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  • Making a sprocket

    What all is involved in making a sprocket on a bridgeport? What I'm making will be a 50 tooth sprocket for a 520 chain. The side surfaces of the sprocket will serve as a brake rotor. So far I haven't found anyone who either stocks, or is willing to make exactly what I need. Also, any suggestions for material would be great. Most sprocket/rotors I have seen are stainless, which I've had little experience working with.

    Also, with the current setup I'm working on, the counter sproket on the motor will only have about 4.4 teeth in contact. I'm worried that the teeth will shear off. Any thoughts on that? Neither changing sprocket diameters or increasing the distance are good options. I was considering adding an idler sprocket on the slack side of the chain to increase the wraparound on the counter sprocket.

    This is all going into a small, forumla style car powered by a 600cc bike motor. Should be making somewhere around 70 to 80 horsepower at the wheels.

  • #2
    Machinery's Handbook has the specs for what the teeth form should be. You can buy (but I've never seen them listed) form cutters for sprocket teeth, just as you can buy form cutters for involute gear teeth. (I know they exist because I have one I picked up in a lot of $5/pound cutters once.)

    You'll need an indexing head. Or you could do it with a vertical rotary table, assuming you can mount the blank in a way so you can get at it to cut it.

    I'd use something like 4140, I think, but I really don't know.....
    ----------
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
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    • #3
      Years ago I used to buy motorcycle sprocket blanks (just the teeth and a 1" center hole) of 7075 aluminum to remachine into chainrings for racing bicycles. Here's a link to their website. Maybe they can help you out.
      http://www.peszbros.com/index.html

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      • #4
        Wow! cool project that raises some design considerations.

        A sprocket that doubles for a disk brake: A few things to think about maybe. How hot does will the disk get? If you use an alloy or high carbon steel for tooth strength, then you may temper the strength away with heat while braking. Also, what about thermal stress. Note that hi performance disk brakes such as on a motorcycle...the disk floats on the hub. This allows the disk to expand freely in the hot section without being constrained by the cooler hub area. Hence the thermal gradient from hot to cold is split by 2 seperate parts and the stress is reduced. In a solid disk/sprocket...you will combine the stess from chain force with the thermal stress. How much is too much? I dunno. It's a great text book design problem that can be investigated with fundamental strength of material and thermodynamic approaches.

        As far as shearing teeth...use the 80 hp to determine determine the max torque, Then using the pitch diameter of the sprocket in question, determine the tangential force. Calculate the shear area for number of teeth engaged then find stress= force/area. This will give you and idea of the shearing stress in the teeth. Then you can spec the sprocket material and heat treat to meet the stresses...add a sfaety factor to account for abuse and transient loads.

        Another approach would be Finite Element Analysis.

        The project sounds like SAE formula car design competition. I did that in '91,'92,and '93.

        Good luck and keep us posted.
        Rob

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        • #5
          Thanks for the input.

          SGW: I've been looking for a cutter as you describe, but no luck. The only company I've found online is out of India.

          Bob: You're right, it is a FSAE car. My second year working on it. It's been a blast, and I've learned so much about machining that I never would have learned otherwise. And thanks for the input on design ideas.

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          • #6
            Will there be a problem with chain lube contaminating the disc surface?

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            • #7
              Hello JDF,

              Several things, but i dont mean to discourage you:
              1) Where did you get this 520 number on the chain? Standard ANSI chains only go up to 240? How did you id this chain?
              2) From Motion System Design Handbook the min requirement for chain sproket is 2 teeth assuming that you have at least 120 degree contact angle. But note do the stress calculation to fiqure out if you would shear the teeth. Note the prefered distance between both drives is 30 to 50 times the pitch. What is your pitch? What is the distance between drives? Also and VERY important DO NOT exceed Maximum HP and Maximum speed/RPM rating for your chain!!!
              3) Also put a guard on the chain in case the chain link fail and come flying off. Been there done that.

              Regards

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              • #8
                Hello JDF,

                Found the 520 chain spec. Its a motorcycle chain class 50. The inside width varies for this type of chain (for more extreme conditions) Also check out:

                http://www.ustsubaki.com/

                For chain reference section. Call them or a motorcycle specialty shop may be able to help you also.

                Regards

                Please post back on your solution

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                • #9
                  In order to keep chain lube from the rotor surface, we will use a lube-free chain. I know many people say you still need to lube these, but the chain will see relativley little run time compared to most bikes, so it should be ok.

                  There is no way to get 30-50 times the pitch diameter between centers. It would add at least ten inches to the wheelbase. Can't happen. Right now the design calls for seven inches between centers, down from 8.5 last year. Trying to move everything closer to cut a few inches off the wheelbase.

                  alumtuna, I've got 117 degrees of contact which is just over 4 teeth. Sounds like it might be okay by what you're saying. The actual calculated shear stress I found for the four teeth is just under 3000psi. Haven't had a chance to find the allowable stresses for the sprocket material yet....test this evening.

                  Thanks again for all the suggestions, look forward to reading more.

                  JDF

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                  • #10
                    JDF,

                    In the mid '60's BSA ran their works Moto X bike with an Airheart kart disc caliper onto an L72 aluminium spec rear sproket. This is similar to what you are looking at I believe. From what I can gather the aluminium sproket was hard anodized, and the caliper floated. What front sprocket size are you looking at ~ Renolds chain recommended a minimum 15T for a driver although I have seen as low as 13T used on occasion.

                    I have a Renold 520 form cutter ~ they do exist! There is a place in the UK that sells these cutters for about آ£15 ($22USD). Let me know if you want their address.

                    RR

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                    • #11
                      RR

                      I'd definatley like the address of the company you mention. Send an email if you don't wish to post.

                      I am looking at either a 13 or 12 tooth front sprocket. Probably the 13. Anything bigger will force the rear sprocket larger, making the rear compartment of the frame bigger than desired.

                      Last year we ran a 7075 aluminum rear sprocket and a seperate aluminum rotor. Not sure of the alloy on the rotor, but it was hard anodized like you said. I guess aluminum may work.

                      I got the idea for the sprocket/rotor from this site:
                      http://www.exilecycles.com/brakes.htm
                      They have a nice product, but I just haven't found a way to make it work.

                      Thanks again
                      JDF

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                      • #12
                        JDF,

                        Try 'Tracy Tools Ltd.' Phone # 001 44 1803 833134 (thats direct dial from the States to the UK). You may wish to email '[email protected]'

                        They have a pretty good range of all sorts of tooling, and do export orders. They even do L.H Acme taps for any feed nut restorers out there!

                        If you phone remember the time difference! We're 5 Hrs infront of Eastern Time. They operate 9am to 4pm.

                        Usual disclaimers ....just a satisfied customer.

                        As for the website ...it looks pretty much how I remember the BSA system. There's nowt new in the world is there!! BSA were pretty hot in the 60's with their metallurgy dept. Pity the 'powers that be' didn't invest more into the company.....usual story about the decline of UK (and now USA) industry.

                        RR


                        [This message has been edited by Ragarsed Raglan (edited 09-30-2002).]

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                        • #13
                          jdf,
                          http://www.linkgear.com/
                          try these guys, if they have a cutter they will probably hob one for you.
                          Most sprockets (and gears) are hobbed or shaped these days.
                          You can drill and bore the holes (at tooth center) on your bridgeport with rotary table, rough slot from holes to o.d.
                          reset 90 degrees and cut the tooth form with a weldon type corner rounder,
                          Between fractional and metric sizes something should be pretty close to size(radius).
                          mite

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                          • #14
                            JDF

                            The site you refer to for the rotor is using stainless - that is why theirs works.

                            A coating that will prevent wear is Diamond plating (Norton Abrasives). Diamond can withstand extreme heat and has the heat tranfer coeefecient of a metal. Only problem I see is the suface may need to be scored with a diamond saw after plating to give the pads (a place for the film to tear off material!) purchase. Carbon metallic pads should work, but may wear fast. Cooling is going to be real important...

                            Something to ponder. If this works I expect steak & beer...

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                            • #15
                              Thrud,
                              Are you suggesting that diamond plating would make using aluminum for the piece acceptable, or are you saying to use it on steel? What kind of price would something like that run? A large part of the FSAE competition is keeping the marketable price of the finished car under $25000, based on a production run of 1000. Just something else I have to keep in mind.

                              As far as the beer and steak, the competition will be in Detroit next May...come on down.

                              RR,
                              Thanks for the information. Going to try to contact them tommorow.

                              Thanks again to everyone here. Many great ideas. Once I get the sprocket squared away, it will almost be time to get down to the shop for the start of the real work! Can't wait!

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