Monday, September 8, 2008

Roll On !

I got the nice needle roller bearings, opened up the roller bores and trimmed them to my original design length. Beautiful, smooth.

In fact, even with my heavy springs fitted they roll well on an oval filament. Grip is good, but needs more so I will experiment with heavier springs, and maybe bigger ribs on the rollers. I also think the pitch angle needs revising. The photo shows the rollers in their 'nut' with radial mounted compression springs, all mounted in the housing. I am using red 'fibre washers' as thrust bearings mainly because I had them to hand. As the rollers are smaller than the tile cutters I was able to make up the brass tube spacers too so everything stays tight, but not tooooo tight and the rollers roll well.

On a different tack - I have been cutting toooooobs to make my repstrap bot structure:

I have some of the 22x22mm aluminium extrusions left over from an old project. It is nice stuff to work with with basic tools. The bearings (not skate bearings, but 70pence each from RS) are on M5 bolts, 3 washers between them and pull down onto special 'T' nuts in the aluminium slots. This allows pressure adjustment.

The bearings run on 6mm steel rods, simply sitting in the slots. The plastic cable tie each end help hold the rods in place whilst I fiddle/assemble. (the carriage is made from 2-slot and the rail is 3 slot extrusions)

This reveals an interesting thing with the bearings - they are designed for radial loads. In this arrangement, they are being loaded partially between radial and axial. Normally ball bearings can run with axial load, but in this case the axial component is ALL on one side which causes the outer race to cant over slightly - there is often a bit of axial slack.

This causes the bearings to 'cog' for some reason (like when you hand-turn a unenergised stepper motor) . It could be the balls passing the loading slot, or something to do with the cages. Anyway, it is very bad when I put 4 washers beetween the bearings and almost perfect with only 2 washers. 2 washers reduce the contact angle on the rods and the carriage shown popped off with only slight side loads when running along. Not good.

I have settled on 3 washers for my Mk.1 and I have skeched a layout with proper 45 degree shafts and radial loading for the Mk.1.5 if it needs to be smoother. 45 degree angles add considerably to the tooling requirements, but now my Dad has got his mill motor re-wound I could do it.

I assume this 'cogging' issue applies to Timothy Schmidt's chunky pipework design too.

Progress is a little slow this week - too much time spent in Jazz clubs and visiting friends for vegetarian curries. Nice.


Bert said...

Hi, love your way of thinking.

Was just wondering: have you considered eliminating the pitch angle altogether and instead using pieces of threaded rod for the rollers? The thread would provide the pitch and plenty of grip, and an integral depth stop as well...

Drilling & reaming chunks of threaded rod might be quite difficult and could require annealing of the parts, though.

Probably would be easier to use a lathe to cut threads in softer rollers instead. This would also allow you to use a non-standard, shallow & fine thread pitch.

Just a thought.

Lampbus said...

Bert, actually that was my first idea - see my comments on an earlier blog...