Metal Universal Joints

[amorti]

Universal joints can be a weak point on crawlers or other RC off-road vehicles where a lot of torque needs to get to the wheel. The old style CV joints aren't much stronger, and worse they are quite severely limiting in terms of working angle.

This can be alleviated by using portal hubs with a reduction or the new planetary hubs, but you may not have those parts or they may not be suitable for your application.

If you have this problem, you might have wondered what can be done to strengthen a 3L universal joint.

You can wrap bands or string around the connections to strengthen them:

This will help to a degree as they won't snap so easily where the one in the first image has, but then you're just looking for the next weak link, and in this case it's the yokes which will spread and spit out the disc never to be seen again.

You might like to try the new CV joints without using a planetary hub reduction. They're definitely stronger...

...but they are still a wear part.

There are some aluminium universal joints listed on AliExpress.

Unfortunately these are pretty janky.

They are made of very thin aluminium (they say, but I'm not sure, maybe even cast zinc? ) and the tolerances are appalling. They are very loose on an axle, more so than an old CV joint.

Right from new they have about 1/16th turn of free-play in them and in use, the steel pins will wear away at the softer bodies creating more and more play, until at around 1/8th turn of play the yokes can touch each other on each rotation. At that point, they don't have long left to live.

Or you can try these better metal joints from Dark Ice Designs.

These are not currently listed on the website  but they do have more listed on their eBay store.

Pictured here compared with CaDa plastic and Chineseium metal U-joints (Yes I am aware that there are only 2x original Lego pieces in this picture, hopefully even the purest purists among us can get past that):

They are basically 4mm steel-bodied RC universal joints chucked up in a lathe, and bored (very cleanly I might add) to 4.8mm. You then use a grub screw to clamp the axle in place, there's no cross-shaped hole here.

There is so little free-play in these, it's basically zero. Certainly, they have less slop than a plastic joint has.

There are some limitations:

• They are just the right overall length, but they are thicker, this may affect fitment in some models. 
• You can't quite get 1L of axle into the joint without it limiting the working angle, which may affect fitment in some models. They should just about take the short end of a 5.5L axle (particularly the newer ones with the slightly shorter stubs) cleanly.
• You can use them with plastic axles, but the axles will be scrap afterwards because they'll have a grub screw indentation in them.
  • You can also order metal axles from Dark Ice. They list them in odd lengths only but can offer in other lengths against a custom machining fee. Be aware the metal axles have a 1mm rounding added to each end to be gentler to plastic pieces, this again may affect fitment in some models.

Even if they're not pure, and even with their fitment considerations... I think this piece solves the problem of broken universal joints once and for all.

Edited July 23, 2020 by amorti

[amorti]

A quick eBay search also found these:

https://www.ebay.co.uk/itm/Durable-Metal-Aluminum-Alloy-Silver-Joint-Propeller-Shaft-For-Lego-MOC-61903/303321237598?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20131003132420%26meid%3Dd8674e24c5c74b52b37eb6e55670e4cc%26pid%3D100005%26rk%3D3%26rkt%3D12%26mehot%3Dco%26sd%3D114240368888%26itm%3D303321237598%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithBBEV1Filter&_trksid=p2047675.c100005.m1851

They say they have an aluminium body, and looks like they're not the same as either of the other two metal options I've listed. Might be worth a try, however the price isn't much less than the DarkIceDesigns joints, and you'd still have the issue of steel pins running in alloy bodies.

And you'd have to wait for these from China.

Edited July 23, 2020 by amorti

[Polarlicht]

^^^These are also made out of aluminium. Not sure if thats strong enough at the holes where the center cross is.

[amorti]

40 minutes ago, Coolusername said:

^^^These are also made out of aluminium. Not sure if thats strong enough at the holes where the center cross is.

Sorry, my edit crossed over with your post. 

Yes, agreed.

They'd be strong enough for sure, but they will of course wear at the pins faster than the steel bodied ones. However any wear may be slow to the point that you never notice it. The AliExpress joints wear fast partly because of the materials but partly because of just how janky they are.

Considering the small price difference, I recommend not taking a chance on the alu bodied joints.