Modern Fixture Methods

A Vibration Fixture is a device that typically interfaces the vibration shaker/shock machine and the test item(s). Its design may range from a very simple plate with a few attachment holes to an extremely complex device either designed specifically for a unique test item or designed with automatic features which allow production testing to occur with the rapid insertion and/or removal of the test item.

Several questions must be addressed before any fixture may be proposed:
1. How many test items must be tested at a time?
2. How many axis of vibration is required? Is there a choice?
3. What is the force capacity of the shaker or the payload capacity of the shock machine upon which these tests must run?
4. What are the test requirement:
- sine, random, sine on random, resonant dwell, shock
- amplitude
- frequency range

Often, for products which do not exhibit any unique mounting characteristics, a universal style fixture is appropriate. Fixtures of this type may be classified as Cube, L or T.

In the case of the cube fixture, there would be 5 faces available to mount test items as the bottom surface would be hard mounted to the test apparatus and not available to mount test item. If the condition of test is to vibrate in each of three mutually orthogonal axes, the cube lends itself as the perfect fixture. One may place product on the top surface as axis Z and on each of the sides for axes X and Y. It would only be required to rotate the test item in relation to each face of the cube to realize all the orthogonal axes. Assuming the test equipment could handle the resulting payload, the cube fixture is capable of testing up to 5 products at a time, thereby minimizing total test time.

For those with 6 axes requirements, a cube may be fabricated with a cavity on the top such that the UUT may be situated inside the cube while mounted on an adapter plate for the Z axis.

The L and T Fixtures are particularly useful in supplying a mounting surface perpendicular to the direction of the test apparatus. The product may be mounted directly to the apparatus for one orthogonal axis, but if for some technical reason, the product does not allow itself to be mounted on its side relative to the position of the apparatus, an L or T fixture would be appropriate. The product would be mounted on the vertical member of the L or T for the second axis and then rotated 90 degrees in order to perform the third axis. Note that the vertical member of the T fixture may accept product on either side to maximize test throughput.

Vibration fixtures are, most often, analyzed from a rigidity point of view. The utilization of geometric shapes as exhibited by the cube, L and T result in the greatest possible rigidity and, therefore, allow for these fixtures to demonstrate the highest possible natural frequencies.

The anticipated natural frequency is based on the following criteria:
a. Analysis of a beam using a free-free mode using its longest feasible length and ribs for maximum depth.
b. Experience with similar applications. (note that it is impossible to predict the precise performance of a fixture from one test apparatus to another since each apparatus has its own peculiar dynamic characteristics, thereby, rendering the performance of each fixture to be slightly different).

Always, a resonant frequency is characterized by an amplification
where the response of the fixture is greater than the input. In very general terms, every animate object has a fundamental resonant frequency. That is why glass shatters when exposed to high frequency and bridges twist when exposed to low frequency seismic vibration or air waves.

When a fixture is determined to have a resonant frequency at some
point, that does not mean that the fixture is flat or without
dynamics to that point. Because of the phenomenon of the
transmissibility curve, the fixture begins to build up to the
resonant point at a frequency approximately 30% below the
fundamental resonance. That means a fixture whose resonant frequency is predicted at 2,000 Hz will demonstrate a gradual increase in amplification starting at 1,400 Hz, and will continue to increase until it reaches its apex at 2,000 Hz.

In order for a fixture to be flat to 2,000 Hz, the resonant frequency of the fixture must be in excess of 2,600 Hz. It probably will be necessary to use any one of several techniques to control the fixture:
1. Multi-point averaging
2. Notching
3. Strategic placement of the accelerometers, i.e. place the control accelerometer in the fixture's corner and the monitor accelerometer in the fixture's center.

The material most often selected for vibration fixtures is magnesium. Most fixtures are, in fact, weldments made from AZ31B magnesium tool plate. Tool plate is available in a wide range of thickness for ease of design. The primary reason for using magnesium is that it is light weight. Note that the density of magnesium is approximately .064 lbs/in/in/in while the density for aluminum is .1 ls/in/in/in and the density of steel is .3 lbs/in/in/in. That means that magnesium is 30% lighter than aluminum. It is, therefore, possible to use a 1.25 inch thick piece of magnesium to produce a fixture which would weigh less than its aluminum counterpart made from 1 inch material! The significance is that thicker translates to more rigid and more rigid provides better performance. Size for size, all materials have the same resonant frequency as the ratio of stiffness to mass are all identical.

Magnesium certainly has its rewards. It also offers some interesting challenges in that, if ignited, a magnesium fire will become enhanced when exposed to oxygen. Therefore, water or other standard fire extinguishing methods will not work. TT6 MET-L-X powder is necessary! The issue with flammability is when a spark is created on the machining center and ignites the chips. The machining process includes a slower spindle speed, high feed rate and no tool bit lubrication except for forced air to blow away the chips and ready access to the fire retardation material. It is noteworthy that we preheat the material before we weld by placing an open flame on the parent material with no fear of creating a hazard. It is the condition of a tool dwelling on a machine surface with no chip load that produces fine powder and a spark to ignite the powder.

Magnesium is considered to be a hazardous material and, as such, must be disposed of according to EPA guidelines. This includes placing and sealing in steel drums and marking the drums DANGER WHEN WET, KEEP FIRE AWAY, MAGNESIUM CLASS NO UN1869, AND FLAMMABLE SOLID. If over 1000 lbs, add HAZARDOUS MATERIAL. It is usually expensive to dispose of this material unless you can find a fireworks manufacturer who will either buy it, pay shipping charges or take it free.

I have been involved in many discussions regarding issues that pertain to sound, fundamental fixture design. Here is a list of FAQs:

Washers: All bolts must have steel washers placed under their heads to prevent gouging the material and to prevent any separation between the fixture and the test apparatus. Separation will cause unwanted dynamics and cause data to be flawed.

Damping: Magnesium is soft, so, it offers natural inherent damping. Much more so than aluminum or steel. If damping is required, M/RAD would utilize a visco-elastic material adhered to the sides of the fixture or foam, placed in cavities, to minimize the amplification factor associated with a resonant frequency. A Q of 40 - 50 is typical WITHOUT the Damping Material. A Q of 20 - 25 is typical WITH the Damping Material. Note that the Damping Material does not eliminate the natural frequency, rather it serves to reduce the amplification factor. It may still be necessary to use multi-point averaging or notching to control the fixture to required tolerance limits, if applicable.

Inserts: As magnesium is soft, it requires a steel insert. Otherwise, bolts and fasteners will simply strip from the fixture.

Corrosion Protection: Especially when placed inside an environmental chamber, a fixture can corrode. The method most commonly used to protect magnesium is HAE per MIL-M-45202C, Class A which ultimately produces a tan coating and is an excellent paint base.

Jeffrey Marshall


Presented at the IEST Annual Technical Meeting, 5/2001 in Phoenix, AZ.