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A Coupling is a device that makes relations with two shafts end to end, while a clutch is a coupling provided with some form of sliding or other arrangement whereby the shafts can be connected and disconnected at will. Couplings generally differentiate into (a) rigid couplings and (b) flexible couplings. The rigid type is used where accurate lineal alignment of the shafts is ensured. Where accurate alignment is not possible, a flexible coupling is used; it allows for a certain amount of misalignment, besides acting as a shock absorber for vibrations and jerks in the torque transmission.

The flanged coupling (Fig.1) is one of the simplest types, contains two halves, each holding a flange mounted on the end of a shaft. The boss of each flange is keyed to its shaft, and the flanges are bolted together, thus connecting the two shafts. The split-type muff coupling (Fig.2) is easier to install and remove because the two halves can be fitted around the aligned shaft ends and clamped by bolting. The muff is keyed to the shafts. A more elaborate form of construction is the serrated coupling (Fig.3), comprising contact surfaces with interlocking teeth that are held meshed together by bolts.

The jaw coupling (Fig.4) differentiates with two flanged bosses with jaws projecting from their inner faces. This coupling allows longitudinal movement of the two shafts in relation to each other, so that it can compensate for thermal expansion or inaccuracies in assembly. If one of the halves is so mounted on its shaft that it can be slid into or out of engagement with the other half, the jaw coupling can serve as a clutch.

The floating-center coupling (Fig.5) basically used in a case where the two shafts are not accurately aligned and have a slight parallel shift in relation to each other. This coupling comprises two flanged halves and a center piece with lugs which engage with slots in the flanges. The lugs are set at right angles to each other and have a sliding fit in the slots, so that compensation for slight axial movements is provided.

The toothed coupling explained in Fig. 6 allows a certain amount of parallel, angular and axial displacement between the two shafts. The two coupling bosses fitted on the shaft ends are provided with teeth which mesh with internal teeth in a coupling sleeve. The teeth have a crowned (convex) shape, thereby permitting some movement in all directions, including angular movement.

A universal coupling is applied for the connection of two shafts that are set at an angle to each other and whose angle can be varied while the shafts are rotating. An arrangement whereby two shafts are interconnected by an intermediate shaft with a universal coupling at each end is referred to as a universal shaft (Fig.1). This principle is in use, for example, in propeller shafts of motor vehicles.

The universal coupling may take a more elaborate form, permitting greater amounts of angular movement, as in Fig.2, where each half of the coupling comprises two swivel pins which so engage with appropriate sockets in a ring that the pins of one half are set at 90 degrees in relation to those of the other half. Essentially the same principle is applied in the ball joint (Fig.3): the ball is provided with four holes which engage with two pins on each half of the coupling.

Consider two shafts as examples, each interconnected by a universal coupling, which are set at an angle ß in relation to each other. If the driving shaft rotates at a uniform speed, the driven shaft will undergo speed fluctuations; i.e., it will be alternately accelerated and retarded according to a sinusoidal pattern (Fig.4). These fluctuations will be accordingly greater as the angle ß is larger.

Such fluctuations can be eliminated by the interposition of an intermediate shaft and two universal couplings. If the two angles ? (Fig.5) are equal, so that the driving and the driven shaft are parallel to each other, these fluctuations will be canceled out. If the angles a and ß (Fig.6) are unequal, speed fluctuations will be transmitted to the driven shaft. In Fig.6 the two universal couplings are moreover incorrectly mounted in that their respective pivot pins are not parallel to each other, as they ought to be to ensure uniform transmission of rotational motion from the driving to the driven shaft.

In a flexible coupling the connection between the two halves is formed by a yielding intermediate element which may consist of rubber, leather, steel springs, or some other flexible material. This element allows small amounts of parallel and/or angular movement of the shafts in relation to each other, besides absorbing impact (shock) due to irregularities in the motion of the driving shaft. Shock absorption may be achieved by storage of energy or by conversion of energy or both. Thus the coil-spring coupling (Fig.1) stores the impact energy in its coil springs when one flange of the coupling undergoes rotation in relation to the other in consequence of a sudden variation in speed or torque.

When the springs successfully return to their original length, they transmit the temporarily stored-up impact energy to the driven shaft. Every resilient mechanism forms an oscillating system whose natural frequency of oscillation will depend on the spring characteristic and the oscillating masses. The extension or the shortening of the springs in the clutch, and thus the angle of relative rotation of the two clutch halves, is proportional to the magnitude of the torque applied (Fig 3a). This oscillating system has a particular natural frequency, and if the coupling is rotated with an impact frequency corresponding to this natural frequency, the phenomenon known as resonance will occur, causing objectionable oscillations of large amplitude.

As against a linear characteristic of the type represented in Fig.3a, the steel-band coupling (Fig.2) has a progressively curved characteristic (Fig.3b): in this the angle of relative rotation is not proportional to the torque to be transmitted by the coupling. When the effective lever arm of the steel bands changes, the natural frequency of the couplings also changes, so no resonance can occur.

The area located under the characteristic line or curve in Fig.3 (abc) represents a certain amount of energy, namely, the impact energy which is absorbed by the yielding system of the flexible coupling and subsequently given off by it. If the coupling, in addition to presenting a curved characteristic, also develops a so-called damping action (Fig.3c), its recovery characteristic (the lower of the two curves) will differ from that of the characteristic for initial deformation.

In general, the energy given off is less than the energy absorbed by the yielding system. The difference between these two energy amounts corresponds to the area between the curves; this lost energy may, for example, be converted into heat by internal friction in the coupling. The flexible coupling illustrated in Fig.4 has a characteristic of this type because of the rubber bushings which enclose the bolts in one of the two halves. A similar effect is achieved by the arrangement shown in Fig.5, where the two halves of the coupling are interconnected by a rubber tire which provides flexibility and shock-absorbing capacity.

In the disc-type flexible coupling (Fig.6), each of the two shafts is provided with a boss (called a spider) having three radial arms set at 120 degrees in relation to one another. Between the two spiders is a flexible disc made of rubber and canvas bonded together. This disc has six equally spaced holes for bolting. The spider arms on each shaft are bolted to the disc, but at different positions from those on the other shaft. Thus give or yield of the disc will occur when power is transmitted.