Shaft Dynamic Balancing 3

[USMechE6]

Hi All,

I am trying to understand if there is a minimum shaft speed above which I should spec dynamic balancing over static balancing. I realize it depends on the specific application but I wasn't sure if there was some general rule of thumb or interpretation from a standard.

I've looked at Iso 1940.1.2003, specifically G6.3. The chart for permissible residual specific unbalance looks to go down to about 70 rpm for common/general use, with the whole chart bottoming out on the axis at 20 rpm. Does either one mean that below these numbers static balancing is sufficient? My specific use is plant process equipment rollers.

One prominent vendor in the industry I've spoken to indicated a general rule of 400 fpm being their rule of thumb, far above these values depending on the roll diameter. Is this common and is the use of a linear velocity common as well?

 

[Tmoose]

something like this ?

http://www.munireng.com/image/cotton.png

Depending on the manufacturing method, well intentioned static balancing can make matters worse.

Do you know what the roll first bending critical speed is likely to be?

If it is a welded, hollow roll, uneven wall thickness can result. Turning the OD true makes it worse. The result can be a whopping heavy spot in the middle which can cause the roll to "whip" when operating even below 50% of the first bending critical. Neither static nor dynamic balancing can be of much help, since what is really needed are for the balance correction weights to be placed more than 25% in from the ends.

Turning the semi-finished core ID true, and even prebalancing the core before afixing the ends is the solution.

 

[USMechE6]

Hi Tmoose,

Thanks for the reply. Yes something just like that, a hollow tube with journals welded on each end, one of which being hollow for a hot-water/oil supply tube. I calculated the critical speed to be about 22000 rpm, way above anything we run across that particular roller and its machine.

That said, we run all different kinds of rollers at different speeds on other machines, and since we typically have vendors make these for us, I guess I'm looking for more of a fundamental rule/ general understanding of what speeds apply to which kind of balancing. There may or may not be one, at least insofar as the ISO Standard goes?

 

[HydroPE]

Hello USMechE6,

I don't know of any rule of thumb for determining whether or not dynamic balancing is required. However, there are ways to help guide you.

Similar to what Tmoose mentioned, the geometry of the hollow shaft may vary, causing heavy spots and likely a dynamic imbalance, even after a static balance. After a static balance, the roller would not "swing" when released from rest, but may wobble at high speeds. Dynamic imbalance will induce oscillating bearing loads at each end. If you're below 10% of the critical speed and the bearings are large and stiff for the application, then dynamic balancing may not be required. If the bearings aren't stiff, then you may notice more vibration and deflection at each bearing. If the bearings aren't sized appropriately, the wobble may significantly accelerate bearing wear. The bearing design will vary among original manufacturers, along with loading characteristics (like on a paper machine), so a rule of thumb may not be appropriate.

One may estimate the effects of dynamic imbalance, like the resulting oscillating loads at a given speed, then determine if dynamic balancing should be considered. The oscillating load is nonlinear with speed since we have centrifugal forces in dynamic applications, so using rotational speed may not be the best rule of thumb.

I use this method in hydroelectric facilities. Large generator rotors and turbines typically generate high loads from imbalance. On longer rotating components, like your rollers, I would have dynamically balanced since it's a relatively inexpensive process.

I hope this helps.


Best Regards,
HydroPE

 

[CouplingGuru]

When it comes to long span items rotating, it is impossible to "balance mitigate" the geometric critical speed limitations of a product. This is because the critical speed limitation is due to the fact that the weight of the part creates a "jump rope effect" The only way to increase critical speed is to decrease weight and/or increase horizontal rigidity (i.e increased diameter)

Now when it comes to your balancing question. The ISO1940.1.2003 spec is a rigid rotors spec. The degree of balancing is predicated on speed of operation. So at a speed of 70 rpm that standard would be utterly useless, this is because it would allow a tremendous amount of imbalance. For that reason ISO1940.1.2003 @ G6.3 becomes virtually useless on fully machined parts when operating under 1800 rpm. Let me qualify that, though. When it comes large diameter parts operating at or near the centrifugal stress limitations then even a small imbalance can make a huge difference. However, I am assuming that isn't your application.

Also in reference to your linear measurement question, the ISO standard is a somewhat linear measurement, G = e * w = constant, e = eper value and w is speed in rad/s. So as speed increases allowable eper value decreases. conversely as speed decreases allowable eper value increases. eper is the distance from center of rotation to center of mass. So the "G6.3" represents a value of 6.3 g-mm/kg rotor weght * rad/s

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[GregLocock]

"When it comes to long span items rotating, it is impossible to "balance mitigate" the geometric critical speed limitations of a product. "

Either badly phrased or wrong.

It is entirely possible to suppress the flexure of long flexible rotors that operate way above critical speed using balance weights. It is routine in some applications, and a consideration in many.

Cheers

Greg Locock


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

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

 

[CouplingGuru]

Phrased badly,

You are referring to safely operating above critical speed in flexible rotor, I am referring to changing the point in which critical speed occurs in a rigid rotor.

Either way pretty sure the OP isn't running into any of those issues with their application.

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[USMechE6]

Thank you all for the input, and I apologize for my delayed response; I've only just now seen your thoughts.