Is it going to be possible to put a sprocket on this motor shaft?

[harrisonpatm]

I am looking to do a small moped conversion as a second vehicle to my larger e-moto. I would like to use a 2400w BLDC that I was able to acquire at the scrapyard for $6. Today I picked up a Trac moped from 1986 that could be a good base, it weighs less than 100 pounds and the motor mounting points look close enough that I could probably fab a motor mount myself without too much difficulty.

The issue is fitting a sprocket. I have never done this before, so I have little to know experience regarding the parts available and how to properly secure sprockets. So before I dive into the project, can someone with more experience tell me if this is at all possible? If not, that's fine, I can ditch the parts and look elsewhere. Just want to know if this is going to become too large of a bottleneck for the project to complete itself.

My goals are around 35mph top speed, it's going to be a neighborhood commuter. Rear wheel is approx 20 inches diameter. Omnicalculator tells me the rear wheel will need to spin at 588 rpm to achieve that speed, so lets call it 600. Rear sprocket is 42t. My motor's specs are hard to track down for sure, but it looks like I can expect a top rpm, loaded, of about 4000 @72v. That's top speed, meaning that I may need to find a rear sprocket with more teeth. But I've already found a few.

My question is, to the experts here, will I be able to fit a 13t or similar sprocket onto my motor shaft? Shaft is 5/8", with the top 1/2" as a 1/2" thread.

I do realize I need to figure out the correct rear sprocket size in order to not burn out my motor. But if I could get someone to chime in and tell me that it's not going to be at all possible to put a sprocket on my motor's shaft, then I don't need to waste my time with the rest of the project. I just don't even know enough about what types of sprockets are available in order for me to search for them.

Current rear sprocket, in case it helps:


Thanks!

 

[harrisonpatm]

Also putting up the only spec sheet I have for this motor, in case that's helpful. I also have three of them, so I don't mind overvolting and experimenting on them, I've got spares. That also means if I need to grind, cut or key the shaft, in an attempt to get a sprocket on it, and I mess it up, I have 2 more tries to get it right.

 

[amberwolf]

Fitting a sprocket, then assuming it's not already made this way, I'd modify the shaft to a double-D or D shape, flatting two or one sides, and using the threaded area for a nut to secure it in place, like on these powerchair motors with gearboxes I used on CrazyBike2:



Depending on the shaft diameter there's already sprocket-hubs availble to bolt them to, or sprockets with hubs built onto them (like for Karts and stuff). I'd wait until you have the sprocket/hub on hand to modify the shaft so you can do it carefully and as close to a perfect fit as possible.

 

[harrisonpatm]

Should I be searching for "D-shaft" sprockets, or something like that?

 

[PaPaSteve]

Grinding a generous flat spot (or two) on the shaft for a setscrew to land on, can work.
Also grind a flat face on the end of the setscrew instead of a cone or point, the reliability will be better.
The highest grade steel for the setscrew. Consider a Allen head bolt if there is room as they are easy to find in high strength.
Sometimes it pays to modify the sprocket for a larger diameter thread for more surface area.

Sprocket

setscrew

 

[harrisonpatm]

Grinding a generous flat spot (or two) on the shaft for a setscrew to land on, can work.
Also grind a flat face on the end of the setscrew instead of a cone or point, the reliability will be better.
The highest grade steel for the setscrew. Consider a Allen head bolt if there is room as they are easy to find in high strength.
Sometimes it pays to modify the sprocket for a larger diameter thread for more surface area.

Sprocket

setscrew

Couple of helpful links, thanks. Do you think 10t on the shaft, to 42t on the rear wheel, is going to be a problem? That ratio would probably work well for my speed/torque, and then I wouldn't need to find a new rear sprocket.

 

[Papa]

Sprockets & Roller Chain | www.surpluscenter.com

Sprockets , Roller Chain,Roller Chain Sprockets,Weld-On Sprockets,QD Sprockets , Bushings,ANSI Roller Chain , Links,Ag , Conveyor Chain , Links,Metric

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On your rear sprocket, measure the 'pitch' distance between two adjacent sprocket teeth (tooth tip to tooth tip) - then measure sprocket width at the base of the teeth.

Avoid using sprockets smaller the 14t (larger is better).

 

[harrisonpatm]

Sprockets & Roller Chain | www.surpluscenter.com

Sprockets , Roller Chain,Roller Chain Sprockets,Weld-On Sprockets,QD Sprockets , Bushings,ANSI Roller Chain , Links,Ag , Conveyor Chain , Links,Metric

www.surpluscenter.com

On your rear sprocket, measure the 'pitch' distance between two adjacent sprocket teeth (tooth tip to tooth tip) - then measure sprocket width at the base of the teeth.

Avoid using sprockets smaller the 14t (larger is better).

Also helpful, thank you. Already determined that the current sprockets are 420.

Are you suggesting I use a 14t motor sprocket and replace the rear sprocket with something more than 42t?

 

[Papa]

Are you suggesting I use a 14t motor sprocket and replace the rear sprocket with something more than 42t?

Because of the motor's 4200 RPMs @48V, you'll need a rear sprocket close to 100t or slightly larger (assuming 14t motor sprocket)

 

[PaPaSteve]

Couple of helpful links, thanks. Do you think 10t on the shaft, to 42t on the rear wheel, is going to be a problem? That ratio would probably work well for my speed/torque, and then I wouldn't need to find a new rear sprocket.

My suggestion is only pointed towards fixing a sprocket to the shaft.

A link to a gear ratio calculator

 

[harrisonpatm]

Because of the motor's 4200 RPMs @48V, you'll need a rear sprocket close to 100t or slightly larger (assuming 14t motor sprocket)

Of course, that's why I was wondering if a 10T motor sprocket is too small, because if not, a 42T or 52T rear sprocket would keep me in a better range between how fast I want the moped to go and what the motor is putting out. Is 10T mechanically unsound for this application?

 

[Papa]

20" wheel RPM @ 35mph = reduction ratio of about 7:1. Assuming motor RPM of 4200.
A 42t driven (rear) sprocket would require a 6t drive sprocket

As sprocket diameter decreases much below 14t, vibration & noise dramatically increases - and drive train wear increases significantly.

 

[harrisonpatm]

20" wheel RPM @ 35mph = reduction ratio of about 7:1. Assuming motor RPM of 4200.
A 42t driven (rear) sprocket would require a 6t drive sprocket

As sprocket diameter decreases much below 14t, vibration & noise dramatically increases - and drive train wear increases significantly.

Thank you. I'll have to fudge some of those numbers to get an appropriate drive ratio.

As for the actually fit, it seems like I'll want a sprocket with hub, 420, with a 5/8" D-type shaft. And flatten the motor shaft accordingly .

 

[harrisonpatm]

I'm also amused that the two users who have responded the most are Papa and PaPa Steve. Thanks dads!

 

[Comrade]

Threaded flange coupling bolted to a sprocket?

 

[harrisonpatm]

Not a bad idea. Is it going to unscrew itself?

 

[PaPaSteve]

It's possible, probably likely, the original drive sprocket was held in place with a taper lock system with the 1/2" thread doing the clamping.
The smooth, polished shaft may be a clue.
This style is popular in the food machinery industry as there is no need for cutting keyways in a long span of shafting.
Search "taper lock sprockets"

 

[harrisonpatm]

It's possible, probably likely, the original drive sprocket was held in place with a taper lock system with the 1/2" thread doing the clamping.
The smooth, polished shaft may be a clue.
This style is popular in the food machinery industry as there is no need for cutting keyways in a long span of shafting.
Search "taper lock sprockets"

I think you're right about the original use being for food industry, when I got it out of the scrap heap, there was some sort of pump attached to it.

So a taper lock sprocket is one where the bore is conical? If so, I get what you mean about the 1/2" thread doing the clamping. That, plus an added grub screw on a flattened portion of the shaft, could do the trick

 

[PaPaSteve]

Taper lock is a two part wedge system.
Sometimes they will have a keyway.
Never seen one with a grub screw as it means two holes need to align and less material for threads.

 

[harrisonpatm]

Thanks for all your ideas and keywords to help in my search guys. My best bet it seems, with my ability and available tooling, is going to be something like this:

Finished Bore Sprockets | Roller Chain Sprockets | Sprockets & Roller Chain | www.surpluscenter.com

Finished Bore Sprockets,Roller Chain Sprockets,Sprockets & Roller Chain,New. ANSI, fixed bore standard roller chain sprockets. Carbon steel

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I can get it in pitch size 40, 10-14T, It'll fit over the 5/8 portion of the shaft, and I just need to machine some way to keep it to sliding further down the shaft somehow. Two grub screw positions to keep rotation in place.