Wire Vibration

[Husam1]

Dear All,
I believe I have a unique problem. To best describe it, I would request you to first have a good look at the attached sketch:
- This machine runs a steel wire at about 3000 RPM.
- The wire needs to continuously run WITHOUT any vibrations.
- The Two green pulleys are mechanically attached and move in Up/Down Directions.
- The wire between the two green pulleys runs smooth, then all of a sudden starts to vibrate for a few seconds then go back to run smoothly.
- We increased the wire tension; but that did not help!
- changing the drive speed did not help!
- We replaced the pulleys; but that also did not help!
- This vibration occurs even if the green pulleys are stationary; but worsens when these pulleys move!

I'm no mechanical engineer and was wondering if this vibration was due to:
- Wheels imbalance (supplier confirmed all are balanced).
- Improper wire tension.
- The configuration/arrange of the pulleys WILL cause Vibration. i.e. This machine was incorrectly designed!!
What is it!!!

I appreciate your help and advise.

Thanks,

Husam

 

[desertfox]

Hi

Just a thought, is the wire clean and dry? I was wondering if its caused by fluctuation in friction between the green pulleys and the wire, for example if there was oil on the wire as it contacts the pulley and then slips until it runs on another part of the wire without oil and no sliping.

 

[MikeHalloran]

It's clearly a wire saw.
I wouldn't expect it to _not_ vibrate, some.

... but I suspect the resonance that's allowing the wire to sing, is actually in the structure that connects the pulleys, especially the carriage connecting the two moving pulleys.

Have you guys ever designed or built a wire saw before?
Did you take one apart to see how it worked?
Why is yours different?



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Husam1,

If my interpretation of your sketch is correct, I believe the wire is rubbing against the sides of the blue & red pulleys shown on the left, as is passes between them. Due to the fact that these two pulley axes of rotation are tilted out of the plane of view, the contact forces between the wire and pulley are not purely radial about the pulley axis. There is also an axial force on the wire that causes it to rub against the sides of the blue/red pulley groove where it enters/exits. This stick/slip sideways rubbing condition causes the wire to vibrate, just like a bow being dragged across the strings of a violin.

You can resolve the problem by changing the position/orientation of all three red/blue pulleys. First, the rotational re-location of these pulleys should only be about a vertical axis passing through the wire center. Second, all of the pulley operational axes of rotation should be parallel to your XZ (horizontal) plane. Third, the center lines of the wire where it enters/exits any given pulley groove should both lie in a common plane that is normal to the pulley's axis of rotation.

Hope that helps.
Terry

 

[Husam1]

Guys,
- DesertFox, the Wire is DRY. Almost clean and Not Slippery.

- Mike, You absolutely right. It is a 2.5 Meter wide Wire Saw machine; but why would you expect it to resonate!

- The Carriage that is shown in the sketch was merely symbolic. i.e. The system is much more sophisticated than this; but I wanted to show that there is a link between the moving pulleys. It is actually made out of guides, non-slip belt, etc. that don't interfere with the path of the wire. Nevertheless, I want to stress that the vibration comes and goes, even if these pulleys are NOT moving! The frequency of the vibration reoccurrence seems to increase as the two pulleys move up and down.

- Terry, the Wire is about 1.5mm in dia. and the pulleys are indeed rotated; but not more than 5 Degrees out of plane. It is made just so those to avoid the wires intersection close the left green pulley.

- The right Blue and green pulleys are all in the same plane and we have used "Pulley Pro" laser alignment tool to verify that. Assuming that the tilts in the other pulleys would cause vibration; wouldn't this vibration be killed as it enters the right blue pulley?

- We have of thought of all these alignment issues Terry mentioned earlier, and have made sure all is aligned; but to no success.


- I was wondering: is it an issue with Forces? I read something about pulleys tension side, and slack side. Do you guys think it's got something to do with it? Also, these are custom pulleys (300mm Outer Dia, 10mm Inner Dia [2 X SKF 6000Z Bearings], and 10mm width). What would I specify for the maximum values when we send it for balancing (or re-balancing)?

Hope this explains my dilemma,

Husam

 

[MikeHalloran]

Yeah, it's an issue with forces, and masses, and springs.

To a mechanical engineer, everything's a spring.

You desperately need one on site.
... not a spring; a mechanical engineer.

One red flag; 2 cheap bearings, 10mm ID x 26mm OD x 8mm wide, supporting a pulley made of 10mm plate of some unspecified material. The bearings can't be properly suppored on the OD unless there's a hub or a shaft you haven't mentioned. There are too many ways for that to turn out badly, and too many details unrevealed to properly evaluate what's going on. That's why you need an ME, right there.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

OK, I can see how a 300mm dia pulley spinning at 3000rpm might produce a dynamic balance problem. I can also imagine there might be a tension problem existing in the slack portion of the wire, due to a combination of high peripheral wire speeds and high polar inertias in the spinning pulleys. Basically, you can transmit force by pulling on a wire, but you cannot transmit much force by pushing on a wire. It seems possible that the pulleys driven by the wire may be unloaded periodically.

 

[Husam1]

Guys, OK.  
It took me a couple of days to think about what you guys are saying. But still, I have a few questions before I call a mechanical engineer:
-How does the pulley material (steel vs.  Alum.) have an effect on the vibration? 

-What about the pulley ID? Is there a way to suggest a better one? I. E. Is 20mm better than 10mm? Is 30mm better than 20mm? Where do we stop? 

-What do we request the maximum value to be when we send the pulleys to be balanced? 

-How does the wire tension affect vibration? And what are the forces in play here? To me, the sum of all forces across all pulleys is zero; but then again how does the tension side and slack side of the pulleys affect vibration? 

- On the bearings side, what a good bearing? I thought a bearing that can go up to 14000RPM is good for 3000!

All these thoughts run through my head and can't find an answer to!! 

Need your thoughts on this! 

Thanks 
Husam 

 

[MikeHalloran]

I was just guessing that you probably bought the cheapest bearings you could find in the selected size. You probably want to go to a higher ABEC number. Even better, call in a bearing application engineer to help you select something appropriate, and help you mount it properly.

I'm guessing about the details, but something doesn't add up, mounting two bearings that are each 8mm thick in what I'm guessing is just a 10mm thick plate.

I'm also guessing that you're driving the whole thing with a 2-pole AC motor, which is going to introduce strong variations in tension with each revolution, in phase with the AC power. If that is the case, I'd make the motor pulley heavier, and all the other pulleys lighter, than they are now.

A few photographs or drawings would help keep us out of speculative blind alleys...



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianE22]

If the spring/mass system is the cable itself then increasing the tension should increase the frequency of vibration (like a guitar string).

Steel tends to dampen vibrations better than aluminum.

I assume you have a splice somewhere in the cable. How "smooth" is it? That section of the cable going over a pulley might be setting of the vibration.

 

[Husam1]

Mike,
Attached is sample pictures of the wheels for your reference. Note that the wheels become thicker towards the center. the bearings are mounted inside machine grooves to the exact size of the bearings OD. The drive motor is high torque 3 phase frequency driven motor.

But what's your opinion on my other questions; Pulley ID's, Balance values to be requested, etc.

Brian,
With respect to the wire splice, I have checked it and found it to be smooth. I don't think that the splice causes the vibrations, as these vibrations are intermittent does not seem to follow a pattern!

Regards

Husam

 

[desertfox]

Hi

Well I think you can rule out bad balance of pulleys otherwise it be happening every revolution and your saying it only happens now and then, I still think it's down to friction and or slipping of wire or pulleys.

 

[Husam1]

DesertFox,

The outer rim of the pulleys are lined with an anti abrasion PU Layer (about 8mm wide X 12mm deep) that is grooved to provide a seating for the wire. The wire is already set to maximum tension. Though I'm not sure if it is slipping of not, HOW can I find out if it is??

Regards

Husam

 

[MikeHalloran]

The pulleys are much nicer than I had imagined.
The bearing bore in the pulley should be a press fit to the bearing OD.
The pulleys do have to be balanced because of the multi-piece construction. I would just ask for balancing at 3000 rpm or above, with a time limit as a tolerance.

WRT slipping, any smooth pulley that's transmitting power to or from a belt or wire must be slipping in order to transmit the power.

Even with a 3-phase motor, extra mass on the motor pulley should help.
... but the irregular nature of the vibration suggests that it's more of a natural frequency thing than a forced vibration thing.

Have you been able to measure the frequency spectrum?




Mike Halloran
Pembroke Pines, FL, USA

 

[Timelord]

I would approach this by first calculating the resonant frequencies of the wire and then search for those exciting frequencies elsewhere on the machine. Have you tried relaxing the wire tension some? This will shift the resonant frequencies down and may shift you out of an exciting frequency range.

Timelord

 

[Husam1]

TimeLord,
How do you calculate the resonant frequencies of the wire, and how do we measure the frequency spectrum?

Regards

Husam

 

[MikeHalloran]

Mechanical vibration of wires is probably covered in the powerlines section of your EE Handbook.

Have you got a smartphone?
There just has to be an app that can record a sound sample and then show you its spectrum.




Mike Halloran
Pembroke Pines, FL, USA

 

[BrianE22]

Does the coming and going seem to be at a constant frequency? If so, you might have two close together frequencies generating a difference frequency that is modulating the vibration.

 

[BrianE22]

Can you remove the cable and run up the drive pulley? See if the vibration is still there.

 

[Husam1]

OK guys. Give me a day. I will check and come back to you.

Regards
Husam