Conclusiones

A KEY is a detail inserted axially between a shaft and a hub to prevent relative rotation of the parts. The hub may slide along the shaft whilst rotating with it, as with the gears in a car gearbox, and in these cases a feather key is used. Keys &re made from steel to resist adequately the high shearing and crushing stresses placed on them.

BS 4235: Part I sets out the forms and dimensions for metric keys which are square and rectangular in cross-section with parallel or tapered faces at the top and bottom. Figure I opposite shows square and rectangular parallel keys. Notice that the key is sunk in the shaft for half its thickness and that it is fitted to the sides of the keyways with top clearance. Three classes of fit are provided for these keys: free, where the hub is required to slide over the key when in use, that is, where the key is used as a feather; normal, where the key is to be in-serted in the keyway with the minimum fitting, as in mass production assembly; close, where an accurate fit of key is required. Here fitting will be necessary under maximum material conditions, that is, when the keyway width is a minimum and the key width is a maximum.

Keys are not normally supplied with the ends radiused as in Forms A and C, or with chamfers. The chamfers are necessary to prevent the corners of the key fouling the radii in the bottom of the keyway. Sharp corners in the keyway would act as stress raisers with a consequent risk of fracture at these points.

Form A keys are used in keyways machined with an end mill, the keyway being situated part way along the shaft. The semicircular ends of the key fit the corresponding ends of the keyway produced by this type of cutter. Form C keys are used in similar keyways cut at the end of a shaft, which have only one semicircular end. Form B keys for are use in keyways cut with a slotting saw.

Parallel keys are used to transmit unidirectional torques where heavy starting loads are not involved, and where periodic withdrawal or sliding of the hub may be required. When a gib head, see below, cannot be accommodated and there is insufficient space to drift out the key from behind, the hub must be withdrawn over the key and a parallel key is essential. Reference to the table on page 223 will show that square keys are used for shafts up to 22 mm diameter. Over this size rectangular

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keys are used.

Square and rectangular taper keys are illustrated in Figure 2, and have a taper of 1: 100 on the top face. They are made as plain keys in Forms A and B and with a gib head in Form C. The gib head makes withdrawal of the key possible by driving a tapered drift between the back of the head and the hub. Taper keys fit at the top and bottom, with side clearance. They are used to transmit heavy unidirectional, reversing and vibrating torques, and where periodic withdrawal of the key may be necessary. They cannot be used as feathers. As with parallel keys, square taper keys are used for shafts up to 22 mm diameter, with rectangular keys being used over this size.

The relations between shaft diameter and key section given in the table on page 223 are for general applications. Smaller key sections may be used if suitable for the torque to be transmitted. The use of larger key sections is not permitted.

A Woodruff key, shown in Figure 3, is in the form of a segment of a circle and fits in a corresponding recess in the shaft. It adjusts itself to any taper on the hub keyway and is widely used on machine tools. It cannot be used as a feather because it may jam, and it has the dis-advantage that the deep keyway weakens the shaft.

Figure 4 shows two types ofround key. The plain type is a length of circular bar which is a driving fit in a hole drilled half in the shaft and half in the hub. Round keys may also be threaded and screwed into position, in which case they usually have a square head for fitting which is machined off when the key is in place.

Saddle keys are of two types, flat and hollow, and are illustrated in Figure 5. They are suitable only for light torques, and the hollow saddle key is used solely for temporary fastenings.

Splines, shown in Figure 6, are projections of rectangular cross-section machined on a shaft. In effect they are several keys integral with the shaft, and fit corresponding recesses in the hub. They are equally spaced round the shaft and vary in number from four upwards. If the angular relationship between shaft and hub is important, one spline may be left uncut on the shaft and one removed from the hub. The shaft and hub will then assemble in one position only. Serrations are similar to splines but are triangular in cross-section instead of rect-angular. Since they are smaller than splines the angular relationship between shaft and hub may be closely controlled. For this reason they are often used to secure levers to shafts, when the lever position is to be altered for adjustment purposes. Splined and serrated shafts are used extensively in the car and aeroplane industries.

The recommended ways of dimensioning keyways are shown in Figure 7. Note that the drawings are not completely dimensioned.

Only those dimensions are given for which alternatives might be chosen.

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Cottered joints

These are used to join rods which carry axial loads only and Figure I on page 136shows a typical arrangement. One rod end fits in a socket in the other and both rods are slotted to take the cotter. This is a flat piece of material with a taper on one side. The slots are a little longer than the width of the cotter, and their positions are such that driving the cotter in pulls one rod into the socket in the other. This is shown by the arrows on the elevation. The clearances on each side of the cotter are important and should be noted. Figure I shows the rod end and socket tapered, but they may be cylindrical. Also the cotter can have square instead of round ends. The ends of the cotter which take the hammer blows are chamfered to reduce spreading and cracking. Cottered joints are easily assembled and dismantled and the details always take up the same positions when the joint is remade.

Other applications of cottered joints are shown in Figures 2, 3 and 4.

Figure 2 shows a cotter pin securing a leverto a spindle. This method is used to secure a bicycle crank to the chain wheel spindle. A rod secured to a plate by a cotter is illustrated in Figure 3. Figure 4 shows a strapped joint secured by a gib and cotter. The gib prevents the lower end of the strap spreading as the cotter is driven in, and allows a parallel hole to be used in the rod end and strap.

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