The most important application of the crank is in the steam-engine,
where the reciprocating rectilineal motion of the piston is converted
into the rotary motion of the crank-shaft by means of the crank and
connecting rod.
At one time steam-engine cranks were largely made of cast iron, now they
are always made of wrought iron or steel. The crank is either forged in
one piece with the shaft, or it is made separately and then keyed to it.
_Overhung Crank._--Fig. 37 shows a wrought-iron overhung crank. A is the
crank-shaft, B the crank arm, provided at one end with a boss C, which
is bored out to fit the shaft; at the other end of the crank arm is a
boss D, which is bored out to receive the crank-pin E, which works in
one end of the connecting rod. The crank is secured to the shaft by the
sunk key F. It is also good practice to _shrink_ the crank on to the
shaft. The process of shrinking consists of boring out the crank a
little smaller than the shaft, and then heating it, which causes it to
expand sufficiently to go on to the shaft. As the crank cools, it
shrinks and grips the shaft firmly. The crank may also be shrunk on to
the crank-pin, the latter being then riveted over as shown in fig. 37.
[Illustration: FIG. 37.]
A good plan to adopt in preference to the shrinking process is to force
the parts together by hydraulic pressure. This method is adopted for
placing locomotive wheels on their axles, and for putting in crank-pins.
As to the amount of pressure to be used, the practice is to allow a
force of 10 tons for every inch of diameter of the pin, axle, or shaft.
Instead of being riveted in, the crank pin may be prolonged and screwed,
and fitted with a nut. Another plan is to put a cotter through the crank
and the crank-pin.
The distance from the centre of the crank-shaft to the centre of the
crank-pin is called the radius of the crank. The _throw_ of the crank is
twice the radius. In a direct-acting engine the throw of the crank is
equal to the stroke of the piston.
EXERCISE 36: _Wrought-iron Overhung Crank._--Draw the two
elevations shown in fig. 37, also a plan. Scale 1-1/2 inches
to a foot.
_Proportions of Overhung Cranks._
D = diameter of shaft.
_d_ = " crank-pin.
Length of large boss = .9 D.
Diameter " = 1.8 D.
Length of small boss = 1.1 _d_.
Diameter " = 1.8 _d_.
Width of crank arm at centre of shaft = 1.3 D.
" " crank-pin = 1.5 _d_.
The thickness of the crank arm may be roughly taken as = .7 D.
EXERCISE 37.--Design a wrought-iron crank for an engine having a
stroke of 4 feet. The crank-shaft is 9 inches in diameter, and
the crank-pin is 4-3/4 inches in diameter and 6-1/2 inches long.
[Illustration: FIG. 38.]
_Locomotive Cranked Axle._--As an example of a cranked shaft we take the
cranked axle for a locomotive with inside cylinders shown in fig. 38;
here the crank and shaft or axle are forged in one piece. A is the wheel
seat, B the journal, C the crank-pin, and D and E the crank arms. Only
one half of the axle is shown in fig. 38, but the other half is exactly
the same. The cranks on the two halves are, however, at right angles to
one another. The ends of the crank arms are turned in the lathe, the
crank-pin ends being turned at the same time as the axle, and the other
ends at the same time as the crank-pin. This consideration determines
the centres for the arcs shown in the end view.
EXERCISE 38.--Draw to a scale of 2 inches to a foot the side and
end elevations of the locomotive cranked axle partly shown in
fig. 38. The distance between the centre lines of the cylinders
is 2 feet.
[Illustration: FIG. 39.]
_Built-up Cranks._--The form of cranked shaft shown in fig. 38 is
largely used for marine engines, but for the very powerful engines now
fitted in large ships this design of shaft is very unreliable, the
built-up crank shown in fig. 39 being preferred, although it is much
heavier than the other. It will be seen from the figure that the shaft,
crank arms, and crank-pin are made separately. The arms are shrunk on to
the pin and the shaft, and secured to the latter by sunk keys. These
heavy shafts and cranks are generally made of steel.
EXERCISE 39.--Keeping to the dimensions marked in fig. 39, draw
the views there shown of a built-up crank-shaft for a marine
engine. Scale 3/4 inch to a foot.