Why Do Most DR4B's have such a high torque ratio?

Ok, I’m just curious- if you can rubber band away all the weight on a DR4B, why do you need such extreme ratios like 1:5 to lift it? by my calculation, a two motor 1:1 lift should have 29.52 pounds of stall torque, which should be more than enough to overcome most friction.

Because even if the motors don’t have to lift the lift itself, they have to be able to stop it’s inertia and reverse it at any time. Also, 1:1 could just be uncontrollable.

dr4bs accelerate upward at the twice the speed of the motors. A 1:1 is really hard to aim and drive properly and having smaller gear ratios decreases dependence on the rubber band and also allows the lift to more easily overcome the static friction of the bands

If you used autostack, though, couldn’t you technically pull it off?

Yeah but they still have to be able to quickly overcome the static inertia of the lift, which needs torque to do. I don’t think 1:1 would have enough torque to do that quickly without eventually stalling the motors

You’re probably right. I still think you might be able to get away with HS 1:5, though.

I agree. Maybe even HS 1:3… probably those are the types of things we’ll see at worlds.

It’s not like someone has publically made a 1:1 turbo lift and has videos of it. Hint hint

Wait someone has? I feel like it was @antichamber … am I right? Also could you link the video? Also I knew it would be possible… but there’s probably a reason that 1:1 turbo lifts are not widely popular

Because he did it with literally nothing on top of the lift, and without picking up a cone, and with highly diminished structure. It would not possibly work for actually picking up cones.

Yoy have to overcome the friction and the not perfect rubber banding of you lift. Then you have to be able to accelerate it without stalling motors. These necessitate a hefty gear ratio.

Think it is very misleading for anyone to say that the rubber band will counter the weight of the lift.

Yes, to certain extent (it will), when you rubber-banded enough, the lift will bounce up… to an equilibrium point. But the weight is still acting down… and if the lift is to go any higher than the equilibrium point, then you will need to do additional work to make it possible. And that’s where the torque required comes into play.

Can you rubber band away ALL the weight? No… cause that will means the equilibrium point will be vertically above the pivot (and you will need to do work to go against the rubber band in order to bring the lift down).

Can the rubber band aid the lifting? Definitely.

As what @Aponthis has rightly pointed out, the only way to reduce the gear ratio to 1:1 is to reduce the weight of the lift.

Seriously? 1:1?? Sure, I think you definitely rubber band a lift to 1:3, but seriously 1:5 is plenty fast, and as many people pointed out, the speed of a 1:1 would make a lift uncontrollable, and autostack cannot easily compensate for play in VEX parts.

29.52 pounds is not a measurement of torque. the 29.52 pounds of force is only available with a lever arm of one inch, since the proper unit is 29.52 inch-pounds. That means at the end of a 10" radius arm, it produces just 2.9 pounds of rotational force, and that is of course at at 0 RPM since you mentioned stall. The optimal power output would be at 50% unloaded velocity, which would output only 1.45 lb of lifting force, which is pretty weak, but that is for a single arm, and not a DR4B.

Additionally, the velocity, using regular motors, would be at 50% of 100 RPM = 50 RPM, which we can round to 1 rotation per second. That means that the arm would travel 300 degrees in 1 second, or around 120 degrees of useful turning in only 0.4 seconds, which makes human reaction time a significant source of error.

And all of those numbers are using your number of 29.52 in-lbs of torque on only a 10" lift. most teams have significantly longer arms and a double arm system requiring double the torque, taking the already measly 1.5 lbs and reducing it further, down to around…

…half the torque per arm at 12" arms… 29.52/(2*12)= 1.23 lbs of lifting force at stall speed, and .615 lbs at the optimal velocity. That isn’t even enough to lift a single arm, even with proper rubber bands (which can multiply the lift strength by up to a magnitude of 2x assuming it takes 100% of torque to lower the arm and stretch the rubber bands and then returns 100% that torque when trying to lift).

A 5:1 ratio gives us 5 times more torque, but travels 5 times slower. That means the 100% efficient time with this 5:1 gearing would be 2 seconds for a 120 degree motion (more than most dr4/6b travels), and the torque would be 3 pounds of lift, with up to 3 additional pounds from the rubber bands.

Again this math is pretty sloppy and doesn’t take into account a lot of things, but it does show that 1:1 is ridiculous, but that a 5:1 is doable with a light enough lift and excellent rubber-banding.

I’ve gone over this about 100 times with students, and I’m glad to see someone else doing the math, and pointing out that you don’t need to rotate an arm at 240RPM to meet your goals. It’s enough to make up for the mispelling of “Aperture.”

Would 1:3 be practical?

So, here’s an exercise – how long would it take to lift an arm, say, 160 degrees with a 1:3 ratio compared to a 1:5? If the difference is important to you, you might try making 1:3 work, which might involve adding additional motors. You can have whatever you want to “pay for” in your design trade-offs.