Improving Balancing Processes

To understand balancing, some definitions are useful: 

Mass Center Axis. The line passing through the center of mass of the part and around which a part naturally spins (for example, axis of rotation of a thrown ball, Frisbee or boomerang).

Axis of Rotation. The line around which a part spins in standard operation.

Balancing. Process of redistributing the mass of a part so that mass is evenly distributed around the part’s rotating axis. The balancing process aligns a part’s mass center with its geometric center. Unbalance causes vibration and heat buildup, which can result in bearing failure. Some dangers for unbalanced pumps can include shaft deflection, excessive vibration, mechanical seal/mechanical packing failures, catastrophic bearing failures and seizing. The goal of balancing is to control the vibration to an acceptable limit when the pump is operated in the field. Unbalance can result from a number of causes. An example is blow-holes and cavities occurring in the rough forging of a pump impeller. Pump users may label a problem as misalignment, bad bearings or overheating, but the root cause is often unbalance in a component or pump assembly.

Pumps commonly have two types of unbalance; single plane or two-plane. Single Plane or “Static” unbalance is a condition that exists when the center of mass is not on the axis of rotation. 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. To correct static unbalance, the unbalance is measured in only one plane. It simply measures the force that gravity creates on the part.

 

Two Plane or “Dynamic” unbalance is the unbalance as measured on two or more separate planes. Both planes will be perpendicular to the axis of rotation and thus parallel to each other. Two plane unbalance can be illustrated by using a cylinder part where the length is longer than the radius, such as a driveshaft or a multistage pump. Correcting plane unbalance requires two unrelated correction weights in two different planes, at two unrelated angles. This type of unbalance can only be measured on a spinning balancer which senses centrifugal force due to the couple component of unbalance.

There are many ways to reduce initial unbalance. Speaking with a casting supplier about monitoring casting unbalance prior to machining could improve initial unbalance. Holding tighter tolerances during the part manufacturing process and mass center monitoring prior to final machining could also reduce initial unbalance. When designing pumps, balancing engineers can complete a part analysis and provide recommendations on part design to minimize initial production unbalance, lowering downstream costs for balance tooling and correction times. Balancing services provides an opportunity to test and balance prototypes during the early stages of development saving you time and money by finding potential issues before going into production.

Once the unbalance of the part has been determined, it can be corrected. There are many ways unbalance can be corrected, and they often depend on the part’s physical features and the quantity of parts that need to be balanced. Pump impellers are generally balanced by removing material via drilling, milling, or grinding. Grinding correction spreads the material removal over a large area with minimum depth.

The goal of balancing is to align the mass center with the geometric axis. Correction tooling accurately holds the part on the geometric axis. Tooling for balancing operations must be well-designed and manufactured, in order to balance the part properly. The more accurately the tooling duplicates the actual mounting surface condition of the part, the greater the likelihood of obtaining the proper balance.

Horizontal overhung balancers are typical for balancing pumps. A horizontal overhung balancing machine has an overhung spindle suspension that can be used for both single and two plane balancing. 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, along with the angle at which the correction is to be made. The process is computerized, and all measurements and calculations are displayed on a monitor. Generally, the balancing machine is also equipped with a brake that makes on-machine correction possible. For heavy parts that require automated correction, a vertical rotating impeller balancing machine is recommended. Automation in balancing machines reduces labor time per part, warranty costs, operator fatigue and potential work related injuries.

How preciously you need to balance depends on your part weight and speed of rotation. Rather than repeating analysis for every part, possibly using different criteria, industries rely on an outside organization to set objective standards. International Standards Organization (ISO) provides guidelines for various “grades” in ISO 1940-1. 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. 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. Balancing, a key component of a quality maintenance process, can also be used to improve plant reliability records. 

There are many variables that can limit the actual achievable balance for a given part. These include the effects of roundness, material of manufacture, concentricity, tapered bores, keyways, setscrews, bushings, dirt and other physical properties in the mounting of parts. When establishing the balance tolerances for parts, it is important that the machining tolerances of the part and the shaft be considered.

Precision balancing and careful assembly of impellers will result in a smooth and quiet product whose quality is recognized by the end user. Regular maintenance will extend the useful life of pumps and reduce the risk of expensive pump system damage. A well-balanced part will decrease vibration, heat, friction, and noise while increasing system life cycle to save time and money.