Step 16: 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.

<p>This 4 year old instructable is STILL a treasure trove of useful relevant information. Thanks for sharing!</p>
A fun build! Thanks for the awesome instructable!
<p>Actually instructive. Though the term is common I had no idea what a brushless motor was or how it worked, and I am an electrical engineer, an old one. Sixty years ago I was wondering why the brushes of the dead motor that I was rewinding were not semiconductor devices. The transistor was then 10 years old. Life took me in other directions so the idea never matured - and suddenly I know. Thanks! Further I now understand something about the DC/AC drive of the Tesla Auto. My interest is, at the moment, in electric propulsion for a nearly finished Sampan sitting in my back yard. Though off the shelf devices are readily available I am hoping for something better. Your remarks about RC devices and propellers seem to apply. The battery info is also helpful. I am struggling with the tradeoff of a low HP gas outboard, at less than 40lb and a 20lb trolling motor with a 50lb 12 volt lead acid battery. You advice about how to purchase motors, batteries and controllers is most pertinent. - Boatmakertoo</p>
<p>Hi My gfs bike has a 12mm rear axle and she hates the 6kg front hub. Id like to use this motor to drive the back wheel. It looks very similar to my internal motor on a mac 10t geared hub motor ie lots of turns. Could I not just use the infineon sensorless controller? http://www.hobbyking.com/hobbyking/store/__54888__9235_100KV_Turnigy_Multistar_Brushless_Multi_Rotor_Motor.html</p><p>Anyway thanks for the article, Dan</p>
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<p>Hi, your<br>article is sooo good!</p><p><a href="http://www.instructables.com/id/The-New-and-Improved-Brushless-Electric-Scooter-Po/all/?lang=es" rel="nofollow">http://www.instructables.com/id/The-New-and-Improv...</a></p><p>I created<br>some links to it at my site <a href="http://www.avdweb.nl" rel="nofollow"> www.avdweb.nl</a></p><p>You've put all<br>topics under each other so they are difficult to localize, it is sooo long now.<br>Can you create a table of contents on the top with links to the separate<br>topics?</p><p>Or create<br>separate items on instructables.</p><p>Kind regards,</p><p>Albert</p>
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What is the pinout for the standard, RC esc sensor connector?
Wonderful instructable. Have been giving it some thought for a bit and wondering why you haven't considered building your own ESC. From your descriptions a sensorless controller suitable for our purposes seems very doable, but not available within the R/C airplane realm. We need a custom, self-starting, sensorless ESC. Other's have made them. <br> <br>Once you have the basics together fine-tuning becomes possible. Seems like the perfect solution to a host of problems. Perhaps the springboard to designs otherwise out of reach.
Thanks for your instructable. It's very interesting and complete but I have a, may be, stupid question. I'm planning to put sensors in an outrunner as well and wanted to know how to tie the sensors to the ESC. There seems to be a standard 6 pin connector shared between the different ESC models. How do I wire the sensors to this connector?
This reminds me of a really cool scooter my best friend made once. While I was just looking for <a href="http://www.voloscooters.com" rel="nofollow">electric scooters for sale</a> he decided to make one himself, he even made it run off of a vacuum motor! I really didn't think it would work, but it sure did! I always admire those who are creative enough to figure out how to do this kind of thing.
This is a pretty sweet little scooter, might make one for my son if he starts behaving again. lol <br> <br>I am legitimately trying to find that gas motor which you showed but did not use. <br> <br>Can you tell me where you found it? I can only find electric models, which are great but not appropriate for my current project. <br> <br>Thanks, <br>-Grey
I found a good place to get thick very flexible 8 and 4 gauge wire is the local car stereo center. Th 8 gauge I used cost $1 per foot. They had 4 gauge but it was much more expensive.
Wow after reading this article now I'm building a scooter. <br>Bad influence I think.. <br>Welding and machining I get but the electrics are not so much my thing :( <br> <br>I bought 2 motors <br> <br>KA63-18L <br>Constant: 259Kv <br>Battery: 10Cell Lipo <br>Operating Current: 25-60A <br>Peak Current: 72A(15sec) <br> <br>Here is the problem.. Do I run one and have a spare or use them both? <br>If both what would be the best way? <br>
If you run both you need a controller for each. Now this may sound easy but you need to be careful to set up each esc the same and use the exact same model of esc and motor or one motor will do more work. Now unless you want to push more than 200 lbs around at 20 mph or more one motor should be sufficient as long as you use the gearing equation from this article as to not burn up your motor. Good luck and feel welcome to ask any questions you want to.
Hobbyking now sells 20C four cell 5 Ah lipo packs from the US warehouse for about $25 per pack: <br>http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631
Could I maybe use a large pvc pipe to hold the hall effect sensors?
How exactly do you back install hall effect sensors in a motor? I am trying to make an electric motorcycle and want to use a kelly controller. I have a Turnigy 80-100-A 180 KV brushless outrunner (the same class that is on the scooter in this instructable) and have had a really hard time figuring out how to install sensors. I am also having a hard time figuring out how this motor is wound. <br>Any help would be greatly appreciated, thanks in advance.
It is extremely difficult to install *internal* sensors onto a mystery motor (of which you do not know the internal winding pattern). The C80 series is also very difficult to discern because they require so much current. <br><br>Usually, you would run current through two phase wires (&quot;line to line&quot;) and use a test magnet to observe which poles are polarized in which directions - they should always be alternating from tooth to tooth and there should always be 8 of them on in total. Using this and the process of elimination you can separate the 12 teeth of the stator into 3 groups of 4 teeth which you can call &quot;A&quot;, &quot;B&quot;, and &quot;C&quot;, which are associated with one of the three phase wires. Then, the Hall sensors are placed *between* 2 teeth of *different* phases.<br><br>Because the C80s have a very low resistance and few turns, you need either a ton of current or a sensitive magnetometer to determine the direction of the stator field. <br><br>Using external Hall sensors on the C80s is possibly the easiest solution. <br><br>
Thank you for the info, I have tried to run current through the different phases but I could not get enough current to get discernible results. I am thinking of just rewinding the motor so I know where the different phases are. Unfortunately I do not have access to anything to make an external mount for hall effect sensors. Do you know of a website that could make a mount for me? I also do not have any experience using CAD software. Another question I have is do all hall effect sensors have the same wiring scheme? The website I got mine from did not tell me which lead is which.
hi <br>a bit for the sake of accuracy: when you refer to &quot;convection&quot;, you actualy mean free convection(no air speed), as opposed to forced convection. <br>other than that petty note, its a great article and im learning why i shouldnt have bought the esc that i have...
Just wanted to say a quick thanks for this instructable. It's the detail all in the one place that i have been searching for, for years. Thanks for confirmation of R/C parts as well and the technical detail.
Can you use the sensored kelly controllers with a sensorless motor?
Not the KBS - Kelly does have the KSL line which is sensorless, but I have not heard vehicle stories about them nor know what settings they can manipulate (e.g. ramp-up time, initial current, etc.). And they're huge - they're full-size Kelly cases.
Great tips, especially about bore size changing! Thanks for taking time and explaining this!
If I'm going to use LiFePo4 batteries from HK, will I need to include a battery management system when discharging and/or charging? Great instructable, by the way.

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More by teamtestbot: How to Build your Everything Really Really Fast Chibikart: Rapid-Prototyping a Subminiature Electric Go-Kart Using Digital Fabrication and Hobby Components The New and Improved Brushless Electric Scooter Power System Guide
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