Difference Between Static and Dynamic Balancing

Static balancing is performed with the object being balanced at rest.

Dynamic balancing is performed with the object being balanced in motion.

Now, what's the real question?

static balancing is done without the part moving

dynamic is done with the part moving it is spun and using stroboscopic flash the heavy bit is higlighted and you then add or remove weighs till it stops vibrating

http://www.universal-balancing.com/en/balancing-machine-range?gclid=CJbv-NGpmKECFclr4wodBAw5NQ

http://www.precibalance.com/

http://www.google.co.uk/search?q=static+balancing+ways&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&redir_esc=&ei=DjfPS8nnH5Tu0wTA_bngDw

"stroboscopic flash" Wow, that's old school.. takes me back. Problem with that was it didn't tell you how much to add or subtract, took forever to get it to stop vibrating..

Unbalance, or residual unbalance is caused by the center of rotation not coinciding with the center of gravity. When the rotor spins, the center of gravity always wants to become the center of rotation.

When you balance a rotor, or tire or mower blade, you can achieve the balance via static, that is to balance the rotor on the center of rotation and either add or remove weight. In the old days I took my car to have my tire balanced and they had a stand with a bubble in the middle, they set the weights on the tire until the bubble was inside a circle on the stand. that is static balancing

Dynamic balancing takes a machine and does the same thing. It's only done by sensing the vibration and matching that with the rotation, any vibration that is not seen as 1x rotation, is noise and filtered out. The computer matches vibration and amplitude to determine the amount of weight to add or subtract to achieve the stated tolerance.

Laby

Whenever the method used for balancing involves object in motion while under test, consider the test "dynamic".

The test is "static" when done while the object is not moving.

In both cases the test results are subsequently used to balance the object.

Do I tape the bubble to the weight, or to the opposite end of the cylinder?

There are three types of unbalance:

1. Static unbalance – is where the mass axis is displaced only parallel to the shaft axis. The unbalance is corrected only in one axial plane.
2. Couple unbalance – is where the mass axis intersects the running axis. For example: a disk that has swash run-out with no static unbalance. The unbalance is usually corrected in two planes
3. Dynamic unbalance – is where the mass axis is not coincidental with the rotational axis. This unbalance is usually a combination of static and couple unbalance and is corrected in two planes

From:http://www.universal-balancing.com/en/balancing-information/what-is-balancing

Hi the static balance is to make the rotor weight uniform around its geometrical axis (centre). Put the rotor on parallel horizontal knife edge and slowly rotate it, the resting position should be random. if its comes to rest at the same position, that is the heavy location of the rotor. you have to add weight in the opposite circumfrance at any location to counter it, such that rest position becomes random. Now the rotor is spun at greater than 600rpm the cerifugal force of heaviness also will take over and produce vibration, if the chosen location of added weight for static balancing is not exactly opposite to the heaviness location. Then the phase angle between the vibration measured at both ends of the rotor may differ by 180deg. eventhough the rotor is perfectly balance statically. Dynamically it will still produce vibration. this condition can be rectified for which, there are lots of tecniques available for carrying out dynamic balancing.

seethasub -- I gave you a GA even though most of us who speak only "USAEnglish" might have to read it twice. You've got the right idea. The 600 rpm number is not always the case; but for much of the rotating machinery we deal with it's around the point at which the dynamic (or two plane) unbalance becomes a problem.

I think the way you explain the difference between "Static" and "Dynamic" unbalance will depend a lot on the technical education and experience level of the audience. It's not easy to explain without having a simple balancing machine right in front of you to demonstrate or at least some good diagrams.

Rather than trying to upstage everyone else I'll just watch for a while. Seems to me the best explanation uses the example of a perfect cylinder on knife edges and two equal weights that can be moved around. The explanations I see on the websites of companies that make balancing machines are pretty poor. Gotta think about this.

Here's a post of mine from 2008 on the subject of balancing. Might be something in there of interest to the rest of you folks.

http://cr4.globalspec.com/comment/292489

Ed Weldon

Balancing of any component depends on the fact as to how its behavior changes under stands still (or for that matter at low speed) to higher speed. This is what is called rotor dynamics. If the behavior does not change appreciably then static balancing may serve the purpose otherwise dynamic balancing would only ensure safe running of the component.

Rotors can be divided in two categories, flexible and rigid depending upon how much they sag when supported of their bearing. If sag is less or negligible then they come under rigid category and static (low speed ) balancing may serve the purpose but if sag is appreciable then dynamic balancing is required.

Behavior of rotor also depends on the type of bearing or you can say on bearing stiffness (bearing deflection per unit load). Whether it is rigid bearing or semi rigid bearing?

Depending upon rigidity and the bearing span (distance between two bearing) the rotor will have its mode of vibration. Mode of vibration could be first mode or second mode. (sorry I am not able to insert figure to show first and second mode of vibration) So depending on the mode its behavior would change and balancing would have to be attempted. Such type of rotor can be balanced by dynamic balancing only. This would also decide the critical speed of the rotor where vibration level would be comparatively high. Some times for best results after attempting rough balancing at rated speed, trim balancing is attempted at critical speed

Pcchatur -- Your description of the behavior of rotor systems in the critical speed range is a useful addition to this discussion. It appears you have some good experience with at least one class of this kind of mechanism where experience has shown that static (single plane) balancing would suffice in many cases and two plane balancing was only required in worse cases.

It would be instructive to all of us if you could gave a brief description of the types of machinery you have gained this experience from. Important in this particular discussion is whether "critical speeds" of the lateral variety (likely what you are observing) are above or below the normal operating speed ranges.

I feel that yours is a special case solution. In the general case the only first order effects on critical speed due to lateral vibrations are the flexibility and dampening of the rotor support system (shaft, bearings, rotor if flexible, and foundations) and the masses involved. Whether the excitation forces are due to a rotating force, reduced by single plane balancing or a rotating couple reduced by two plane balancing is immaterial as a first order effect. However, when one addresses higher vibration modes the issue becomes more complicated.

Ed Weldon

ED Weldon: Thanks for interest in my comment. I am retired electrical engineer with a back ground of large size Turbo Generator design. Our rotors are cylindrical and have long length (4-5 meters) and diameter around 1.0 to 1.2 Meter. These rotor have slots milled in its 2/3 periphery for the purpose of winding assembly. This makes rotor stiffness in two axes different and brings rotor under "flexible rotor" category. Normal running speed of these rotors is 3000 / 3600 RPM. Some rotors have their first critical speed below (1200-1600) rpm and second critical speed above running speed. Here I am talking about only the generator rotor critical speed and not the combined critical speed of Turbine and generator. Some larger rating sets (800MW- 1000MW) generators run between second and their critical speeds. For these rotor dynamic balancing (two plane rather three plane if you take three dimensional view i.e longitudinal and peripheral and at both the ends) is a must and cannot be balanced just by static balancing. I hope information would be useful