Static vs Dynamic Balancing

In static balancing, you are only balancing the torques. In dynamic balancing, you are balancing couples and, depending on exactly what you're doing, also balancing out of plane torques (That's not a very elegant answer but it's early and I can't think of a better way to say it).

Speed is also a judgement. If you're doing dynamic balancing on a gear, for example, you should be able to get by with a much lower speed. I've worked on large motor rotors where dynamic balance had to be done in situ at rated speed. So, I think, the question will be: What is your testing fixture capable of and how good are you at understanding the scaling of the speed?

If the part is short it can be assumed that the disbalances are almost in same plane so that a simple "static" balancing will eliminate the resulting centrifugal force.

Now let us consider a long part. You make a static balancing and it is ok but the disbalances are not in the same plane and when you rotate the part the centrifugal forces multiplied by the lever arm between them generate a turning torque which of course will over load bearings, generate vibrations, noise and so on. This is the reason why depending on the part geometry one is obliged or not to make a dynamic balancing.

Static would involve the non movement of the item being tested and corrected. Example would be the old bubble balancers they used for tires and bearings. They checked it by spinning them up it was just part of the process. The test and correction was done static. The check was done dynamic.

The 600 rpm may be enough for a rotor turning at 3000 rpm. The balancing speed would depend on the application of the motor.

Normal Thumb rule is, if the balancing been done with in 100 rpm, that is called static balancing, Depends up on the part diamter and the accuracy of balancing, the speed of the balancing will vary. I will try to get more details.

I have a 36 inch diameter wheel (laminated garolite sandwiched around expanded polystyrene) that I need to spin to 2000 rpm. I've spun non-laminated EPS (same diameter) to 1100 rpm without any real balancing trouble. I need to static balance this new wheel (it has a 1-1/8" arbor) and I'm trying to do it inexpensively. I was going to buy a 1-1/8" shaft and two self-aligning bearings with mounts, but I thought maybe someone may have a better (cheaper??) solution in regard to balancing.

Dear Zatau,

Please visit the web-site : www.irdbalancing.com & read the technical paper on

Static and Dynamic Balancing .

Hope this info. will be interesting & useful.

Regards,

P.Rangasamy

Static balancing is crude and useful in single plane balancing. Depends on the skill of person doing it.

2000 RPM is a good speed. Dynamic balancing is preferred.

The requirement of balancing will depend on vibration that can be tolerated and the life required of equipment.

Here is a simple example. Suppose you have a balanced wheel on a horizontal shaft. Attach a weight to the right side near the rim and an equal weight on the left side diametrically opposite and equidistant from the axle. It will still be statically balanced in that it will not want to turn about the axle. However, if it is spun on the axle, you will see it is no longer dynamically balanced.

Static Balance will be perform to balance the weight of the rotating parts in one plan and in statice mode without motion parts by their axel and should be perform in closed room with clim air, by additing required weight balance to equilibrate the mass center of the all system like weight scale.

Dynamic Balance will be performed after the static balance by rotating the system, to correct the differeness in the weights because of the mistakes values in Static balace process, which achive the vibration in one plan during rotating the system, the vibration occured by imbalance of their centerfugal forces and that will correct by adding balance weights for light parts or remove balancing weight from the havy parts till equilibrate and balance the acts of centerfugal forces.

STATIC BALANCING : This is done by holding the part at its AXIS, then compensating

( by removal or addition of MASS ) for the " HEAVY " side. During Static Balancing the

part is NOT ROTATING. Hence it is called STATIC. This is done on FLAT products or on parts have alarger Dia. to axial length ratio.

Eg.: FANS, PULLEYS, WHEELS.

DYNAMIC BALANCING : Ti is done on parts thatare long compared to their DIAMETERS

Eg. Rotor Assemblies. These parts require balancing to be done on 2 planes, since

the actual imbalance will intersect the center-line. Unless both ends are balanced,

Mass Imbalance will continue to remain.

Specify Requirements clearly for better results :

* RPM should be at a min.,the max.service RPM

* balance " Grade "

* The drawing should specify the location for adding or removing of mass.

STANDARDS : the Std. that relates to Balancing is the ISO 1940-1 : 2003

This can be obtained from ANSI- Pl. refer web-site : www.ansi.org

Hope this will be further useful.

P.RANGASAMY

I hv been in a motor manufacture ages ago, hehe. now I forget either its winding and connection, but the balance I was impressive.

the principle is find mass center, in otder that it cannt produce extra vibration by a extra centripetal force. this vebration can be expressed by x y direction in math.

not motor , many things like car, all shift object all have to correct this ballance.

how much rote speed? depend on precision. the higher speed, the easy you can detect the vibration. because the force is positive ratio to speed.

I can only speak theory. just take it as reference/

Much thanks to you all. I read through the white paper and it was helpful. You have convinced me that this is one area I should not nickel and dime. Though my similar 1000 RPM wheel worked "okay", the speed was well below the theoretical rotational structural limit (as determined by FEA). 36 inch diameter x 6 inch thick (weighing 12 pounds) spinning at 2000 RPM --- field dynamic balancing from a professional --- sounds good to me! Thanks again.

Dear Mr.zatau,

Your Question - If rated speed of a rotor is 3000 rpm and balancing is carried out at 600 rpm whether it is OK OR not ?

It is OK - that is while running at 600 RPM, the UN-BALANCE is measured and it is CORRELATED to actual working speed - for the simple reason that the Centrifugal Force is proportional to SQUARE of the Speed. Variation.

DHAYANANDHAN.S