Shock, awe, and vibration

Associate Editor

The real estate inside portable handheld electronics continues to shrink leaving designers no option but to pack sensitive electronics and tiny mechanical components more closely. Within these tight quarters, however, there's often little room left for shock mounts and vibration isolators. These components protect printed-circuit boards, LCDs, disk-drive components (spindle motors, heads, head arms, and discs), as well as other delicate electronic and mechanical components from g forces and vibrations. These forces can greatly reduce the life expectancy and reliability of the device.

For shock protection, designers often turn to a class of materials tailored with properties of both liquids (viscous solutions) and solid elastic materials. That's because these so-called viscoelastic materials have a hybrid of qualities.

Purely elastic materials store energy during loading then fully return when the load is removed.

In contrast viscoelastic materials have a high damping coefficient that returns a portion of the stored energy and dissipates the rest as heat after a load is removed. This hysteresis (energy loss) effect reduces the amount of "bounce back" caused by unwanted vibration. Additionally, the fluidlike viscous response of these polymers lets them deform uniformly under load. They transmit applied forces in all directions and distribute a small amount of pressure over a large area.

These viscoelastic properties let Sorbothane, a proprietary polyurethane, couple the structural support of a solid with the low-pressure comfort of a fluid. Sorbothane is known for reducing unwanted energy in the form of shock, vibration, and noise in a wide range of mechanical applications and comes from Sorbothane Inc., Kent, Ohio. The high damping in this polymer reduces the impulse peak of a shock wave and distributes it over a longer time frame. Impact forces are reduced up to 80% as payloads are gradually decelerated to rest.

Fluid-based shock absorbers or foam products can degrade under cyclic loading. In contrast Sorbothane's long fatigue life reportedly absorbs shocks for millions of cycles. The material has excellent memory and returns on its own to its equilibrium position.

Energy translates away from normal to the axis of incidence and its effect is pushed nearly 90° out of phase from the original disturbance. This phase shift (Tan Delta) is a measure of the material's damping effectiveness. The higher the Tan Delta the greater the damping efficiency.

The way to help ensure that portable electronics survive abuse is to first determine which components are the "weak links" in the design. Then the trick is to find whether the destructive damage will come via drop shock g forces or destructive resonating vibrations.

BEATING DROP SHOCK
Portable electronics must be robust enough internally to withstand the shock of being dropped from heights of up to 6 ft. Protection against drop-shock g forces is, however, often only an afterthought once the basic electronic and mechanical designs have been finalized. Additionally, sensitive electronics often sit in thin-walled injection-molded housings. If these aren't designed initially to accommodate shock mounts it may be impossible and expensive to build them in towards the end of a design cycle. It's important, therefore, for designers to investigate drop-shock constraints as early in the design as possible. To do so, it is helpful to determine four important parameters:

Fragility or the maximum g force that the various components can take during a drop without irreparable damage. For example, a typical hard drive can sustain damage to its spindle motor, heads, head arms, and disks if not adequately protected. Drivemakers give a g-force rating during operation as well as for when the device is shut off. A typical operating spec may be on the order of 200-g maximum input with nonoperating specs hitting 1,000 g. Designers must use the lowest g-force rating (i.e., 200 g) and design the shock mounts to meet that spec using G = 2h/x where G = fragility, dimensionless; h = drop height, in.; and x = sway space, in., needed to effectively dissipate energy. Proper material selection coupled with a good corner mount design let one OEM drop the acceleration profile of a ruggedized computer from 350 to 40 g. Likewise, g forces on a portable X-ray machine dropped from 6 to 0.4 g with the addition of custom stud mounts.

Drop height (h) specifications are also needed to calculate the amount of sway space (deflection) required in the shock-absorbing layer. The shock-absorbing layer should not deflect (compress) more than 50% of its free height. Compressing the layer beyond 50% will make material properties deteriorate and will drastically reduce the isolator's useful life.

Space restrictions inside the housing must be gauged as early in the design as possible. If the shock mount is denied adequate sway space, designers may have to make a trade-off between g-force ratings and how high the device can drop and still functionally operate. As a general rule, the greater the sway space, the better shock mounts dissipate shock-impact forces that would otherwise transmit to sensitive components.

Mass of the device is also important. A component that is not heavy enough will also deny the shock mount adequate sway space to dissipate unwanted shock energy. Designers may have to "add" weight to a component by mechanically forcing it to compress into the shock mount via a clamp or other mechanism. Space constraints again will dictate whether such a device is practical. Shock-absorber Durometer may also serve to help ensure adequate sway space. A 30 Shore 00 material will have a lower spring constant and support less weight (i.e., compress more) than a 70 Shore 00 equivalent that can carry heavier loads.

ISOLATOR TIPS
A good vibration isolator lowers the natural frequency of a system to below the excitation (or disturbing) frequency. Keeping these two frequencies "out of sync" eliminates resonance and greatly reduces the problems of vibration.

The first step in selecting an isolator is to determine the load (weight) of the component needing isolation. This value is divided by the number of isolators to be used and will determine the load/isolator. The calculation will help determine the best geometry and Durometer combination for isolating a component. Different Durometers allow for different spring rates. Load ratings assume a 10 to 20% static deflection of the material. Deflection can come from component weight alone or a combination of component weight and innovative clamping methods.

For example, multiaxis isolators (corner pads) moderately shock-mount hard-disk drives or LCDs in portable electronics. The Sorbothane is encapsulated between two brass housings and attenuates shock and vibration in all directions. Load ratings range from 0.5 to 1 lb at 30 Shore 00 to 0.75 to 2 lb for 50 Shore 00, and 1.5 to 4 lb at 70 Shore 00.

Another option may be to use two bushings together or a bushing and a washer(s) to create a floating bolt. This technique isolates components from any metal-to-metal contact. Isolation pads with and without a pressure-sensitive adhesive backing are yet another option for use in tight quarters. They come in a range of thicknesses and Durometers to meet nearly infinite load and frequency conditions.

The next step is determining the lowest critical or excitation frequency of the system. Motor rpm, cylinder stroke rate, and the output speed of a speed reducer all produce excitation frequencies. Divide minute-based frequency information, such as rpm by 60 to convert to Hertz (cycles/sec).

Next, select an appropriate isolator by using information charts supplied with each product. Generally, the appropriate isolator will create a system natural frequency at least one third lower than the excitation frequency. For example, if the source of vibration is a 1,800-rpm motor, then the excitation frequency is 30 Hz (1,800/60). The resultant, damped system natural frequency therefore must be 20 Hz or less. Do not oversize isolators. Oversized isolators raise the natural frequency and reduce the effectiveness.

Although it's not rocket science, elastomers like Sorbothane are designed for use in spaced-based and countless other applications. Basic laws of physics allow designers to determine the desired parameters. Published data (or Sorbothane's online designer guide) will help optimize the design.

MAKE CONTACT
Sorbothane Inc.,
(330) 678-9444,
www.sorbothane.com