2, 5.5's vs 1, 11w

VEX V5 General Discussion
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So me and my team are designing our robot next year and we aren’t sure if 2, 5.5’s are better than 1, 11w.
I see that the torque theoretically is higher with 2 motors, but I would like to make sure first that their aren’t any downsides.

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5.5 watt motors can only use low strength axles while 11watts can use high strength. The torque is the same but 5.5watt motors can only spin at 200 rpm unless gears are used. With 11watts, you have way more customization than 5.5watts. If you are building a lift or a slip gear, it would be better to use 11 watts since high strength axles can be used. Otherwise, it’s your choice.

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2 5.5Ws are usually worse than 1 11W, a 5.5 has a stall torque or .5 Nm and a green cartridge has a stall torque of 1.1 Nm so they technically have a lower max torque, along with the additional friction of a gear train to connect the two motors.

One, scenario where it might be better to split it into two motors would be an intake where you want both sides of the axle to be supported so you don’t end up with a twisted axle.

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(2) 5.5 will cool more efficiently than (1) 11 watt.

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So it would be better to use 1, 11w motor for a dr4b than 2, 5.5w’s?

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A lot of really good points were made in this thread. Read them, then evaluate what you think is best.

This would make a great entry to the notebook. Weigh your solutions and decide on the one you deem best. Then test it!

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It depends on if you have a reason to. For me, I plan on using 2 5.5W motors because I want to use this mechanism.

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Has there been any testing that shows this?

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This mechanism is what we called a 4-bar.
you can also easily use 1 x 11W instead of 2 x 5.5W as well.

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It’s a four bar differential… It needs motors to spin the input gears in different directions to work… I’m getting the impression that you didn’t watch the video and just assumed based on its appearance…

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I did watched the video.

What I am suggesting is that there is no need to have a differential system for this game, and also, even if there is a need to have differentiate, it is still not a must to use 2 motors on it, i.e. you still can have a differentiate system using 1 motor.
There is no need to make things complicated when a simpler system will work as well.
The main purpose of differentiate system is to have different torque outputs. So unless there is a reason why you need to have this feature?

and btw, more gears and more motors normally mean more friction and inefficiency.

And also - be open-minded to hear what others have to say and don’t be too quick in assuming you have the best solution and become defensive.

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I wanted to use the power from the differential to power an intake that can be raised and lowered to score the higher wall stakes, climb bottom tier. I should have clarified that in my original post.

I mostly just didn’t want to use pneumatics for a PTO and accidentally found this design on youtube and it was cool and fit my needs from a mech like this,

I actually didn’t know that one motor differential systems exist (or do they use pneumatics?) If you could show me an example of one, that would be nice. Also, the reason I suggested 2 5.5 W motors was to basically use one 11 W with no major drawbacks.

Yeah, I’d probably use as many screw joints as I could.

Sorry, my earlier reply was a little rude. I assumed that you assumed that I didn’t know what a four bar lift was, and assumed that you didn’t watch the video when you said that you could easily use one 11w motor for the differential, when I doubt you could get full functionality out of the differential with only one motor. I definitely assumed too quickly what you were trying to say when I read your reply, and also should have explained my idea more clearly when I originally linked the video.

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Not that I’m aware of. That said… you’ve got same overall power and more or less twice the surface area exposed to the air between both motor groups.

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Are you referring to raising and lowering the intake or to power the intake itself?
If it is the former, then all the functions can be done with a simple 4-bar, i.e. if there is enough torque to climb, then there will be enough torque to raise and lower the intake.

So yes - there isn’t a need for PTO as well.

PTO is generally useful for differentiate transmission, eg. pushing the driving gear from one system to another.

But for your purpose, there is really no need to do that.

This is an excellent post (not by me) on differentiate gearing - Vex Differentials Explained - VEX Robotics Competition Discussion / VRC - VEX Forum

As for the need to 2 motors or 1 motor, well - instead of 1 motor on each gear, having just 1 motor on one of the gears will work too (since the gears are meshed together).
Normally we will prefer 2 motors as it will provide more torque.
But since your idea is to have 2 x 5.5W, then it is not much different from just using 1 x 11 W.

PS. the only thing that I am not thrilled about the gearing system (in the linked post) is how messy the wiring can be.

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Both. I wanted the intake and the 4 bar lift to be powered by a full motor, and preferably by the same motor which pretty much requires a pto or a differential

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I am not sure what sort of intake system are you planning to do - I can’t give any suggestion.

But you might want to take a look at the differential gear provided by VEX as part of the advanced gear kit.

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even if there is a need to have differentiate, it is still not a must to use 2 motors on it, i.e. you still can have a differentiate system using 1 motor.

Sorry, maybe I’m just being dense, but the post you cite specifically says a differential relies on two inputs: “Two inputs control two outputs, based on the difference in their speeds.” That’s the OP’s point. Can you point to an example of a differential system using a single motor as input?

I get that you’re also saying you don’t think a differential is needed for High Stakes, and I’m not addressing that.

Thanks!

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Not the OP. I was simply giving an instance of 2 5.5 watt motors being better for the sake of the discussion.

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Oops, yes, I stand corrected on the attribution.

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Thought this requires a more detailed explanation.

The generic or the original intent of differential gear system is about transmitting the force/torque from one motor (or input) to 2 or multiple outputs. The most common example will be cars - 1 engine, 2 wheels.

Differential gear | Types, Uses & Benefits | Britannica

And here’s a wiki link that shows some other variations (and limitations) of differential gear system.
Differential (mechanical device) - Wikipedia

And if you look at the official differential gear system by VEX, it sticks to the original intent of 1 input to 2 outputs - Gears - VEX Robotics (look under Advanced Gear Kit).

I guess this should address your question on examples of differential gear system that has 1 input.

As usual (and rightly so), VRC teams adapted this idea of differential gear system to their needs and figure out some other variations as well.

But in the earlier years, the main focus was about shifting gear ratio (without using pneumatics transmission, and hence the development of planetary gear system), rather than having 1 input to 2 outputs (of different gear ratio).

This is one example of the work done by some of the old old timers on planetary gear - Planetary Gearset - VEX V5 General Discussion / General Forum - VEX Forum
Because it is such an old post, you might need to scroll down to post #9 to see the pics.

Here’s another interesting variation on planetary/differential gear system - VEX Planetary Guide - VEX V5 General Discussion - VEX Forum
This is a good example of how things have evolved and adapted to VRC context.

I do like this post - because it has good discussion on the pros and cons of such system (which interestingly, some of the limitations are also mentioned in the wiki page), especially those points that were mentioned by @technik3k

And do read his last post in that thread as well - it gives a good summary on when it makes sense to use differential and when it doesn’t.

Back to this thread, my points are:

  1. there is really no need for differential for this game
  2. if there is a need to use 2 motors (or inputs) for 2 outputs, then it might make a lot more sense to just dedicate 1 input to each separate output instead.
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