For convenience of making and handling, shafts used for transmitting
power are generally made in lengths not exceeding 30 feet. These lengths
are connected by couplings, of which we give several examples.
[Illustration: FIGS. 21 and 22.]
_Solid_, _Box_, or _Muff Couplings._--One form of box coupling is shown
in fig. 21. Here the ends of the shafts to be connected butt against one
another, meeting at the centre of the box, which is made of cast iron.
The shafts are made to rotate as one by being secured to the box by two
wrought-iron or steel keys, both driven from the same end of the box. A
clearance space is left between the head of the forward key and the
point of the hind one, to facilitate the driving of them out, as then
only one key needs to be started at a time. Sometimes a single key the
whole length of the box is used, in which case it is necessary that the
key ways in the shafts be of exactly the same depth.
The half-lap coupling, introduced by Sir William Fairbairn, is shown in
fig. 22. In this form of box coupling the ends of the shafts overlap
within the box. It is evident that one shaft cannot rotate without the
other as long as the box remains over the lap. To keep the box in its
place it is fitted with a saddle key.
It will be noticed that the lap joint is sloped in such a way as to
prevent the two lengths of shaft from being pulled asunder by forces
acting in the direction of their length.
Half-lap couplings are not used for shafts above 5 inches in diameter.
It may here be pointed out that the half-lap coupling is expensive to
make, and is now not much used.
As shafts are weakened by cutting key ways in them, very often the ends
which carry couplings are enlarged in diameter, as shown in fig. 21, by
an amount equal to the thickness of the key. An objection to this
enlargement is that wheels and pulleys require either that their bosses
be bored out large enough to pass over it, or that they be split into
halves, which are bolted together after being placed on the shaft.
_Dimensions of Box Couplings._
D = diameter of shaft.
T = thickness of metal in box.
L = length of box for butt coupling.
L_{1} = length of box for lap coupling.
_l_ = length of lap.
D_{1} = diameter of shaft at lap.
+---------------------------------------------------------------+
| D | 1-1/2 | 2 | 2-1/2 | 3 | 3-1/2 | 4 |
+-------+--------+--------+---------+-------+----------+--------+
| T | 1-1/8 | 1-5/16 | 1-1/2 | 1-3/4 | 1-15/16 | 2-1/8 |
| L | 5-3/4 | 7 | 8-1/4 | 9-1/2 | 10-3/4 | 12 |
| L_{1} | 4-1/8 | 5-1/4 | 6-3/8 | 7-1/2 | 8-5/8 | 9-3/4 |
| _l_ | 1-7/16 | 1-7/8 | 2-5/16 | 2-3/4 | 3-3/16 | 3-5/8 |
| D_{2} | 2-5/16 | 3 | 3-11/16 | 4-3/8 | 5-1/16 | 5-3/4 |
+---------------------------------------------------------------+
+----------------------------------------------+
| D | 4-1/2 | 5 | 5-1/2 | 6 |
+-------+---------+--------+--------+----------+
| T | 2-5/16 | 2-1/2 | 2-3/4 | 2-15/16 |
| L | 13-1/4 | 14-1/2 | 15-3/4 | 17 |
| L_{1} | 10-7/8 | 12 | -- | -- |
| _l_ | 4-1/16 | 4-1/2 | -- | -- |
| D_{2} | 6-7/16 | 7-1/8 | -- | -- |
+----------------------------------------------+
Slope of lap 1 in 12.
EXERCISE 20: _Solid Butt Coupling._--From the above table of
dimensions make a longitudinal and a transverse section of a solid
butt 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.
_Flange 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 flanges forged on them, which are turned along with the shaft.
These flanges butt 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.
[Illustration: FIG. 23.]
Sometimes a projection is formed on the centre of one flange 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 flanges, being sunk
half into each, for the purpose of diminishing 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 flange | 6 | 19 | 24 | 32 | 35 | 38 |
|Thickness of flange | 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 flange coupling is shown in fig. 24. In this kind of
coupling a cast-iron centre or boss provided with a flange is secured to
the end of each shaft by a sunk key driven from the face of the flange.
These flanges 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
flange of the other.
In order that the face of each flange 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 flanges should be made thicker, and be
provided with recesses for the nuts and bolt-heads.
[Illustration: FIG. 24.]
_Dimensions of Cast-iron Flange Couplings._
+--------------------------------------------------------------------+
| |Diameter| | |Depth | |Diameter|Diameter|
|Diameter| of |Thickness|Diameter| at |Number| of | of bolt|
|of shaft| flange |of flange| 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 Flange Coupling._--Draw the views shown in
fig. 24 of a cast-iron flange coupling, for a shaft 4-1/2 inches in
diameter, to the dimensions given in the above table. Scale 4 inches
to a foot.