Hi, we tried quite a few things and we are out of ideas. We have a very big sprocket to rotate the turret on a turntable. It is McMaster Carr part # 18635A52. The sprocket came out to be 250 teeth. The CAD model was generated in Inventor. The sprocket was made using waterjet and is 0.090 inch thick. The problem is the #25 chain cannot wrap around it. It seems the pitch of the sprocket is slightly under 0.25 inches. After about 50 teeth, the chain does not want to seat in the sprocket anymore. For most normal size sprockets this would not be a problem because it will only engage 40 teeth or so at the most. Our chain will need to engage about 150 teeth.
Does anybody have experience in making such large sprockets? Did you experience these type of problems? Please help. I have attached the CAD model.
It’s difficult for roller chain to stay on pitch over long distances of continuous tooth engagement, as slight tolerance variations can really add up. Generally using sprockets with more than 100-120 teeth should be avoided because of this.
One way to get around this tolerance limitation of large sprockets that do not have to rotate continuously is to use a toothless sprocket and physically lock the roller chain to some point on the sprocket; torque is transmitted through a combination of friction and the locked position.
I’m going to guess you want to do 360 + rotation & don’t want to fasten the chain to it but want to keep on revolving. That being said, if I was faced with your problem I would apply a wire wheel to the teeth at frequent intervals to “eat away” the excess tooth width. Don’t agonize about the tooth geometry it doesn’t have to be perfect it just has to work.
Good luck. Regards.
Yes we do not want to be limited too much on amount of rotation. Another reason is having most of the chain engaged in the teeth will ease my concern about the chain drooping down.
What is wire wheel? Is it just wrapping wires around the chain? Do you have a picture?
I suggest you consider “segmenting” your sprocket.
Lets say you plan to divide it into six pieces, and you know the saw cuts will allow the segments to collapse inward just enough to get the rollers back down into the roots of the teeth.
You lay out your six slice lines and you mark out three mount holes per segment to be drilled through each segment and through whatever the sprocket mounts to. You drill your mount holes BEFORE you slice up the sprocket into segments.
Now by slightly oversizing the three through holes in each segment, relative to the mounting fastener OD, you can shift the segment locations radially, thus shrinking the effective sprocket diameter enough to maintain decent chain wrap engagement across all the segments throughout the full circle.
I would use shoulder screws for mounting the sprocket segments and an adjustable reamer to gradually increase the sprocket mount holes’ size until your chain seats fully into all six of the segment arcs (well, only three at a time really).
Because you have such a large contact area you could probably get away with removing or shortening some of the teeth this will turn it into a many sided polygon with a perimeter less than that of a circle, that is if the chain is too short. If the chain is too long, then you could paint or powder coat it several times to increase its diameter.
Here is what I would do: I would abandon the chain idea and try to drive the sprocket like it was a gear. Take a small segment of #25 chain and then wrap it around a flanged hub of some sort, securing the ends to the flange of the hub with a pair of pins. Basically, this provides you with a “pinion gear” to install on your drive motor and power your “sprocket gear”. You mentioned the #25 chain engages the sprocket fine in small intervals so driving it like a gear would only mean you are contacting the sprocket along two or three teeth.
Generating an accurate CAD model for something like this is always a pain in the @$$. Last year it took us about 15 tries to finally produce a working sprocket for our drivetrain despite using the FIRST CAD library. I assume that at this stage of the game the chances of you getting a new sprocket cut in time is slim to nil.
A wire wheel is a disc that you would install on a pedestal grinder that looks like this: http://www.wolfcraft.us/images/photographs/WireWheel-8In.jpg
It is used for cleaning up parts and removing small amounts of material. I remember trying this last year when my team was trying to produce their own sprockets and if you are off by much it won’t work and I wouldnt suggest it.
Thank you to all your suggestions. All of them are possible solutions. Based on the amount of time left, this is what I am going to do. Jim Zondag came to the rescue again with this idea. It is similar to what Wayne suggested. I have a motor attached to the driving sprocket at one corner to turn the big sprocket. I am going to put an idler sprocket on the opposite corner to pull the chain out to reduce and split the number of teeth engaged into less than half of what it used to be. I will also mount this idler sprocket on something that has a slot for adjustment until the chain sits properly on both sides. This seems so simple and elegant. I learn something new every day.