Proof Testing

The Situation

Proof testing is performed as definitive “proof” that a rotating component will not fail at its maximum operating speed. Sometimes called Overspeed testing, a proof test is typically conducted at greater-than-operating speed to provide a margin of safety. When manufacturing prints require spinning and balancing, precision machine shops are often required to provide this qualification testing for customers.

Why It Matters

Manufacturing turbomachinery components for demanding OEM’s requires a testing partner that can handleparts quickly to meet production schedules, while providing process control and/or aerospace certification (if required). Tooling design and analysis is also important for high-value parts.

What You Can Do

Manufacturers can mitigate the risk associated with the design and production of high-speed components with Proof Testing (also known as Integrity Testing). Test Devices offers component manufactures two methods of Proof Testing, each of which offers a range of situation-specific variations:

Overspeed spin testing verifies high-speed component integrity and provides objective evidence regarding the capabilities of a component at design limits and beyond.

Production spin (also known as Pre-Spin) testing elicits a specified amount of plastic radial growth to the residual stress state of rotor forgings prior to final machining.

Test Devices has expertise and experience in the specialized field of Proof Testing. We have invested in top-flight equipment, instrumentation processes, data recording, and personnel to provide our customers with the crucial information they require to ensure that their high-speed rotating components perform as designed.

Applications for Proof Testing

Jet engine components Gears Compressor rotors
Turbine rotors Centrifuge rotors HVAC fans
Turbofans Impellers Metallic flywheels
High-speed fans Turbochargers Composite flywheels
High-speed machine tools Electric motor rotors Grinding wheels

Advantage of Proof Testing

Verified Integrity: Components that are Proof Tested are typically accelerated to between 120% – 150% of operating speed, and then held at speed as briefly as several seconds or as long as a few minutes. During one excursion above the normal operating speed, the integrity of a finished component can be thoroughly tested. Design and analysis, material selection, manufacture, and inspection all can be examined in detail. In addition, information needed to meet ISO9001 / AS9100 quality requirements to validate that a component design actually meets its intended performance can be measured and recorded.

Higher Quality Components: By spinning a rotor forging past its design limits prior to final machining, centrifugal stress is imparted on the rotor to preferentially yield lower strength areas and equalize global material properties. The result is a more homogeneous rotor. Spinning is also used to “pre-grow” rotors to achieve a desired degree of plastic radial growth before final machining. This pre-growth helps to avoid in-engine plastic rotor growth, which can lead to problems including excess vibration and unintended contact between parts.

Dedicated Spin Chambers: Due to the large amount of kinetic energy stored in high-speed components, and the risk of damage from a component failure, Test Devices conducts Proof Tests within vertical spin chambers designed for this application. Spin chambers are robust steel vessels that provide containment in the event of a high-speed component failure. An air turbine or electric motor, mounted on top of the spin chamber, drives the test piece within the spin chamber. A vacuum system reduces frictional drag in the spin chamber, reducing test cycle time and improving drive efficiency while testing bladed components.

Data Acquisition: Test Devices uses an on-line, high-speed, digital data acquisition system to record all test parameters throughout the duration of all Proof Test procedures. This system replaces paper data recorders traditionally used in the industry, allowing large amounts of data to be stored, transferred and analyzed easily. All test data are included in the final report together with all other project paperwork, including certificates of conformance, balance, heat calibrations, and tooling material certificates. Test data is also archived at Test Devices for the customer’s future reference.

Proof Test Options

 

Standard Proof Test

Test components are accelerated to predetermined speeds, dwelled at maximum speed for a specified period of time, and decelerated to 0 rpm

Data Includes

Speed

 

Elevated Temperature

Test Devices offers customers the option of performing proof tests at elevated temperatures. Proof tests can be run at isothermal temperatures up to 1500°F (815°C), with special applications to 1600°F (871°C). Tests performed at operational temperatures better simulate the conditions the rotor will actually experience during use, producing a worst case scenario for the component during testing.

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Elevated Temperature

Thermal Gradient Option

Controlling the temperature of individual sections of a rotating assembly allows for better correlation with finite element stress models and real world conditions. Temperature gradient simulation during testing adds additional stress on rotating components. Differences in temperature (non-isothermal), both radial and axial, create stresses in an assembly that rival centrifugal stresses.
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Cryogenic Temperature Option

Test Devices also provides proof testing services at cryogenic temperatures. Spin tests of rocket components often require test temperatures well below ambient, and Test Devices has developed techniques to test components at cryogenic operating temperatures below -300°F.

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Strain

Strain Measurement Option

Sensors mounted to the test component are used to monitor the strain resulting from the centrifugal stress. Customers are able to select specific high stress areas of interest on the test component and obtain strain data at speed. The strain data is most often used to validate CAE analysis.
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Growth

Elastic/Plastic Growth Measurement Option

Proximity sensors are used to measure the growth of a component bore and/or rim at speed. This is particularly important when component growth becomes plastic. Growth measurements provide customers with the ability to determine a component’s position on the stress/strain curve during a test.
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