[HELP] Save the U-joints!

[KirTech LAB]

Hello!

Today I want to share my idea how to reduce the U-joint stress in transmission. I haven't try it yet so maybe somebody was trying it and it won't work at all. So I need your opinion!

The basic task: to transmit the torque from motor to wheels with reduction through cardan shaft. If the wheels are big it can result in u-joint damage.

We know that the chain "motor -> cardan -> REDUCTION -> wheels" is better than "motor -> REDUCTION -> cardan -> wheels" because in the first case less torque is applyed to u-joint.

So what if we add two more stages of gearing: gearing UP before cardan and equivalent gearing DOWN after it? Does it reduce the u-joint load in comparison with the first case? The final chain: motor -> gearing UP -> cardan -> gearing down -> reduction -> wheels.

What do you think? Is it good idea or just one more perpetuum mobile? :)

[MaxSupercars]

It must work because cardan will be then rotating faster with lower torque and then will be speed lowered and torque increased... similar as by front steered wheel with U-joint and then with reduction in portal hub...

There are disadvantages...

1.) first any 2 gears can cause addition problems with skidding

2.) second there will be higher friction due additional gears what causes losses...

3.) additional weight due additional elements because the gears have to be well attached to avoid movements and skidding

Max...

Edited September 16, 2015 by MaxSupercars

[KirTech LAB]

I think it can be one more disadvantage - high rotation speed will result in fast wear of axles (probably)

[Lipko]

Gearing-up means higher friction than with gearing-down with the same ratio.

Friction is (simplification) proportional to the contact force between the gears.

Sorry, I'm sleepy and both my engineering thinking and English fail me at the same time. I just can't explain it, but it's sure you will have a much higher friction with gearing up and down than with a 1:1 then 1:1 gear train. The friction could be reduced by using bigger gears (smaller contact force needed to transfer the same torque), but most of the times that's not space efficient.

Edited September 16, 2015 by Lipko

[KirTech LAB]

The friction could be reduced by using bigger gears (smaller contact force needed to transfer the same torque), but most of the times that's not space efficient.

Yes the bigger gears the less the friction. And we can place twice gears to reduce friction too. I was doing that in my Arctic Truck and in my racing boat - this system works very good!

[brunojj1]

What kind of vehicle are you building or thinking of? Or is the question just theoretical?

I remember the older universal joints breaking, but the new ones are much better and not so easy to break. I have destroyed or worn some cardan joints and now I use the u - joints instead with heavy torque at the wheels without breaking them (1/10 scale, 2 kg car). Primarily it depends on instant torque and weight of the vehicle, I think. The axles lenght are playing some role, too.

The experiments will show how far we can go ;-)...

[KirTech LAB]

What kind of vehicle are you building or thinking of? Or is the question just theoretical?

I remember the older universal joints breaking, but the new ones are much better and not so easy to break. I have destroyed or worn some cardan joints and now I use the u - joints instead with heavy torque at the wheels without breaking them (1/10 scale, 2 kg car). Primarily it depends on instant torque and weight of the vehicle, I think. The axles lenght are playing some role, too.

The experiments will show how far we can go ;-)...

Are you talking about 3L cardans or about CV-joints?

I have an idea of vehicle but I'm interesting in theory too.

[brunojj1]

Are you talking about 3L cardans or about CV-joints?

I have an idea of vehicle but I'm interesting in theory too.

Yes, I meant the CV joints were worn after some time, precisely the dark grey part of it. In general I think gearing up and then down doesn´t make sense theoretically, of course they will reduce the total torque power simply because of more friction which also doesn´t make sense to me.

Another unknown factor which makes a huge difference - the motor(s)! XL or even RC motors provide the highest torque and combining them to get more power output is dangerous for the plastic !...

You don´t have to reveal your vehicle idea, but I am curious about it already

Edited September 16, 2015 by brunojj1

[Erik Leppen]

The extra friction of fast-spinning axles is only a problem if you gear up by huge factors. But you don't have to. Gear up 5:3 and the torque on your U-joint is already almost halved. Gear up 3:1 and torque is reduced 67%. In many situations this is enough to not damage those parts.

Also, gearing up doesn't have to take extra gears. A drive train already needs gears, so if you need them, just use different sizes (20/12 instead of 16/16) to gently incraese/decrease the rotation speed every step, without having to add more gears.

[Saberwing40k]

I would not think that gearing up, then gearing down would actually do anything. If gearing up adds stress to a U-joint, and gearing down removes it, if you gear up and then gear down, you get a net reduction of stress of zero, just like you get a net zero change in speed. Or, do I have this wrong?

[BusterHaus]

I would not think that gearing up, then gearing down would actually do anything. If gearing up adds stress to a U-joint, and gearing down removes it, if you gear up and then gear down, you get a net reduction of stress of zero, just like you get a net zero change in speed. Or, do I have this wrong?

The stress depends on the torque being applied, which can vary through the system. I think the extra gears offer more protection to the universal joints, but not only by reducing the torque. The extra gears also introduce slack and flex to the system (resulting in energy loss and less efficiency), which allows the system to better absorb sudden changes in the torque like acceleration from a stop or reversal of direction. It's kind of like having a very primitive clutch.

Edited September 17, 2015 by BusterHaus

[bonox]

I would not think that gearing up, then gearing down would actually do anything. If gearing up adds stress to a U-joint, and gearing down removes it, if you gear up and then gear down, you get a net reduction of stress of zero, just like you get a net zero change in speed. Or, do I have this wrong?

if you gear up then down by the same amount, then torque on the joints outside the intermediate shaft won't change (neglecting losses). Only the torque in the intermediate shaft will change. You are correct in a nett zero analysis. Regarding Erik's comments about stages, i've not done any analysis on it, but it's possible that due to the radial loads you get from gears, that the friction in the 'bearings' could sum to different values based on differing magnitudes of change at each gear pair. Would be interesting to investigate. In general though, I believe tribological studies show that for equal power transmission, higher speed equals more friction even though the coefficient vs speed curves tend to taper off with increasing speed.

[brunojj1]

Gear up 5:3 and the torque on your U-joint is already almost halved. Gear up 3:1 and torque is reduced 67%.

I disagree with that.

“The magnitude of torque depends on three quantities: the force applied, the length of the lever arm connecting the axis to the point of force application, and the angle between the force vector and the lever arm. In symbols:

(symbol...) torque = r x F

(symbol...)is the torque vector and the magnitude of the torque, r is the displacement vector (a vector from the point from which torque is measured (typically the axis of rotation) to the point where force is applied), F is the force vector” source: Wikipedia

Conclusion: the reduction or increase of the gearing doesn´t affect the torque vector THEORETICALLY. Reducing the displacement vector means increasing the force vector and the other way around. Do I understand it right or did I miss something important in the discussion?

Edited September 17, 2015 by brunojj1

[PKW]

I used this gearing up to save chains in my 4x4 tank transmission for a lego razer (from robot wars, with two wheels and two omniwheels) it works very well, but in that project it helped also to reduce the space and to avoid the use of an high number of gear

[bonox]

Conclusion: the reduction or increase of the gearing doesn´t affect the torque vector THEORETICALLY. Reducing the displacement vector means increasing the force vector and the other way around. Do I understand it right or did I miss something important in the discussion?

I disagree with that. ( )

The point here is where you put a universal joint in a drivetrain. I believe there is an implication of a certain change in motor speed by the time you get from the motor to the output (a wheel for example). You are correct in your assessment for equal torque. ie a smaller offset requires a larger force for the same moment sum. Where you're going wrong is in the torque multiplication of gearing in exchanging torque for speed (or vice versa) depending on whether you're gearing up or down. If your output shaft speed is lower than the input speed, for the same input/output power, both the torque and shaft speed change in proportion. In this case, the argument is putting the uni joint at the point of lowest torque (and consequently maximum shaft speed).