A clutch is any
coupling that allows shafts or other rotating parts to be
connected or disconnected at will i.e., without the removal
or refitting of any components. Clutches are useful in devices
that have two rotating shafts. In these devices, one shaft
is typically driven by a motor or pulley, and the other shaft
drives another device. In a drill, for instance, one shaft
is driven by a motor, and the other drives a drill chuck.
In the claw clutch (Fig.1) one half of the clutch can slide
on its shaft, so that the claws can be engaged or disengaged.
This type of clutch can be engaged only when the shafts are
stationary or rotating at low speed.
geared clutch (Fig.2) is widely used in machine tools and
motor vehicles. The two clutch bosses are each supported with
external teeth which can mesh with a sleeve that has corresponding
internal teeth and can be slid over both bosses so as to establish
a positive connection between the two shafts. To permit engagement
of the clutch while the shafts are rotating, the sleeve and
the shaft end to be coupled are respectively provided with
friction surfaces which are brought into contact with each
other and thus equalize the speed of the rotating parts before
the teeth on the shaft and inside the sleeve are brought into
mesh. Friction clothes transmit power through contact friction
surfaces on the two halves to be connected.
types of friction clutch are illustrated in Fig.3. In the
disc clutch (or plate clutch), the cone clutch, the boss of
the movable part slides in longitudinal grooves in the shaft
on which it is mounted. The movements for engaging and disengaging
the clutch are performed by the action of a lever whose forked
ends fit into a circumferential recess in the boss of the
clutch plate or cone. The internal-expanding shoe-type clutch
comprises an outer shell attached to one shaft and two semicircular
shoes which are mounted on arms attached to
a sliding sleeve on the other shaft and which can be brought
internally into contact with the shell. The forks of the clutch-operating
lever engage with a recess on the sliding sleeve.
type of friction clutch commonly used in machine tools and
other motor vehicles is the multiple-disc clutch (Fig.4).
It is based on the principle that a series of discs or plates
alternately connected to the driving and the driven shaft
will increase the power-transmitting capacity in proportion
to the number of pairs of contact surfaces. In the form of
clutch illustrated in Fig.4 the boss mounted on the driving
shaft is provided with external teeth with which the internal
teeth on a series of thin steel plates engage.
outer shell of the clutch is mounted on the driven shaft and
has internal teeth with which the external teeth of a second
series of plates (alternating with those of the first series)
likewise engage. When the clutch lever shifts a collar to
the left, the plates are pressed together and thus transmit
power by friction. Multiple-disc clutches in machine tools
usually operate immersed in oil; those in motor vehicles are
usually of the dry type. A magnetic clutch is a friction-disc
clutch that is engaged by the energizing of a magnet coil,
which attracts a set of steel friction discs and thus establishes
A double-acting clutch based on this principle is illustrated
in Fig.5. When the coil a is energized, the discs b is compressed
together by magnetic attraction, thereby connecting the gear
c to the shaft d. When the coil e is energized, the discs
f is pressed together, so that now power is transmitted from
the shaft “d” to the gear g.
automatic clutch is often installed between the driving shaft
of a motor and the machinery it drives. It does not allow
the shaft to reach a predetermined speed before engagement
is effected and is especially useful in a case where the driven
machinery requires a high starting torque.
such purposes a centrifugal clutch (Fig.1) may suitably be
utilized. It comprises two or more shoes which,
when the driving shaft on which they are mounted has reached
a certain speed, overcome the pressure of restraining springs
by the action of centrifugal force and move outwards to press
against the inner surface of the rim mounted on the driven
shaft. In this way the transmission of power to the driven
shaft is gradually and automatically increased, so that smooth
engagement is effected.
speed at which engagement takes place can be increased by
fitting the clutch with more power-restraining springs. When
the shafts are not rotating, the shoes are retracted and not
in contact with the rim. Various other types of automatic
clutch are likewise based on the centrifugal principle. Freewheeling
clutches drive in one direction only and permit free movement
when the speed of the driven shaft exceeds that of the driving
shaft. In the grip-roller type of freewheeling clutch (Fig.2)
each roller is gripped, i.e., jammed, in the wedge-shaped
space as soon as the movement of the outer race in relation
to the inner race causes the roller to move into the shallower
part of this space.
friction produced at these faces will depend on the contact
pressure exerted by the springs. If the pressure is low, the
friction will also be low, so that slip in the clutch will
occur at a low value of the torque. By means of a screw it
is possible to increase the spring pressure and therefore
the friction, so that the clutch will be able to transmit
a greater torque without slipping. The torque can thus be
adjusted to a predetermined value, and the clutch can serve
as a safety device against overloading of the driven machinery.
A simpler safety device for this purpose is the shear-bolt
coupling (Fig.4). It comprises two flanges connected by bolts
that are designed to fail in shear (i.e., to break
off) when the torque exceeds a predetermined value.
o CONTACT US |