You want to run vibration tests on your new product. The shaker table can handle your product’s weight with some room to spare and operate over the proper range of frequencies and amplitudes. You need one more thing—a very important element—to get valid, repeatable test results.
You need a well-designed, well-manufactured interface between the product and the shaker table. The proper test fixture is a device that can’t be found on the shelf or in some vendor’s catalog. It’s the missing link in many programs.
An improperly designed test fixture can distort your vibration test results beyond recognition. If the fixture has a resonant point within your frequency range, you can shake the product more than you planned. And then even good products can fail the test. If the fixture has low transmissibility, it can isolate your product from the full force of the shaker table, showing the product to be good when it may not be. Either problem can make it impossible to repeat your results on a different table with a different fixture.
Where can you get details on how to design and build a valid fixture? The Institute of Environmental Sciences and Technology (IEST) has prepared IEST-RP-DTE013.1, “Vibration and Shock Test Fixtures.” This document, compiled by a committee chaired by Wayne Tustin, can help a test engineer or fixture designer:
Translate a statement of work, test standard, or test specification into detailed requirements for interfacing a unique product to a specific shaker table.
Generate design criteria for a fixture to meet these requirements.
Design a fixture, or work with specialists who design it.
Fabricate the fixture.
Evaluate the fixture experimentally.
Use the fixture properly.
Maintain it.
Document 013.1 focuses on fixture design for a wide variety of basic shock and vibration requirements. It covers most test scenarios, including vibration applications to 2,000 Hz. It also addresses the basic mechanical and crash safety shock levels, but not unusual tests such as pyrotechnic and other specialized shock applications.
Fixture Criteria
Successful fixture design starts with a thorough and accurate definition of the test program. This includes a determination of the test level, including tolerances. The weight and center of gravity of the unit under test (UUT) must be determined. Will the UUT be functional during the test? What is the test philosophy or goal? Should the fixture be rigid, or should it simulate the actual mounting of the UUT in its end use? How will the UUT attach to the fixture?
When a rigid fixture is used, vibration applied to the UUT is approximately equal to the vibration level of the table. Fixture transmissibility, the ratio of the response signal at the UUT to the input signal from the table, is near one if the test is run well below the fixture’s lowest natural frequency fn .
Figure 1 shows transmissibility of a hypothetical fixture, where the natural frequency fn is evident for five different values of the fixture’s damping factor x . For proper vibration testing, the forcing frequency ff, the highest test frequency, should be no more than 59% of the natural frequency fn . Expressed another way, the transmissibility should not exceed two.
The other extreme is the case where a fixture is used well above its natural frequency and effectively becomes a vibration isolator. A test should not be run at a transmissibility of less than 0.5.
In contrast to a rigid fixture, a simulation fixture includes or simulates part of the structure on which the product will be mounted in use. Here the natural frequencies of the fixture approximate those of the in-service structure to which the UUT will be attached. This allows the unit to respond to test conditions in the same way that it will respond to its actual field environment.
Fixture weight also is critical. The lighter the shaker armature, fixture, and other necessary moving parts, the more shaker capacity can be devoted to the UUT. The fixture can be machined from solid-stock cast magnesium or aluminum to make an optimum test arrangement. When possible, leave at least 25% of the shaker force-pound rating as spare capacity.
Detailed Design
A fixture usually is bolted to the shaker to eliminate any motion between the two surfaces. Large loads or test frequencies above 1,000 Hz require that the fixture be bolted at every available hole in the shaker table.
A machined steel flat washer should be permanently installed under each bolt head. This gives more structural integrity and longevity than allowing the bolts to turn against relatively soft magnesium or aluminum.
A rigid fixture should mimic end-environment mounting. That is, the UUT should mount on the test fixture with the same bolts and clips in the same locations that will be used in actual service.
Review the configuration and geometry of the UUT, particularly its weight, center of gravity, and mounting provisions. If it has no mounting provisions, a clamping fixture can suffice. When all the attachment hardware is on one plane, a flat fixture may be optimum.
Several other classical shapes also have been used successfully. These include cubes, tees, ells or bookends, and boxes. Section 6.2.3 in Document 013.1 shows examples of these and other shapes.
The fixture should be arranged so the combined center of gravity for it and the UUT is located along the axis of symmetry of the shaker armature or the carriage of a shock machine. This will decrease the probability of carriage rocking during test.
If a fixture will be used for environmental stress screening (ESS), the test philosophy is different from that for new-product development testing. Section 6.4 of Document 013.1 defines these differences.
The goal of ESS is to stimulate the UUT to precipitate latent defects if they exist. Mounting is not as critical. Transmissability does not need to be near one for the frequencies of interest, and random vibration is the preferred dynamic excitation. A good fixture helps to excite all resonances within the UUT.
In cases where a combined temperature-vibration environment is required, the fixture design must consider control of heat flow, insulation of the UUT from the fixture/armature mass, and control of any condensation. Special care also must be given to securing hoses, harnesses, and electrical cables.
Fixture Fabrication
Most high-vibration-frequency fixtures are welded, cast, or machined from solid-stock magnesium or aluminum. This yields stiff fixtures with high natural frequencies and is preferred over plates that are welded or bolted together. The fabricator should be especially concerned about relative motion between parts, bolt layout to avoid rocking, excessive length of the bolts and their diameter, and the flatness of the mating surfaces to within ±0.001″.
Each new fixture should be permanently identified with a control number. An index must show all details relating to its design and construction plus reports of data recorded during fixture evaluation and recommendations that will be useful in future designs.
Fixture Evaluation
A survey should be performed on each new fixture prior to its use as part of a test. This will establish response characteristics and determine the natural frequencies. The survey should include the answers to these questions:
Are the in-axis and cross-axis responses correct?
Are the torque values for the fixture and the UUT mounting bolts adequate?
Should the fixture have more mounting bolts?
Does the fixture need to be stiffened?
Is there a center-of-gravity problem?
Results of the tests and analysis must be kept with fixture records to help analyze test results and provide information for designing the next fixture.
Fixture Use
Before bolting the fixture in place for a test, check for rocking between the fixture and the shaker table. If the two surfaces are not properly mated, remachine the problem surface as soon as possible. If you use shims as a temporary fix, recognize that your tests are not repeatable, and a permanent fix is imperative.
Installation of the fixture on its shaker table and installation of the UUT on the fixture are critical to proper performance of an environmental test. As a step in this process, a calibrated torque wrench should be used. Section 9.1 of Document 013.1 recommends torque values for several bolt sizes.
Evaluate the placement of the control accelerometer before a test sequence. To ensure repeatability, it is important to place this pickup device at the same point for all tests. If your system uses multipoint control, follow the manufacturer’s instructions carefully when placing the accelerometers.
As you proceed with the test, compare your results with analytical predictions. You can learn about your UUT and your fixture as you perform tests.
Fixture Maintenance
Your fixtures are a very important part of your test system, so protect them from damage during use and storage. This is especially important for fixtures such as magnesium and aluminum. Return each fixture to its assigned, clean storage site and rest it on wooden blocks; cover it with clear plastic sheeting; spray or anodize it to help prevent corrosion; wrap or install covers on all threaded studs; leave bolts in their holes during storage but do not tighten them; and inspect the bolts before each use.
Reference
Document IEST-RP-DTE013.1, Vibration and Shock Test Fixturing, 1998, Institute of Environmental Sciences and Technology, 940 E. Northwest Highway, Mount Prospect, IL 60056, (847) 255-1561. Web site: www.iest.org. E-mail: [email protected].
Copyright 1999 Nelson Publishing Inc.
September 1999