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.
[Illustration: FIG. 25.]
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.
[Illustration: 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 = 1.05_d_ + .5.
Length of base = 3.6_d_ + 5.
Width of base = .8_l_.
" block = .7_l_.
Thickness of base = .3_d_ + .3.
" cap = .3_d_ + .4.
Diameter of bolts = .25_d_ + .25.
Distance between centres of cap bolts = 1.6_d_ + 1.5.
" " base bolts = 2.7_d_ + 4.2.
Thickness of step at bottom = _t_ = .09_d_ + .15.
" " sides = 3/4 _t_.
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_ = 1.5_d_; others make _l_ = 2_d_.
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.
[Illustration: FIG. 27.]
_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 manner as is shown, for the attachment of the bracket to
the column.
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.
[Illustration: FIG. 28.]
_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.
[Illustration: FIG. 29.]
[Illustration: FIG. 30.]