Spacers in FIRST gearboxes

The latest exploded diagram on the FIRST doc pages shows "spacers" being used along with the locking clips and key stock to assemble the gear boxes and make sure the gears don't drift off the shaft. Has anyone found a good way to make these spacers if you dont have a machine shop? Iwas thinking maybe 3/4 inch pvc or other plastic pipe. A whole bunch of washers is messy and adds too much weight.

Also is it just me, or is mounting the gears with the supplied keystock a bear? Seems the key stock is harder than the shaft but softer than the gears - is this the way it is supposed to be, or is there some magic tempering trick?

[Moshingkow]

There is no need for a machine shop for any of this. Just a little elbow grease. All you need to do is cut the spacers with a hack saw, and make sure you file it out afterwards. It doesnt need to be perfect, but make it as close as you can to the actual measurments. The stock for the spacers was provided in the cardboard box with the 2x4 aluminum extrusion. As for the keys, just cut them with a hacksaw (slightly smaller then the width of the gear) and hammer them into the shafts keyway. then slide the gear over and use a mallet to hammer it down the shaft. Thats what our team did. We have a machine shop, and the gearboxes needed no machining. The thing that needs the most machining in my opinion would be the shafts, which I chamferred and faced off, it makes it much easier to get the gears and bearings on the shaft.

[jrgrim12]

Sometimes filing a little start on the keys help the get started the the key slot. make sure you don't have any burrs on the mating pieces. If it is to loose just use a drop of super glue to hold it. It will hold it in place and can easily be knock out if you need to remove the key.

Oh, so that is what that aluminum tube was for!

BTW I think 3/4 inch pvc will also work the inside needs to be reamed slightly, or cutting a slot on one side would work. A 3 fot piece of PVX 3/4 pipe costs 36 cents.

[DougHogg]

Does anyone have the lengths for the spacers and axles for FIRST's drive train as shown at http://www2.usfirst.org/2003comp/Drive_Train-expII.pdf.

I am trying to get some students to assemble that drive train while we work on our other one.

[jskene]

You need to be aware that helical gears like the ones supplied for the gearbox generate considerable side (axial) thrust. That is, the gears try to slide sideways along the shaft when they are subjected to a load. The equation for this axial thrust (from Boston Gear's website: ) is:
Axial thrust load = (126,050 x HP) / (RPM x Pitch diameter)

So, for the large gear when driven by the drill motor in low gear, RPM = ~450, HP =~ 250watts/746 watts per HP = .34, Pitch Diameter of the 1224 gear = 2"
Axial thrust load = (126,050 x .34)/(450 X 2) = 190 lb. This suggests that you would be wise to use the aluminum tubing provided, and not PVC pipe.

I think that the key and the lock washers would do more to fix the gaers in place on the shaft would provide considerable "grip" on the gear. The spacers are mostly for spacing, and really don't provide a lot of structural strength.


As to precise measurements for spacers, the aluminum is soft enough to work easily, so a hacksaw and a file should be adequate to cut(slighty big) and fine tune the spacer. So I think FIRST is correct in saying the Chasis could be easily built with handtools. The only reall hard part is making a hole for the axel through the square aluminum
tube. Hard to drill a hole that big with hand tools. A small hole could be drilled and then enlarged by hand filing, While a spurred wood spade bit in a drill press would work - I don't feel that is a very safe method.

I think we will get 5fps with the FIRST supplied drive in low gear.

[Dick Linn]

I would not depend on banging in keystock and having the gears stay put and in proper alignment. We initially hacksawed spacers just a wee bit long, after using a magic marker and the jaws of a vernier caliper to scribe a line at exactly the right length. The trick is sawing part way through, all the way around, right next to your scribe line. Otherwise you don't get a straight cut. With practise, I could get within a few hundredths, but it's a bit of work. Then the trick was to file the ends parallel and make each pair the same length. Still, we managed to get a number of them to within, I'm guessing, .010 of what they should be. Later we got a cheap $4 tubing cutter, which makes it a whole lot easier to cut the tube straight, although you still have filing to do. I used a short piece of 5/8 inch wooden dowel clamped vertically in a vise and let the spacer protrude just a bit above when filing.

I measured the gearbox and the tubes are nominally .750 for the short ones and 1.1875 for the long ones. This is without any spring preload washers. I still do not know how much shorter to make the spacer tubes in order to properly preload things using the spring washers that came in the kit. (These are the slightly cone-shaped steel washers.) They are too large in diameter for the small gear but can butt up against the large gear. If the spacer tubes are too long, these axle assembly will push too hard against the bearings and create a lot of friction. For now, we're trying to use only the tubes, make each pair on a shaft exactly the same length and end up with little or no slop.

If you do get spacers of equal length but get them a wee bit short, you may be able to find extremely thin shimming washers to add to each end to take up any slop.

I didn't say "don't use spacers." I said the aluminum is soft enough to be easily worked by hand. If the keystock is cut too long, the aluminum can be easily notched. Once you know how do it, it is possible to build a working chasis and drive system in a day with only hand tools. (2 hours is pushing it!)

Mounting the gear boxes and motors on the aluminum takes patience. The wheel mounting plate can be used as a guide. It is notched to fit the square alu0..000000000(that was my son's kitten typing) aluminum. use a center punch to locate the holes then use a drill press. I suggest step drilling (start smaller then go bigger 3/16, 1/4. 5/16) Use washers and lock washers. When the gear box is mounted and snugged up, make sure it still turns freely. Andy binding would be a source for concern. I would mark and mount the motor after the gearbox was in.

[ROB at RAGE]

Buy a step drill bit, they are sold in many catalogs as well as any good hardware store. They make drilling holes in aluminum (and plastics go almost like butter (espically lagre ones). Use a little oil on the bit for metals also.

[tenfour]

To avoid using a spacer, I would suggest that you look for a shaft collar. If the axial loads are too great to be held by a shaft collar, then my next choice would bve to use the Al tubing. If you cut it to a rough length, you can file it to the right square length. This does take some time, but taking shortcuts doesn't pay off with drive systems. As for the PVC, if you use Schedule 40, it will probably be OK. It is good under compression.

Hope that helped!! Good luck!

[ROB at RAGE]

If you got a pipe/tubing cuter use it. You can also cut it with chopsaw for wood if it has a carbide tiped blade, coating the blade with some bees wax keeps it form getting loaded with aluminum. Clamp the tubing down and don't use your hands with the chop saw. A band saw with a metal or wood blade usually cuts aluminum well also.

If you have to hacksaw it, you can square it up on a belt or disc sander.