sprocket

[abhishek502]

hey guys,

we have designed a double stranded 19 tooth driver sprocket with a 9.525 chain pitch after calculating forces on the tooth and having proper static analysis F.O.S is found to be '5.8' with aluminium 7075-T6. shaft dia is 22.6 and key way dimensions are
length:4.89793
width:5.6125
depth:4.6
All dimensions are in m.m

generally,what is the preferable material and F.O.S for the driver sprocket for static and fatigue analyisis??

[AxelRipper]

Generally the preferred material is something that you can acquire easily and will stand up to the loads that you are putting to it. This could range from steel, aluminum, CFRP, magnesium, titanium, rubidium, chromium, or really any other material on earth, as long as the design can withstand the loads being applied.

[Owen Thomas]

I'm not sure what "proper static analysis" means, but if you're looking this much into sprocket design you'll want to look at the contact stresses (and the assosciated fatigue), not just the static loading of the whole thing.

We use steel drive sprockets because that is what is available, but if I had to justify it over aluminum it would be because the mating spline is steel and because the number of stress cycles is ridiculously high (3-4 times the aluminum rear sprocket), leading to excessive wear and fatigue in aluminum. Also 19 teeth on a drive sprocket is huge, you might want to just make sure it will fit on your engine (it wouldn't on our CBR without some questionable block grinding).

[abhishek502]

thanks for replying....
@owen thomas,we are making an electric car for F.S event first time this year.
'proper static analysis' means that we have calculated the contact tooth forces and the factor of safety by applying these static forces on sprocket teeth.Fatigue analysis is also to be done.
F.O.S found to be 5.8 but i'm not sure that what should be the minimum F.O.S required for the driver sprocket.
As we are not using an engine, our driver sprocket teeth and pitch are not fixed so we have to optimise it according to our design and packaging with required factor of safety.
I am also confused that how the high speed bike engines(15 teeth or less) are designed while having so much high load,speed and forces without considering polygonal effect or chordal action having required factor of safety.

[Dash]

Well, If you use the instantaneous highest torque the motors can output in your math. Then a FOS of 1.00001 would be sufficient for a static case. I'd probably lean closer to the value of 2-3 since you might not know the numbers exactly.

For the fatigue analysis, you could probably go down the same rope. How long do you want the sprocket to last? I'd assume at least the distance of 5 or 10 endurances so you don't have to carry spares around with you all the time. So if you design for that stress, then FOS could be 1.00001 as well. Maybe just bump it up to 2 though if you like. But you must remember, if you want it to last forever, aluminum doesn't have a fatigue limit.

If I was to design a sprocket, I'd probably make some kind of hybrid. Steel teeth to keep contact stresses close down to the endurance limit. Then bolt the teeth to something that is a lot softer ( and lighter ) to adapt it to whatever motor/shaft/diff you are using.

You could also just take the easy way out. Buy some sprockets used on 1000cc bikes and bolt em up!

[Owen Thomas]

Originally posted by Dash:
Well, If you use the instantaneous highest torque the motors can output in your math. Then a FOS of 1.00001 would be sufficient for a static case. I'd probably lean closer to the value of 2-3 since you might not know the numbers exactly.

For the fatigue analysis, you could probably go down the same rope. How long do you want the sprocket to last? I'd assume at least the distance of 5 or 10 endurances so you don't have to carry spares around with you all the time. So if you design for that stress, then FOS could be 1.00001 as well. Maybe just bump it up to 2 though if you like. But you must remember, if you want it to last forever, aluminum doesn't have a fatigue limit.

+1. Your biggest problems are going to be shock loading when (if?) changing gears, and any misalignment/compliance in your sprocket carriers which would cause some axial load on the sprocket. Both of these are very hard to predict, so a FOS of 1.0 might not be wise for the static case.

The reality is that sprockets will not last forever in almost any scenario, and changing them every so often is considered acceptable. With most designs it is far more likely to have the teeth worn down to the point of replacement before the fatigue limit is ever reached (chain rollers are really hardened). I would figure out the number of cycles you think gives an appropriate lifespan and design to that. Also make extras, just in case.

Good on you for actually doing the analysis before asking for opinions too, by the way http://fsae.com/groupee_common/emoticons/icon_smile.gif