Vibrations stem from a behavior that's inherent to all mechanical structures. Under load, all structures slightly deform and act springlike, causing motion to ripple to and fro as energy waffles between kinetic and potential. Vibrations are as complex as these structures they run through, and are an intricate composite of movement, especially rotating motion, where both amplitude and frequency change dynamically with cycling deformations caused by misalignment, play, and impact forces.

In industrial machinery, at higher rpm, mass unbalance can cause vibration that degrades manufacturing quality — and are often mistaken for control problems. For example, unbalanced milling spindles can lead to elliptical cutting tool error hard to distinguish from slide motion errors. Resulting impulses, high speeds, load, and resonance — these are the things that can shake systems to the point of error, or even failure.

Now there are several approaches to dealing with this nonlinear problem: Systematically ensuring that each system component, from motor to interface, is balanced; cutting vibration off at the pass with forgiving couplings; containing it with various damping schemes; and continually adjusting auxiliaries to keep vibrations near an acceptable level.

Beating backlash

One common source of vibration in rotary linkages is backlash. Backlash opens connected components up to slamming which propagates through the system, causing micromovements in shafts and attached components, as well as noise, short-lived designs, and detrimental inaccuracies. To illustrate: On gears where backlash is required, heavy loads exaggerate tooth stiffness irregularities and cause varying mesh deflections. These changing deflections then cause transmission errors and a periodic inertial load supplement — a noisy and eventually destructive proposition.

Switching to helical gears is one solution; they're usually better than spur gears at being quiet. Used in everything from automobile transmissions to presses, helical-profile gears create a spiral engagement to soften tooth meshing and minimize noise. “Helical gears have increased contact ratio compared to spur gears, and are less noisy,” explains Georg Bartosch, president of Intech Corp., Closter, N.J.

However, spur gears offer unique simplicity and efficiency. What's the solution if someone includes them in a design for, say, cost savings? To cash in on spur gearing benefits, power train engineers often specify them with reliefs cut into tooth roots and tips. These cuts often reduce transmission error at certain roll angles and loads, but unfortunately, they increase it at others. Approaching the problem with tuning, on the other hand, substantially eliminates mesh stiffness variations under all load conditions. How? With crowning, in which gear teeth are approached as sets of tuning forks. To make them sound better, the gear shapes are tweaked so that the meshes produce pleasant sounds, exact octaves apart.

A different approach is necessary to tackle looseness (backlash) on chains and belts. Here, the component's compliance is sufficient to allow engaging and disengaging. Compensating generally requires the addition of tensioning elements on the slack side. “But newer tensioner types replace multiple tensioning elements with just one part that compensates for three kinds of slack — that from torsion, pivoting, and vibration,” says one source at Lovejoy Inc., Downers Grove, III.

Wedges resist vibration

For shaft/hub connections, pounding out of keyways leads to failure which goes far beyond the keyed component itself. “In fact, it would take books to address all vibration issues, especially because they normally concern not single components but entire systems,” says Gunther Zwick, president of Gerwah Drive Components, Atlanta. Here, frictional locking assemblies and shrink discs do better under vibration and shock. “The worst scenario by far is extreme shock load, for which machine components are not designed,” he adds. “Shock load can damage bearings, couplings, even gearboxes and motors.”

Smaller connections also benefit from wedging to hold fast. Fasteners with standard 60° threads experience vibrational loosening because of the gap between the crest of male and female threads. To combat this problem, a locking fastener from Spiralock Corp., Madison Hts., Mich., adds a 30° wedge ramp at the root of the thread of the standard 60° thread form. Still mating with standard 60° male fasteners, the wedge ramp allows the bolt to spin freely relative to female threads until a clamp load is applied, which improves resistance to vibrational loosening. In dynamic and static testing at Goddard Space Flight Center, for example, the nuts stayed tight even under sine excitation of 24.7 Hz at 2g and random excitation of 20 to 400 Hz at 2g rms.