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X-Y-Z: Vibration Control Ensures Accurate Positioning

September 1, 2006
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Automated assembly systems are alive with robots, linear actuators and cam-driven mechanisms moving up and down and side to side at high speeds. An unfortunate byproduct of all that starting and stopping is vibration, and the Beach Boys to the contrary, there's nothing good about it.

Assembly systems plagued by excessive vibration will have difficulty meeting accuracy and repeatability requirements. Moreover, excessive vibration can cause bearings and other moving parts to wear out prematurely. To prevent these problems, engineers need to isolate sensitive mechanisms from vibration, as well as eliminate or attenuate sources of vibration.

There are many ways to isolate machinery from vibration, ranging from simple levelers tipped with rubber pads to sophisticated, motorized tables that actively compensate for vibration. Which to choose depends on the weight, size and center of gravity of the machinery; the source, magnitude, frequency and direction of the vibrations; and the accuracy demands of the application.

Unwelcome vibrations can originate both inside and outside an assembly system. Inside sources include motors, drive belts and pneumatic cylinders. Outside sources can be obvious, such as a compressor, or subtle, such as street traffic. Vibrations can be small and constant, as from a motor, or large and intermittent, as from a stamping press. In most assembly plants, the floor vibrates vertically with an amplitude of 2.5 microns and a frequency of 4 to 30 hertz, and horizontally with an amplitude of 2 microns and a frequency of 4 to 20 hertz.

To prevent vibrations from affecting an assembly tool or instrument, isolation components can be placed between the vibration source and the machine base, between the tool and the base, and between the base and the floor. These resilient components absorb and dissipate vibrational energy. They also store energy from shock loads, transforming large, sudden vibrations into smaller, smoother vibrations that gradually decrease over time.

Vibration isolators come in a variety of designs. Leveling mounts, channel mounts and base-plate mounts consist of a resilient material, usually rubber, mounted in a metal frame. Wire rope isolators consist of metal cables threaded through two aluminum retaining bars. An advantage of this design is that, unlike rubber, the cables are unaffected by temperature extremes, chemicals and ozone. With air springs, the vibration absorbing material isn't rubber or steel, but air trapped inside a fabric-reinforced balloon. An advantage of air springs is that they can also be used to raise or lower the height of the machine by adding or subtracting air. In some cases, it may be desirable to avoid special mounts altogether and simply cover the floor beneath and around the assembly system with a vibration-absorbing rubber mat.

When choosing isolation mounts, engineers should check the maximum compression and shear loads that the mount can support; the minimum vibration frequency that can be absorbed effectively by the mount under various loads; and the transmissibility of the mount, or how well it absorbs and transmits vibrations.

For an isolation mount to work, it must be free to deflect vibration. The more a mount can compress and expand, the better it can deflect vibration from equipment. To ensure adequate deflection, engineers should avoid overloading the mounts. They should also design the support structure above and below the mounts to be as stiff as possible. If the structure is too soft, it will deflect some vibration instead of the mount. As a rule of thumb, the structure should be at least 10 times stiffer than the mount.

It's also important to note that isolation mounts are a bit like poison ivy treatments. They ease the symptoms, but they don't address the underlying problem. To reduce the adverse effects of vibration, engineers may need to reassess the operating parameters of the assembly system. Adjusting the speed, acceleration and deceleration of a SCARA robot, for example, can significantly reduce vibration.

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