Page 401 - Analysis and Design of Machine Elements

The pressure on the friction surface satisfies Couplings and Clutches 379
4F
p = ≤ [p] (13.4)
2
2
(D − D )
o
i
where K is service factor and can be selected from Table 2.1 using the median values.
A
The value of coefficient of friction f and allowable pressure [p] can be found in design
handbooks [8, 9].
Friction clutch design demand that the friction torque be produced by an acceptable
normal force. In clutch design, the initial value of outside diameter D and inside diam-
o
eter D are initially selected as [7]
i
D =(1.5∼2)d, D =(1.5∼2)D For friction clutches operate dry
i o i
D =(2 ∼ 3)d, D =(1.5∼2.5)D i For friction clutches operate wet
o
i
Then the normal force presented to the clutch F can be obtained from Eq. (13.4), and
the number of friction surfaces z from Eq. (13.3).
Unlike most machine components, such as bearings, gears and many others, minimiz-
ing friction is desirable for reducing energy loss and wear. In contrast, friction clutches
rely on friction to transmit torque. One of the aims of friction clutch design is to maxi-
mize the friction coefficient and keep it uniform over a wide range of operating condi-
tions, while at the same time minimizing wear [6].
Suitable material for friction surfaces should have a high and stable coefficient of fric-
tion to transmit required torque, good wear resistance to guarantee an acceptable period
of operation and high thermal conductivity to dissipate friction-induced heat. In a typ-
ical friction clutch, one contact surface is metal, the other is a high-friction material
called a liner. Contact and friction occurs between a replaceable lining and metallic sur-
face [2]. The metallic surface is commonly made of grey cast iron or steel, while the
replaceable lining is usually made of composite materials in which hard particles are
embedded in a matrix of thermosetting polymer. The commonly used reinforcing par-
ticles are fibreglass and sintered particles that include brass, zinc, ceramic and so on
[3].
Friction clutches are designed to operate either dry or wet with oil. Automotive
clutches operate dry, using circulating air to dissipate frictional heat. Dry operation
implies a high coefficient of friction and high torque capacity, yet high temperature [2].
Nevertheless, they have simple structure, low cost and short starting times. They are
widely used in situations requiring instantaneous engagement or overload protection.
Most multiple disc clutches can operate wet. Friction discs are soaked in an enclosed
box of oil. The oil serves as an effective coolant during clutch engagement, leading
to a reduced coefficient of friction and torque capacity, yet smooth engagement and
less wear.
Friction clutches can be engaged or disengaged mechanically, electromagnetically,
pneumatically and hydraulically. Once the torque capacity of disc friction surfaces is
exceeded, the clutch will slip, protecting machines from overload damage. Friction
clutches have a large frictional area, resulting in large heat generation and wear.
For special requirements of automatic engagement or detachment, it is demanded that
when torque, rotational speed or rotational direction reach the preset value, clutches can
engage or disengage automatically. Safety couplings and clutches, centrifugal clutches
and overrunning clutches are examples of these devices.