Vibration analysis for electronic equipment pdf


 

VIBRATION ANALYSIS FOR ELECTRONIC EQUIPMENT - Ebook download as PDF File .pdf), Text File .txt) or read book online. VIBRATION ANALYSIS FOR. 2 Vibration of Electronic Equipment in an Airplane .. 13 .. Analysis of the electronic units will be presented and %medical-site.info Dave S. Steinberg's Vibration Analysis for Electronic Equipment is a widely used reference in the aerospace and automotive industries. Steinberg's text gives . equivalent probability density function (PDF). Rainflow Cycle.

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Vibration Analysis For Electronic Equipment Pdf

Electronic components in vehicles are subjected to shock and vibration Dave S . Steinberg's Vibration Analysis for Electronic Equipment is a widely used. Structural vibration analysis of PCBs and their electronic components can be performed using either FEM or a vibration test. In this study, the natural frequency . VIBRATION ANALYSIS. FOR ELECTRONIC. EQUIPMENT (3RD ED.) By D.S. Steinberg, John Wiley and Sons,. Inc., This book is a very practical book con.

These torque values for steel screws are given in Table 1. The screw head should permit a positive nonslipping grip for the driving device and should also withstand the driving forces. A slottedhex-head machine screw seems to be, for general purposes, the most satisfactory. In all applications involving through-holes, locknuts should be used instead of lockwashers. Most of the standard steel locknuts are satisfactory. Blind-tapped holes should be avoided, if possible. When they are necessary, locking devices such as lockwashers should be used under the screw head in order to prevent the screw from backing out during vibration. The tightening torque should be increased by an amount equal to the resisting frictional torque of the locking device. The fasteners for a unit should be distributed so that the failure of one fastener will not free the unit or cause it to malfunction. Even for very small components there should be a minimum of two fasteners. Lockwashers and screw-locking inserts should be used with care in electronic systems that will be used in the zero-G environment of outer space. These two devices create a binding friction in the screw by biting into the metal during installation. This action will very often shave metal particles from the screw. If these particles are not removed, they can float around in a zero-G environment and create electrical problems.

To put things in perspective we will consider what random vibration is and then follow the step-by-step approach outlined below to solve this problem. For the purpose of clarity, a simplified explanation is being used. Grms OUT 5 Approximate deflection based on the above Understanding Random Vibration Random vibration is specified for acceptance tests, screening tests and qualification tests as it more closely represents the true environment in which electronic equipment must operate.

Random vibration is nonperiodic with all of the frequencies within a given bandwidth present all the time, and at any instance of time. This means that over a frequency bandwidth of 20 to Hz, all of the structural resonances within the same bandwidth, at the box, board and component levels, will all be excited at the same time.

This helps to identify failures that can not be duplicated in a sinusoidal environment. The probability of a given instantaneous acceleration follows the Gaussian distribution curve.

Typical white noise curves are shown in Figure 1. Figure 1a. White noise input curve with constant input PSD. Figure 1b. White noise input curve with positive and negative slopes.

Epoxy glass is the most common material with laminated layers of copper. The manner in which PCBs are supported determine their dynamic response. Three scenarios are possible: 1 free edge; 2 supported edge; and 3 fixed edge.

ISBN 13: 9780471633013

The natural frequency of a PWB can be approximated as a plate problem. Figure 2.

This air must be conditioned before it can be used for cooling, because the air temperature from the first stage is usually greater than F. Liquid-cooled cold plates are usually used to cool electronic equipment on spacecraft or very-high-flying research airplanes.

The cooling liquid can be a mixture of ethylene glycol and water, or some other liquids such as FC, a fluorocarbon or Coolanol 45, a silicone fluid. The liquid-cooled cold plates are usually made part of the spacecraft airframe structure instead of the electronic box structure. Then, when the electronic box must be removed from the spacecraft, it is not necessary to disconnect fluid lines, which can become quite messy.

Since the cold plate stays with the airframe, the heat dissipated by the electronic box is usually transferred to the cold plate through a flat interface on the mounting surface of the electronic box that makes intimate contact with the cold plate.

Vibration Analysis For Electronic Equipment

The trend in commercial and military electronics is toward the line replaceable unit LRU , with which it is possible to replace a defective electronic box right on the flight line in a matter of minutes. This is accomplished by providing all of the required interface connections, both mechanical and electrical, at the back end of the electronic box. The box becomes similar to a printed circuit board PCB that can be plugged into its receptacle. If there are several large electrical connectors on the back end of the electronic box, it may be quite difficult to insert the box and engage the electrical connectors properly.

Some connectors may require a force of 0. When there are 8 connectors, each with pins, there is a total of pins, which will require a lb force to engage them. The plug-in electronic box is usually engaged and locked into position by some mechanism at the front of the box. Since the connectors are at the rear of the box, this means the force required to engage the connectors must pass through the box.

When this type of electronic box is subjected to vibration, in many instances the vibration loads must be added to the installation loads to determine the total load acting on the structure. There has been an attempt to standardize electronic equipment used in military and commercial airplanes by establishing certain sizes for modular electronic units. These modular units are then mounted in a standard air transport rack ATR , which provides rear-located dowel pins and connectors and a quick-release fastener at the front.

The electronic components are usually mounted on panels and in sliding drawers. Panels are generally used to support dials, gages, manual controls, and test points. Only small masses are mounted on panels, because they are fastened to a frame or rack in the cabinet and they cannot withstand large dynamic loads.

Drawers are often used to support the more massive electronic components such as those normally used in power supplies. The drawers are mounted on telescoping slides to provide access to the equipment.

For safety, the drawers usually lock in the open and closed positions.

Vibration Analysis For Electronic Equipment | Electronics Cooling

For convenience, the drawers may also tilt to improve access in tight spaces. The vibration frequency spectrum for ships and submarines varies from about 1 to about 50 Hz, but the most common range is from about 12 to about 33 Hz. The maximum acceleration level in this range is about 1 G and appears to be due to vibrations set up by the engines and propellers. In military ships, shock is an important factor, due generally to various explosions, which can do extensive damage to electronic equipment, unless proper consideration is given to the design and installation.

For example, it is not desirable to have a very rigid structure supporting the electronics, because a very rigid structure may not deform enough to absorb much stain energy. Theoretically, any structure that does not deform when it is subjected to an impact load will receive an infinite acceleration.

A large displacement is desirable, since it can substantially reduce acceleration levels. Either this displacement must be confined to the structure or shock mounts must be used. In either case, provisions must be made in the design and installation to make sure parts will not collide and equipment will not break loose [36]. If shock isolators are used, they should be designed to deflect enough to absorb the shock energy without transmitting excessive loads to the electronic equipment.

Vibration Analysis For Electronic Equipment

The shock mounts should have a minimum resonant frequency of about 25 Hz [2, Ideally, the resonant frequency of the electronic components should be at least twice that of the shock mounts, but never below 60 Hz. If the electronic components have resonances substantially below 60 Hz, it might bring them into the range of the most common vibration forcing frequencies, which, as previously mentioned, are as high as 33 Hz.

If this should happen, the electronic components would be driven continuously near their resonance and could suffer fatigue failures. When the forcing frequency of the ships structure is near its higher frequency limit around 25 Hz , the resonant frequency of the electronic equipment cabinet will be excited, since the shock isolators also have their resonance at 25 Hz.

The cabinet support structure will, however, have to withstand the dynamic vibration loads. These loads will be determined by the amplification characteristics of the shock isolators during vibration.

Shock isolators are available that provide a vibration amplification of about 3 for the conditions described above.

Since the general vibration acceleration input level in these frequency ranges is normally quite low, an amplification factor of 3 for the isolators does not result in high stresses in the equipment cabinet. It is generally not desirable to increase the resonant frequency of the electronic equipment as high as possible. If the equipment is very stiff, it may be good for the vibration condition but poor for the shock condition. A very high spring rate may result in very high shock stresses because of the high acceleration loads.

Lee [5] recommends a maximum resonant frequency of about Hz and a maximum acceleration of G on the electronic components. On tall narrow cabinets the load-carrying isolators should be at the base and stabilizing isolators should be at the top. A rigid structure must be used to support the stabilizing isolators at the top. If there is excessive deflection in the top support structure, it can change the characteristics of the entire system because of severe rocking modes.

If shock isolators are not used, the shock energy must be absorbed by deflections in the electronic equipment cabinet and in the structure of the ship supporting the cabinet. In this case the natural frequency of the assembly, which consists of the cabinet and the ships structure, should be about 60 Hz.

The natural frequency of the electronic components mounted on the equipment cabinet should be twice that of the assembly, or about Hz. The ships structure must provide a good part of the deflection required to attenuate the shock force, or the dynamic stresses in the equipment cabinet may be high enough to cause structural failures.

When the vibration forcing frequency in the ships structure is near its normal maximum limit of 33 Hz, the dynamic loads in the equipment cabinet will not be amplified to any great extent since the resonant frequency of the cabinet, at 60 Hz, is almost twice the forcing frequency.

Furthermore, the electronic components mounted in the equipment cabinet have a still higher resonant frequency Hz , so their dynamic vibration loads should be relatively small. The materials that are best suited for shock are ductile materials with a high yield point, a high ultimate strength, and a high percentage elongation. In general, metals that are mechanically formed are more desirable than cast metals, which have a relatively low percentage elongation.

Since acceleration forces become progressively smaller as they propagate into the interior of the equipment cabinet, the electronic components that can withstand the highest G forces should be mounted near the exterior of the cabinet. Electronic components that cannot withstand high G forces should be mounted at the maximum elastic distance from the application points of the shock load.

This will usually be at the center of the cabinet. The load path for each mass element in the system should be examined closely to determine the path the load will take as it passes through the structure. For example, a large transformer should be mounted close to a major structural support in order to reduce the length of the load path.

This will result in smaller deflections and stresses. Electronics are being used in their antilocking brake systems, fuel-air mixture control, radios, ignition systems, air conditioners, heaters, automatic transmission shifting points, rear view mirrors, door locks, instruments, global positioning systems GPSs , windows, sun roofs, cruise control, air bags, television, telephones, and many other features, with more being added every year. Anticollision systems are being developed that will automatically engage brakes when vehicles are dangerously close to each other at high speeds.

There are plans for computerized car trains that will link several automobiles together going to similar areas. Antitheft systems such as Lojack are available that can be activated by radio to send out a silent alarm to police when an automobile is stolen.

Automobile and truck electronics must operate in harsh environments that include wide temperature swings, high under-hood operating temperatures of "C, high humidity with condensation, high-velocity splashing water, and subfreezing temperatures with only moderately high vibration and shock levels for the automobiles.

Trucks are often required to travel over unpaved country roads for deliveries in outlying areas. Test data on a 2. At truck speeds above about 35 mph over rough roads the frequency appears to be random in nature. Diesel electric trains have been making use of electronics on their drive wheels to sense when the wheels are just reaching the slipping point on the tracks.

A large engine may have six pairs of driving wheels. The electronics can sense when any one pair of drive wheels will slip. A single diesel electric engine with this new feature can go up steeper grades and pull three times as many cars as previous diesel electric trains. Deep drilling at 30, feet can encounter temperatures as high as C. The electronics are usually located in a 6-foot long tube section just above the cutters.