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Alex

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This is a question for Cat 10 analysts only. [biggrin] I am thinking about the diameter scale principle (hard bearing balancing machine). Everyone who had worked with this type of machines knows the diameter scale is not linear but goes with the circle equation. At first it was logic to me but more I think about it, more sceptic I am. I know the linear scale doesn't fit the principle that is a fact. There is some other thing that is bothering me.

What is the basis from which the scale should be made from. Do we just need to have the center of rotation on the same height all the time? Is that it? I think the answer is no. Let me explain why I think so. I am thinking about the force that gets from the rotor to supporting wheels. I suppose the machine measures horizontal force component only. But the contact angle between the shaft and the wheels is different with different shaft diameter. So the horizontal force component is not the same with different shaft diameters. 

I hope I described the issue in a way someone could understand the question [smile] Thanks in advance! Alex

 
John from PA

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Is this what you are asking about?

Capture.jpg 

Alex

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John I am asking about the scale on the machine itself. The supporting wheels are fixed on the plate which can be moved up or down regarding the rotor shaft diameter. The scale shows where to fix the plate for each rotor shaft diameter.
OLi

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Is it just to get the shaft center to always be at the same distance above the force transducers as it was when it is calibrated? Or if it is a drive shaft machine to be centered to the drive shaft?
I worked on a drive shaft machine 30+ years old last year and I can't remember seeing a scale for that but they rarely used any variation of shaft diameter either.

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John from PA

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Quote:
Originally Posted by Alex
John I am asking about the scale on the machine itself. The supporting wheels are fixed on the plate which can be moved up or down regarding the rotor shaft diameter. The scale shows where to fix the plate for each rotor shaft diameter.


Can you supply a picture of what you are talking about?
Alex

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Oli, it was a Cemb under resonance balancing machine with cardan drive. The cardan and the rotor were always in the same line when set to diameter scale. But I wonder if that is all to be aware about or there is some catch with the rotor and wheel contact angle.

John unfortunately I don't have any photo. 
Alex

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The machine was similar to this one:

https://www.machinio.com/listings/22126037-cemb-balancing-machine-in-mogliano-veneto-italy
Walt Strong

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Based on Olov response and the photo, I would assume that the support wheels would be lowered or raised to accommodate larger or smaller shaft with diameter, respectively. The goal would be to maintain the same elevation of the shaft centerline above the force sensors. If this is the case, then the change in elevation could be calculated from geometry. The triangle would have a base equal to the span between roller centers, and the length of the two sides of triangle would equal the sum of the roller radius and shaft radius.

Walt
Alex

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Yes Walt I agree on that. But when you think about the centr. force (unbalance) transfered from the shaft to rollers...When you have a large shaft, the triangle you mention has larger side angles. When you have a small shaft, the triangle has lower side angles. I think that the force going from the shaft to rollers is different in horizontal direction in both mentioned cases when you have let's say 100 g unbalance in both cases.
OLi

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My son say that it is only convenient in production feature or when you know the shaft diameter to preset it even before the rotor arrives so all is set and ready to run
and to load the rotor and start running w/o fiddling further with the setting of the rollers and maybe also to satisfy the magical range that seem to exist with where the shaft
is running relative the rollers position, at least in soft machines. I am not that versed in the inner life of stiff machines. What do sort of surprise me is that the sensor(s) calibration
is only done by running 1 test rotor once with a defined, known unbalance and that should be basically only calibrating the force reading from the sensors when commissioning or I may be wrong,
there may be some magic I don't understand. 

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Walt Strong

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"I think that the force going from the shaft to rollers is different in horizontal direction in both mentioned cases when you have let's say 100 g unbalance in both cases."

The rollers are merely an extension of the rigid support, so that the force on the load cells is a function of shaft nominal centerline height and unbalance. If you adjust the elevation of the rollers, then the centerline height can be normalized to the test rotor/shaft diameter. In short, I believe it does not matter how deep the shaft sits into the rollers (within design/specification limits) as long as the height is adjusted to maintain calibration centerline elevation. I have not used such a machine, so my comments may be incorrect.

Walt
John from PA

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Reply with quote  #12 
Here's a better picture of one of the supports.

To the OP, yes it is desirable to have the rotor approximately near horizontal, or level.  I am however puzzled when you state "Everyone who had worked with this type of machines knows the diameter scale is not linear but goes with the circle equation."  Can you elaborate on what you mean by "not linear"?

Capture.jpg    

Walt Strong

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Alex,

Here is my description that would be easy to make an Excel SS, if you are trying to do very precise balancing.

Description for "Hard Support" balance machine:

Two rollers on balance machine support the rotor to be balanced at each shaft mounting location (stands or pedestals). The rollers have a fixed diameter and distance between centers. The height of each roller pedestal can be adjusted vertically to accommodate rotor shafts of different diameters. The height is adjusted so that the elevation of the shaft centerline can be maintained the same as the settings for the calibration rotor (reference standard). The balance machine specifications should include minimum and maximum shaft diameters along with minimum and maximum rotor weights and rotational speed.

 

"Pythagoras' Theorem" can be written in one short equation:

a^2 + b^2 = c^2

 

From <https://www.mathsisfun.com/pythagoras.html>

 

Triangle Definitions:

a = base of triangle = 1/2 of RC; where RC is center distance between support rollers

b = vertical height from triangle base (support roller centers) to center of shaft (SD) axis

c = radial distance between shaft center and roll center = 1/2 (SD + RD); where SD is shaft diameter and RD is roll diameter.

 Note that RC and RD are constants/fixed as determined by machine design, and SD (both ends if different) is measured at time of rotor balancing.

The objective is to solve equation for b as a function of SD, compare value to calibration value, and adjust roller elevation as necessary.

Walt
OLi

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Reply with quote  #14 
You may also adjust rollers center to center distance for various shaft diameters not just the elevation since there are a optimal range of "angle of attack" to get a stable rotation on the rollers. I have seen someone give the details on that somewhere. I am not sure it change anything anyway.
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Alex

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Reply with quote  #15 
John, the scale is not linear. Lets say you have 3 rotors, the first one with 20 mm shaft diameter, the second with 30 mm diameter and the third one with 40 mm diameter. The supporting height is not equally different for those three cases. That is because the smaller shaft goes deeper between the rollers.
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