Special partially threaded screw joints and HS gear bearings using modified 393 output gears

With the V5 limit of just 8 motors worth of power (even applicable for VEXU) more teams are considering sharing motors between multiple subsystems. There are various options for doing that using ratchets, differentials, or transmissions, but most are facing the same obstacle of extra power losses to friction due to the increased axle count and complexity of the build.

There are several solutions to reduce the friction and improve efficiency of such designs. One is to use partially threaded screws instead of square axles and another is to employ drilled out internal gears from the legacy 393 motors as bearings for the high strength sprockets and gears. The latter, in addition to reducing the friction compared to square axles, allows to have independently rotating coaxial mechanisms without doubling the friction penalty.

Legality of using internal gears outside of the 393 motors have been ruled and confirmed in the past seasons and, according to <R11>, any commercially available screws under 2" long and between M3 and #8 in diameter are allowed.

Partially threaded screws

They are available from multiple manufacturers in various sizes: https://www.mscdirect.com/browse/tn/?searchterm=partially+threaded+cap+8-32


To maximize the utilization of the smooth thread-free part you could drill into a flat bearing such that it holds the screw cap.

For more advanced use you could modify the screw to have threads on both sides:


Above is the sequence of steps (from bottom up) taking the 2.5" screw, cutting its cap off, and grinding other end to lock it in the vise. Then threading the smooth side with a #8-32 die, cutting it to legal 2" size and finally combining two of them into a long axle replacement with the aid of the standoff. Using blue threadlocker inside the standoff is highly recommended.

This method was used to build a custom hub. The “axle” both let the sprockets on the green inserts run smoothly and provided additional structural supports connecting chassis on both sides.


Drilled out 393 gears

Here is a picture of the 30T sprocket on a 393 gear bearing:
The sprocket was “drilled” with a standard screwdriver bit and now it fits both 393 gears and normal high strength 1/4" axles and inserts. Just make sure to drill it perfectly perpendicular to the plane of the sprocket.

To drill 393 output gears you will need hard metal cobalt bits (4.0 and 4.8 mm) and an access to a drill press. In theory, you can do it with just a titanium bits and a handheld drill, but it will take you 10x longer and you will have to re-sharpen the bits countless number of times.

The best drilling sequence seems to be to, first, drill with 4.8 to the bottom of the square hole, then use 4.0 to drill the gear through and finally use 4.8 to enlarge the hole to its full size.


There are several ways to attach 393 gears to your structure, but the best seems to be the modified c-channel couplers, which let you mount them almost everywhere. You can use conical sheet metal drill bit to make holes precisely centered and matching gear diameter with a final touches of the round file.

Few more pictures of the parts:



More details of the differential built using drilled out 393 gears can be found here: Differential transmission - power takeoff from the drivetrain motors


Wish I had more hearts to give to this post. Very nice engineering going on there.


Technik3k Confirmed OP


If anyone is wondering if there are any good uses for the drilled out 393 gears, besides building differential transmissions, the answer is yes.

You can use them anytime where you need two independent coaxial mechanisms and don’t want the same axle to support both.

Here is an example from Team 91A of the adjustable hood, which rotates around the same axis, but without adding extra friction to the high speed flywheel axle. This is the mount point, where flywheel axle could freely pass through the 393 gear:

And this is the video of the robot in action:

In the following video, in addition to adjustable hood, two more 393 gear bearings are used to support a movable bracket with the flywheel motor:

This allows a single motor to power the flywheel (when it spins forward) and to act as an indexer (when it pulses backward).

Mounting motors such that they are not fixed to the chassis opens up a lot of exciting possibilities. Passing axles through the drilled out 393 gears allows to avoid extra friction that otherwise would have resulted from supporting the motor assemblies.

Another example where coaxial motor mounting could add efficiency is this clever motor sharing setup between the intake and indexer from Team 10E:

When motor runs:
Clockwise: intake runs at 1000 rpm, indexer 0 rpm
Counterclockwise: intake 333 rpm, indexer 1000 rpm

Do you see how it works?


Always great to see innovation with limited resources. I am going to point out that cutting down 2.5" long cap screw to 2" was ruled illegal in a previous season. Not that any robot inspector would ever notice but perhaps be ready to produce engineering drawings of the 2" maximum length screws you start with :wink:


Interesting, I never saw that ruling and quick search of Official Turning Point Q&A did not yield any results. Either new Q&A portal is not working or I don’t know the right keywords to look up clarifications for R7.c

However, I don’t believe rules that are not realistically enforceable make any sense.

If a hypothetical inspector, who is expert in material science, hardware, software, and everything vex, could take your robot apart, test every part and every bit of hardware and software, would this even be detectable? What would be the positive effect of enforcing the rule that screws on the robot that are under 2" legal length limit, were never originally sold in the 2.5" length?

I would want to reward students for being resourceful and fully documenting their build process, rather than create an environment where not being truthful has any benefits.


Forget what season it was but some time ago. Where you can gain an advantage here is obtaining a longer non-threaded round part which of course becomes a much better pivot or shaft than 1/8" square or #8 threaded screw. It’s no different than only VEX cable ties (and those of the same dimension) being permitted and teams using whatever they buy locally.

Ask an official Q&A about >2" screws being cut down to 2". I would bet the answer is not permitted because it was never a 2" long COTS part to start with.


Ok, so I searched the forums and found an answer all the way back from 2013:

My VRC motor memory tells me to accept @Karthik’s word as the final, but for the sake of the argument I must resist. :smile:

@wdr_mentor I understand your argument about longer non-threaded section.

I did more research and found 2" brand name screw at $50/100, that has really long non-threaded section: https://www.mscdirect.com/product/details/70222443

In their value collection, MSCDirect sells 2" screws for as low as $26/100 but none of those have similarly long non-threaded section. Apparently, you have to pay premium for the rarely used parts.

However, if I expand my search to 2.5" screws, I could find a screw in the value collection with non-threaded section similar to the premium screw but at an affordable price of $26/100: https://www.mscdirect.com/product/details/85326924

If I cut that screw to 2" legal length limit, it will be functionally identical to the brand name screw, but at about half the price.

My understanding of RECF/VRC mission is that its goals include making the program affordable and accessible to the wide student audience to maximize the outreach of the STEM education, and also to provide fair and level competition environment for all kids.

It is important that everybody plays by the same rules, but wouldn’t it make sense to write and interpret the rules in a way that makes more resources accessible to more teams and not only to those who could afford to pay premium prices for the exquisite COTS hardware?


Ah, well that’s another topic. Who’s using titanium screws? :money_mouth_face: VRC rules aren’t complex and long winded for a reason.

My own viewpoint is that once you’re pushing the rules to this point a team should be looking at more open materials competitions like FRC and FTC. VRC is constrained and relatively simple in comparison. It fills a niche and being constrained is actually quite a fun part of the competition.


Yes, titanium screws could get very expensive, and I am glad VRC is not competitive to the point when people would start buying them. Solid game strategy and planning for plenty of driving practice on a good enough hardware would always beat even the most loaded robot finished in the last minute.

@wdr_mentor, I am 100% in agreement with you that constrained environment could both: provide even playing field for teams with access to various levels of resources, and spark creativity to search for the new solutions within the limited set of available parts.

For each aspect of the game you need to hit a sweet spot (or range). If you constrain it too much - people will struggle to do basic tasks. And if you make it too easy - people will get lazy and throw unlimited resources onto the problem, instead of learning how it works and consider balanced alternatives.

I was somewhat disappointed when, with the introduction of V5, they let VEXU teams use unlimited number of motors. Just when the students are mature enough to understand more advanced concepts and will have access to precision CNC equipment, they removed one of the major incentives to learn deeper and try harder.

For example, during ITZ season, one of our students was putting a lot of effort into building fully automatic differential transmission. It could have been done if we had just a few more weeks before the Worlds…

Long story short - he graduated and, after it was announced that VEXU had no motor limits for V5, any plans to keep working on the transmission had evaporated in an instant. This is the latest prototype that was made (original post here):

Worm based differential

I could only imagine how much more it could have been improved with the access to VEXU legal materials, good CNC equipment, and a lot of motivation. Plus, all the educational benefits of trying to learn the theory behind designing custom gears, CNC machining techniques, etc…

So, I got pretty excited when I read this post:

I wish it had been better communicated to VEXU teams in the beginning of the last season. It adds a major incentive to think harder about game strategy and robot design, instead of just mindlessly adding extra motors regardless of their relative contribution to the final score.

Few days after that “revelation” was discussed on the unofficial VEX Discord server, I noticed increased traffic to several of my transmission prototype videos, and a few people had inquired about them over DM.

This was my main motivation to create this and its sibling thread: Differential transmission - power takeoff from the drivetrain motors


It is a well known “secret” among the veteran VRC teams that it is best to use single bearing screw joints when building 4B, 6B or DR4B/6B lifts.

With them you can achieve minimum slop, which translates into more stable lift that could go higher with more precision and less wobble.



When assembled correctly nylock nut is tightened almost all the way, but not fully such that you could still rotate the white spacer with your fingers.

To reduce slop and increase precision further you can use longer (2") screws. Below is a picture of the 2" screw tower connection for DR4B:


However, you have to be careful and need to regularly check those screws, because if they get bent you will end up with more friction losses than benefits.

Similarly, to reduce wobble you could increase the width of the arm/gear assembly in the mid-section. Here,1.5" screws hold an extra bearing on 3/8" nylon spacers (note that spacers are slightly rounded with a grinder wheel to fit the shape of the newer 60T gears):


On the other side of the gear keps nuts are used for better overall stability:


Washers are added under and over the keps nuts to get the perfect distance where c-channel is almost touching the gear.


This fits perfectly with 2" screw that acts both as a gear axle and rigidly connects both sides of the DR4B mid-section. Same as with the tower connection you need to tighten the nuts such that there is no visible slop, but not too tight, to avoid any unnecessary friction. After the final assembly you can use a small ziptie to add some white lithium grease lubricant under the bearings and onto the gears.


As you can see on the side view there is no need for any additional connecting hardware. Four 2" screws used as axles also connect the sides of the mid-section.


You can assemble top and bottom parts of the DR4B separately and then connect them at mid-section to get the optimal alignment between the gears.



<R11>… The intent of the rule is to allow teams to purchase their own commodity hardware without introducing additional functionality not found in standard VEX equipment. It is up to inspectors to determine whether the non-VEX hardware has introduced additional functionality or not.

So I may need to clarify what qualifies as a additional functionality. the GDC has confirmed that any commercially available bolt can be used, including set screws and screws with captive washers,

… but not threaded rods or eye bolts. However this is spread across the last 5 years worth of Q&A.

While I love this design, I am left to ask if the thread free part of your screws introduces functionality not found in standard vex hardware. If the functionality is limited to “gear spins on it” then is a better spin additional functionality, or just superior functionality. I think this should be dropped into an official Q&A, because I would love to know for sure with this added clarification.

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How would you differentiate, as an inspector, between a bolt manufactured with a longer unthreaded section, and a fully threaded bolt with the threads carefully filed off to the desired location?


I would recommend more clarification from the GDC. However, the rules allow for larger bolts, like , which could be machined down to the appropriate size, increasing the cost and exclusivity, which is counter to what I believe the GDC has in mind for this rule.


Theoretically a student could chuck some high strength shaft on my metal lathe and turn most any fastener up to 12" long and 1/4" diameter. I’d have the students take pictures to prove the material came from legal VEX parts.


How good is the high strength shaft material for bolts? Vex lists the material type as 1018 steel, but I am not very familiar with the various types of steel

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1018 is a pretty typical low carbon steel for non-heat treated fasteners and stuff. The “10” means plain carbon (no alloy) and the “18” means 0.18% carbon.


Very good!

Congratulations on creating the transition system for the gear set.
Would you like to know more details about this construction, do you have images with more details of the construction or some 3D drawing that could be shared?


This looks very solid, however, I have a few questions about it. Is there any reason why you put a gear on the bottom bar, and is there a benefit to not stacking the two mid-gears? Also, wouldn’t it be sturdier to sandwich the bars connected to the tower rather than attach them on one side?


I’ve seen people using partially threaded screws in VRC for years. And that would be perfectly consistent with the original wording of <R7c> which became <R11> this season:

<R11> Certain non-VEX screws, nuts, and washers are allowed. Robots may use any commercially available #4, #6, #8, M3, M3.5, or M4 screw up to 2” (50.8mm) long (nominal), and any commercially available nut, washer, and/or spacer (up to 2” / 50.8mm long) to fit these screws.

The most relevant official Q&A that makes such screws legal is this post from May 2013:

Also, this answer from Aug 2013:

I would love to, but RECF & GDC made it especially hard for people to ask questions in the new Q&A format - I will not be able to do this until the fall in the earliest. They are likely to miss a bunch of useful questions that could help them make corrections and clarifications to the game manual early in the season.

Also, I just realized that you cannot search Q&A from multiple seasons with the new interface. Forum based Q&A format worked so much better in so many different ways.