Shaft Couplings. A box, or muff coupling, is shown in the figure. It consists of a cast iron cylinder which fits over the ends of the shaft. The two ends are prevented from moving relatively to each other by the sunk key. The key-way is cut half into the box and half into the shaft ends. Quite commonly the ends of the shafts are enlarged to allow the keyway to be cut without weakening the shaft. The key may be proportioned by the formula already given. For the other dimensions take 1 = 2 d2". EXAMPLE.-Find the dimensions of a muff coupling for a shaft 2" in diameter. SOLUTION. -For the key we use the formula previously given, b = d = 12 d +2 = 2 × 2+2 = 81. t = .4d+.5" = .4 x 2.5" = 11". A flange coupling is shown in the following figure. Cast iron flanges are keyed to the ends of the shafts. To insure a perfect joint the flange is usually faced in the lathe after being keyed to the shaft. The two flanges are then brought face to face and bolted together. Sometimes the ends of the shafts are enlarged to allow for the key-way. To prevent the possibility of the shafts getting out of line, the end of one may enter the flange of the other. The following proportions may be used for this form of flange coupling : d = diameter of shaft. The proportions for the key have already been given. The names pedestal, pillow block, bearing and journal box are used indiscriminately. They are all a form of bearing, and indicate a support for a rotating piece. A form of journal box frequently used for small shafts is shown in Fig. 1. It consists of two parts: (1) the box which supports the journal, and (2) the cap which is screwed down to the box. In this journal box the seats are of babbitt, or as it is commonly expressed, the box is babbitted. The cap is held in place by what are called cap screws. This is invariably done in small pedestals. The proportioning of a pedestal is largely a matter of experi ence. Few or none of the parts are calculated for strength. All the proportions of the pedestals which follow are based on the diameter of the journal d as the unit; the length of the seats is the same as that of the journal. For the journal box shown in Fig. 1, the following proportions may be used for sizes of journals from 2" to 2" diameter inclusive. The diameter of shaft d = the unit. In Fig. 2 is shown a common form of pedestal, which is used for somewhat larger journals than the one shown in Fig. 1. It consists of (1) a foundation plate which is bolted to the foundation on which the pedestal rests; the plate is essential when the pedestal rests on brickwork or masonry, but may be dispensed with when the pedestal rests on the frame of the machine; (2) the block which carries the seats and supports the journal; (3) the cap which is screwed down over the seats. The bolt holes in both foundation plate and block are oblong, so that the pedestal may be readily adjusted. The following proportions may be used for this kind of pedestal, having journals from 2" to 6", inclusive. An oil cup having a 1/4" pipe tap shank may be used on pedestals for journals having diameters from 3" to 4", and " pipe tap shank for larger sizes up to 6" diameter. NOTE. The shanks of oil cups and grease cups bought in the market are made with a ", 4", 3/8" or 11⁄2" pipe thread. The amount of oil or grease the cup holds when filled is usually expressed in ounces The diameter of journal d = the unit. n = 1.25 d. r = .25 d. u = .75 d. v = 1.375 d. z = .0625 d. Fig. 3 shows a pedestal suitable for the crank shaft of a horizontal engine with journals from 8" to 20" in diameter. The block may be complete in itself, as shown in the figure, but more often it forms part of the engine bed. The seats are in three parts, and may be adjusted horizontally by means of the wedges W. The lower seat may be raised by placing packing pieces under it. To obtain its dimensions, use the following proportions, which are based on the unit d the diameter of the crank shaft journal. q' = 1.5 d. S = .9 d. t = .15 d.375". t'.9 d. u = 1.5 d. v = .25 d + .375". W w' 1.45 d. 1.47 d. W1 = 1.75 d. X = .1 d. y3d.75". y1 = .2d.5". z' = 2.5" (constant.) Taper of adjusting wedge, 1: 10. |