Motion System Design
Waving goodbye to vibration

Waving goodbye to vibration

Equipment vibration can fatigue and even injure workers; it can also transmit into and damage surrounding components, as well as degrade accuracy. All moving machines must be protected.

From heavy-duty tool presses to light and zippy semiconductor manufacture, all machines are subject to vibration. Minimizing oscillation boosts system accuracy and repeatability. Case in point: Controlling cutting-bit chatter speeds operations and boosts accuracy. Likewise, accurately positioning a part without needing to dwell (and let the part settle) boosts throughput.

Damping approaches

Well-designed motion machinery does not vibrate at its natural frequency during regular operation. Otyherwise, amplitudes quickly grow near natural frequency and its multiples.

Instead, the operational frequency of a well-designed machine swiftly passes through its natural frequency during startup or shutdown only.

One way to reduce a machine's natural frequency (to a value suffiently low for safety) is to add mass. Though it increases stress on floor mounts, increasing mass (commonly with steel or concrete in subfloors or machine bases) proportionally decreases vibration amplitude, particularly when a machine's operating speed carries it through natural frequency.

Another solution is passive dampers, snubbers, and vibration isolators that absorb or attenuate vibrations. These must have the load capacity to support application load or equipment weight. Rated for a natural frequency under that particular load, these often only activate under abnormally dramatic motion, or shift vibration to another frequency. Note that particularly soft isolators can increase motion magnitude, unhelpful constricted areas.

Semi-active systems (such as powered hydraulics or magneto-rheological or MR fluids that are detailed on page 20) damp at system natural frequency while still isolating like undamped machines.

Active vibration controls are sensors and motion actuators that measure problematic vibrations and then apply equal but out-of-phase frequency force to cancel or minimize it. Some active vibration controls can be adjusted automatically; designers simply put these units at critical locations.

Case in point: Passive air tables don't isolate laser and optical equipment (such as scanning probe microscopes) enough to allow highest-resolution imaging. The lowest possible noise (on the order of an Angstrom) requires active isolation.

Negative-stiffness mechanism or NSM isolators from Minus K Technology, Inc. Inglewood, Calif., enables laser and optical instruments (and micro-hardness testers and optical profilers) to operate under severe vibration that render passive and even electronic isolators impractical. The isolators can also independently adjust vertical and horizontal resonant frequencies.

Points to remember

  • Good isolators shift system natural frequency as far as possible from excitation frequency: The former must be no more than one third excitation frequency.

  • Where vibrations are transmitted to equipment through the floor, handheld or mounted sensors can reveal what kind of isolation the machine will require.

No play, no sway

Single-turn wave springs are suitable for preloading bearings at the outer race, to take up play. Preloads reduce bearing damage and wear. The wave spring shown here from Smalley Steel Ring Co., Lake Zurich, Ill., also prevents sliding between the rolling elements and reduces assembly vibration and noise.

Arcousaflex couplings from Ringfeder Corp., Westwood, N.J., are used on machines powered by diesel engines and strong pumps, which run less smoothly than electric motors. These soft couplings isolate driven systems from their less-than-steady power sources — particularly helpful where cardan shafts connect the two. Why? Cardan shafts allow shaft displacement and longer distances between driving and driven components, but their torsional flexibility can allow resonant excitation.

As explained by Sarah McChesney, chief engineer at Ringfeder, most isolators, including the Arcousaflex, don't prevent vibration generation: “Instead, these devices dampen vibration and lower natural machine frequency to below lowest operating speed.” Of course, the other approach to lowering natural frequency is to add mass: “We've helped designers applying reciprocating compressors with tuning by both isolation and mass,” explains McChesney, “because these machines have terrible vibration. Often, a vibrating engine powers the compressor, which compounds the vibration issue with its own excitation. In fact, depending on system specifics, these compressors can only be applied successfully with both isolating couplings and increased mass.”

Nixing pain in the you-know-what

Comfort is a safety requirement (not a luxury) for those who operate heavy-duty dump trucks, construction vehicles, or tractors. To address the issue, design engineers are creating seat and cab suspension systems that manage rather than just control vibration. By reducing operator fatigue, the improved environment increases operator productivity and lessens health problems. Leading ergonomic seating designer Sears Mfg. Co., Davenport, Iowa, a maker of suspension and nonsuspension seating for heavy mobile equipment, has been working since 1993 with LORD Corp., Cary, N.C., manufacturer of mechanical motion and vibration control, and now uses Lord Corp.'s magnetorheologic (MR) fluid in a myriad of seats.

A fast-acting fluid for damping or moving variable loads, MR fluid is a blend of designer magnetic powder in oil, water, or glycol that can instantly change properties. More specifically, MR fluids reversibly change from a free-flowing liquid to semi-solid with controllable yield strength when exposed to an electromagnetic field. In an automotive primary suspension system, MR fluid replaces traditional hydraulic fluid in each shock absorber. As sensors monitor road and vehicle conditions, a controller modifies the damping characteristics up to 1,000 times per second for greatly improved ride comfort.

LORD Corp.'s Motion Master Suspension system features a MR fluid damper, a controller, a suspension position sensor, and ride-mode switch. The system adjusts the strength of the magnetic field based on realtime data such as operator body weight and changing levels of shock and load vibration, to induce the appropriate level of fluid viscosity — for variable compensation ranging from soft to firm depending on operating conditions.

Most recently, Sears and LORD Corp. introduced a seating system with longer-stroke MR dampers and improved control software, the VRS2000-LS, for heavier-duty applications. “The longer 3-in. MR damper excels where terrain and ride are really tough,” says Tom Brodersen, Sears Director of R&D. “Applications include construction and mining vehicles, articulated dump trucks, agricultural sprayers, tractors, and forestry equipment — anywhere with rough terrain and higher operating speeds.” In fact, Sears is redesigning several new and existing seats with the larger damper.

According to Brodersen, most Sears suspensions operate in 6 in. of travel. “The 2-in. stroke damper gives 2 in. of travel for 6 in. of motion,” notes Brodersen. “That 3:1 damper ratio really taxes the dynamic capabilities of the current shock absorber,” says Brodersen. “In contrast, a 3-in. damper lessens required damping force and boosts net damping.” Testing on a six-axis Sears ride simulator with real-world ride profiles aided in development and tuning of the LORD controller to leverage the longer damper's dynamics, especially end-stop control. Firm, positive end-stop control is key: “The better one ‘puts on the brakes’ during a severe terrain event, the softer one can leave the ‘normal’ ride,” said Brodersen. “The end result is overall reduction in harmful vibration transmitted to the operator.” The new seat reduces vibration transfer by 40% for less fatigue and the ability to operate equipment about 15% faster.

Sears VRS customers include Case-IH, New Holland, Hitachi, and McCormick. John Deere and Sears also both offer aftermarket VRS seats, and Sears is developing new VRS-LS-based seat suspensions for use in more vehicles. “Ergonomically designed seating translates into a comfortable working environment for all heavy-duty equipment operators,” said Brodersen.

Thanks to Jim Toscano of Lord Corp. and John Koutsky, V.P. of product development at Sears Seating for technical information.

Vibration surveillance

In another application, Sporian Microsystems, Lafayette, Co., is now working with the US Air Force to develop modular sensors for solid rocket motor health assessment. One project focuses on sensing potentially damaging vibrations and impacts using Sporian's AssetOverseer SS accelerometers.

“The system should increase operational readiness and reduce cost of ownership by extending service life and reducing reliance on prescheduled replacements. We expect to develop a low-power sensor network that will show the evolving health of these systems,” says Dr. Brian Schaible of Sporian.

The Space and Missile Propulsion Division Motor Branch of the US Air Force Research Laboratory at Edwards Air Force Base in California is monitoring Sporian's developments; ATK Launch Systems, Brigham City, Utah, is providing physical design requirements for retrofit and forward-fit SRM deployments, as well as data requirements for ATK physics-based simulation models to predict damage and remaining life from sensor inputs.

For more information

IMI Division of PCB Piezotronics
(800) 959-4464 •

Ringfeder Corp.
(888) 746-4333 •

Smalley Steel Ring Co.
(847) 719-5900 •

Minus K Technology Inc.
(310) 348-9656 •

Sporian Microsystems
(303) 516-9075 •

Lord Corp.
(877) 275-5673 •

PCB Group Inc.
Piezotronics Div.
800-828-8840 •

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