Saturday, 6 February 2010

A Chain of Events

Today I went to a couple of bicycle shops to check out chains and sprockets as an alternative to threaded rods. Prices vary, but some sprocket sets can be had for $35, and bicyle chains for $25. Normal bicylce sprockets seem too large and difficult to mount to a stepper motor, but the (pardon my new French) "shimano rear derailleur jockey pulley set" has two small plastic sprockets that might do the job. eBay had some chains etc but I wasn't too excited.

BicycleHQ gave me very good service on a busy Saturday, and on top of that a used sprocket for free. I asked the guy if I could bargain any further, but he refused.

I am old, but I still have my teeth...

My colleague pointed out Little Bird Electronics who offer a 960mm Ladder-chain & Sprocket set for $23. It looks like plastic and has no specifications on their web site, so I sent them an email.

I also discovered Penhalluriack's Building Supplies, a hardware shop with a surprising range of products in an unexpected location (aluminium, screws, bolts, fasteners, wheels, metal sheets, etc). In particular, they have an assortment of threaded rods. Based on size, weight, and pitch factors, I'll go with the 0.5" (12.7mm) diameter. Pitch is 1/12" (~2.1mm) according to the (pardon my new English) British Standard Whitworth (and my ruler).

Threaded Rod vs Sprocket Chain
  • Threaded rods cost between $6 (600mm) and $8 (910mm). In addition I will need a coupler for the stepper motor, ball bearings to hold the rod in place near the motor, a thrust bearing, and an end-stop bearing, plus bearing housings. No idea about pricing for those items.
  • Sprockets seem to cost around $10 to $25 (or free if I'm lucky), plus chains for $25.
  • Movement with threaded rods is very slow. at 2.1mm pitch, a half-step by the stepper motor moves a theoretical 0.005mm. With the sprocket system a stepper motor half-step moves 0.35mm (chain element length 12.7mm, 11 sprocket teeth).
  • Linear positioning with threaded rods will be very stable. I don't know how rock solid this will be with chains.
  • Threaded rods may have a small pitch of 2.1mm, but their pitch drifts considerably over distance. Potentially, this could be sorted out via software. They also have a tendency to swing during faster rotation.

So, who's the winner? Maybe the x and y axis could be done with chains, and the z axis with a threaded rod. Looks like some experimentation is in order.

If you should find your parked bicycle without chain and pulleys, don't call me.

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