In vibration testing, engineers use fixtures to mount the test article in the required orientation, distribute load, and adapt the device under test (DUT) to the shaker interface. Shaker fixtures provide mechanical support, and well-designed fixtures are necessary for reliable vibration test results.
The fixture is not just a way to hold the product, but also a structural component of the test system. From the shaker’s point of view, the fixture and the DUT are a single dynamic system.
Article Overview
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- Determining the necessity of a shaker fixture
- Key design principles for effective fixtures, including resonance and stiffness
- Importance of fastener selection and maintenance
- Verifying fixture behavior
- Fixture design checklist
Do I Need a Shaker Fixture?
In most cases, a vibration testing setup requires a shaker fixture. The alternative is mounting the test item directly to the shaker head, which introduces several potential issues:
- Shaker may not support the required axis of excitation
- The base of the DUT may not align with the shaker
- Direct bolting may introduce uneven force distribution
- Overhanging DUT may introduce resonances
- Heavy or unbalanced loads may cause off-axis forces
- Consistent mounting may be difficult
Unless the test article matches the shaker interface directly, a test setup will likely necessitate a fixture for accuracy and repeatability.
Tips for Shaker Fixture Design
A well-designed shaker fixture provides an accurate response, minimizes unintended resonances, and protects the DUT and test equipment. Ideally, the DUT will be mounted on the fixture in the same orientation as its end-use environment.
Fixtures should transmit vibration energy from the shaker to the DUT without amplifying or attenuating certain frequencies. It should not resonate within the test frequency range, nor should it dampen the vibration energy.
Natural Frequency
If the fixture has a resonance within the control bandwidth, the controller will attempt to drive through it. This behavior leads to control instability, excessive drive levels, or damage to the shaker system.
As a rule, design fixture natural frequencies well above the test bandwidth. The common guideline is 2-3x the highest DUT resonant frequency. If a random vibration test reaches a maximum of 2,000 Hz, the first fixture mode should be at least 4,000-6,000 Hz.
It can be difficult to design fixtures with high resonances, so the engineer may need to limit the test bandwidth, if appropriate, or use notching to work around unwanted fixture resonances (see note about notching under “Detecting Fixture Resonances”).
Stiffness vs. Mass
A fixture should not have any motion of its own in any axis. It must be properly mounted to the shaker table, and no parts should be free to move or vibrate.
A fixture that is too flexible becomes a test problem. When its natural frequencies fall within the test bandwidth, it amplifies some frequencies and attenuates others, invalidating the test. Use simple, stiff load paths and symmetric designs whenever possible. Think of the fixture as an extension of the shaker: it should transmit, not distort, energy.
Stiff and lightweight is preferred over heavy and flexible. Ribbed aluminum is a widely used material because of its stiffness-to-mass ratio and predictable behavior. Steel fixtures can be appropriate in some cases, but its mass adds inertia and lowers natural frequencies. A heavier fixture does not automatically mean a better test. In fact, it often means a more difficult one to control.
Additionally, the fixture must be designed and constructed to prevent its damage during testing. Again, this reliability cannot be achieved by over-engineering a massive fixture. Additional mass causes the shaker to work harder and limits its responsiveness to changing control signals.
Symmetry
Symmetry is another important but sometimes underrated aspect of fixture design. Symmetrical fixtures transmit vibration evenly and reduce cross-axis motion. Asymmetric designs can introduce unintended rotations or torsional modes, which can complicate control and lead to false failures.
Geometry and Overhangs
Long overhangs and tall structures are common sources of test error. They introduce bending modes that are easily excited, especially during random vibration. These modes often lead to high responses at sensor locations that are unrelated to product behavior in the field. “Mysterious resonances” often derive from these responses.
Fastener Response
Shaker fixtures are often bolted down. Fasteners are susceptible to vibration, so consider using vibration-resistant fasteners, thread lockers, safety wire, or bonding, and perform regular checks and maintenance.
Detecting Fixture Resonances
The next step after designing a fixture is to verify that it behaves as intended, both before running the test and during testing.
Before the Test
Before building a fixture, finite element analysis (FEA) can be used to estimate its natural frequencies and mode shapes. This action helps avoid obvious issues, such as bending modes within the test bandwidth. After the fixture is built, a simple impact test or modal survey can confirm if the as-built hardware matches the analysis. This step often reveals unexpected modes caused by joints, fasteners, or manufacturing tolerances.
During the Test
During vibration testing, fixture resonances often reveal themselves indirectly. One of the first signs is a rising drive level, indicating the shaker has to work harder to maintain control. Low coherence between the drive and response signals can also indicate fixture dynamics, slipping interfaces, or nonlinear behavior rather than true product response.
Design vs. Compensation
While notch filters can be used to manage isolated resonances, they should be applied sparingly. Heavy notching often masks poor fixture design rather than addressing the root cause. If a fixture resonance dominates the test, redesigning the fixture is almost always the better long-term solution.
Boundary Conditions
Fixture design should control stiffness, mass, and geometry. Nonetheless, even a well-designed fixture can behave poorly if the boundary conditions are incorrect.
Mounting defines the boundary conditions, and boundary conditions define stress. Over-constraining the DUT artificially stiffens it. Stiffness can raise natural frequencies, change mode shapes, and reduce potential fatigue damage compared to the field.
On the other hand, under-constraining the product permits motion that does not occur in use. This behavior often leads to connector fretting, fastener loosening, or cable failures. Engineers may mistakenly blame these issues on the design instead of the setup.
The goal is to ensure that the test is representative. The use of every fastener, torque value, gasket, or isolator should be purposeful. Documentation becomes critical at this step because small differences in mounting between builds can change results.
Even if the fixture is perfect, improper contact or uneven bolt torque can change the force transmitted to the DUT.
HALT vs. Electrodynamic Shaker Fixtures
Many test issues come from applying HALT instincts to electrodynamic testing or vice versa.
HALT (repetitive shock) machines and electrodynamic shakers are fundamentally different systems, even though both apply vibration. HALT is designed for rapid, multi-axis, and highly accelerated stress. The tables are compliant, the excitation is broadband, and the fixtures are intentionally simple. Engineers accept some ambiguity in the exact stress levels because the goal is discovery: they are looking to find weak links quickly.
Electrodynamic shakers, on the other hand, are precision instruments. They apply controlled input in a defined axis, with known amplitudes, frequencies, and durations. The fixture must be stiff and well-characterized so that the controller drives the product, not the fixture.
A fixture that works perfectly in HALT may be inappropriate on an electrodynamic shaker. For example, compliant or open fixtures that are acceptable in HALT can introduce severe resonances, control instability, or unrealistic stress on an electrodynamic system.
Fixture design is not universal. It must match the machine, the control method, and, most importantly, the test objective. Understanding this distinction helps teams avoid false failures, poor correlation, and wasted test time.
Summary
An optimal shaker fixture securely mounts the DUT in its proper orientation and provides mechanical support. A fixture must accurately transmit shaker inputs without inducing unwanted resonances or motion, and its mass must not affect shaker performance. Attention to fastener integrity and regular maintenance helps prevent fixture degradation over time. By combining these design principles, engineers can incorporate fixtures that facilitate efficient testing.
A well-designed shaker fixture has the following characteristics:
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- Mounts the DUT to the shaker system in the orientation of end-use
- Transmits the forces from the shaker to the DUT without resonances or interference
- Serves to quickly mount and test multiple samples of the DUT
- Has minimal mass so the shaker can function with minimal force
- Performs many tests without sustaining damage
- Does not have any independent motion in any axis