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Clutches and Brakes
Brakes and clutches
• Clutch is a device that connects and disconnects two
collinear shafts.
– Similar to couplings
– Friction and hence heat dissipation
• Purpose of a Brake is to stop the rotation of a shaft.
• Braking action is produced by friction as a stationary part
bears on a moving part.
– Heat dissipation is a problem
– Brake fade during continuous application of braking due to heat
generated
Brakes and clutches are essentially
the same devices. Each is
associated with rotation
• Brakes, absorb kinetic energy of the
moving bodies and covert it to heat
• Clutches Transmit power between
two shafts
Types of Brakes
• Band
• Rim/Drum
– Internal shoe
– External shoe
• Disk
• Cone
• Many others
Braking
• Forces applied
• Torque regenerated to ‘brake’
• Energy is lost :HEAT
• Temperature rise of brake materials
Energy Considerations
• 𝑇𝑜𝑟𝑞𝑢𝑒 = 𝑇 = 𝜃 𝐼𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 = Fr
• ∆𝐾. 𝐸.=Work
• ∆𝐾. 𝐸.= 1
2𝑚 𝑣𝑓
2 − 𝑣𝑖2 + 𝐼 𝜔𝑓
2−𝜔𝑖2
𝑊𝑜𝑟𝑘 = 𝑇𝑑𝜃
• Work=MC(ΔT)-Heat loss
Brake Friction Materials
• Sintered metal
– Cu +Fe+ Friction modifiers
• Cermet: sintered metal + ceramic content
• Asbestos
– (not used any more in general applications)
Characteristics:
• High friction f=0.3 to 0.5
• Repeatable friction
• Invariant to environment conditions
• Withstand high temps
– 1500F cermet
– 1000f sintered metal
– 600-1000 asbestos
• Some Flexibility
Model of Clutch/Brake
Remove relative rotation
Clutches: Couple two shafts
together
An Internal Expanding
Centrifugal-acting Rim Clutch
Fig. 16–3
Clutch: How much force we need to
stop the relative rotation
Basic Band Brake: How much force
is needed to stop the Drum rotation
Alternate Band/Drum Brake
Cantilever Drum Shoe Brake
External Cantilever Drum Brake
Common Internal Drum Brake
Internal Friction Shoe Geometry
•
Fig. 16–4
Internal Friction Shoe Geometry
Fig. 16–5
p is function of θ.
Largest pressure on the shoe is pa
Pressure Distribution Characteristics
• Pressure distribution is
sinusoidal
• For short shoe, as in (a),
the largest pressure on the
shoe is pa at the end of the
shoe
• For long shoe, as in (b),
the largest pressure is pa
at qa = 90º
Fig. 16–6
Force Analysis
Fig. 16–7
Force Analysis
Shigley’s Mechanical Engineering
Design
Self-locking condition
MN is the Normal Moment (opens brake),
Mf is Frictional Moment ( assists closing brake)
F is actuating force
Force Analysis
Shigley’s Mechanical Engineering
Design
Force Analysis
Basic Band Brake: How much force
is needed to stop the Drum rotation
Notation for Band-Type Clutches and Brakes
Shigley’s Mechanical Engineering
Design Fig. 16–
13
Force Analysis for Brake Band
Shigley’s Mechanical Engineering
Design
Force Analysis for Brake Band
Common Disk Brakes
Geometry of Disk Friction Member
Shigley’s Mechanical Engineering
Design Fig. 16–
16
Uniform Wear
Shigley’s Mechanical Engineering
Design
For uniform wear, w is constant, so PV is constant.
Setting p = P, and V = rw, the maximum pressure pa
occurs where r is minimum, r = d/2,
Uniform Wear
Shigley’s Mechanical Engineering
Design
Find the total normal force by letting r vary from d/2
to D/2, and integrating,
Uniform Pressure
Shigley’s Mechanical Engineering
Design
Comparison of Uniform Wear with Uniform
Pressure
Shigley’s Mechanical Engineering
Design
Automotive Disk Brake
Shigley’s Mechanical Engineering
Design
Geometry of Contact Area of Annular-Pad Brake
Fig. 16–19
Analysis of Annular-Pad Brake
Shigley’s Mechanical Engineering
Design
Uniform Wear
Shigley’s Mechanical Engineering
Design
Uniform Pressure
Shigley’s Mechanical Engineering
Design
Example 16–3
Shigley’s Mechanical Engineering
Design
Example 16–3
Shigley’s Mechanical Engineering
Design
Example 16–3
Shigley’s Mechanical Engineering
Design