"The Noise That Annoys," Feature Article, December 2003

FEATURE FOCUS: Electric Motors

the noise that annoys

A bearing maker develops a tool to design the racket out of products with electric motors.

by Dan Snyder
and Elisabeth
Smith

The demand for quieter products has increased in recent years. Quietness adds value to consumer products, office equipment, and factory machines. Workers want equipment that makes less noise than even the levels mandated by OSHA; they want the same noiselessness in their appliances at home.

Increasingly, quiet products equate to quality products.
Noise, to many ears, means that something is wrong. Noisy products often have electric motors and other rotating parts. In such items, vibrations can be transmitted from rotating parts through surrounding components to the outer structures, and cause them to shake.

While all sound is caused by something that excites air molecules—a physical vibration—the musical note plucked from a guitar pleases the ear more often than the unwanted buzz of a motor housing. Some products—motorcycles, for instance—exploit their sounds as distinguishing characteristics. For electric machinery, that's a rarity.

Noise From Motors

The perception that a noisy machine is a damaged machine can be correct. A damaged bearing can cause noise or it can simply act as a transmitter.

Noise does not always indicate a defect. A motor can run at a speed that matches the resonance frequency of one of its own parts or that of a driven component, such as a fan or pump, and cause vibration.

The best way to ensure that users do not perceive a product as damaged or inferior is to design the noise out of it—build it to run quietly. A designer faced with designing the noise out of a system must consider many sources of system and bearing noise.

Consumers increasingly perceive quiet as a sign of quality.

An unbalanced shaft or eccentric rotor might cause system noise. Mechanical looseness or misaligned shafts, shoulders, housing seats, and other parts can generate unwanted sounds.

Housing materials and geometries, and their resonance frequencies play noise-making roles, too. Sources of bearing noise include slack internal clearances, inadequate loading, raceway mounting defects, and out-of-round, wavy raceways. Bearings can also make cage noise, sometimes described as "whirling" or "chirping."

Just as problems in other parts of the body are often detected by monitoring the heart, bearings can function as transmitters or receptors of information about what is happening in other parts of a machine. As such, they "sense" the health of a machine.

For example, the skidding of rolling elements causes noise in a bearing, but indicates that there is inadequate loading on the bearing. The noise is the sound of rolling elements transitioning from unloaded to loaded to unloaded states.

Shafting out of alignment will transfer vibration into the bearing and create noise. The sound will be of a different kind at a different frequency than that caused by inadequate loading.

Tool for Silence

Until now, designers of equipment using electric motors faced arduous tasks when they attempted to build silence into their products. They had to design and construct prototypes, measure these systems for vibrations, change them based on their findings, test again, and so on. Designers expert in bearing analysis could shorten the process because bearings reveal the health of a system.The simulation program Orpheus, developed by SKF, brings bearing analysis and rotating equipment expertise to engineers trying to predict potential noise problems in their proposed designs. Using the software, technicians working in a virtual environment can control or even eliminate noise at the design stage. The software accelerates new product development.

Technicians can use the software to predict how a housing will bend and deflect, to determine how the critical speeds of individual elements will affect system resonance, to specify fits for shafting and housings, to calculate the number of bearing elements in contact and loaded at any one time, and to select bearings that will meet criteria for emitted sound. The software can also be used as an analytical tool to pinpoint the source of noise in rotating machines. A company wanting to analyze a motor-driven equipment design provides SKF analysts with a drawing of the drive.

Then, the analysts create a model of the device and simulate the vibrations it creates and the possibility of damage.

Designs can be tweaked to eliminate vibrations and noise in final products.

The software develops bearing models from equations of motion, accounting for friction, the relationship of speed to vibration frequencies, and other factors. The software simulates surrounding structures using finite element analysis, breaking large, complex components into small, simpler, more manageable bits.

The result of a simulation is a total system response based on user-defined input of speeds, loading, and so forth. The software generates a 3-D computer animation of the system to reveal the effects of vibration. Of course, the analysts performing the simulation are able to vary inputs and components to optimize the design.

Validating the Program

The code writers and engineers who developed Orpheus know that the virtual prototypes generated by the program faithfully represent the products to be created because of a special rig they built. They use the rig to investigate the vibration that bearings generate and transmit. The rig, a large steel block perforated by many openings, provides numerous mounting points for instrumentation, rotors, bearings, and end shields.

The test rig model included stiffness and damping of its support.

Using the test rig, technicians can control bearing preload and misalignment, and run at speeds as high as 30,000 rpm. Instruments can measure vibration modes of the shaft under varying conditions. A special device may be used to excite a shaft into well-defined vibrations.

In various tests, technicians modeled system variations in the software. They also produced the same variations physically within the test rig. The close agreement between calculated and measured dynamic behaviors validated the software's capability for reducing noise and vibration in rotating equipment.

SIDEBAR: MARSHALING THE PARADE OF PARAMETERS

Dan Snyder is director of applications engineering at SKF USA Inc. in Norristown, Pa. Elisabeth Smith is an applications engineer with
the company.