Overstress

Low Density Gases. A fan may have to operate on low density gas because of temperature, altitude, gas composition (high water vapor content of the gas can be a cause of low density), reduced process pressure, or a combination of such causes. To develop a required pressure, the fan has to operate at a considerably higher speed than it would at atmospheric pressure, and hence it must operate much closer to top wheel speed. Bearing life is shorter, and the fan tends to vibrate more or can be overstressed more easily by a slight wheel unbalance. Abrasion of the blades from dust particles is more severe. Therefore, a sturdier fan is needed for low density gas service. 

R. Walker, Electrical Overstress j Electrostatic Discharge Symposium, EOS /FSD Association, Rome, N. Y., 1983. 

External-pressure failure of shells can result from overstress at one extreme or n om elastic instability at the other or at some intermediate loading. The code provides the solution for most shells by using a number of charts. One chart is used for cylinders where the shell diameter-to-thickness ratio and the length-to-diameter ratio are the variables. The rest of the charts depic t curves relating the geometry of cyhnders and spheres to allowable stress by cui ves which are determined from the modulus of elasticity, tangent modulus, and yield strength at temperatures for various materials or classes of materials. The text of this subsection explains how the allowable stress is determined from the charts for cylinders, spheres, and hemispherical, ellipsoidal, torispherical, and conical heads. 

All loads that may overstress the generator windings fall in this category and call for special consideration. The manufacturer must be consulted for the right choice of machine. 

Besides the three basic parameters noted above, the incoming machine must also be running a little too fast compared to the existing source, i.e. /, >/j or />, at the instant of synchronization. If it is not. it will further stress an already overstressed source. When the incoming... 

Series capacitors have also proved to be an easy way of relieving an already overstressed distribution netw ork to meet ever-growing load demands, particularly when it is not practicable to add another line for reasons of cost or space. 

Through careful management of the reactive pow er. making use of shunt and series capacitors and reactors, we can provide support to an overstressed LT or FIT stipply system, and achieve optimum utilization and a higher level of stability. 

Graham, R.A., Overstress Effects in Shock-Induced Electrical Signals from Ferroelectric Cermaics Implications for Impact Fuzing, Sandia National Laboratories Internal Memorandum No. 1131/82/2, Albuquerque, NM, 6 pp., December 1982. 

Moke end plate thicker (increase t in eqn. 28.10). Plate would have to be 70 mm thick heavy difficult to weld to collar might overstress plate-shell weld collar would become irrelevant. 

Effects of Water Hammer. Water hammer has a tremendous and dangerous force that can collapse floats and thermostatic elements, overstress gauges, bend mechanisms, crack trap bodies, rupture fittings and heat exchange equipment, and even expand piping. Over a period of time, this repeated stress on the pipe will weaken it to the point of rupture. 

This test is better left to the vendor for interpretation unless the user representative is quite experienced in the design ot centrifugal impellers, The vendor can be asked to identify the critical areas prior to the test if there is time to check his analysis. Actually, the liability to the vendor is much too high for him to risk an overstress failure just to pass a test. Unfortunately, failures come in the unexpected situations, where the hedge is hind sight. ... 

Overstressed piston rods due to unbalanced or dynamic forces. 

Accuracy and reproducibility are of vital importance to industry. Quite apart from the effect of these factors on the final product, several plant items are frequently links in a continuous chain of production processes. A sizing error in one machine, for example, could overstress and damage the succeeding machinery. Similarly, an error in a press may increase stress on the tool and could necessitate an additional operation to remove excessive flash . Wear in a material preparation unit could allow oversize material to be passed to a molding machine, creating an overload situation with consequent damage. 

Poor assembly can also limit the life of a gear. If a pinion is clumsily pressed onto a shaft, for example, it is likely to operate in an overstressed condition and fail at an early stage, even through running well within its designated rating. 

Cleanliness and good housekeeping in machine shops do much to avoid bacterial infection, and their importance cannot be overstressed. Various techniques such as heat treatment, centrifuging and filtration can be used to advantage, although economic considerations may restrict their use to systems containing large volumes of soluble oil. Chemical sterilization with bactericides can be more convenient. 

The requirement for safety precautions when operating or working on all types of machinery cannot be overstressed, in that the likelihood of an accident occurring on moving machinery is invariably high. It is essential that all these who work on such machines are adequately trained as to the operation and the safeguards fitted. Regulations that must be complied with include ... 

Thus a strong bond is not always desirable. We can see this from Table 7 and 8. The authors of [100] interpreted their experimental data as follows the rigidity of specimens increases with increasing PVC-filler interaction as a result the rate of relaxation of stresses arising at interphases in the course of deformation decreases. The overstressed states at the interphases may, in the authors opinion, promote separation of the polymer from the filler surface. That is, it is more desirable that the matrix-filler bond is not rigid but labile. 

On the basis of the above data it has been hypothesized that the conductivity of PFCM is due not to the contact between the filler particles but the current passes across the thin (less than 1 -2 microns) polymer interlayers. The conductivity arises when a spontaneous pressure exceeding the threshold value develops in the material. The overstresses apparently arise as a result of PP crystallization in the very narrow gaps between the filler particles [312], Since crystallization must strongly affect the macromolecular conformation whereas the narrowness of the gap and fixed position of molecules on the filler prevent it, the heat released in the process of crystallization must, in part, be spent to overcome this hindrance, whereby a local high pressure may arise in the gap. This effect is possible only where there are gaps of the size comparable with that of macromolecules. The small gap thickness will also hamper pressure relaxation, since the rate of flow from such a narrow clearance should be negligibly small. 

The most common conditions of possible failure are elastic deflection, inelastic deformation, and fracture. During elastic deflection a product fails because the loads applied produce too large a deflection. In deformation, if it is too great it may cause other parts of an assembly to become misaligned or overstressed. Dynamic deflection can produce unacceptable vibration and noise. When a stable structure is required, the amount of deflection can set the limit for buckling loads or fractures. 

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