I decided to look at how thick the material in the motor inserts for both 393 and V5 motors are and I found the following things:
The greatest diameter of a 8-32 screw is 4.16mm
The greatest diameter of a 6-32 screw is 3.50mm
According to the CAD files for the relevant parts on vexrobotics.com:
The outside diameter at the pilot of the V5 motor insert is 4.52mm
The outside diameter at the pilot of the 393 motor insert is 4.55mm
Now this means that the thinest part of a V5 motor insert is less than 0.18mm
and a 393 motor less than 0.525mm
That means a V5 motor insert has thin sections that are 2.9 times thinner than the thinnest parts of a 393 motor inserts. However due to tolerances in manufacturing and allowing space for screws to move inside the inserts it is likely that there are parts of the inserts that are even thinner than that.
I feel like this might not fix the problem entirely, you would still need the pilot part of the insert to have space for the thickest part of the screw which would still mean the insert would have parts that are 0.18mm thick. However, this could improve the quality of the inserts as there will be less force on them during normal operation, reducing the chance of breakage. I also expect it would be easier to make a constant sized hole more precisely than a screw thread which would reduce the potential for weaker inserts.
Overall I feel the best solution to fix this issue would be to revert back to 6-32 screws for motors as this would allow for the motor inserts to be the same thickness as the 393 motor. Personally I have never broken a 393 motor insert so I think I would be great if the V5 ones could be up to the same standard.
@Download Complete, don’t throw the broken standoff yet. You could “repair” it by using a 1" coupler screw to keep semi-broken end in place (use threadlocker to secure coupler) and essentially turn it into something like this to mount with a nut instead of a screw:
I afraid that under high loads the current design of the motor mounts will lead not only to shredding of the thin end of the standoffs but also to fracturing of the plastic ends of the motor mounts.
Traditionally VRC rules prevent any modifications of the internal motor components. So having to buy a new motor every time standoff or motors’s plastic is damaged sounds like an expensive proposition.
However, since the standoffs are fully removable the right call would be to give people some freedom and let them pick any off-the-shelf hardware for this purpose, unless VEX is going to sell both 8-32 and 6-32 threaded versions of the replacement standoffs in the very near future.
@technik3k that’s an interesting suggestion, however I am hoping that we can just use a normal standoff In place of our broken insert, we’re still waiting for the Q&A to be answered on the legality of this.
I am hoping that we will be able to buy replacements soon, making 6-32 inserts available would be a great option as well.
The old motors have bottom plastic part injection moulded over the metal mounts, which guaranteed very good load distribution between metal and plastic. With the new motor design, and especially if the tip of the metal standoff is missing, you have unevenly distributed loads concentrating on the end of the plastic motor case.
If you are going to use a regular aluminum standoff as is (or even machined to match the broken part) and its end’s shape is not matching the shape of the plastic that it is contacting 100% - then you would risk concentrating stress loads on the plastic even further, which could easily lead to its damage (most likely fracturing of the plastic).
If such damage is going to be typical for v5 motors I would consider getting custom 6-32 inserts and supergluing them to the motor’s case to spread out the load.
We had V5 motors for 4 weeks and our local teams report about half of the inserts showing various degrees of damage with a few completely unusable. It is not an option to order new motors just because inserts are damaged and spares are not available from VEX.
Below is the picture with V5’s 8-32 motor inserts on the left and 6-32 replacements on the right. Leftmost insert is in the good condition and second from left started shredding some threads just after 3 mounts/dismounts. On the right is the commercially available replacement as it is shipped (it is slightly longer than original) and second from right is when it filed to have shorter length and with rounded bottom edges to match the shape of the V5 originals:
And this is comparison of the 8-32 originals vs 6-32 replacements when inserted into c-channel:
Somebody with Q&A posting privileges would need to ask that question (I don’t have such privileges).
I could understand the intent of VEX to simplify classroom usage of V5 by going with a single 8-32 screw diameter, but current V5 motor inserts clearly could not withstand normal usage patterns.
I would say that according to G2 they should be legal, because it is a common sense to replace defective (and easily damaged) inserts with commercially available alternatives if VEX itself doesn’t sell replacements:
However, if official Q&A will rule them illegal - it would tell me that it is time to abandon VEX brand as that would be an indication that they are losing common sense in their decision making.
Person who is an administrative contact for our club is busy with a lot of other important tasks at this time - it should get better in the future.
That’s true… and switching the process for Q&A to something more complicated is another data point.
I could see how all those changes (from tournament structure to V5 components’ design) were intended to make something better, but too many of them seem to have multiple side-effects that make the end result worse off.
We all do stupid design mistakes from time to time (I certainly do), but we expect larger entities (like companies, standard committees, industry associations, federal regulators, etc…) to have formal design and review process in place that ensures that any critical decisions that would be costly to fix or change in the future are properly discussed and verified. The more the potential costs are the more in-depth review there will be and the broader set of experts (and end users) will be consulted.
For the past year, I am just baffled trying to understand what kind of review process is there in place at VEX.
Formal design process is part of the VRC curriculum and a component of a judging criteria for Design and Excellence awards.
I would assume that between IFI and RECF they have people with many years of experience in the industry following the process.
And by following the process I don’t mean just filling out the ton of paperwork and attending design review meetings, but actually understand the potential future costs of the critical decisions - both tangible and intangible. Such as time, customer goodwill, employee morale, etc…
I haven’t really had any problems with V5 (except batteries, but they updated the firmware pretty quickly), and since they allow you to use standoffs instead of the default inserts I think this isn’t an issue. Would be nice if on any new V5 motors they changed the mount design though.
EDIT: I just remember this can make it really hard to unscrew the motors so before mounting any sketchy looking motors maybe take the insert out and try putting a screw in it.
we’ve also had the issue - including at least 1 motor shipped with smashed insert threads. threading a screw from the inside cleaned it up enough to be usable, but there’s no longer a lip outside the casing, so it doesn’t snap cleanly into the holes.
The standoff option is less than ideal for this same reason.
Yes, you could use a regular 0.5" standoff and for very light loads that will likely work fine. But, I afraid, that for high shock loads when V5 motors run at full power during the competition it will quickly ruin plastic ends at the bottom of motor stands. If I had 10 spare motors laying around I would not complain.
However, if you break one of your motors it could take several weeks before you could get a replacement. Would you expect VEX to let you use superglue to repair broken motor or it is more likely they will tell us that the only way to stay competition legal is to pay $40 and wait for 8+ weeks for the new motor to arrive?
I wish to be wrong in my opinion that use of regular standoffs will lead to a lot of broken motors, but we now have only (6) V5 motors per team and only vague estimates when the rest of the motors will arrive.
I wouldn’t complain as loud about software or electrical issues with V5 that are discussed in other threads, because I simply don’t know enough details about them. But, unlike other issues, this one is purely mechanical and should be relatively easy for everyone to grasp.
Getting third-party inserts for about 25 cents a piece seems like a great insurance policy to avoid potential lengthy and costly replacement process both for us and VEX. For me it would be a no-brainer to let each team solve this potential problem in whatever way they feel comfortable and take at least one issue off the table.