MECH1200
Clutches, Brakes, and Backstops
MECH1200
Functions
• Clutch – Go (or limit torque)
• Brake – Stop (or slow down)
• Backstop – Go only one direction
• Primary function: Stop, slow, or prevent
reversal of load in a mechanical system
MECH1200
Activation Methods
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Centrifugal
Mechanical
Electrical
Pneumatic
Hydraulic
MECH1200
Applications
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Control Torque
Disconnect loads
Starting
Stopping
Prevent load reversal
Tension Control
MECH1200
Clutch-like operations by Electrics
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Soft start
Servo drives
Variable speed drives
Positioning systems
MECH1200
Application Guide
Brake
Description
Notes
Band
Operates by the action of a fixed belt over a rotating cylindrical surface(drum). Action not harsh.
Band
Drum
Works by bringing curved metal plates, lined with friction material(brake shoes) against an external or internal rotating
cylindrical surface.
Drum
Disc
Disc brake consists of a disc rotating between two friction pads. When the pads are forced together under hydraulic,
pneumatic, electrical or magnetic action then the disc is very effectively stopped.
Disk
Block/Shoe
Probably the first design of brake based on a shoe/block which is forced against the outside of a rotating wheel. A simple
Block/Shoe
design visible on many old vehicles
Fluid
A smooth progressive brake action resulting from rotating plates, or vanes in a retained fluid. When the fluid is prevented
from flowing the viscous resistance retards the rotation.
Magnetic
Particle
This type of brake includes a fixed cylindrical enclosure housing a coil wrapped around a fixed cylinder. Within the
cylinder is a cylindrical rotor mounted on the rotating shaft. The annulus between the fixed cylinder and the rotor is filled
with magnetic particles. If there is no current in the coil the rotor is relatively free to rotate. On applying a current the
particles coalesce and prevent relative movement.
Electric
Regenerative
Shafts driven by an electric motor often using regenerative braking. When the power to the motor is removed. The motor
effectively becomes a generator driven by the shaft. The shaft is slowed as the kinetic energy is converted into electical
energy. The recovered energy is often stored when used in electric vehicles.
MECH1200
Selection Criteria
• Function of the application
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Stopping
Holding
Braking
Slowing
Speed of shaft
Horsepower of prime mover
Torque requirement
Accelerate or Stop time required
Cycle rate
Environmental conditions
Mounting method
Maintenance and servicing required or available
MECH1200
General Categories
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Positive mechanical engagement
Sliding or friction engagement
Electrical engagement
Pneumatic or hydraulic engagement
Torque limiting
Centrifugal actuation
One way motion control
MECH1200
Positive Engagement Clutches
• Relies on engagement of a dog or latch to
connect two shafts
• No slippage is possible once engaged
• Shock loads are directly transferred.
• Tapered surfaces can
permit slow engagement
• Tend to become replaced
with better equipment.
MECH1200
Friction Clutches and Brakes
• One surface has friction lining
• Surfaces brought into contact by
electromagnetic, mechanical,
pneumatic, or hydraulic
pressure.
• Radial engagement
– Band
– Expanding shoe
• Axial engagement
– Disc caliper
– Plate
– Centrifugal
MECH1200
Friction Materials
• Asbestos no longer used
• High coefficient of friction
• Heat resistant
Coefficient of Friction
Material Combination
Wet
Temp.(max)
Dry
Pressure (Ma
x)
Deg.C
MPa
Cast Iron/Cast Iron
0,05
0,15-0,20
300
0,8
Cast Iron/Steel
0,06
0,15-0,20
300
0,8-1,3
Hard Steel/Hard Steel
0,05
0,15-0,20
300
0,7
Wood/Cast Iron-steel
0,16
0,2-0,35
150
0,6
Leather/Cast Iron-steel
0,12-0,15
0,3-0,5
100
0,25
Cork/Cast Iron- Steel
0,15-0,25
0,3-0,5
100
0,1
Felt/Cast Iron- Steel
0,18
0,22
140
0,06
Woven Asbestos/Cast Iron- Steel
0,1-0,2
0,3-0,6
250
0,7
Moulded Asbestos/Cast Iron- Steel
0,08-0,12
0,2-0,5
250
1,0
Impregnated Asbestos/Cast Iron- Steel
0,12
0,32
350
1.0
Carbon-graphite/Cast Iron- Steel
0,05-0,1
0,25
500
2.1
Kelvar/Cast Iron- Steel
0,05-0,1
0,35
325
3,0
Electrical Clutch and Brake
• Electrically activated
friction elements
– Uses electromagnet to
apply force
• Magnetic particle
– Magnetic particles
drawn by coil
• Eddy current
–?
MECH1200
MECH1200
Wet and Multiplate Clutch
Multiple plate clutch
This type of clutch has several driving members interleaved or "stacked" with several
driven members. It is used in race cars including F1, IndyCar, World Rally and even
most club racing, motorcycles, automatic transmissions and in some diesel
locomotives with mechanical transmissions. It is also used in some electronically
controlled all-wheel drive systems.
Wet vs. dry
A wet clutch is immersed in a cooling lubricating fluid which also keeps the surfaces
clean and gives smoother performance and longer life. Wet clutches, however, tend
to lose some energy to the liquid. Since the surfaces of a wet clutch can be slippery
(as with a motorcycle clutch bathed in engine oil), stacking multiple clutch discs can
compensate for the lower coefficient of friction and so eliminate slippage under
power when fully engaged.
MECH1200
Pneumatic and hydraulic
• Air pressure expands
diaphragm or piston to
exert force on the friction
material.
• Springs are used to
disengage
• Hydraulic actuation can
supply higher pressure.
MECH1200
Torque Limiting Clutches
• Provide overload protection
• Pressure is applied to friction discs by spring
pressure and adjusting nuts.
• Intended as an overload protection, not for
repetitive starts
MECH1200
Fluid Clutch
• Function to provide soft start, delayed
engagement, reduced starting load, and
overload protection.
• Uses a fluid torque converter.
• Consists of an impeller and running vanes in
an oil filled housing.
• Dry fluid clutch involves the use of shot
between the housing and impellers.
MECH1200
Overrunning clutches
• Terms – cam clutch, free-wheeling clutch,
backstop, sprag clutch, roller ramp clutch, onway clutch, or single direction clutch.
• Allows rotation in one direction and prevents
rotation in the opposite direction.
MECH1200
Overrunning clutch types
• Pawl and ratchet
– Most primitive
– Pawl catches on backside of teeth
• Ramp clutch
– Spring loaded ball engages outer ring via flat ramp
• Sprag
– Special shaped locking cams engage inner and
outer ring
– Garter spring keeps cams centered
• Wrap spring clutch
– Hubs fit inside spiral wound spring and tighten one
direction, slip the other.
MECH1200
Overrunning Clutch Applications
• Overrunning
– Two motors (main drive and inching drive) connected
to same equipment
• Indexing
• Backstopping
– Safety mechanism
– Prevent load reversal
– Can mount within gearbox
• Holdback
– Prevents inclined conveyor from reversing
MECH1200
Stopping Torque
MECH1200
Stopping Torque
MECH1200
Stopping Torque