Shaking free of vibration

July 1, 2006
Vibrations stem from a behavior that's inherent to all mechanical structures. Under load, all structures slightly deform and act springlike, causing motion

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.

Isolating with plastics

Besides avoiding vibration generation, another approach is to squelch it. “In shaft-to-shaft connections, many coupling types can produce vibrations when connecting improperly designed or misaligned parts. To address this, servo-insert couplings feature polyurethane spiders between the coupling's hubs to dampen vibrations,” saysZwick.

Plastic cam followers reduce noise in the same way; for example, metal components clinking together on dry-cleaning garment tracks can be incredibly noisy. “While plastic cam followers will reduce noise of a dry-cleaner garment track by 6 to 10 dB by absorbing shock and cushioning the ride, industrial applications require cam followers with precision runout and high load capacity,” says Bartosch. He adds that in addition to significant noise reduction, cam followers with plastic outer races can carry high load.

Similarly, plastic gears absorb energy for noise reduction of 3 to 6 dB. “A gear made of stable, tension-free materials (and not injection molded) reduce noise even further,” says Bartosch. A cast-in metal core reduces thermal expansion of the gear by up to 50% to further increase stability. In practical terms, a stable material allows for tighter backlash and higher quality class gear. For example, using stable plastic gears, a CNC gear hobbing machine can consistently produce AGMA class 11 metal gears.

That said, when plastics are at risk for swelling due to moisture, backlash must be increased to accommodate the dimensional change. “The bigger gap between the teeth of mating gears can produce clicking, especially if the gear speed changes or in stop and go applications,” adds Bartosch.

What if a metal component does its job well, save for its conduction of vibration? “Linear ball bearings exhibit metal-to-metal contact, which allows transference of vibration, and fretting corrosion caused by vibration,” says Ray Stojonic of Pacific Bearing Corp., Rockford, Ill. The area of each ball in contact with the shaft is a small pinpoint, measured in thousandths of an inch. The balls cut through oil and grease film, dent the shafts, and cause metal-to-metal contact with the shaft. “The high load on both makes for high stresses, so that eventually metal particles fret off, accelerate wear, and even cause catastrophic failure,” says Stojonic. There is a solution to this problem, however, by adding a coating to the screw's inner channels. The coating absorbs vibration while still allowing designers to benefit from ballscrew precision. “Teflon-based lamination materials on the aluminum parts absorb vibration and prevent it from transferring through equipment,” Stojonic explains.

Cushioning with fluids

Fluid, a natural cushion, is another tool that can be used in the battle with vibration. Damping modules for pneumatic semiconductor isolation tables, for example, provide good isolation from vibration even at frequencies above 5 Hz. Similarly, oil buffers vibrations running through bearings. Oil-supported clearance in bearings is usually only around 0.1 to 1µm. “But under pressure, oil takes on the properties of a kind of temporary solid, enabling it to separate and protect the rolling elements,” says Lee Culbertson, president of Royal Purple Ltd., Porter, Tex.

Abnormally high vibration signatures — from metal-to-metal contact — appear when oil-film thickness is insufficient to completely separate rolling elements. When that happens, surfaces become damaged, bearing surface roughness increases, and it becomes even more difficult for the oil to separate balls from the race. As the surface damage progresses, the severity of the measured vibrations steadily increase.

One way to address this is with additives that form a chemical film on surfaces, increasing film thickness due to viscosity alone. “The additives are ionic and tenacious, making the oil's film tougher,” Culbertson explains. “Instead of allowing rougher surfaces to self-generate, these lubricants actually promote contacting surfaces into lapping one another,” he adds.

Decoupling and correction

One way to deal with imbalance, as well as isolate shock loads, is with zero-backlash couplings. “These do a good job compensating for a limited amount of vibration resulting from shaft misalignments,” says Zwick. But taking it a step further are safety couplings and torque limiters, widely used to prevent unexpected shock load from transmitting down the drive chain — by interrupting the connection altogether. “For example, nowadays bottlecapping machines are often equipped with magnetic hysteresis couplings that disengage smoothly without producing vibrations that would otherwise affect the service life of the entire machine,” Zwick adds.

Besides disengaging from the vibration source completely, another option is to cancel out vibrations with opposing forcing functions. This is especially effective against vibrations caused by imbalance. The negative effects of imbalance are a function of rotational speed, squared. So consider a machining robot: It must be run at low processing speed or risk overexciting robot arm resonance and moving in a manner not controlled by the primary motion control. “Every mechanic who has been involved with balancing these understands that it is a repetitive process with an undefined number of iterations. But active balancing eliminates ground equipment, and mechanic training,” says Andy Winzenz, of Lord Corp., Cary, N.C.

What is active balancing? The application of forces that literally cancel others causing resonant vibration. Vibration and other sensors provide feedback to adaptive controllers; this sensor information is then used by the controllers to shift balance rings permanently mounted to the rotating machine. Active damping technology takes motion from the primary motion control system's sphere of influence in applications where disturbance forces cause problematic resonant response.

These technologies also calculate when more balance correction is needed than is available from system capacity. At that point the controller warns the operator and indicates the actual correction magnitude and phase angle required to optimally balance say, a rotor. The operator can then schedule maintenance to bring rotor balance back to within active system capacity. Besides on robots, this is handy in the machine-tool industry, where high-speed tools incorporate fast linear slides to yield the full benefits of high-speed machining.

Shock: A belt example

Warped belts, out-of-round sheaves, misalignment — or any combination of these — can also cause shock. It's best to realign sheaves first because it's an easy fix. Once that's done, other causes can be investigated.

“Suppose a belt has a bad seam that makes a lump at one location. Think about what this is going to do,” says L. Robert Pyle, president of Systemaitec Inc., Wichita, Kansas. “Once per revolution of the belt, this lump goes over each sheave. The rise of the belt is going to increase belt tension and put a hard downward force on the sheave. We would expect, then, to find vibration frequency at 1x the belt-pass frequency.” This forcing function's frequency is below shaft-speed spikes. Note, though, that on a loose machine, the small forcing function from a bad seam can excite every harmonic, even out to high frequencies. “But don't get misled by that; always look very carefully at sub-shaft-speed spikes,” advises Pyle. “For example, the rotor speed and the first belt-pass frequency harmonic can appear as one if they're only separated by a few cycles per min.”

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