belts, clutches and brakes - mercer...

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