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GB High-Accuracy Moment of Inertia Series

Product Details

Gas bearing-supported table for objects up to 8,800 lb / 4,000 kg with 0.1% accuracy. The GB series Moment of Inertia (MOI) measurement instruments are designed to measure large mechanical components such as re-entry vehicles, spacecraft, automobile engine parts, and complex structures.

Standard models are available for measurement of test objects weighing up to 15,000 pounds (6,800Kg) with an accuracy of +/-0.1 percent. In addition to their use as a design tool, these instruments are fast enough for use on production lines for manufacturing quality analysis.

Most rotating machinery and all space vehicles require the calculation of Moment of Inertia of components and complete assemblies. While these calculations have become faster and more accurate through the use of computers, the exact value and extent of manufacturing variations cannot be known without making measurements.

Raptor Scientific GB Series instruments measure the MOI of any object regardless of its size or shape in a fraction of the time required for the MOI calculation and permit rapid accumulation of data for variation analysis.

GB Series instruments use a flat gas thrust bearing to support the test part weight, a cylindrical gas bearing for precise centering, and a torsion rod as a rotational spring. The lower end of the rod is attached to a flexure which prevents changes in torsion stiffness as a function of small variations in gas bearing gap and thermal expansion of the machine frame.

The instrument operates on the principal of an inverted torsion pendulum. To measure MOI, the test table is rotated through a small angle and held against a fixed stop. When released, the table will oscillate freely since the load is supported and centered by nearly frictionless gas bearings. A digital timer on the instrument determines the period of oscillation. This procedure is first carried out with the bare table and any necessary locating fixture, and then again with the test part mounted. A simple computation using the tare period, the period with test part, and a calibration constant, converts this data into moment of inertia of the test part.

Special Instruments

In addition to our standard line of instruments, we have designed and manufactured special instruments for test parts in excess of 25,000 lbs. and have achieved accuracy as great as 0.01%. These machines can also be fully automated to meet special needs. Contact our sales department, if a standard instrument will not meet your requirements.

For smaller moment of inertia instruments you may consider:

  • XR series for payloads up to 250 lb (including fixture)
  • XKR series for very accurate measurement of small payloads
    • Options & Accessories
      • Air supply
      • Custom fixtures
      • Spare parts
      • MOI through CG software: calculate MOI passing through the CG without knowing the CG location
GB550AX10 lb-in(0.003 Kg-m2)550 lbs (250 Kg)0.12 in. (3.2 mm)0.1%
GB3300AX50 lb-in(0.015 Kg-m2)3300 lbs (1500 Kg)0.25 in (6.6 mm)0.1%
GB8800AX125 lb-in(0.036 Kg-m2)8800 lbs (4000 Kg)0.5 in (12.7 mm)0.1%
GB15000AX500 lb-in(0.146 Kg-m2)15000 lbs (6800 Kg)3.0 in (76.2 mm)0.1%

Gas supply: A source of pressurized nitrogen or dry clean air capable of delivering 2 CFM (57 L/min) at 75 PSI (.52 MPa) must be provided for the operation of the gas bearing. For locations which do not have a permanent nitrogen line or oil-less dry air line, this gas can be supplied from a Space Electronics Instrument Air Supply. (Shop air usually contains oil and water and is not acceptable.)

Choosing a Location: Care should be exercised in choosing the proper site for the instrument in order to achieve full rated accuracy. The foundation on which the instrument is mounted must be rigid enough to support the total weight of the instrument and test part without altering the level condition of the instrument. A means must be provided for gently lowering the heavy payloads in place without damaging the instrument. For maximum accuracy, short term temperature variations should be less than +/- 2o F (1oC). Drafts should be avoided.

These instruments are supplied with calibration weights, a calibration weight locating system (beam or table with calibration holes), a 6 digit period counter with crystal controlled time base, and a pocket calculator.

Computer Interfacing Option: This an IBM style computer, VGA color monitor, printer, RS232 interface, and operating software. The software provides prompts to the operator for calibration, tare, and part measurement procedures, reads timer data, averages period readings, calculates MOI, and generates a report.

Moment of Inertia Instruments


Aircraft Control Surface for MOI Testing View Product
XKR Moment of Inertia Small-Payload View Product
XR Moment of Inertia Production Series View Product

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