U-joint non-linearity demonstration

[oracid]

Thank you for all this !

Here the U-joint, https://en.wikipedia.org/wiki/Universal_joint#Equation_of_motion

Here the CV joint, https://en.wikipedia.org/wiki/Constant-velocity_joint#Double_Cardan

[aeh5040]

Now I see why some of the mocs seam to be driving in "waves" or with some kind of hasitation

In most cases I don't think U-joints are responsible for that. It is more likely backlash plus elasticity of various kinds.

[grum64]

Thank you for bringing to my attention a problem I had no idea about in such an interesting way.

[flyboy01]

Thank you for bringing to my attention a problem I had no idea about in such an interesting way.

yes, I will use this from now on in my builds!

[pgplay]

In most cases I don't think U-joints are responsible for that. It is more likely backlash plus elasticity of various kinds.

Thx for clearing it out, I think:)

Still a bit new to all this, learning as I go

[Technyk32231]

Do you say about this joint? TLG call it CV joint, but in fact it and U-joint have the same principle, so it is not correct to call it "constant-velocity" joint.

It is perfectly correct to call this a constant-velocity joint, since it does provide constant velocity.

[Limga]

It is perfectly correct to call this a constant-velocity joint, since it does provide constant velocity.

Two of these joints are constant-velocity joint (so there is CV-joint between differential gear and rear wheels in 42029, for example), but one of them has the same cinematics as an U-joint. Look how parts are connected between each other, the principle is the same as in U-joint. I will make a video demonstrating "CV"-joint's non-linearity.

[aeh5040]

It is perfectly correct to call this a constant-velocity joint, since it does provide constant velocity.

Two of these joints are constant-velocity joint (so there is CV-joint between differential gear and rear wheels in 42029, for example), but one of them has the same cinematics as an U-joint. Look how parts are connected between each other, the principle is the same as in U-joint. I will make a video demonstrating "CV"-joint's non-linearity.

I think both of you are wrong here. (Sorry to point it out! )

I do not think the Lego "CV" joint is constant-velocity. Imagine what would happen if it were at an angle of very nearly 90 degrees (if the casing were long enough to allow that). The casing on the left would hardly rotate at all for most of the rotation of the insert on the right. You are probably thinking of something like a Rzeppa constant-velocity joint. But that is very different. It needs free-floating ball bearings that slide in grooves in BOTH axles, not just one of them.

On the other hand, it is definitely not equivalent to a (Cardan) U-joint. You can see this immediately from the fact that there are two moving parts, not three. And there are many other differences (sliding versus pivoting, etc...)

[Limga]

I think both of you are wrong here. (Sorry to point it out! )

On the other hand, it is definitely not equivalent to a (Cardan) U-joint. You can see this immediately from the fact that there are two moving parts, not three. And there are many other differences (sliding versus pivoting, etc...)

Well, I'm still thinking that it is equivalent to U-joint. The missing cross-piece in "CV"-joint is substituted by towball joint. If nobody agrees with me, I will have to refer to math and write some movement equations of both joints

Edited December 16, 2015 by Limga

[keroro]

For U-joint the oscillation of the angular velocity it's due to the shape of the mechanism, in particular it's due to the pivot that isn't able to slide....

For CV-joint, the pivoting mechanism it's different (possibility of sliding) .

Totally with a U-Joint the lever-arm change continuously during the revolution of the primary shafts and this imply an oscillation of the secondary shafts, for the construction of the CV-joint this fact doesn't happen and the speed of the two shafts is constant.

[Sylvian]

Never knew this actually,... thanks alot! 'Little' things like this can really make my day. (sort off :P)

[Kumbbl]

thank you guys for this very interesting topic and explanations...

Now my problem/question: Can there also be a non-linearity between ONE lego u-joint and ONE lego CV-joint (connected by one axle)?

Background of this question:

Just rebuilt this really cute and outstanding (concerning the operating) excavator: http://www.eurobricks.com/forum/index.php?showtopic=90485&hl=%2Bhydraulic+%2Bexcavator

Here is the rebrickable page where in one of the 5 comments is a link to professional stepped pdf-lego instructions (very well done btw)...http://rebrickable.com/mocs/yannmar/motorized-hydraulic-excavator

The second big LA of the arm is now driven by this mentioned combination of 1x u-joint (directly connected to the LA) + 2x axles (joined with an axle joined) + 1x CV-joint (which is finally driven by the gearbox and the motor). The gearbox works flawless,ie. all is running very smoothly, so the power comes without vibrations and in great "quality" to the CV-joint.

Now the problem: this drivetrain acts now exactly as the non-linear connected 2 u-joints in the video above (made by splat): the whole arm-drivetrain for this second LA has strong vibrations - if i try to turn this drivetrain by hand then there is exactly 1 point in one turn where the resistance is quite strong and needs much more power than during the rest of the turn...in addition this behavior depends slightly from the extraction-position of this LA (which influences slightly the angle of the connected u-joint)...

Therefore: how to connect 1 u-joint and 1 CV-joint in a linear manner? In case this is the reason for the problem...

Hopefully my description is understandable...

Here is the rebrickable page where in one of the 5 comments is a link to professional stepped pdf-lego instructions (very well done btw)...http://rebrickable.com/mocs/yannmar/motorized-hydraulic-excavator - also linked is a lxf file - so anybody should be able to check the construction/design of the arm...

Thanks for your help in advance!

[Zerobricks]

Kumbbl YES and combination of a CV and U joint is the same as U joint alone with no compensation. I have a same issue on my small 4x4 I'm making which uses U joints and CV+hubs and I had to reduce the suspension angle in order to limit the vibrations.

[Kumbbl]

Thank you Zbij, but the angle at the CV-joint is really quite wide - can't imagine that this angle is too narrow... maybe the angle at the u-joint?

but it is somehow weird that the vibration becomes less if the angle of the u-joint becomes slightly narrower....

Therefore: can a u-joint and a cv-joint be linearized?

EDIT: Thanks Zbij, the term angle was the right hint... following the BI the axle between CV-joint and u-joint is a 5.5 + 5 axle - which is slightly too long so there is too much strain on the u-joint - replaced the 5.5 by a 5L axle and now it is performing very well with almost no vibrations...

Edited December 28, 2015 by Kumbbl

[Zerobricks]

The name CV means Constant Velocity. All lego's CV joints (8880 ones and the new-ish ones) are trully constant. All U-joints are NOT constant. No matter how you combine a single U-joint with a single CV joint, there will be always vibrations at angle.

There are only 2 ways not to have (notable) vibrations in the system:

1. Only use CV joints or gears to transmit torque at an angle

2. Use pair (or pairs) of alligned U-joints - and all alligned U-joint pairs have to be at the same angle. (of course there will still be a slight vibration in the interconnected axle, but the mass is usualy really low)

Edited December 28, 2015 by Zblj

[AOW]

The name CV means Constant Velocity. All lego's CV joints (8880 ones and the new-ish ones) are trully constant. All U-joints are NOT constant. No matter how you combine a single U-joint with a single CV joint, there will be always vibrations at angle.

There are only 2 ways not to have (notable) vibrations in the system:

1. Only use CV joints or gears to transmit torque at an angle

2. Use pair (or pairs) of alligned U-joints - and all alligned U-joint pairs have to be at the same angle. (of course there will still be a slight vibration in the interconnected axle, but the mass is usualy really low)

could you show an example (picture) of such setup ? please ?

[Zerobricks]

could you show an example (picture) of such setup ? please ?

Sure, here's an example from my older MOC:

As you can see all 4 drive axles use U joints and they are all parallel to each other.