Wheel mounting
 

Well, last year, we had some serious problems with wheel mounting. Our teacher said that we should use pins that go through the shaft, to fix the wheels on the shaft. This did not work out all too well. One of the pins broke on one side of the shaft, and so it was wobbly, and on the verge of breaking during competition.

Is there any way to fix wheels on to a shaft, which is not permanent, which will keep it on there? All the tools i have are 40 year old non-cnc lathes, drill presses which are not too accurate, a milling machine, a bansaw, and various powertools.

I would also like to tell you guys that we have had some bad experiences with set screws, and unless you have a way to make them better, weve already been down that path

thanks,
tenkai

Keys, keyed shaft, and keyway bores. You don't even need any tools really except if you wanted to custom turn the ends of your axles to fit in smaller bearings. Much stronger than pins and less permanent too.

We usually use key stock and set screws just to make sure

In the words of Woodie, "Set screws inhale audibly." Set screws=bad. At least for attaching sprockets, etc. We use the key method, too. It works great, though it's not flawless. We've broken keyed shafts before, but only under extreme circumstances (huge forces on 3/8 shafts). Most of the time, keyed shafts are just right for whatever you need to attach. We don't actually attach our wheels to the shaft (because of how we do our drive, they're free-spinning), but that's how we attach sprockets.

Oh yeah, I forgot to mention the use of set screws to keep the key in place and shaft collars to keep the wheel/sprocket/etc. in place.

Ahh. yea, i know set screws are pure crap. (pardon my french) but let make sure i understand keys. Is it when you have a shaft with a small piece attached, and on the wheel, its taken away, so that they fit together? if so, is the stock you buy already extruded with the keyway on it?

tenkai

oh, also, can this method be used for gears? thats a problem we have too.

Yes. The shaft has the key, and whatever you are attaching has a hole for the key.

No, you have to make a key and have trench-like section removed from the shaft for the key.

Sure can. We did on our transmission.

I hope those answers were helpful. Let me know if you need clarification.

so the keyway is the trench like thingo on the wheel/gear and an lump on the shaft. ok. i get that. But one problem i forsee would be attaching the keyway. Would it just be like a flat piece of metal screwed on to a shaft, or would it have to be welded? Also, cutting out trench from the wheels/gears would be a daunting task to do accuratley. I think that I would use the miller, but i dont know if it will be precise enough to be effective in transfering the power without vibration.

if anyone has a link to pictures of this, that would help me immensley.

tenkai

I hope this clears up any confusion. The shaft is sold with the "slotted out" side. The key is a small piece of metal (usually only as long as the thickness of the sprocket or gear hub) that fits in the slot and portrudes up into the keyway (slot) in the sprocket. Keys can be bought in key stock (long piece) or cut and precision ground into keys of predetermined length.

You can check out the "power transmission" section near the bottom left of the www.mcmaster.com homepage.

here's a pic of a keyed shaft

here's a pic of a key (images not proportional. Key is much enlarged)

This was taken from a powerpoint that was given by Andy Baker (i think) at some Indiana FIRST Team thingy...

Gear and Sprocket Mounting
-Trantorque (for medium to low loads)
-collar clamp on a split hub
-drill and tap through on a hub
-dutchman's pin
-weld
-NEVER use set screws for this

Gear or Sprocket to Wheel
-skyway - pin wheel to large shaft, or capture spokes with a hub
-using 3-4 holes, attach to wheel, close to perimeter

here's a pic of a keyway bore sprocket

And last here's a pic of a common type of shaft collar.

Ooooooh. Ok I think i get it now. the keyway is on both the gear/wheel and the shaft, and the key is just a piece of stock that is inserted. That does seem like a good idea. I the only problem I have would be machining a keyway inside of a hub for a wheel or the bore of a sprocket. any ideas? I have, as stated before, a non-cnc lathe, drill press, milling machine, band saw, and various hand power tools

tenkai

also, if anyone has sources where you can buy pre-keyed stock from, that would be greatly appreciated.

Two sources for keyslotted shaft stock are MSC and McMaster-Carr. And there are lots of others.

The most common method to cut the keyway on the interior of a gear or wheel hub is to use a broach set. This is basically a specialized set of cutting blades (think of it as a very wide saw blade with just a few, large teeth) that are forced through the bore opening with an arbor press (a manual press will work just fine). Two passes with the broach, and you are all set. You can buy individual broaches, or complete sets, from either of the sources mentioned above. If you can't afford broaches (a full set can be $150 - $300), you can cut keyways with the lathe, it just takes some patience. Put the gear in the lathe chuck, and lock the lathe spindle with the backgear so it doesn't rotate. Mount a small square cutter in a boring bar, and put the bar on the lathe cross slide (sort of like a sharp tooth on the end of a stick). Advance the cross slide carriage toward the spindle, and let the cutter just skim the inside of the gear bore. Retract the carriage, advance the cross slide a few mils, and do it again. Repeat until you have cut a keyway of the correct depth (1/2 the width of the key). It takes a while, but it works.

Note that if you cut a shaft keyway that is too large, you may substantially weaken the shaft/axle. The industry rule of thumb is that the keyway width should be no more than 1/4 the diameter of the shaft.

Most common keystock falls in the range of 35,000-50,000 psi shear strength. Based on the dimensions of the key you use, and the radius of the shaft, you can calculate the maximum loads the key will withstand before it deforms and shears. Make sure you size the key appropriately, otherwise you may end up shearing through them just as you did with the pin mounting (although at a much higher load level).

-dave

sounds good to me! but keep in mind that there are two different loads stock can take: a safe load and an ultimate load (i think thats the right term). The safe load is usually advertised as the maximum load. But that is usually about 1/4 i believe of the ultimate load where the piece will be permanently deformed. I dunno how much stress is exerted on a shaft, but i guess i could calculate it. (gotta dust off the strength of materials notebook).

Anywho, thanks for all the help! If anyone has anything to add, please do so.

tenkai