as for the bearing size, no I was not suggesting anything, as it is not really my field of expertise, I was just curious.
Maybe Riff_Raff or Xpensive can make some suggestions from a tribology PoV.
But I did look up some bearings, and have seen that you can get smaller ones, as you say 5mm OD is possible and you could go for 1,5mm shafts etc.
As for the shrinking, it goes along the same lines as a pressfit.
You would oversize your shaft in relation to the ID of your bearing.
Then to be able to assemble it, you can either press it onto the shaft, which has some drawbacks and risks such as fretting etc, or you can make use of the CTE. Which means you would heat up your bearing (depending from the material and seals/lubrication used ~100°C perhaps for a steel bearing) and cool down your shaft (in liquid nitrogene (-196°C - be very careful!!) or dry ice (~-56°C, easier and safer to handle).
The temperature difference will affect your tolerances/clearance, so that you are able to slide your bearing onto the shaft in place.
After both parts return to ambient temperature the bearing will clamp onto the shaft.
The clamping force will depend on the tolerances (oversize) you choose.
You will need to look up the correct tolerances in a book, it will also define your bearing preload.
For your application a pressfit should be possible and I would secure it with some Loctite as Marcus suggested.
But its difficult to get Loctite between the bearing and the shaft if you pressfit it, using the shrinking method is easier, because the Loctite does not get scraped off as much while you press the bearing on.
Some other radom thoughts, not sure if it is possible, just wanted to mention it, and then let you run with it.
As I understand it, you use one solid axle going from one side to the other, and have the bearings inside the wheels.
There is the one bearing per wheel vs. the two bearing per wheel argument with sideforces due to tilting of the wheel in mind.
Maybe you can put your bearings inboard (into the body) and fix the wheels ridgid onto the shaft instead.
So the shaft turns inside the bearings, but you have no tilting forces on the bearing, and can use two bearings without any drawbacks.
Not sure if this agrees with your "no foreign objects or voids in the balsa block" rule, but the axle across is a foreign object as well - IMHO.
As for the axle material if you want/or need to keep your layout.
Mind about wood, I know it sounds crazy at first, but it is an quite versatile material, with some good characteristics if used correctly.
I played around with a 2mm tooth pick while at dinner last night, it´s pretty strong, and I don´t think, that this was any "special" wood.
That would make the press fit of the bearing easier to achieve.
If you go for steel, maybe a tube instead of an shaft will help safe some mass.
It´s difficult to drill a 0,5 m hole through a 2mm shaft, but I was thinking about a injection needle or something along these lines.
Maybe you can find something in the correct size, may need to harden the material a bit, as a injection needle is probably quite soft, to prevent it from breaking off.
Just some radom thoughts and brain storming, feel free to ignore the parts which are maybe too crazy.
Good luck anyway
Last edited by 747heavy
on Thu Dec 09, 2010 7:27 pm, edited 3 times in total.
"Make the suspension adjustable and they will adjust it wrong ......
look what they can do to a carburetor in just a few moments of stupidity with a screwdriver." - Colin Chapman
“Simplicity is the ultimate sophistication.” - Leonardo da Vinci