  Fig. 25. Fig. 25. An example of a very simple form of bearing is shown in fig. 25, which represents a brake shaft carrier of a locomotive tender. The bearing in this example is made of cast iron and in one piece. Through the oval-shaped flange two bolts pass for attaching the bearing to the wrought-iron framing of the tender. With this form of bearing there is no adjustment for wear, so that when it becomes worn it must be renewed. Exercise 25: Brake Shaft Carrier.—Draw the elevation and sectional plan of the bearing shown in fig. 25. Draw also a vertical section through the axis. The latter view to be projected from the first elevation. Scale 6 inches to a foot. Pillow Block, Plummer Block, or Pedestal.—The ordinary form of plummer block is represented in fig. 26. A is the block proper, B the sole through which pass the holding-down bolts. C is the cap. Between the block and the cap is the brass bush, which is in halves, called brasses or steps. The bed for the steps in this example is cylindrical, and is prepared by the easy process of boring. The steps are not supported throughout their whole length, but at their ends only where fitting strips are provided as shown. As the wear on a step is generally greatest at the bottom, it is made thicker there than at the sides, except where the fitting strips come in. To prevent the steps turning within the block they are generally furnished with lugs, which enter corresponding recesses in the block and cover. Fig. 26. Fig. 26. In the block illustrated the journal is lubricated by a needle lubricator; this consists of an inverted glass bottle fitted with a wood stopper, through a hole in which passes a piece of wire, which has one end in the oil within the bottle, and the other resting on the journal of the shaft. The wire or needle does not fill the hole in the stopper, but if the needle is kept from vibrating the oil does not escape owing to capillary attraction. When, however, the shaft rotates, the needle begins to vibrate, and the oil runs down slowly on to the journal; oil is therefore only used when the shaft is running. Exercise 26: Pillow Block for a Four-inch Shaft.—Draw the views shown of this block in fig. 26. Make also separate drawings, full size, of one of the steps. Scale 6 inches to a foot. Proportions of Pillow Blocks.—The following rules may be used for proportioning pillow blocks for shafts up to 8 inches diameter. It should be remembered that the proportions used by different makers vary considerably, but the following rules represent average practice.
The length of the journal varies very much in different cases, and depends upon the speed of the shaft, the load which it carries, the workmanship of the journal and bearing, and the method of lubrication. For ordinary shafting one rule is to make l = d + 1. Some makers use the rule l = 1·5d; others make l = 2d. Fig. 27. Fig. 27. Fig. 28. Fig. 28. Exercise 27: Design for Pillow Block.—Make the necessary working drawings for a pillow block for a shaft 5 inches in diameter, and having a journal 7 inches long. Brackets.—When a pillow block has to be fixed to a wall or column a bracket such as that shown in figs. 27 and 28 may be used. The pillow block rests between the joggles A A, and is bolted down to the bracket and secured in addition with keys at the ends of the base of the block, in the same Exercise 28: Pillar Bracket.—Fig. 27 shows a side elevation and part horizontal section, and fig. 28 shows an end elevation of a pillar bracket for carrying a pillow block for a 3-inch shaft. Draw these views properly projected from one another, showing the pillow block, which is to be proportioned by the rules given on page 32. Draw also a plan of the whole. Scale 4 inches to a foot. Hangers.—When a shaft is suspended from a ceiling it is carried by hangers, one form of which is shown in fig. 29, and which will be readily understood. The cap of the bearing, it will be noticed, is secured by means of a bolt, and also by a square key. Exercise 29: Shaft Hanger.—Draw the two elevations shown in fig. 29, and also a sectional plan. The section to be taken at a point 5 inches above the centre of the shaft. Scale 6 inches to a foot. Wall Boxes.—In passing from one part of a building to another a shaft may have to pass through a wall. In that case a neat appearance is given to the opening and a suitable support obtained for a pillow block by building into the wall a wall box, one form of which is shown in fig. 30. Exercise 30: Wall Box.—Draw the views of the wall box shown in fig. 30, and also a sectional plan; the plane of section to pass through the box a little above the joggles for the pillow block. Scale 3 inches to a foot. Fig. 29. Fig. 29. Fig. 30. Fig. 30. |
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