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U-joint connection

U-joint connection

U-joint connection

(OP)
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!

RE: U-joint connection

Hi
Any chance of a picture?

RE: U-joint connection

(OP)
And here is the design that features only one u-joint.
Notice how close the transmission is to the tiller head.

RE: U-joint connection

Hi

Thanks for the information, vibration normally comes from misalignment or out of balance rotating parts.
Universal joints are usually provided in pairs when two parallel shafts are not aligned, I cannot see a problem with the set up assuming all the calculations for the bearing etc have been analysed

RE: U-joint connection

(OP)
In this case we need to connect 2 unparallel shafts. So are we going to get any vinrations by using a single u-joint?
Will it be better and more smooth if we use a pair of u-joints?
Thanks!

RE: U-joint connection

Yes you will experience vibrations only if the input shaft angle is significantly different from the output. With 2 U- joints, this sinus vibration is canceled out.


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

RE: U-joint connection

The problem is that u-joints are not uniform transmitters of rotation. See http://www.cs.cmu.edu/~rapidproto/mechanisms/chpt8.... The reason they are usually used in sets with the input and output parallel is to undo at the final joint the non-uniform acceleration introduced at the first joint. CV joints are called constant velocity to distinguish them from u-joints.

The design team should have presented the expected loading from the non-uniform effect.

I also see in the CAD image that the input shaft is not parallel to the output shaft. If the angle change between input and output in the new design is the same as that in the old design, the load on the u-joints because of misalignment will be the same. Further examination indicates the CAD design is to allow the tiller to tilt relative to the fixed motor, which would vary the amount of non-uniformity. At least the new design is constant.

RE: U-joint connection

I found some of the derivations. I attached just the results of the kinematic derivations.

Notably, the results indicate a sinus vibration that is directly proportional to the differences in shaft angle.

I have also derived the inertial torque. If you are interested let me know.


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

RE: U-joint connection

(OP)
@3DDave
By telling "new design" you mean the picture and "old design" you mean the CAD image?
If so, in the new design (picture) the tiller is allowed to tilt, although not apparent in first view. And i assume that the axis of tilt should be the same as the axis of the u-joint cross to be able to tilt.

By definition, the tiller head should be able to tilt. This is a given. So we must find a way to eliminate the vibrations caused by misalignments.
In my design (CAD image) i tried to cancel out the vibrations caused by misalignment (which will always be present as the head should be able to tilt) by using a shaft with 2 u-points featuring opposite angles (see image below).



The 2 shafts will never be perfectly aligned because the tiller head will be able to tilt and the operator (theoretically) will not be able to tilt the head perfectly to align it with the angle of the transmission shaft. The question is: Will we have less vibrations with the double u-joint design?

RE: U-joint connection

The vibration is only proportional to angle for cases where theta is approximately equal to sin(theta).

The ratio varies from cos(theta) to 1/cos(theta) for theta is the angle between the shafts over each rotation.
For 5 degrees this is .996 to 1.004.
At 15 degrees this is .966 to 1.035, too much for most drivelines, probably OK for low RPMs.
30 degrees is .866 to 1.155, which is very noticeable.

RE: U-joint connection

"The question is: Will we have less vibrations with the double u-joint design?"

Depends on how much misalignment there is. But in general, assuming similar angles between the single and double u-joint arrangements, there will be less in the double arrangement. This can be easily proven with the kinematics I provided.


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

RE: U-joint connection

(OP)
@ FeX32
In the single u-joint design the misalignment angle (between transmission and tiller shafts) will always be greater.
Could you please make a sample of calculations for me? Whatever you need please ask me. Thanks a lot!

RE: U-joint connection

Dave, that link you provided is useful. However, it's analysis assumes the angle between the input and output (beta) is the same. This is not likely a good assumption for the OP's application.
Consequently, the analysis is inaccurate.
Although, the conclusions (in general) drawn are for sure valid.


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

RE: U-joint connection

If you make the tilt axis the same as the gearbox input shaft axis then the tilt of the tiller will not change relative to the output of the gearbox. You can move the gearbox by adding a double-pulley (at least that's what the input looks like now) and a belt from there to the gearbox. Costs more, more finger catchers, but eliminates u-joint vibrations.

I thought you were reviewing in-house designs with old-costly-two u-joints vs new-cost-saving-one u-joint. It's early.

RE: U-joint connection

(OP)
I made a modification to the initial design. I made the 2 shafts parallel. And given that the tiller head will most of the times be working horixzontally i think that this is the best solution. As i have told before, we will always have a bit of vibrations for the reason that the tiller head is designed to tilt.
I attach an image of the new setup at the bottom of the post.

If we use a single u-joint, even with the tiller head in horizontal position the misalignment will be high. There is no way to cancel the vibrations out.
What do you think?

RE: U-joint connection

I agree with Dave's statement.

Also, here is what you asked for. The kinematics of a single u-joint. I coded it in Matlab.
https://dl.dropboxusercontent.com/u/18923918/u_joi...
You can change the angle of Beta at the top and re-run the code to compare.

If you don't use Matlab let me know and I can save the plots.


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

RE: U-joint connection

(OP)
@FeX32
No, i don't use Matlab unfortunately.

RE: U-joint connection

(OP)
@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.

RE: U-joint connection

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

RE: U-joint connection

(OP)
@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?

RE: U-joint connection

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

RE: U-joint connection

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.

RE: U-joint connection

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

RE: U-joint connection

(OP)
@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?

RE: U-joint connection

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

RE: U-joint connection

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.

RE: U-joint connection

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.

RE: U-joint connection

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


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RE: U-joint connection

Indeed, such a machine is no BMW wink


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

RE: U-joint connection

(OP)
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)?

RE: U-joint connection

Quote:

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

RE: U-joint connection

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

RE: U-joint connection

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.

RE: U-joint connection

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

RE: U-joint connection

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.

RE: U-joint connection

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.

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