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drive_shaft_issues 3

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vanuta

vanuta

Mechanical
Aug 1, 2009
15
MY
Hi,

I am having some drive shaft issues. I have posted this before. I did further testing and came up with some new results and thought i will share this with the community and at the same time I hope to get some direction in this forum.

I have a PTO driving a Centrifugal pump. There are three shafts between the pump and PTO. All the shafts are slip-yoke type with universal joints at the ends. One of the shafts goes through a wall.

On one of the tests, i discovered that the bolts on the PTO housing (face) were sheared and the housing flange came off. Please note that the PTO that i have used in the attachment is not the actual one that i used. It is just there as a representation. Further analysis showed that the bolts broke in tensile action.

Vibration analysis on the wall showed that the wall was vibrating excessively in the axial direction (at least 1 mm).

My postulation is since there was excessive axial vibration, this might have caused the PTO shaft to be be pulled. When this shaft gets pulled, it also pull the housing flange which have caused the bolts to fail in tensile.

What i dont understand is, even if there is axial movement, only the shaft should slide axially in the bearing ( between the shaft and the flange bearing) right ? how would it cause the housing on the PTO to move??

Some causes of vibration could be:

1) the high universal joint angles at the position pointed out in the attachment

2) All the three shafts are yoke -slip type? does it mater how they are assembled...as in whether the tube side should be connected to the driver or the driven? is there a best practice on this? if so, why is it so?

3) The slip on the drive shaft (between PTO and the wall) is only 4 mm and the axial vibration is about 1 mm. Should the slip be longer?



Is there anything else i should check out?

Thank you.
 
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Hi vanuta

Have a look at this site it states that to large and unequal joint angles can cause vibrations


I would check with the U joint manufacturer what the maximum operating angle for your joints are.
Any chance that you can upload a picture of the failed bolts?
looking at the fracture faces of the bolts can often tell the cause of failure.

desertfox
 
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desertfox,
you are right, the large angles are one of the problem....and i mentioned that the bolts failed in tensile..which means that there is a high possibility that there is an axial force acting on the housing that caused the bolts to fail.. Vibration analysis at the wall also shows that there is a significant displacement in the axial direction...my question will be..generally in a PTO,between the PTO housing and the shaft, there is a bearing, so rightfully, the shaft should be freely sliding in the bearing, rather than pulling housing...which would have caused the bolts to fail in tensile... I do not understand why this happens...

 
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Hi vanuta

How easy is it for the shaft to slide if held at an excessive angle.
Again can you provide a picture of the failed bolts, also how about the shaft in question are there any notable marks on it and/or the bearing.

desertfox
 
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Hi Vanuta,

Have you tested the wall motion with a grounded dial indicator while turning the pump by hand yet? If there is much motion at all there are some basic mechanical problems with the installation.

"the shaft should be freely sliding in the bearing"
Typically for rotating shafts on equipment with service lives greater than a few hundred hours, no. The shaft will creep in the bearing, and wear, creep faster, wear even more, etc.

So Axial motion must be accommodated by other means, like a spline in the shaft, or a non-fixed bearing that can slide in its housing, or mount flexibility. Don't you have a spline on each shaft section?
"All the three shafts are yoke -slip type"

Can you post a picture of the "wall" detail?

Does your wall bearing look like this?
The sketch says "bearings" so I'm assuming there are 2 of this type bearing on the thru-the-wall-shaft.

More vibration info is REALLY needed, to arrive at a solution with any efficiency. Amplitude AND Frequency, readings on the PTO, pump, beam, and around the "wall, a bump test on the wall to check for resonance.
If the vibration frequency is twice rotating speed, then u-joint angularity >>can<< be part of the problem. If the vibration is at 1X rotating speed then excessively tight u-joints can be part of the problem.

Without frequency info, and an understanding of the vibration direction and locations, strategies to correct
excessive vibration" might just as well be made using a device shown here.

I think the axial force necessary to break the cover bolts would do terrible things to the bearing the cover is retaining, if that is the "fixed" bearing for that shaft of the PTO. Bolt failure is often a sign of insufficient preload (~torque), which is not easy to obtain with a gasketed joint. You may find the PTO's fixed/thrust bearing is elsewhere on the shaft.
 
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Tmoose,
Thanks for your input. Could you please further elaborate on the statment
"If the vibration frequency is twice rotating speed, then u-joint angularity >>can<< be part of the problem. If the vibration is at 1X rotating speed then excessively tight u-joints can be part of the problem".

Is there any reference book that illustrates this, so that i might be able to understand more on this.

Could you please elaborate on excessively tight U-joints?

The part on "slip yoke" shafts, What really puzzles me is the fact that if it is slip=yoke, it should allow for axial movement and should not affect the housing, unless the spline is seized in the tube for some reason...do you think it could be a possibility.

As for the wall bearings, it is similar to the picture you sent, except that it is a square flange bearing with four bolts.

Thank you.
 
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Two things that are easy to check. A shaft with two U-joints needs to be in phase and make sure the slip joints are not bottoming out or worn excessively.

 
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"As for the wall bearings, it is similar to the picture you sent, except that it is a square flange bearing with four bolts."

How many of those bearing units support the shaft section passing thru the vibrating wall?

Dan T
 
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Many times slip yokes lock axially when torque is applied to the shaft. They are not dynamic adjustment devices under load and will transmit axial forces from one device to the other. You also cannot get angled operation for 'free'...the consequence of angle is a restoring force acting on the bearings of the components connected by the shaft. Depending on shaft angle and torque, the forces can be unexpectedly high and cause damage to the components.
Additionally, as Unclesyd mentions, the phasing (rather the joint angle) should be the same relative to the shaft axis, otherwise, due to the nature of U-joints, a twice-per-revolution vibration will be induced into your rotating system, as mentioned by desertfox and Tmoose, if the joint angles are not equal.
All that aside, Vanuta, you state that the shaft should be moving axially inside the bearing...why do you believe this? From your sketch of the components, it seems the vibration source could also be the PTO...could whatever is driving the PTO also be shaking the wall?
 
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Maybe switch to a CV joint for higher angular misalignment.

Russell Giuliano
 
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Suggestions to verify
vanuta

Are all components (PTO, flange bearing, input shaft) are parallel in both axis.
Are the drive shaft angle Less than 3 degrees, seems to be a magic number.

Are all the drive shafts dynamically balanced?

Are the u-joints phased as mention before?

What is the runout of the drive shafts when assembled?

MfgEngGear
 
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Make sure that the shaft's of the driver and the driven device are parallel. Even if one or both are set at an angle, make sure they are on the same angle.

rmw
 
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In the supplied diagram, the joint angles at each end of each of the three driveshafts are equal,
>>but of opposite sign<<,
so the shafts have to be 'quartered'.

I.e. the axes defined by the bores in the ears on the ends of each shaft should be at right angles to each other, not parallel, which is the more common arrangement.



Mike Halloran
Pembroke Pines, FL, USA
 
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Somehow, I learned that cases 6.1 and 6.2 of DynoGuy's reference, i.e., parallel and intersecting shafts, required different intermediate shafts.

Not true; intermediate shaft is the same whether phi is 0 or 180 degrees in Figure 14.

I was wrong.

It happens. Write that down. Thanks to DynoGuy for the gentle correction..





Mike Halloran
Pembroke Pines, FL, USA
 
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You may have to resort to a double Cardan joint (CV) if you are too far out in your alignment.
Even though these are made auto after market you can get the same configuration for other uses. The BAMF joint looks interesting.

 
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