when the temperature of a ferromagnetic or ferrimagnetic material is increased, the added thermal energy increases the mobility of the domains, making it easier for them to become aligned, but also preventing them from remaining aligned when the field iis removed. Consequently, saturation magnetization, remanance and the coercive field are all reduced at high temperatures. If the temperature exceeds the Curie Temperature (Tc) ferromagnetic or ferrimagnetic behavior is no longer observed. Instead, the material behaves as a paramagnetic material. The Curie Temperature which depends on the material, can be changed by alloying elements. The dipoles can still be aligned in a magnetic field above the Curie temperature, but they become randomly aligned when the field is removed.
(Askeland D.R., Phulé P.P., The Science and Engineering of Materials, pg.737, Kayra Ermutlu)
Percussion Welding
The process appears in several forms but in every variation a short-time high-intensity arc is formed by the sudden release of energy stored egnerally, but not invariably, in capacitors. Subsequently very rapid or "percussive" impacting of the workpiece to form the weld is required. The difference should be noted between this process and the capacitor discharge resistance welding method in which a capacitor is dischared into the primary of a transformer and heat is generated by resistance and not an arc.
The main point of difference between the variations of percussion welding is in the method of initiating the arc. Three methods are used: (1) low-voltage with drawn arc; (2) high-voltage breakdown; (3) ionization by a fusing tip. With each method the energy source can be a bank of capacitor, possibly as part of delay line, which is charged by a variable voltagetransformer/rectifier unit. Both charging voltage and capacity are variable so that the energy available and the shape or the current-time curve can be adjusted.
(Houldcroft P.T., Welding Processes, pg.95, Kayra Ermutlu)
Upset Welding
A resistance welding process in which the weld is produced, simultaneously over the entire area of abutting surfaces or progressively along a joint, by applying mechanical force (pressure) to the joint, then causing electrical current to flow acress the joint to heat the abutting surfaces. Pressure is maintained throughout the heating period.
(American Society for Metals, ASM Metals Referance Book, pg.94, Kayra Ermutlu)
Oldham Coupling
It is used to connect two parallel shafts whose axes are at a small distance apart. Two flanges, each having a rectangular slot, are keyed, one on each shaft. The two flanges are positioned such that, the slot in one is at right angle to the slot in the other. To make the coupling, a circular disc with two rectangular projections on either side and at right angle to each other, is placed between the two flanges. During motion, the central disc, while turning, slides in the slots of the flanges. Power transmission takes place between the shafts, because of the positive connection between the flanges and the central disc.
(Narayana K.L., Machine Drawing, pg.124, Kayra Ermutlu)
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