Balancing Pump Impellers
What Is Balancing?
Balancing is the procedure by which the mass distribution of a rotor is checked and, if necessary, adjusted in order to ensure that the vibration of the journals and/or forces on the bearings at a frequency corresponding to service speed are within specified limits.
In practical terms, balancing corrects unbalance. There are two types of unbalance, single and two plane. Single plane unbalance can be pictured by imagining a disc shaped part, such as a bicycle wheel, with a weight taped to the rim. When the bike is lifted off the ground, the wheel rotates and comes to rest with the weight at the bottom. If you were to spin the wheel, the bike would shake as the wheel tried to rotate about the center of the wheel's mass, which is no longer located at its axle. The center of mass is displaced from the geometric centerline. Another way to illustrate this effect is to place a lump of modeling clay inside a Frisbees rim and throw it. Instead of flying normally, the disc will wobble as it spins around the shifted center of mass.
Unbalance of this type is sometimes called force or static unbalance. It can be corrected by removing the weight or by adding an equal weight directly opposite (180 degrees from the unbalancing weight. Either measure would move the center of mass back to the centerline of the part.
Two-plane unbalance can be pictured by imagining a cylindrical or drum-shaped part, such as an automobile wheel's rim, with one weight attached at one end of the cylinder and another attached at the other end, but offset 180 degrees from the first weight. Note that the ends of the cylinder are in different planes. If the rim were raised off the ground, it would not rotate as the bicycle wheel did. Spinning the automobile wheel, however, would cause it to wobble as it sought to rotate about the axis of this mass, which is no longer parallel to the geometric axis. Two-plane unbalance is sometimes called couple unbalance. It can be corrected only by adding two correction weights at an axial distance from each other (see figure 1).
When both single and two-plane unbalance are present in a rotor, rotor diag. the condition is called dynamic unbalance. To correct this type of unbalance, one must compensate for both eccentricity (caused by static unbalance) and wobble (caused by couple unbalance). In practice, any dynamic unbalance can be corrected by making adjustments in two axially separated planes. However, as the planes get close together, couple correction weights become very large.
How Do I Balance?
As mentioned above, rotors that have significant "thickness (as opposed to disc shaped rotors,) need to be balanced dynamically. For rotors such as pump impellers and blowers, a horizontal overhung balancer can make the balancing process simple and easy. The rotor is mounted on a horizontal shaft held by a motor-driven spindle. The spindle incorporates a pair of vibration transducers that make it possible to measure dynamic unbalance.
The operator mounts tooling that will accept the part to be balanced and enters the part's dimensions (essentially the axial location of the two planes where corrections will be made and the radius at which weight will be added or removed). When the part is spun, the balancer indicates the amount of correction to be made in each plane in terms of weight to be added or removed and the angular location of the correction.
The process is computerized, and all measurements and calculations are displayed on a large color CRT. The part is rotated manually until the computer angular position display matches the present angle with the angle of unbalance correction for one of the two planes. The operator then knows precisely where to make the correction for that plane. The ergonomics of the machine allow the user to learn how to operate the machine in a matter of minutes. The machine is also equipped with a brake that makes on-machine correction possible (Figure 2 and Figure 3).
How Well Do I Need To Balance?
The International Standards Organization (ISO) has issued guidelines with regard to a number of different kinds of devices. A pump impeller has a recommended "balance grade" of G-6.3. In practical terms, that grade is achieved by balancing a rotor that is to run at 3600 rpm to a value of 0.01 ounce-inches per pound of weight. As an example, assume an impeller weighs 1000 ounces (62 pounds). The tolerance would be 0.62 ounce inches. This means that the center of mass and geometric center must be aligned within 0.00062 inches. If the balance correction radius is 5 inches, the correction weight must be selected accurately to within a tenth of an ounce, about 2.8 grams. Actually, this is a slight simplification. The ISO standards contain detailed methods of calculating different static and couple unbalance tolerances that are dependent on the ratio of the part's diameter to its length.
How Well Can I Balance?
How well parts can be balanced depends on several limiting factors. Our HO-100 (100 pound capacity) overhung balancer, for example, has a specified repeatability of +/- 0.01 ounce inches. This limitation is one of "pulling the signal out of the noise." There are two major types of noise: thermal and mechanical. Thermal noise is produced by electronic devices and transducers. Moving elements such as belts, motors, spindle bearings and windage as the part is spun produces mechanical noise.
Repeatability of 0.01 ounce inch would at first glance seem to indicate that a one pound impeller could be balancing on this machine. Another limitation, however, is simply a physical one. It was just stated that we are aligning the mass and geometric center of a 62 pound part to within 0.00062 inches. Though this can be accomplished, the part must be mounted on exactly the same center in the final assembly or the balance will not be as good as that achieved on the balancer. Suppose, for example, that the worst case fit of the impeller on the shaft in the final assembly is 0.002" clearance, which will be taken up by one or more setscrews. This moves the part off center by 0.001", causing a balance change of 1.0 ounce inch due to the mass center shift. Maintaining a precision balance in the final assembly requires precision fits. A precision balance is a wasted effort if the part is not centered as well in the final assembly as it is on the balancing machine.
How Well Can Tooling Center A Part?
How well can a part be centered on a balancing machine? We have found collet tooling to be capable of repeated centering within _+/- 0.00002 inches. In the case of the previously discussed 62 pound part, this centering accuracy limits the balance repeatability when the part is removed from the collet and reinstalled to +/- 0.02 ounce inches. If that is added to the basic machine repeatability of +/- 0.01 ounce inches, the uncertainly becomes 0.03 ounce inches. If the part is rather "long", the ISO tolerance is split in half for each correction plane, (i.e., 0.31 ounce inches per plane.) Thus, a balancer using standard collet tooling is capable of resolving about 1/10 of the part tolerance, which is adequate to allow rapid balancing to tolerance without "chasing" the balance correction around the part. The advantage of an expanding collet is that it takes up the tolerance in the bore size and still maintains the centering of the part. Collets in the range of an inch diameter can expand 0.010 inches or more.
In some applications where an impeller is to be run at low speeds, there is a lower cost, alternative tooling method - solid post tooling. The part is places on a post and locked down with a drawbolt and cap. The limitation of this type of tooling is that the post must be smaller than the smallest allowed bore. Clearance 0f 0.0002" to 0.0005" is required. If the tolerance on the bore is 0.002" a maximum clearance of 0.0025 may be encountered, and repeatability with part removed from and replaced on the tooling can be +/- 1.25 ounce inches! That is, a total shift of 2.5 ounce inches can be observed.
At the other end of the range of possibilities are special grease filled arbors. These have a very thin section that can be expanded by tightening a setscrew that compresses the grease. Such devices are capable of two or three times better centering than a standard collet that is expanded by sliding it on a tapered locator. These devices are, however, specialized and expensive.
How Can Unbalance Be Corrected?
Pump impellers are generally balanced by removing material, by drilling, milling, or grinding. Horizontal overhung balancers can be fitted with grinding correction (Photo 1). This method spreads the material removal process over a large area with minimum depth. Impellers can also be balanced by drilling or milling, if the material permits. Machines can be supplied with correction devices and automatic correction cycles. On-machine correction is fast and accurate.
Does Machine Configuration Make Any Difference?
Items such as pump impellers have been balanced on cradle balancing machines by placing them on a balance mandrel. That is, the part is mounted on a shaft with a pulley. The shaft and part assembly is placed on a pair of bearings on a cradle balancing machine. The bearing spacing needs to be adjusted from part to part. A belt is placed on the drive pulley, and the part is balanced. Distances from bearings to correction planes must be precisely maintained in order to achieve an accurate display of corrections at both planes. On completion, the part must be removed from the mandrel. In addition to the several steps required, there is the drawback that mandrels can become damaged. A bent mandrel causes the part to be improperly balanced. Damage or correction debris at the bearing surfaces can cause false readings or poor repeatability of readings, making balancing much more difficult. The above discussion regarding the limitations of solid post tooling also applies to a mandrel.
We believe that the horizontal overhung balancer is the answer to these concerns. The part is simply slid onto the tooling, the tooling locked, the part unbalance measured and corrected, the tooling unlocked, and the part removed. Some smaller impellers can be loaded directly by an unassisted operator and conveniently balanced on a vertical spindle machine. Photo 2 shows a CNC balancing machine with grinding correction that automatically balanced impellers from 5 to 25 lbs., to very find tolerances in one or two minutes (mass center to geometric center of 0.00001").
Conclusion:
Precision balancing and careful assembly of impellers will result in a smooth and quiet product whose quality is recognized by the end user. A two to five minute operator investment ($5.00) to balance a part to a very fine tolerance can result in many days or even months of added "uptime".
Click here to see Hines Pump Impeller Balancers.
Click here to see Hines Horizontal Overhung Machine.
References:
International Standard ISO 1925, Balancing (Second Edition).
NOTE: Gordon Hines introduced the HO balancing machine to the pump industry in 1981. It was designed and built for pump companies because of the difficulty of using the cradle balancer to balance impellers. Hines has been involved in the design and manufacture of balancing machines since the early 1960’s.
