May 25, 2018
At Elite, every product tested for vibration or mechanical shock has to be properly secured, or “fixtured,” to a vibration or shock machine. The easiest way to fixture is using bars, threaded rods, clamps, or other tools that can grip the test item and secure it to the table. The preferred way is to create a custom assembly, called a “fixture,” which provides a mounting surface and tapped holes for holding the Device Under Test (DUT). The base of the fixture also has attachment points that connect to the vibe/shock machine surface.
A custom-made fixture is especially important for devices that have complex shapes which do not allow for a simple clamping solution. A fixture is also helpful when the DUT must be held in a particular orientation for all axes of testing and to hold multiple samples for a single test run.
Generally speaking, there are two types of fixtures; those that “simulate” the mounting conditions, and those that “stimulate” vibration into the DUT.
Simulate-style fixtures are intended to replicate the actual DUT mounting bracket or mounting surfaces. They are unique structures with their own mechanical response. A simulated bracket fixture will likely alter the characteristics of the vibration/shock dynamics when passing from the machine through the fixture and into the DUT. In some cases, this is desirable, for example when the goal of the test is to provide fatigue input representative of how the DUT is mounted in the actual end-use environment. The simulated fixture will have its own mechanical resonant characteristic which may amplify the input vibration and create a more severe vibration environment for the DUT.
Stimulate-style fixtures tend to be designed with thicker more robust elements and structural members. They are arranged to provide stiffness while being lightweight. Stimulate-style fixtures are intended to transfer the applied vibration/shock input from the table directly into the DUT. A perfect stimulate-type fixture has input transmissibility that is very flat, meaning that the frequency range, amplitude, and phase of the vibration environment are applied directly to the part with no modification to the input vibration environment.
The design of a fixture should first account for being a simulate vs. stimulate purpose. However, the design should also be lightweight to achieve maximum vibration test levels and should allow access to the DUT mounting points in order to easily attach the test item. The design needs to consider future modifications and re-tooling. It also has to balance performance with cost.
Elite engineers take all of these parameters into consideration when developing fixtures for client testing. They have the benefit of years of expertise through hands-on testing and vibration fixture development but are also able to apply the latest CAD and vibration simulation tools, like those from Solidworks.
Once a test item CAD design file is received from a client, it can be loaded into Elite’s Solidworks tool and the fixture structural design can begin. Engineers create the initial design taking into account all the particular details of the test item shape, mounting configuration, attachment and accessibility, and cost.
The next step is to run a full dynamic simulation using the vibration modeling features of Solidworks. From the simulation, we can visually and quantitatively evaluate areas of the fixture that are resonant and redesign the fixture to reduce the resonance magnitude or even shift the resonant frequency beyond the test range.
It is critical to reduce fixture resonant frequencies that coincide with the vibration input profile. When vibration is applied at the fixture resonance conditions large displacements and accelerations (high G levels) are produced and test items can be overstressed. Similarly, when the input vibration and response align out of phase the applied vibration can result in “null” acceleration conditions and under-test the DUT.
Elite’s Steps to Vibration Fixture Optimization
1. Develop an initial fixture design considering the following:
- Fixture purpose: Stimulate or Simulate style fixture.
- Single or multiple samples
- DUT orientation
- Accessibility for DUT mounting and table mounting
- Weight, length, width height considerations
- Single-use fixtures or future adaptations
2. Run the simulation and identify the following:
- Identify natural resonant frequencies, mode shapes, and relative resonance magnitude.
- Consider options for reduction of resonance, weight, stiffening elements, and moment of inertia (length, width, height).
- Repeat simulation and optimize results for performance, machinability, and least cost.