Quote:
Originally Posted by GeeTwo
If you're already thinking of turning a hex shaft down to fit into a round bearing, I'd recommend using a small enough bearing that you get 100% contact. The sheet metal on bearings is mostly meant to hold the bearing together when it is between assemblies, not to distribute the forces tangentially.
A less desirable alternative would be to add a spacer with a hex bore and an outer diameter that fits into a larger bearing. This would also need to be held in place with a shaft collar or similar.
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This would entirely defeat the purpose of ThunderHex. The idea with ThunderHex is to have a round bearing, yet still be able to drive hex components of the same size on both sides of the bearing. An alternative that achieves full bearing contact and still allows for hex components on both sides of the bearing is to take a 1/2" hex shaft and turn it down to 1/2" round for the width of the bearing, and then machine a 7/16" hex geometry on the other side of the bearing. This takes intensive machining resources for the shaft however and an odd size broach for the 7/16" hex components.
It's important to understand the design criteria likely used to develop or decide on the size for ThunderHex bearings and shaft stock. In a typical West Coast Drive, bearings for a press fit in a 1.125" hole are used. When I looked at metric bearing sizes, I see that 13 mm ID ball bearings generally are a good size for a 1.125" press fit and 14 mm ID ball bearings are almost exclusively well sized for a 1.375" press fit. This indicates to me that somewhere in between 13 mm and 14 mm a transition in acceptable press fit holes for the bearing size occurs. A 13 mm hole would be unacceptable because ThunderHex with a 0.512" round bearing wouldn't really work that well (barely any hex shape at that point), and I guess they felt 13.75 mm was an appropriate size and found that they could source bearings of that size which would press fit nicely in a 1.125" diameter hole at a reasonable price.
In addition, I'm fairly certain teams have used 3/4" hex with far less bearing contact and not had issues (Thunder Chickens --> ThunderHex???).
All this to say that I doubt it makes much difference from a loading stand point if you go with 14 mm bearings. As long as you are willing to use 1.375" bearing holes it should work out fine. It's just different than what VexPro came up with to make ThunderHex.