SE8913 Small Payload Center of Gravity

Product Details

SE8913 devices use a dual-axis center of gravity instrument for high accuracy measurement of the CG location of objects weighing up to 33 pounds. This is achieved by measuring the unbalance moment due to the test item’s CG displacement from the center of the rotary table. The manual version requires the operator to calculate the CG location from the moment readout. The automatic variety converts raw data into two-axis CG reading on a Windows computer interface. A third CG axis is obtainable through refixturing. Note that the instrument does not measure moment of inertia or weight. This type of instrument is more accurate than a 3-point weight and CG machine and does not measure moment of inertia or weight.

The SE8913 Small Payload Series for CG Measurement

  • Measures 2-axis Center of Gravity (third axis can be measured by refixturing)
  • Typical Accuracy with maximum weight part: ±0.005 inch
  • Concept: Moment reading (measures CG only). A separate scale must be used to measure weight.
  • Interface: 4.5 inch diameter rotary table with threaded inserts.

Calibration Standard: A precision weight is supplied to calibrate the instrument.

Test Fixture: A fixture is required to support the test object at a known location relative to the instrument. Since there is no motion during measurement, this can be a simple light-weight bracket. Contact our sales department for more details. We offer a number of standard fixtures.

Measurement Principle: This simplified drawing illustrates the basic theory of operation. A rotary table is mounted on a precision spindle bearing. Crossed web flexures provide a frictionless horizontal pivot axis. A torsion rod in line with this pivot axis creates a linear torsion spring around this axis. When the test part’s CG is displaced from the measurement axis, an unbalance moment is created, resulting in displacement of the LVDT mounted at the end of the moment arm. Rotating the test part 180 degrees on the rotary table eliminates systematic errors such as incorrect leveling, zero drift in the readout, or displacement of the rotary table axis from the pivot axis. Since the weight of the part is supported by the web flexure pivot and only the unbalance moment is sensed by the LVDT, high sensitivity is possible. The output of the LVDT is digitally filtered in the moment readout, which eliminates vibratory and other high frequency components. To measure CG offset, the operator rotates the table to 0° and records the value of unbalance moment. He then rotates the table to 180° and records a second moment. CG is computed by subtracting the two moment readings and dividing by the test part weight.

Model SE8913AModel SE8913BModel SE8913C
Max weight (test part and fixture):2.2 lb11 lb33 lb
Max CG height:3 inches5 inches5 inches
Maximum moment without damage to instrument:5 lb-in10 lb-in20 lb-in
Full scale moment:0.25 lb-in2 lb-in10 lb-in
Moment sensitivity:0.0005 lb-in0.002 lb-in0.015 lb-in
Resolution:0.0001 lb-in0.0002 lb-in0.001 lb-in

Model SE9707 Computer Readout Option: To use the basic manual version of the instrument, the operator must record four moment readings. CG is calculated by subtracting two moment readings, multiplying this number by a calibration constant, subtracting a tare offset, and dividing by the test object weight. This calculation must be done for each axis. We offer a computer option which reads the data automatically through an RS232 interface, performs the calculations to yield CG offset in inches, and prints a report. This program runs on a Windows-based computer.

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