CV/universal Joints: Is there a rule of thumb?

[BlindBuilder]

Hi. Given that lego has such a (relatively) low rev/torque/load-baring rate in relation to real world machines, does it matter whether I use a universal joint or a CV joint? Likewise, when is it right/wrong to use either 2 bevel gears or 2 knob wheels? Are there any rules of thumb when connecting axles in Technic/Mindstorm? Or, at this small level, is it simply a question of aesthetics?Any pointers welcomed on the topic of when to use which joints. Thanks.

[Zerobricks]

More info would be nice... Where is the joint going to be used? Will the angle of the axles keep changing (liek in suspension) or is it static? Is the joint held on both sides?

Here are some of my personal pointer I can give you.

Cv pros:

Cv joints are usefull if you have low torque application (like steering) and you need a bit of axle sliding movement.

Cv negatives:

As soon as you out too much torque they simply pop out

They have a limited angle (some 25 degrees) aafter that they pop out

U joints pros:

They are stronger than Cv's.

They dont slip on the axles as much as Cv's.

They allow higher angles than Cv's

U joints negs:

The axle holes tend to crack.

At higher angles they create vibrations.

If the angle ot torque is too high the cross part pops out, or the axle hole breaks

Bewel gears pros:

They allow for a smooth transmission of the power

If the surrounding frame is built correctly they can whitstand the torque of an xl motor

Bewel gears negs:

The half 12 tooth ones tend to break a lot, especially in differential cases

Once they slip at high torque they become deformed and prone to even more slipping

You need a very good housing to keep them from slipping at high torque

Knobs pros:

They are simple and reliable.

They can carry extreme torques even in not-so good gear casings

Knobs negs:

They dont spin smoothey, just like an U joint at a higher angle they create vibrations

They tend to squeak

Their efficiency is lower due to only 4 tooth used (larger intermeshing surface = larger friction)

Hope these tips help you.

One more thing.

A U joint or a Cv joint can carry more torque if both ends are fixed.

A free-floating U or Cv joint has more room to oscillate, slip out or break.

Edited November 29, 2014 by Zblj

[Dalafik]

Zblj, your answer is the most valuable post that I've ever read on this forum. Thanks!

[BlindBuilder]

Brilliant! That^^ was *exactly* the sort of information I was after. Thank you so much.

[Zerobricks]

Zblj, your answer is the most valuable post that I've ever read on this forum. Thanks!

Brilliant! That^^ was *exactly* the sort of information I was after. Thank you so much.

Thank you guys!

I'm glad my assistence is appreciated.

If you have any specific questions just poke me

EDIT one more tip:

When using two U joints, alway align them so they counteract each other's oscilations. This way onle the connecting axles oscilates, but nothing else. (the U joints have to have a simillar angle for this to work, example car axle and driveshaft)

[miguev]

I didn't know what a CV joint was, and still don't know why that name

Searching for images of these, I found this strong joint from Zblj, how does it compare to the others? (apart from taking up more space)

[Zerobricks]

I didn't know what a CV joint was, and still don't know why that name

Searching for images of these, I found this strong joint from Zblj, how does it compare to the others? (apart from taking up more space)

I think this video may help you clear up the things:

That C joints while 3x bigger was able to whitstand HUGE torque at high angles without slupping. This was made in time I didnt come up with portal hubs yet and there were no other alernatives.

[miguev]

I think this video may help you clear up the things:

That C joints while 3x bigger was able to whitstand HUGE torque at high angles without slupping. This was made in time I didnt come up with portal hubs yet and there were no other alernatives.

Great video! Thanks a lot, I love how much I learn every day in this forum

Portal hubs lead to terrible steering geometry while your old solution looks like it does a lot better, why/when would you ditch it favor of portal hubs?

Trial trucks (with live axles) is the one application I'm thinking of, and not sure which one to use.

[Zerobricks]

I no longer use the big CV because a portal axle allows me to:

Have a higher ground clearance

More gearing means less stress on diff or other bewel gears

Less chance of something breaking.

[miguev]

I see. I take it that worse steering geometry (and needing more room inside mudguards) is the least bad.

I was dreaming of using the 42000 portals in a trial truck, but it seems I should stop dreaming (and start building 0:)

[Technyk32231]

Thank you guys!

I'm glad my assistence is appreciated.

If you have any specific questions just poke me

EDIT one more tip:

When using two U joints, alway align them so they counteract each other's oscilations. This way onle the connecting axles oscilates, but nothing else. (the U joints have to have a simillar angle for this to work, example car axle and driveshaft)

what do you mean by both ends are fixed? Do you mean that they are not in motion as in a suspension?

[Zerobricks]

I mean fixed like---You know the Ball joint? It holds the U joint on both sides. But a live axle usually only hold joint on one side at a time.

[BlindBuilder]

Here's another joints-based question for you all. I've noticed a growing trend in latter day technic models whereby single long axles are replaced by 2 shorter axles joined by those ridged connecters. Is there any genuine mechanical advantage to this or is this a case of Lego cynically bumping-up the parts count to make models appear more sophisticated?Surely one long axle will be stronger than 2 shroter ones joined end-to-end or am I missing something?

[Epic Technic]

Here's another joints-based question for you all. I've noticed a growing trend in latter day technic models whereby single long axles are replaced by 2 shorter axles joined by those ridged connecters. Is there any genuine mechanical advantage to this or is this a case of Lego cynically bumping-up the parts count to make models appear more sophisticated?Surely one long axle will be stronger than 2 shroter ones joined end-to-end or am I missing something?

Actually axles have more flex than a bunch of connectors. I'm not sure why lego is using connectors instead of long axles since there the sets don't have much torque and friction. I will try to find a a video explaining.

[bonox]

at a guess, there is very limited application for 16 or 32 stud axles - it's much simpler from a production standpoint to keep the shorter axle moulds running and join them with another common piece.

edit: or you could be talking about a length that is not satisfied by the existing sizes - example, they'll add up to 24 studs, which is simpler to achieve with joined existing units than making a new element.

As for strength, the limiting factor is the material and minimum polar moment cross section. It doesn't matter where that section appears, that's where it'll break.

As for stiffness, which is what epic above is referencing, the joiners/collars act as local stiffeners for a short distance and the more you put in, the less rotation per unit applied torque - ie higher stiffness.

Note that strength and stiffness are two quite different things.

Oh, and i've thought of a third one - perhaps TLG have a high return of bent axle32's in boxes. I know i've had a few new 42009 and 8285's with bent 32's.

Edited November 30, 2014 by bonox

[5imon]

I can imagine that sometimes it's easier to assemble when you don't have to slide in a very long axle in a complex construction. Or if you put in the axle in an early building stage and it's in the way afterwards.

Another explanation might be that it reduces the variety of different parts in the set and thus saves cost for TLG.