Machinery's Handbook, 31st Edition
Keys and Keyseats 2561 Chamfered Keys and Filleted Keyseats.— In general practice, chamfered keys and fil leted keyseats are not used. However, it is recognized that fillets in keyseats decrease stress concentration at corners. When used, fillet radii should be as large as possible with - out causing excessive bearing stresses due to reduced contact area between the key and its mating parts. Keys must be chamfered or rounded to clear fillet radii. Values in Table 5 assume general conditions and should be used only as a guide when critical stresses are encountered. Depths for Milling Keyseats.— Table 11 on page 2562 has been compiled to facilitate the accurate milling of keyseats. This table gives the distance M (see illustration accompany ing table) between the top of the shaft and a line passing through the upper corners or edges of the keyseat. Dimension M is calculated by the formula: M S S E 2 1 2 2 = − − ^ h where S is diameter of shaft, and E is width of keyseat. A simple approximate formula that gives M to within 0.001 inch is M = E 2 ÷ 4 S . Keyseating Machines.— The machines which are designed especially for cutting keyse ats or keyways in the hubs of pulleys, gears, etc., are generally known as keyseaters. Ma- chines of this class usually have a base or frame which contains the mechanism for impart- ing a reciprocating motion to a cutter bar, which moves vertically for cutting a keyseat in the work. There are several types of machines which are used for internal keyseating operations in addition to the machines designed especially for this work. Broaching ma- chines as well as slotters are commonly used, and keyseating is also done to some extent in shapers and planers. Other Key Types.— The sunk key is the most common type and is of rectangular section that engages a groove or slot formed both in the shaft and hub of the gear or pulley. The width of an ordinary sunk key ordinarily is equal to about one-fourth of the shaft diameter and the thickness, when a flat key is preferred to the square form, is usually about one-sixth of the shaft diameter; these proportions are varied somewhat by different manufacturers. The flat key is a rectangular shape which bears upon a flat surface formed on one side of the shaft. The draw or gib key is a sunk key which has a head by means of which it can be removed. The round tapered key is simply a taper pin which is driven into a hole that is partly in the shaft and partly in the hub; this form is used for light work. The name feather or spline is applied to a key which is fixed to either a shaft or hub, as when a gear must be driven by a shaft, but at the same time be free to slide in a lengthwise direction. The taper of American Standard square and flat keys is 1.8 inch per foot. The saddle key does not enter a slot in the shaft. It has parallel sides and is curved on its under side to fit the shaft. It is slightly tapered on top so that, when it is driven tightly in place, the shaft is held by frictional resistance. This key should be fitted so that it bears lightly on the sides and heavily between the shaft and hub throughout its entire length. As the drive with this type of key is not positive, it is only used where there is little power to transmit. It is an inexpensive method of keying, as the shaft does is not machined. Effect of Keyways on Shaft Strength.— See SHAFTS starting on page 295 and Effect of Keyways on Shaft Strength starting on page 301. British Standard Keys and Keyways.— See Keys and Keyways in the ADDITIONAL ma- terial in the Machinery’s Handbook 31 Digital Edition . Cotters.— A cotter is a form of key that is used to connect rods, etc., that are subjected either to tension or compression or both, the cotter being subjected to shearing stresses at two transverse cross sections. When taper cotters are used for drawing and holding parts together, if the cotter is held in place by the friction between the bearing surfaces, the taper should not be too great. Ordinarily a taper varying from 1 ⁄ 4 to 1 ⁄ 2 inch per foot is used for plain cotters. When a set screw or other device is used to prevent the cotter from backing out of its slot, the taper may vary from 1 1 ⁄ 2 to 2 inches per foot.
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