sprocket

hello , I want to have a go at making a sprocket for a go.kart 70 teeth, 3/8 pitch . I am thinking that the layout will be the same as for a 70 T gear , except that I can drill the holes then,turn off the excess and file the tops . I have the use of a mill and rotary table ( but not the experience !!!)

regards Phil

Hi Phil,

The tooth form for a sprocket is quite involved, rapid chain wear may result if the form is not quite right. If this is for a high performance kart I think it would need to be made fairly accurately here is a link to one site with details. if it is for a low performance machine you might get away with what you propose.

Mike

If you look at bike sites on the Internet, people make custom bike sprockets, and IIRC sometimes with much less equipment. Have a Google round.

Yes it can be done, I've done it myself but only for a much lower tooth count and smaller pitch chain. You work out the pcd by assuming a 70 sided polygon with each side 3/8". The angle subtended at the midde is 360/70, and the pitch circle radius is (3/8*1/2)/sin (360/70*1/2), which I make 4.1792", but check it out before cutting metal. You then file the sides until a piece of chain moves smoothly into and out of engagement. This is 140 edges, so very very tedious. Unless you're a masochist I'd buy one ready made, or find a gear cutting firm and get them to do it.

to be honest Duncan most of that is way over my head , except that the dia of the pitch circle of the sprocket will be 8.3784 in , making the circumference of pitch circle 26.24 in . So if I drill 70, 3/8 in holes at a radius of 4.1792(ish) That should be right , needless to say it is a very low speed off road kart for grandson , which is why I need a big sprocket to slow it down , I have an 85 tooth one bit it is too big to fit so I thought I could use that as a blank

thanks for help Phil

can't you find a bicycle chainwheel in the right size?

I make a fair few sprockets but always buy in a blank from a local bearing/power transmission place. Even a biggish sprocket is around the £30 mark, they have a pilot bore ready for you to machine the rest to suit. I did once make a 42 tooth sprocket for a vintage bike but never again, far to time consuming doing the maths !

I'm not sure where you get 8.3784 from, 2* 4.1792 = 8.3584. Unless the wheel is pretty good, the chain will not mesh smoothly which could result in it coming off all the time, or snapping. Snapped roller chain whirling round next to grandson isn't an attractive prospect. Obviously you don't need to worry about the 2/10s of a thou, but 'ish' probably isn't good enough. I very much doubt that 3/8 pitch chain has 3/8" diameter rollers, depending on what the exact spec is it could be 5mm, 0.2", 6mm, 0.25"

You can get 71t sprocket and a length of chain for <£20 from this outfit

http://www.ebay.co.uk/itm/Kart-Gorilla-O-Ring-Chain-Sprocket-Offer-Best-Price-Rotax-TKM-/130962346758?var=&hash=item64289b7af2

I've no idea what pitch it is, but got to be worth a phone call.

I have bowed to superior experience . Bought a 74 t with the right pitch for £8 and put the big one back on ebay Thanks for the messages everyone

Phil