An Introduction to Machine Drawing and Design Part 4

EXERCISE 20: _Solid b.u.t.t Coupling._--From the above table of dimensions make a longitudinal and a transverse section of a solid b.u.t.t coupling for a shaft 2-1/2 inches in diameter. Scale 6 inches to a foot.

EXERCISE 21: _Fairbairn's Half-Lap Coupling._--Make the same views as in the last exercise of a half-lap coupling for a 3-inch shaft to the dimensions in the above table. Scale 6 inches to a foot.

_f.l.a.n.g.e Couplings._--The form of coupling used for the shafts of marine engines is shown in fig. 23. The ends of the different lengths of shaft have f.l.a.n.g.es forged on them, which are turned along with the shaft.

These f.l.a.n.g.es b.u.t.t against one another, and are connected by bolts.

These bolts may be parallel or tapered; generally they are tapered. A parallel bolt must have a head, but a tapered bolt will act without one.

In fig. 23 the bolts are tapered, and also provided with heads. In fig.

14, page 17, is shown a tapered bolt without a head. The variation of diameter in tapered bolts is 3/8 of an inch per foot of length.

[Ill.u.s.tration: FIG. 23.]

Sometimes a projection is formed on the centre of one f.l.a.n.g.e which fits into a corresponding recess in the centre of the other, for the purpose of ensuring the shafts being in line.

Occasionally a cross-key is fitted in between the f.l.a.n.g.es, being sunk half into each, for the purpose of diminis.h.i.+ng the shearing action on the bolts.

EXERCISE 22: _Marine Coupling._--Draw the elevation and section of the coupling shown in fig. 23; also an elevation looking in the direction of the arrow. Scale 3 inches to a foot.

The following table gives the dimensions of a few marine couplings taken from actual practice.

_Examples of Marine Couplings._

+--------------------------------------------------------------------+ Diameter of shaft 2-3/8 9-3/4 12-7/8 16-1/2 22-1/2 23 +--------------------+------+-------+--------+-------+--------+------+ Diameter of f.l.a.n.g.e 6 19 24 32 35 38 Thickness of f.l.a.n.g.e 1 2-3/4 3-1/8 4-1/4 6 5 Diameter of bolts 3/4 2-3/4 2-11/16 3-1/2 4-1/4 4-1/4 Number of bolts 3 6 6 8 9 8 Diameter of bolt circle 4-1/8 14-1/8 18-13/16 25 28-3/4 30-3/8 +--------------------------------------------------------------------+

All the above dimensions are in inches.

EXERCISE 23.--Select one of the couplings from the above table, and make the necessary working drawings for it to a suitable scale.

The cast-iron f.l.a.n.g.e coupling is shown in fig. 24. In this kind of coupling a cast-iron centre or boss provided with a f.l.a.n.g.e is secured to the end of each shaft by a sunk key driven from the face of the f.l.a.n.g.e.

These f.l.a.n.g.es are then connected by bolts and nuts as in the marine coupling.

To ensure the shafts being in line the end of one projects into the f.l.a.n.g.e of the other.

In order that the face of each f.l.a.n.g.e may be exactly perpendicular to the axis of the shaft they should be 'faced' in the lathe, after being keyed on to the shaft.

If the coupling is in an exposed position, where the nuts and bolt-heads would be liable to catch the clothes of workmen or an idle driving band which might come in the way, the f.l.a.n.g.es should be made thicker, and be provided with recesses for the nuts and bolt-heads.

[Ill.u.s.tration: FIG. 24.]

_Dimensions of Cast-iron f.l.a.n.g.e Couplings._

+--------------------------------------------------------------------+ Diameter Depth Diameter Diameter Diameter of Thickness Diameter at Number of of bolt of shaft f.l.a.n.g.e of f.l.a.n.g.e of boss boss of bolts circle D F T B L bolts d C +-------- -------- --------- -------- ------ ------ -------- --------+ 1-1/2 7-1/4 7/8 3-1/2 2-5/8 3 5/8 5-1/2 2 8-7/8 1-1/16 4-3/8 3-3/16 4 3/4 6-3/4 2-1/2 10-5/8 1-1/4 5-5/16 3-3/4 4 7/8 8-1/8 3 12-3/8 1-7/16 6-1/4 4-5/16 4 1 9-1/2 3-1/2 13-1/8 1-5/8 7-1/8 4-7/8 4 1 10-5/16 4 14 1-3/4 8 5-7/16 6 1 11-1/4 4-1/2 15-5/8 2 8-7/8 6 6 1-1/8 12-1/2 5 17-3/8 2-1/8 9-13/16 6-5/8 6 1-1/4 13-13/16 5-1/2 18-1/4 2-5/16 10-3/4 7-1/4 6 1-1/4 14-3/4 6 19-7/8 2-1/2 11-5/8 7-3/4 6 1-3/8 16 +--------------------------------------------------------------------+

The projection of the shaft _p_ varies from 1/4 inch in the small shafts to 1/2 inch in the large ones.

EXERCISE 24: _Cast-iron f.l.a.n.g.e Coupling._--Draw the views shown in fig. 24 of a cast-iron f.l.a.n.g.e coupling, for a shaft 4-1/2 inches in diameter, to the dimensions given in the above table. Scale 4 inches to a foot.

VII. BEARINGS FOR SHAFTS.

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 f.l.a.n.g.e two bolts pa.s.s 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.

[Ill.u.s.tration: 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 pa.s.s the holding-down bolts. C is the cap. Between the block and the cap is the bra.s.s bush, which is in halves, called _bra.s.ses_ 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.

[Ill.u.s.tration: FIG. 26]

In the block ill.u.s.trated the journal is lubricated by a _needle lubricator_; this consists of an inverted gla.s.s bottle fitted with a wood stopper, through a hole in which pa.s.ses 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 workmans.h.i.+p 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.