Mechanical Guru's

STRAIN GAUGE Many devices have been made for measuring deformation (or strain). Strictly speaking, since gauge lengths are always finite, we measure deformations. While we frequently calibrate our output signal to give average strain over the gauge length, we are in reality sensing finite deformation over a finite gauge length. Deformation measuring devices include innumerable devices employing mechanical, optical, electrical resistance, electrical inductance, electrical capacitance and piezoelectric principles. The large variety of devices which have been developed to measure deformations (or strain) is much too extensive to even mention here. We shall consider in some detail the electrical resistance strain gauge. Fig - Strain Gauge Electrical Resistance Strain Gauges The use of mechanical extensometers (either dial gauges or LVDT type) gives us a reusable measuring device for determining deformation over fairly long gauge lengths (typically 2" or 8"). A

CAVITATION Cavitation   is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of  pressure   that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shockwave. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause  cyclic stress  through repeated implosion. This result in surface fatigue of the metal causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. I

COUPLINGS In engineering applications there arise several cases where two shafts have to be connected so that power from driving shaft is transmitted to driven shaft without any change of speed. Such shafts are normally coaxial with slight or no misalignment and can be connected through devices known as couplings. Permanent couplings, often referred to as couplings, are the connectors of coaxial shafts and cannot be disengaged when shafts are running. On the other hand, those couplings which can be readily engaged or disengaged when driving shaft is running are termed as clutches. The power is transmitted when a clutch is engaged and not transmitted when clutch is disengaged. In this unit only permanent couplings will be considered. Figure 6.10 shows one such coupling connecting the shaft of an electric motor with the shaft of a worm and worm wheel reducer. Several types of couplings are used in practice. A few are described here.  Muff or sleeve coupling is shown in Fig