Passive transmission


I’m assuming by “no motors” you mean no more motors than those which are on your drive train. This is a prototype I drew about two months ago to see how simple you can make a differential drive train that can power both the front and back wheels (it’s harder than you’d think) this design would work with both four or six motors, mounted on each twelve tooth gear, and is meant for a tray bot but could be adapted for another purpose. I don’t think it would have a great deal of friction if you managed not to mount the bars on the same axle as the gears. (The reason it is sketched is because since I am no longer part of a vex team I don’t have any access to parts so, oof)(My plan was to design a bot that could have a six motor 240 rpm base while still having an effective tray)

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You could probably adapt it to fit your 14 inch bot if you offset the entire gearing upwards by an inch or three then moving the wheels together.

How exactly would that design switch between powering the tray and powering the drivetrain?

It works by the large (60 tooth) gears locking (you do this by having the motor that isn’t able to rotate around another gear not move at all) and the gear on the bar that does move being powered.


Or, you use two bars in this configuration where the motors spin the same direction to power the tilter

Summarized, in the first picture the motor on the bar spins and the other doesn’t to power the tilter (they spin in opposite directions to power the drive for both designs). In the second picture both motors spin in the same direction to tilt the tilter.

Feel free to use these designs


So the tilter tilts when only the two rear motors are going, and the drivetrain moves when all 4 motors move?

Yes, that is what is happening in the first one. The only downside is it only has the power of two motors.

The tilter tilts whenever the large gears are incapable of rotating. In the first picture, where you have one motor on a rotating bar and one on the chassis, the powered gear on the chassis prevents the large gear from rotating which lets the other gear make the bar rotate on it. In the second picture, it works by the gears rotational directions locking up the large gears and making both of the rotating bars rotates.

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So for the first design, if you turned the motors on the chassis, then you could use all 4 motors to drive.
And on the second one, if you run all 4 you can power the drive, and running only the 2 in the back will tilt the tray.
(Sorry for all the questions I just really want to implement this)

Basically it is:

(One rotating bar)
One motor locked one rotating= tilter active
Back motors running forward, front motors running backward= drive

(Two bars)
All motors running forward= tilter
Back running forward, forward running backward= drive


Oh, I get it! Thanks for the explanation and your time! I’m probably going to rework my drivetrain now :slight_smile:

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Here’s a physical example of what @1498A was talking about.
20190906_205101 20190906_205036


I am sad to say this, but a quick mockup shows that in it’s current form, the setup is unusable as it is too large for my chassis.
I may make modifications to the design mid season to fit it in, but I dont have time right now :frowning:

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If your chassis is too short, you can simply move the transmission higher and subsequently forward. The transmission can fit into 5 holes wide, so it shouldn’t be too wide to fit in your chassis.


Another way to use less motors, which is a little off topic is ratcheting systems. There were a bunch of really cool designs from last years.

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People were able to abuse ratchets last year because all three shooting mechanisms only required a motor to spin in one direction. This year however, most mechanisms we’ve seen require motion in both directions, so there aren’t as many opportunities to use ratchets.


It is great that many teams have experimented with various motor saving / sharing techniques over the summer!

Strict power limit of 8 motors in 1 motor increments is one of the best challenges that VRC got to offer for its most inquisitive minded competitors. :slightly_smiling_face:

I think, power transmission between drivetrain and tilter might work very well this season, but you have to be cautious which specific differential design to use.

As @Deicer correctly pointed out, the biggest problem with DMD style differentials is that they could introduce a lot of extra friction. Axles of 60T or 84T gears have to continuously rotate transferring your driving power. They also have to carry any force associated with the arms on which the motors are mounted. If those arms twist out of alignment, you end up with double or triple friction cost on the power that flows through the geartrain.

Designs that @1489A and @Anomalocaris have linked above, were used by a number of teams during ITZ and, with a good build quality, worked relatively well for some.

It was easier to do with 393 motors, where not all drive power was going through the differential, and some motors were direct driving the wheels.

For example, most 2131 teams had 4 motors direct driving the robot and only two motors were used on the Mobile Goal lifts, providing extra drivetrain power through the half-sided differential.

Also, 81k had 4 direct drive and 4 differential motors. If you take a closer look at their robot you could also see that motors were not mounted on differential arms and 84T axles did not have to carry extra friction load. Even with reduced friction half of the drivetrain power still had to go through the several extra gears.

With V5 motors it would be very expensive to send all your drivetrain power through the extra gears and axles required for DMD differential. However, there is an alternative solution as animated starting from 0:07 in this video:

When motors are powering drivetrain there is no extra gearing involved, similar to the regular drivetrains. Only when you power secondary function, then entire motor assemblies rotate, which requires some extra gears. An example of such transmission could be seen here:


@technik3k , can you actually fit that differential inside the profile of 22 holes width-wise? Also, if you’ve had any experience driving these, do you know how much friction these kinds of differentials would have?

Yes, as you can see from the picture above, that prototype design, while being liberal with spacing, could still fit perfectly into 22 holes.

If you cut down on any unnecessary spacing, then this kind of differential with any associated gears could fit in 20 holes width and, depending on the choice of the wheels, entire chassis could be as narrow as 25-27 holes.

This kind of the differential transmission could have friction losses equivalent to the regular drivetrain design, where all 4 motors are located on the back of the chassis, with 2 directly powering back wheels, and other 2 connected by the chain to the front wheels.

The main trick to the low friction is that axles coming out of the motors do not support 60T gears that motors are attached to. Instead, 60T gears with motor assemblies are supported by the drilled out 393 output gears and, when everything is aligned correctly, motor output axles are passing through without even touching them.


Cool, that might make this transmission viable if 3 motor intakes become necessary, and if the differential is done right with little to no extra loss from friction. However, I just have a couple questions regarding the prototype. What is the main point of the plate attached to the 60t high strength gear? is it just to keep the golden motor standoffs from bending/breaking? and what are the standoff/collar assemblies that are also attached to the 60t used for? The amount of thought and testing that goes into these things still amazes me, and often times I can’t catch up :sweat_smile:


Yes, the plate on 60T gear is there to provide stable platform for the motor and additional structural support for 6" standoffs connecting left and right sides. It may have been an overkill - simple 1x bar under motor may be enough support to make it stable.

Standoffs on collars were used for power takeoff for the secondary function. For this season it could be used for raising the tilter.

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