Proof & Overspeed Testing & Safety Containment in a Spin Tester
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Proof Testing, Overspeed Testing & AnalysisIntroductionUnlike burst tests that are intentionally spun to destruction, Proof Tests are typically run on parts at a higher RPM than operational speeds but lower than burst speeds to prove the parts are suitable for use. The Proof Test (also know as an Overspeed Test) will show that three important aspects are suitable for operation: the design, the materials used and the manufacturing processes employed. All critical production parts should be 100% proof tested while less critical parts may only be sample tested. Other interesting things happen during a high-speed proof test. For example, weaker areas of a spinning part will move ever so slightly and essentially work (strain) harden until they get as hard as the harder sections. The net result is a more homogeneous and capable product. In fact, an off-shoot of proof testing is Bore Pre-stressing where spinning becomes part of the production process adding strength to the parts under test. This is covered more fully below. We can perform many different types of tests depending on the goal and information needed. The intention of a proof test is to extract information about the part while maintaining the part in good working order, however, it can burst at any time releasing substantial destructive energy. We discuss the energy stored and the dangers of a rotating part on our Burst Test page. The same issues are involved in testing of high speed rotating parts and one must always consider the safety aspects. Test Devices is expert in this field and has invested in the equipment and personnel that, together, provide the highest level of safeguards for damage and personal injury. Some applications for Proof Testing include:
What Are the Advantages of Proof Testing?1. Proof testing will provide evidence that the design, material and process are sufficient. Any flaws that could affect the performance or life will show up more readily when spun at a speed higher than at normal operating speed. The exact proof speed should be determined based on the amount of headroom between operating speed and burst speed. (Test Devices will be pleased to work with you to determine an appropriate test speed for your unique parts.) 2. Validates that the part will perform at operating speeds without undue concern for the safety of surrounding people or equipment. Proof testing provides information to meet the ISO9001 requirement to validate that the design actually meets its intended performance. 3. Validation that the processes used to produce the part are robust. For example, in general a material may be quite sufficient to meet the requirements of the operational speed, however, a flaw in the material anywhere in the process from the original metallurgical heat to final machining could cause the rotating piece to burst prematurely as the stresses concentrate around the flaw. 4. To understand the deformation of plastic strain or creep. Should a part grow and not recover, this will have an affect on the life and functionality of a part. Growth could accumulate such that the clearance between the part and any surrounding items decreases which could cause product failure. 5. Laminated rotors, such as composite flywheels, are subject to failure when the glue between the laminated layers fails. Strength usually comes from the tensile strength of fibers wrapped circumferentially around a hub. These fibers generally will not break, but can stretch slightly creating relatively large radial movement causing the composite glue to un-bond from the fiber. Proof testing will quickly expose any flaws in the bonding between layers. 6. Always in nature there are phenomena that we do not yet understand that can affect the best theoretical calculations. Proof testing, particularly strain proof, can be used to verify FEA models. By comparing empirical results to modeling results, one can refine the model to produce future predictable output. 7. When designing a new piece of equipment, a lot of money can be tied up in the prototype apparatus. Proof testing can be inexpensive insurance to protect the large investment of time and money in the prototype from being destroyed by an unexpected burst. 8. Machining, by it very nature, will change the internal stresses as material is removed. For highly precision parts, the resulting distortion can throw a part out of its design tolerance. One process for enhancing accuracy and stability is rough machining and then subjecting the part to high speed rotation. The stresses will be rearranged into a more stable configuration and the part can then be final machined to an extremely high level of accuracy without distortion. Types of Tests
Simple Proof Test Run up to pre-selected speed, dwell and spin down. Data includes:
Stepped Proof Test Run up to pre-selected speeds, dwell and spin down. Data includes:
Heated Proof Same as simple or stepped proof but with test specimen maintained at a specific temperature. Capability from ambient to 1150 °F (special applications to 1600 °F). More closely represents real operating conditions. Data includes:
Figure 1
Cooled / Cryogenic Proof Same as simple proof but with test specimen maintained at a specific temperature below ambient. Capability from ambient to 320 °F. More closely represents real operating conditions. Data includes:
Progression Proof This is a very specialized test that is essentially a combination of Heated Proof and Cooled Proof where a part can be run up to speed and held while the temperature is either incremented or decremented from one extreme to the other. Data includes:
Gradient Proof Rarely in the real world is a part at the same temperature over it entire area at the same time. To better replicate real operating conditions, TDI has the ability to spin a part while different sections of the part are at different temperatures. There are constraints on the delta temperatures and how near two points can be which are dependant mostly on the parts configuration and properties. Data includes:
Strain Proof Sensors mounted onto the test specimen to monitor the strain resulting from the high-speed stresses. This test will monitor what is happening in the locations that you are most interested. This can be done in combination with any simple, heated or cooled test. This test is most often used to validate CAE analysis. Data includes:
Radial Growth a profile of the changes in dimensions of a test specimen as it speeds up. Can be performed at ambient or at controlled temperatures. See Radial Growth (link to RG page) page for more details. Data includes:
Bore Pre-stress This is a production process performed by Test Devices where strength is added to a part much like heat treating or quenching. Typically the greatest stress is at the hub (center bore) of a rotating component and is the area most subject to failure. By running a part up to a specific speed (different for each part configuration), this process causes the weaker sections of the part to work harden up to the same level as the stronger sections creating more homogeneity resulting in a more durable part. Because both the bore and OD grow, the arbor for holding this part during a test is critical as it must hold the part securely as the part dimensions change and it must do this without affecting the process with it own stresses. Test Devices can establish a test plan to closely replicate your operating conditions and provide data on your test specimen. A typical test consists of the following steps:
DefinitionsWhat is a Containment Vessel? Should a part not make the pre-selected speed and burst, this is an apparatus that will prevent the shrapnel from leaving the immediate area of the burst and protects both people and equipment nearby that would be subject to damage. There are numerous technologies to accomplish this and which can be tested in addition to the rotating part in our test chambers. Currently, there are no computer programs that accurately model potential failures in containment. What is Documentation? - Includes all information agreed upon for the test plan and could include plots, certifications, videos, TIF files, photographs, etc. Test Devices, Inc. provides a formal certification document for each article or lot as required. All test documentation and report material is archived at Test Devices, Inc. and is available for inspection or duplication. What is Burst Contingency & Repair As discussed earlier, rotating parts store large amounts of energy and, by definition, all that energy will be released during a test piece burst. While our test systems are very robust, there will be some minor damage by the very nature of the burst. Damage to the turbine or other equipment due to the burst during proof testing is assessed on a time and materials basis up to the limit of the burst contingency. The burst contingency should be accounted for in the testing budget and indicated on the purchase order. Standard PartsThese customer part numbers have short lead times. Test Devices has the following part numbers tooled and in production testing regularly. As a result, we can offer volume pricing for small quantities with short lead times. Please call us today for a quote on:
For additional information or answers to specific questions, please contact us.
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