Instruments for Dynamic Balancing Using the 4-Run Method
As suggested by the name, the 4-run method for dynamic balancing of rotating equipment requires measuring vibration levels at four unbalance conditions. Therefore, an instrument is needed for interpreting the signals from a vibration transducer such as an accelerometer. Then, the steps and calculations for performing the method are discussed here:
1. Attach the accelerometer at a location and in a direction that is dominated by vibration due to rotating unbalance such as on a shaft bearing in the radial direction.
2. Operate the machine at steady-state and measure the baseline vibration level, A0.
3. Specify and mark on the rotor or fan the 0°rotational position. Then, measure and mark the 120°and 240°rotational positions. A trial weight, Wt, will be attached at each of the three angles and at a constant distance, e, from the rotational center.
4. Select and attach the trial weight such that the trial weight does not induce a force at operating speed greater than one-tenth of the static force due to the weight of the fan or rotor, Wr.
g – acceleration due to gravity where
e – eccentricity, distance from the trial weight to the center of rotation
ω – speed of rotation, 2 x π x revolutions per time
5. For the trial weight attached at each of the three angular positions, measure the vibration levels, A1, A2, and A3. The counter (corrective) weight amount, Wc, and the radial position, Φ, at a distance, e, for balancing the equipment are then determined from the following relationships:
Scantek offers multiple instrument models capable of supporting the 4-run method for dynamic balancing. Models that feature this capability include the GTI Predictive Technology iPad Vibration Analyzer, the RION VA-12 Vibration Analyzer, and the Norsonic Nor140 Sound Analyzer.
GTI Predictive Technology offers an entire suite of software for machinery predictive maintenance built on a versatile iPad platform. Dynamic rotor balancing and vibration trend analysis are capabilities already integrated into the GTI vibration analyzer. GTI's latest vibration analyzer software release, VibePro 6i, stands at the forefront of predictive maintenance technologies with a focus on routes and data trending that is simple to understand. To learn more about the GTI Predictive Technology iPad Vibration Analyzer to here.
The RION VA-12 is a portable vibration analyzer for equipment diagnosis and on-site measurements. The VA-12 allows simultaneous measurement of acceleration, velocity, displacement, and crest factor, and offers the FFT analysis function with up to 3200 spectral lines. The VA-12 offers real-time analysis to 20 kHz and time waveform display. To learn more about the RION VA-12 go here.
The Norsonic Nor140 is a full-featured precision integrating sound level meter to IEC 61672 Class 1. The Nor140 offers real-time octave analysis, FFT analysis, and a 120 dB dynamic range. Software integral to the Nor140 is available for applications ranging from environmental noise, building acoustics, source identification, to vibration measurement To learn more about the Norsonic Nor140 go here.