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.
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.
| Diameter of journal | = d. |
| Length of journal | = l. |
| Height to centre | = 1·05d + ·5. |
| Length of base | = 3·6d + 5. |
| Width of base | = ·8l. |
| ” block | = ·7l. |
| Thickness of base | = ·3d + ·3. |
| ” cap | = ·3d + ·4. |
| Diameter of bolts | = ·25d + ·25. |
| Distance between centres of cap bolts | = 1·6d + 1·5. |
| ” ” base bolts | = 2·7d + 4·2. |
| Thickness of step at bottom | = t = ·09d + ·15. |
| ” ” sides | = ¾t. |
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.