Universal Joints & drive shafts

[CF MRC]

I had a conversation last weekend with a visitor at Ally Pally regarding drive shafts and UJs.  He was adamant that the two ends of the UJ should be at 90 degrees to each other. I had a strong recollection that 45 years previously a very good engineer at the MRC, Alan Cruikshank,  had told me that the yokes should be in line with each other - that is the way they were arranged on the Hymek diesels, as an example. The input and output shafts should also be in line or parallel to each other. Needless to say, all my previous engines over the last 40+ years have had this feature. 
 

Wind forward to 2022  and I thought I would try making the UJ for the Raven 4-6-2 class at 90 degrees, as an experiment. It worked, but was perhaps a bit noisy. As is their want, this drive shaft was propelled in to deep space when I accidentally powered up the tender motor with the other end non-constrained. The next shaft was made in-line and performed much better.  
 

This link & video shows why:

https://www.rccaraction.com/tech-center-matter-way-yokes-face-telescoping-driveshaft/

 

Hope this is of interest.  When demonstrating at shows, it is always fun chatting to people who may / may not be knowledgeable.

 

Tim

Edited March 25, 2023 by CF MRC

[queensquare]

That’s really interesting Tim. I remember reading somewhere that the ends of the UJ should be at 90 degrees but never really knew why - I now know they shouldn’t be! I’ve  tended to be a bit random with mine but will now go over my locos and make sure they are in line/ parallel - particularly on locos that are a little noisy.

Jerry

[Caley Jim]

1 hour ago, queensquare said:

..... I remember reading somewhere that the ends of the UJ should be at 90 degrees...... 

I recall reading that somewhere too. The loops on my (KISS principle) drive shafts are generally in line and they tend to flail about a bit anyway. I've never been sure whether the noise was coming from the shaft or the gearing. 

 

Jim

[CF MRC]

If the loops don’t engage freely in the slots and become wedged then I find the noise and vibration go up massively - I suppose because one end is effectively fixed. 
 

Tim

[Izzy]

I've Just looked up what is said about u/j's in my ancient copy of Machinery's Handbook - 15th ed/1956 ( published since 1914 and often said to be the toolmakers bible) and given to me by my late father-in-law. There is just over a page on the subject - out of the 1,880 - but it's quite interesting. I have scanned it and will send a copy via PM to anyone interested, just ask. Can't of course post it here due to copyright infringement.

 

I'm not sure how much applies to what is used in our models, as ever it depends on the intended use but the basic principles are quite clear. Mostly follows that of the link Tim gave. But our shafts and joints are inevitably not 'precision' in the general sense compared to how they are mostly used. So much slack and give that probably mostly overcomes the issues that arise in full size precision applications such as car drive shafts etc.

 

Bob

[Caley Jim]

1 hour ago, CF MRC said:

If the loops don’t engage freely in the slots and become wedged then I find the noise and vibration go up massively - I suppose because one end is effectively fixed. 

Mine are quite the opposite! Very loose in the slots and around 1mm play fore and aft! 

 

Jim 

[CF MRC]

I think we’re saying the same thing, Jim!

 

Tim

[t-b-g]

Having made around 8 or 9 drive shafts for my 2mm Dean Goods (I lost count after the first 5) I ended up with a smooth running loco with the ends of the drive shaft in the same plane.

 

There are several different factors that come into play. Lengths of drive shafts, amount of free play in the joints and the alignment of the output and input all come into play.

 

I think if your input and output positions require a drive shaft to run at an angle rather than pretty straight, then I can see the benefit of the 90 degree option. If both ends were running in slots which are horizontal at the same time, then there is a greater opportunity for tightness and binding. With the 90 degree option, at least one end can take up the angled alignment more easily. With most model railway mechanisms I have seen, the angles involved are quite shallow or the outputs and inputs are almost in line and I have seen (in 2mm and other scales) enough that run well with both options to think that it isn't the deciding factor in how well something runs.   

[CF MRC]

Seems pretty unequivocal
https://www.google.co.uk/search?q=car+drive+shaft+angle&tbm=isch&ved=2ahUKEwi37Z7zs_f9AhX8iv0HHfyzBvQQ2-cCegQIABAA&oq=car+drive+shaft+angle&gs_lcp=CgNpbWcQDFCICFiTD2CnI2gAcAB4AIABRogBgAOSAQE2mAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=fRQfZPe6HfyV9u8P_OeaoA8&bih=1302&biw=1381#imgrc=IRA-FnR1KjFAQM
 

Tim

[Izzy]

I think the fact that with tender driven loco drive the input and output shafts cannot remain parallel at all times, otherwise the loco would be restricted to dead straight track, kind of explains why noise and vibrations may sometimes happen with our models. Interestingly the MM page I mentioned has a formula related to the angle of the drive regarding maximum rpm with something like a 20% reduction needed at 45degrees. With the ball & socket type mostly used in models you have of course to ensure the socket isn’t too long to restrict angular movement too much. All very interesting stuff.

 

Bob

[Chris Higgs]

On 26/03/2023 at 02:10, CF MRC said:

Seems pretty unequivocal
https://www.google.co.uk/search?q=car+drive+shaft+angle&tbm=isch&ved=2ahUKEwi37Z7zs_f9AhX8iv0HHfyzBvQQ2-cCegQIABAA&oq=car+drive+shaft+angle&gs_lcp=CgNpbWcQDFCICFiTD2CnI2gAcAB4AIABRogBgAOSAQE2mAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=fRQfZPe6HfyV9u8P_OeaoA8&bih=1302&biw=1381#imgrc=IRA-FnR1KjFAQM
 

Tim

 

Including that we should never use drive shafts for what we do, going around corners. 😔

Chris