_Keys_ are wedges, generally rectangular in section, but sometimes
circular; they are made of wrought iron or steel, and are used for
securing wheels, pulleys, cranks, &c., to shafts.
[Illustration: Fig. 20.]
Various sections of keys are shown in fig. 20. At (_a_) is the _hollow_
or _saddle key_. With this form of key it is not necessary to cut the
shaft in any way, but its holding power is small, and it is therefore
only used for light work. At (_b_) is the _key on a flat_, sometimes
called a _flat key_. The holding power of this key is much greater than
that of the saddle key. At (_c_) is the _sunk key_, a very secure and
very common form.
The part of the shaft upon which a key rests is called the _key bed_ or
_key way_, and the recess in the boss of the wheel or pulley into which
the key fits is called the _key way_; both are also called _key seats_.
With saddle, flat, and sunk keys the key bed is parallel to the axis of
the shaft; but the key way is deeper at one end than the other to
accommodate the taper of the key. The sides of the key are parallel.
The _round key_ or taper pin shown at (_d_) is in general only used for
wheels or cranks which have been previously shrunk on to their shafts or
forced on by great pressure. After the wheel or crank has been shrunk
on, a hole is drilled, half into the shaft and half into the wheel or
crank, to receive the pin.
When the point of a key is inaccessible the other end is provided with a
_gib head_ as shown at (_e_), to enable the key to be withdrawn.
A _sliding_ or _feather key_ secures a piece to a shaft so far as to
prevent the one from rotating without the other, but allows of relative
motion in the direction of the axis of the shaft. This form of key has
no taper, and it is secured to the piece carried by the shaft, but is
made a _sliding fit_ in the key way of the shaft. In one form of feather
key the part within the piece carried by the shaft is dovetailed as
shown at (_f_). In another form the key has a round projecting pin
forged upon it, which enters a corresponding hole as shown at (_g_). The
feather key may also be secured to the piece carried by the shaft by
means of one or more screws as shown at (_h_). The key way in the shaft
is made long enough to permit of the necessary sliding motion.
_Cone Keys._--These are sometimes fitted to pulleys, and are shown in
fig. 32, page 38. In this case the eye of the pulley is tapered and is
larger than the shaft. The space between the shaft and the boss of the
pulley is filled with three _saddle_ or _cone keys_. These keys are made
of cast iron and are all cast together, and before being divided the
casting is bored to fit the shaft and turned to fit the eye of the
pulley. By this arrangement of keys the same pulley may be fixed on
shafts of different diameters by using keys of different thicknesses;
also the pulley may be bored out large enough to pass over any boss
which may be forged on the shaft.
_Proportions of Keys._--The following rules are taken from Unwin's
'Machine Design,' pp. 142-43.
Diameter of eye of wheel, or boss of shaft = _d_.
Width of key = 3/4_d_ + 1/8.
Mean thickness of sunk key = 1/8_d_ + 1/8.
" key on flat = 1/16_d_ + 1/16.
The following table gives dimensions agreeing with average practice.
_Dimensions of Keys._
D = diameter of shaft.
B = breadth of key.
T = thickness of sunk key.
T_{1} = thickness of flat key, also = thickness of saddle key. Taper
of key 1/8 inch per foot of length, _i.e._ 1 in 96.
+---------------------------------------------------------------+
| D | 3/4 | 1 | 1-1/4 | 1-1/2 | 1-3/4 | 2 | 2-1/4 | 2-1/2 |
+-----+-----+-----+-------+-------+-------+-----+-------+-------+
| B | 5/16| 3/8 | 7/16 | 1/2 | 9/16 | 5/8 | 11/16 | 11/16 |
| T | 1/4 | 1/4 | 1/4 | 5/16 | 5/16 | 5/16| 3/8 | 3/8 |
|T_{1}| 3/16| 3/16| 3/16 | 3/16 | 1/4 | 1/4 | 1/4 | 5/16 |
+---------------------------------------------------------------+
+-------------------------------------------------------------------+
| D | 2-3/4 | 3 | 3-1/2 | 4 | 4-1/2 | 5 | 5-1/2 | 6 |
+-----+-------+-----+-------+-------+-------+-------+-------+-------+
| B | 3/4 | 7/8 | 1 | 1-1/8 | 1-1/4 | 1-3/8 | 1-1/2 | 1-5/8 |
| T | 3/8 | 7/16| 1/2 | 1/2 | 9/16 | 5/8 | 11/16 | 3/4 |
|T_{1}| 5/16 | 5/16| 3/8 | 7/16 | 1/2 | 1/2 | 9/16 | 5/8 |
+-------------------------------------------------------------------+
+-------------------------------------------------------+
| D | 7 | 8 | 9 | 10 | 11 | 12 |
+-----+-------+-------+-------+--------+--------+-------+
| B | 1-7/8 | 2-1/8 | 2-3/8 | 2-5/8 | 2-7/8 | 3-1/8 |
| T | 13/16 | 15/16 | 1 | 1-1/16 | 1-3/16 | 1-1/4 |
|T_{1}| 11/16 | 3/4 | 7/8 | 15/16 | 1-1/16 | 1-1/8 |
+-------------------------------------------------------+