A couple of questions about rotor balancing 1

[AnCheng]

In a few months, we will be overseeing an on-site rotor balancing job for one of the blowers in our plant. And since I haven’t worked on balancing before, I have a few questions:

1. What is the point of installing a trial weight? Even though the heavy spot can be calculated without it.
2. The frequency reading will give us the angle at which trim weight should be added/subtracted. But how can we determine the radius at which to place it?

Any help would be greatly appreciated.

 

[GregLocock]

1) Typically your balance standard will be much tighter than anything you can calculate.
2) you are working in units of g cm or whatever, so you can use half as much weight at twice the radius.

Cheers

Greg Locock


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

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[Tmoose]

"the heavy spot can be calculated without" the trial weight.
Are you counting on the 1X vibration peak-to-peak "displacement" equalling the eccentricity of the center of mass?

"The frequency reading will give us the angle at which trim weight should be added/subtracted."
Did you mean "phase" ?

Was this blower balance previously so the "influence coefficients" are known?

Ever see and work with something like this ?

https://roymech.org/images/freq_curve1.gif

 

[GregLocock]

Is your device expensive to run? For instance if you are balancing a steam turbine in a power station the balancing process is hugely expensive, so learning on the job by trial and error is not feasible (there are other considerations).

If not then why are you worried about the extra run(s) for the trial mass?


Cheers

Greg Locock


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

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[MintJulep]

The trial weight provides a known (measured) imbalance cause and the trial provides a known (measured) effect.

The balancing machine uses that information, together with data from the "as-found" measurement, to calculate the necessary balancing weight and position.

 

[geesaman.d]

"Heavy spot" is only relevant to a single-plane balance.

Most field balancing equipment performs two-plane balancing. The trial weight is part of the process of two-plane balancing.

In my experience, having a balancing specialist bring in their equipment means they will place trial weights and reduce the imbalance condition by telling you finally how much weight to put where. If there are radial locations to be used or avoided, tell them that. While it's generally ok to trade radius for weight in a correction, it's nice to have performed that exact trial weight run.

 

[MintJulep]

A nice explanation of balancing:

Introduction to rotor balancing

 

[spciesla]

Does your blower have fluid film or rolling element bearings? What kind of vibration transducers are you using to monitor and balance the unit. It is recommended to use a pair of transducers or orthogonally (90 degrees apart) mounted at each radial bearing to quantify the vibration.

The synchronous or running speed (1X) vibration amplitude and phase tells us the high spot for a specific measurement location. This response is going to be dependent on the speed of the rotor relative to resonance. For large(r) turbomachinery, its not unusual to encounter 2 or maybe 3 resonances. The machine can have different heavy spots for each mode of vibration.

a) I'm not sure what you mean when you say that the heavy spot can be calculated. Typically we will measure transient (start-up or shut-down) vibration data and with knowledge of rotordynamics we can infer the relationship between the high spot (1X vibration - measured) and heavy spot.

b) Depending on the machine, it may have specific balance planes and/or specific balance weights. If not, then engineering judgement needs to be used to select a safe location and method to attach balance weights. Is there a location to add weights where the blower blades are attached to a hub? Basically the machine design is going to somewhat dictate the radius where you place the weights. The trial weight will calibrate the product of the balance weight and radius for the correction weight.

 

[AnCheng]

If not then why are you worried about the extra run(s) for the trial mass?

Our device isn't particularly expensive to run, however my question is more about the benifits of using a trial mass. Furthemore, it makes sense that halving the mass would necissitate doubling the radius, but what is my starting point. Does the balancing machine suggest an angle, weight, and radius?

 

[AnCheng]

"the heavy spot can be calculated without" the trial weight.
Are you counting on the 1X vibration peak-to-peak "displacement" equalling the eccentricity of the center of mass?

"The frequency reading will give us the angle at which trim weight should be added/subtracted."
Did you mean "phase" ?

Was this blower balance previously so the "influence coefficients" are known?

Ever see and work with something like this ?

https://roymech.org/images/freq_curve1.gif

I watched this playlist on balancing, which doesn't mention trial weight until after identifying the heavy spot location.

I did mean phase.

The blower has not been balanced before.

Personally I've never worked with something like that.

 

[geesaman.d]

The benefit is providing required information for the balance correction algorithm.

Otherwise you'll do no better than a glob of clay and the friction in the bearings.

 

[Strong]

You first have to know whether a Hard support or Soft support balance machine is being used. A Hard Support balance machine is typically calibrated so that the unbalance location, correction mass x radius, and angle are calculated directly. A Soft Support balance machine requires a Trial Mass for each balance plane. The Mass/weight amount, radial location, and angle are judged/calculated from the Baselone Run, rotor configuration and weight,and operating speed. The Trial Weight should preferrably reduce vibration levels during the Trial Run(s), otherwise the Trial Weight should at least change the amplitude and phase angle from the Reference Run so that the Correction Mass/weight can be calculated.

You have 3 choices:
1) Learn Balancing Procedures from online materials (articles/videos) or books.[/li]
2) Practice balancing with a small machine; like a bench grinder.[/li]
3) Hire an independent engineer or trained technician to assist you with the technical details.

Walt

 

[mfgenggear]

I would recommend to hire some one, a pro to train.contact the manufacture of the machine
And ask if they can send a tech to train you.
Don't try to wing it, balancing is done for a reason.out of balance can cause a failure.