Step 16: Good Mechanical Ideas

Picture of Good Mechanical Ideas
This section will address some small mechanical details that can be headaches to those who are not machine shop endowed. If you do have metalworking tool access, then attaching two things together or changing bore and shaft sizes is generally a trivial operation. But I started building vehicles when I did not have extensive shop access, so the following advice is mostly aimed at the starving vehicle hacker.

Set screws may suck, but they work well if done properly

An old adage in the combat robot community I have been part of is "set screws suck". They rattle and come loose, they mar and damage shafts, they strip out, and are generally horrible to deal with. However, I contend that a large part of the disdain that robot builders have towards set screws is caused by them doing it wrong .

While the average set screw you may receive with a pulley or sprocket can be tiny - the common thread on a 5M-15 motor pulley being #8-32 or #10-32 (metric 4 to 5mm) - what's more important is how the set screw is seated in the shaft.

If a set screw only has a round shaft to grip, then all of the transmission forces must pass through a very tiny tangential contact patch. The result is the familiar marking and ring-shaped groove that forms when (not if, when ) the set screw slips due to shock torque or overload. To effectively use a set screw, it must have a flat or a dimple in the shaft to grip . A flat can be filed or ground and a dimple can be drilled using the pulley or sprocket's set screw hole as a template.  It doesn't have to be deep nor expansive - any concavity will do. The contact patch is changed from a small tangential deformation to a much larger solid material interference fit. Now, instead of just overcoming a small ring-shaped patch of metal, motor loads have to essentially crank the entire set screw through the shaft.

Another important but often overlooked detail with set screws is that they must be as large as possible to obtain as much face contact area with said flat or dimple as possible. Too small a screw can just shear off or indent the shaft and become loose. While having a flat or dimple is more important, I often take the opportunity to "upgrade" the set screw size on a pulley or sprocket - especially pulleys, because sprockets tend to come with keyway bores I can take advantage of.  I've done ridiculous things like use a 5/8" set screw on a 3/4" diameter shaft, and just about the minimum I use on a standard 10mm diameter shaft is 1/4"-20 (or M6 coarse thread).

Bore changing - large to small

One of the hardest parts about putting together the power transmission system of a vehicle is how to attach the initial chain sprockeet or pulley to the motor. For those with a shop at their disposal, bore adaptation and remachining is a trivial matter. But otherwise, you'll have to pull some tricks and hacks to adapt the bore of the sprocket or pulley to be mountable on a motor shaft.

If the shaft is smaller than the sprocket or pulley bore, it's relatively easy. Plain bronze bushings ("bronze bearings, oilite bushings") can be used as a shaft diameter adapter. You'd have to find one with an OD that fits the pulley or sprocket bore and an ID that fits the motor shaft. If both are metric, which is often the case, this is relatively simple. Metric bronze bushings are available now from most industrial suppliers, including McMaster-Carr .

If the pulley or sprocket has a simple cross-drilled set screw, then you can use the existing set screw hole as a template to manually drill through the inserted bushing. This hole is important, since having the set screw press against the bushing is an ineffective way to transmit power - a solid mechanical connection must be made between the pulley or sprocket and the motor shaft.

Aluminum can shims

The boring soda can is in fact a very precise source of shim material. Can wall thickness generally varies from 0.003 to 0.004" near the center to 0.006 inches near the base (but not the rolled base itself). So, if you cut a sliver of drink can and stuff it into a misfitting bore, you can take up slack of around 0.01" diameter.

Bore changing - small to large

For operations where the bore of the pulley or sprocket needs to be enlarged , this is a much more difficult affair to the non shop endowed. Drilling out a bore is almost certainly guaranteed to render it off-center and wobbly. There is no technique that I know of which can use garage tools (up to and including a drill press) to create an on-center non-wobbly new bore in an existing old one. If you know one, please share it.

One option is to use reamers if the diameter is close - for instance, 3/8" to 10mm (0.375 to 0.393 inches), or 5/16" to 8mm (0.3125 to 0.3145 inches). The multiple straight flutes of the reamer tend to keep the reamer centered in the existing bore, but this still only works to a limited degree if it is not started straight to begin with (i.e. you're chucking it in a  hand drill). However, the reamer itself can be expensive - $15 to $30 depending on type and size

For large bore size changes, like 8mm to 12mm - very common for fitting a 5M-15 HTD pulley to a large outrunner, metalworking machinery is practically indispensable. You may be better off asking a friend with machine tools or getting to know an area machine shop. Such a large change requires multiple "passes" of larger drills and reamers, and without a fixed center on a metal lathe, each pass causes more concentricity loss.

Imperial drill bit sizes that are almost metric

These drill and reamer sizes are close enough such that you can fudge around and make a fit if the correct size is not available.

A #9 drill is 0.1960 and is a spot on slip fit for 5mm shafts.
A 15/64" drill is 0.2340 and creates a tight press fit for 6mm (0.2360) shafts
Likewise, a B drill is 0.2380 and creates a loose slip fit for 6mm shafts
5/16" is within 0.002" of 8mm (0.3125 vs 0.3145 )
An O (that's the letter O) size drill is 0.3160 and therefore will make a loose slip fit on 8mm shafts
15/32" is almost 12mm , but the difference is 0.004" and is too great to create a press fit.
19mm is almost 3/4" (0.748 vs 0.750 )

Here's my favorite wire gauge and drill size chart for those of you interested in the hard numbers.

Gap-filling retaining compound is your friend

If you're in a situation where there is a few thousandths of an inch of diametric gap, it's not quite the end of the world. Thick retaining compound is beneficial here. The difference between threadlocking compound and retaining compound is that the latter is designed for use in situations where there aren't threads. Loctite type 635 and 638 are very thick retaining compounds that can fill gaps of 0.01" or more.  Type 609 is very thin and my favorite for sticking bearings in pockets where they should be.

This does not mean you should rely solely on Loctite for power transmission - a set screw or similar should still be used, but the Loctite, once set, will bridge the gap and make the joint less prone to wobbling.

bratan3 years ago
Great tips, especially about bore size changing! Thanks for taking time and explaining this!