Balancing of Rotating Parts at Test Devices

Balancing Services

Introduction

An important consideration in the design and manufacture of any high speed rotating equipment is balance of each component and assembly. Unbalance in high speed rotating components can cause noise, component wear, excessive vibration, and reduction in bearing life. Balancing, which "is the process of attempting to improve the mass distribution of a body so that it rotates in its bearings without unbalanced centrifugal forces”¹ is critical to the performance of any high speed equipment.

Test Devices offers a full range of balancing services including production, precision, and "at speed" balancing.

Production Balancing
Production balancing is typically done for Test Devices' customers who fabricate lot size quantities of rotating turbo-machinery components requiring balancing prior to delivery to the ultimate customer. Each rotor is balanced to meet customer specifications and requirements. The balanced parts are returned to the customer or shipped directly to the ultimate customer with Balance Certifications.

Precision Balancing
Precision balancing is offered to customers requiring extremely accurate balancing of high speed components. Many customers utilizing this service are in the R&D phase with a high speed component and need a very precise balance before installing the component in the turbo-machinery equipment for testing.

At Speed Balancing
At Speed or Operational Speed Balancing is a service offered by Test Devices for high speed rotating components where standard "low speed" balancing (less than 3000 rpm) is not sufficient. Typically, at speed balancing is required when additional vibration modes (critical speeds) or radial growth of components are encountered at operational speed. An example where low speed and high speed balancing differ would be a high speed part shaped asymmetrically like an automotive crankshaft. Because of its geometry, the asymmetrical component can be in perfect low speed balance; however, at higher speeds the centrifugal forces acting on the asymmetric lobes would cause the profile to take on a multiple “S” shapes, distorting it and causing premature failure. Test Devices provides this service by use of its spin pit chambers, under vacuum, and in concrete test cells for speeds well above 10,000 rpm.

Explanation

Unbalance is caused by the displacement of the mass centerline from the rotor's axis by an eccentricity in the distribution of the rotor mass. In simpler terms, due to centrifugal force, the "heavy" point of a rotor exceeds the centrifugal force exerted by the light side of the rotor and pulls the entire rotor in the direction of the heavy point. Balancing is the correction of this phenomena by the removal or addition of mass to the component to compensate for centerline error.

There are two general forms of balancing: "Static" and "Dynamic". Static balancing involves installing the component into a balancing machine and measuring the "heavy" point in relation to the centerline, while the part is rotating. If the required balance correction is at a single axial point on the rotor the balance is said to be "Single-Plane". Single plane balancing is adequate for rotors which are short in length, such as pulleys and fans.

Figure 1

Dynamic or "Dual-Plane" balancing is required for components or assemblies of significant length. Rotors with some axial length can have two "heavy" points at opposing ends of the component, acting independently on the mass center line. In order to balance the component, both planes must be corrected for center line error. Dynamic balancing is required for components such as shafts and multi-rotor assemblies.

Figure 2

Balance Correction Procedures

Balance correction is typically done per customer supplied specifications. Test Devices can perform balance correction on materials ranging from plastics to aerospace superalloys, by the following methods:

• Abrasive Material Removal - Grinding or air powered sanding equipment is used to remove mass at the appropriate location.
• Drilling or Milling Material Removal - Drills or endmills are used to plunge to specified depths at the appropriate location.
• Mass Addition - Mass is added by the addition of epoxy, welding metal strips, or by adding mechanical hardware (set screws, washers, etc.) if provisions are included in the design.
• Understanding that the final finish is critical to the performance of many components Test Devices' technicians make every effort to blend and polish material removal surfaces without negatively effecting the aesthetic appearance.

Balancing Capabilities

Test Devices offers customers both Static (single-plane) and Dynamic balancing services. Based on the appropriate equipment and years of turbo-machinery experience Test Devices is able to offer balancing services which meet the ISO 1940 specification for high speed components (G2.5, G1, G0.4). The normal description is gram • inch, but that is relative to the mass of the test specimen. Balancing service is offered for components with diameters from 0 to 45 inches (1140 mm) and weights from 0 to 10,200 pounds (4560 Kg).

Test Devices also maintains its own in-house capability for design and manufacture of balance tooling. Design of balance tooling can be critical to producing precisely balanced components and Test Devices' engineers have years of experience in designing balance arbors.

Over-speed or "Proof" spin testing services are also offered at Test Devices. Many specifications for high speed components call for, not only balancing but also over-speed spin testing prior to final installation of such components. Test Devices maintains 3 spin test systems and provides a full range of spin testing services with a quick turnaround. Normally, routine over-speed tests are shipped back to the customer within just a few days after they have been received at TDI.

Typical components balanced at TDI include:

Below are some examples of components balanced at Test Devices facility.

Figure 3
View of Precision Balance Machine

Benefits and Applications

Test Devices has provided customers with expert, quick turnaround balancing service for over 20 years. Our balancing services are provided by well trained technicians, with extensive experience in component and assembly balancing, supported by our full service engineering department. We balance parts ranging from simple single plane steel pulleys to dual plane exotic alloy assemblies of complex geometry, in quantities of one to production lots.

Because Test Devices is in the high speed rotating equipment field, we understand how critical balance is to equipment operation. Knowing this, all customer components/assemblies are handled with the utmost care and are returned in superior condition.

For additional information of answers to specific questions, please contact us:

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¹(ISO 1940/1 Mechanical vibration - Balance quality requirements of rigid rotors)

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