U-joint connection

[TeoAlfa]

I am in the process of designing an agricultural machine which uses the power from the tractor to give rotation and torque to an inclinable tiller head.
During the process i have reviewed several designs. One of them made me sceptical. They connected the transmission (just to let you know that the tractor connects to a transmission box which then give rotation to the tiller head at 90 deg. angle) to the tiller head by using just one u-joint. Do you think this solution will cause vibrations and lastly bearing failure at the end?
As a solution it's simple but i would like your views on that. I know that a universal joint system includes two seperate u-joints. Have you ever seen a system with only one u-joint? Thanks!

 

[TeoAlfa]

@Greg
Yes, i have considered this. Ypu mean homokinetic joints, right? Problem is that these are quite expensive.
The other problem is that by using a single joint (either u-joint or double cardsn joint) the rotation axis of the tilt will need to be axactly the same with the rotation axis of the joint, otherwise it will be impossible for the head to tilt. By using a double u-joint setup you can use 2 hollow shafts so the length of the saft will be free to change as you tilt the head.

 

[FeX32]

TeoAlfa,

Here is a document with what you would like. The example calcs are for 30 deg.
I have it as a word doc so you can use and modify it as you wish. Luckily, I did that many years ago as part of a larger project. I no longer use word.

http://goo.gl/pCYTAI

"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[TeoAlfa]

@FeX32
Thank you very much for all the help!

So, if i use a double u-joint setup with the input and output shafts aligned, will it be better than using a single u-joint setup with the shafts misaligned. Is that right?
Just to note that the rotatinal speed is quite low (370rpm) so maybe the vibrations will not be so serious?

 

[FeX32]

Yes you are quite right TeoAlfa.

As the others suggested too. There will always be a little bit of vibration unless the shafts are perfectly aligned.
However, you significantly reduce this using your setup as opposed to a single u-joint.

Ideally, you should use some sort of constant velocity joint as Greg pointed out. But, if costs are a limitation then you will be fine with your design. I would imagine that there will be vibration regardless considering what the machine is actually doing. Also, U-joints are more reliable than most CV joints. In a case where you might have shock loads (like in yours) u-joints are likely a good choice.

Cheers,


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[Compositepro]

I am not clear on whether the u-joints are on the tiller shaft or on the PTO shaft. Single u-joints are used where the drive shaft is used as part of the alignment of the driven article. If alignment is controlled by other structures then two u-joints must be used.

 

[MikeHalloran]

Two u-joints works only when the articulation angles of the joints are equal.
If the pto and tiller shafts are parallel, the intermediate shaft's u-joint ears must be phased at 90 deg to each other.
If the pto and tiller shafts are not parallel, but the intermediate shaft is at an angle intermediate to them, the intermediate shaft's u-joint ears need to be in phase with each other.
There is no other situation in which two (cardan) joints will be satisfactory.

That's why the double cardan joint is a superior choice; the angles between pto and intermediate shaft and between intermediate shaft and tiller shaft have no effect, because the double cardan joint has its own intermediate shaft and a mechanism for equalizing its interior joint angles.
Similarly for Rzeppa and other forms of 'constant velocity' joints.

CV joints and double cardan joints would also allow use of just one joint assembly, but I wouldn't recommend it because you can't guarantee that the joint's center intersects the hinge external to the joint.




Mike Halloran
Pembroke Pines, FL, USA

 

[TeoAlfa]

@MikeHalloran

Thanks for the info!
For this type of machine, 80% of the time, the tiller head will be parallel to the pto shaft. The rest of the time there will be a maximum angle of about 10degrees.
So, for this situation and if we are using traditional u-joints, do you recommend phasing the two shafts at 90deg?

 

[MikeHalloran]

Phasing the ears at the ends of the intermediate shaft at 90 degrees should work for parallel shafts. Ten degrees of misalignment will produce severe vibration and torsional stress throughout the drivetrain; even two degrees will be more than noticeable, even on a tiller.

If you are intending to offer a warranty on this product, I strongly suggest that you reconsider CV joints, double cardan joints, or a hydrostatic drive (which would also eliminate the right angle gearbox and the jointed shaft between the PTO and the gearbox input).



Mike Halloran
Pembroke Pines, FL, USA

 

[JStephen]

In something like a car, you have the inertia of the car maintaining a constant speed, the inertia of the motor maintaining a constant speed, and if you have a linkage in between that varies the connection through the cycle as discussed above, you have problems.

In your case, the tiller part doesn't have the momentum that a moving vehicle does. It is quite likely rotating much slower than an automotive drive shaft at speed as well, which lessens dynamic effects as compared to torque transmission. So my take is that, IF a single U-joint can be used, your type of application is one where it is most likely to be successful.

 

[Compositepro]

A u-joint at each end of a telescoping pto shaft is the way that most farm implements are connected to tractors. A standard speed is 540 rpm. I think the u-joints are an insignificant source of vibration compared to a tiller or bush hog, where the vibrations can be measured in inches.

 

[GregLocock]

I'd just add that agricultural engineering is not just a field of enterprise, it is also a term of abuse, for good reason. I've seen machines driven via single (large) UJs at 30 degrees of articulation. So be careful that you don't aim for a better balanced solution than is necessary.

That being said the rest of your transmission will be much happier if you do try and get the UJ phasing roughly right.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[FeX32]

Indeed, such a machine is no BMW


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[TeoAlfa]

As said before, an agricultural machine is not a BMW but my real concern is to eliminate vibrations in order to preserve the transmission bearings at both ends of the shaft (gearbox and housing at the end of the tiller head). You don't need a Jaguar to work on the field, but you need a trouble-free machine which will need low maintenance. Warranty is another issue as you don't need to be charged with parts often.
Adding another u-joint to the setup is not a big deal on the whole project cost so i think that it's worth the effort.

Another issue is the phasing of the shaft. Do you think that the shaft should be phased at 90deg or not phased at all as i have designed it (look the drawing above)?

 

[FeX32]

Adding another u-joint to the setup is not a big deal on the whole project cost so i think that it's worth the effort.

If I were you I would use a double-cardan joint. Essentially, the same design you have except keep the length of the intermediate shaft very short. This would almost eliminate the kinematic variations that cause unwanted vibration.

Attached is some information on the subject. JMD is considered by many to be the top journal in mechanical engineering.


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[FeX32]

If you can't shorten the length of the intermediate shaft, then just keep the design you have. (assuming you don't want a Thompson joint or the like)


"Simplicity is the ultimate sophistication." L. da Vinci
- Gian

 

[TheTick]

Hoo boy! I've been through this exact problem multiple times.

You simply MUST have a double (or more) u-joint if you wish to transmit torque through rigid shafts at multiple angles. Single u-joint just doesn't work. Management creatures and counters of beans like to save money with single u-joints, but it comes at the cost of a design that no worky.

 

[TeoAlfa]

What do you think about the phasing?
90deg phasing or no phasing?

 

[ornerynorsk]

What Tick said. It's simply not an option. There are other designs of couplings which will take up the space of a single u-joint, but will allow for a certain amount of misalignment.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[3DDave]

One the one hand, the dual u-joint system is required to allow for the telescoping section to work, so it doesn't seem like much escape there. On the other hand variation from a single u-joint is small for relatively small angles compared to the shock loads the tiller gets in moving the dirt.

As part of the engineering process, gathering shock load data either from literature or from experimentation in the field (pun!) is a necessary part of the task. That way all the loads can be accounted for, not just the internal inertia loads.