes Chinese and Imperial are about the same..... So get yer calculator and do 3/32= then multiply the answer by 25.4 That gives you mm. All those measurements on the smb site just need to be multiplied by 25.4 (the number of mm in an inch) to get metric equivalents. So take your axles and with a digital micrometer measure them. Is that result smaller than the internal diameter of the smb bearing? If yes ok. If not chuck away the axle and buy drill blanks of the appropriate size. Real 3/32 axles wont go inside real 3/32 bearings! Drill blank companies often want to sell 10+ of any size.... Try www.tapdie.com in the uk, they sell all the sizes and will sell singles. Their chart lists all the sizes so is very handy. It shows its number #42 that sits between 3/32 and 2.35! Good luck Andi
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Post by slo on Oct 11, 2012 19:20:53 GMT -5
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Post by nuvolari on Oct 11, 2012 19:52:19 GMT -5
Thank you all for your reply, and your proffesional help.
Cheers, Danny
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Post by maxrossmassler on Nov 22, 2012 3:42:13 GMT -5
www.vxb.com/page/bearings/CTGY/SCBI got some pretty good BBs from this site in the USA. I haven't done anything like A/B testing or anything. There seem to be several grades & of course therefore, prices.
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