Clutches Flashcards
positive contact
- depend on interlocking parts for engagement and do not allow slippage
- starts with a bang right away
Purpose of clutches
-to engage or disengage a driven machine without starting or stopping the prime mover.
Types of positive contact clutches
-square jaw, spiral jaw, multiple-tooth design
Square Jaw
- bidirectional, one jaw is splined or keyed and can move axially to engage the other
- engaged only at very low speed 10 RPM
Spiral Jaw
- engaged at higher speeds 150 RPM
- one direction only as a rotation in the opposite direction causes the clutch to disengage aka over running
Multiple Tooth
- use a large number of small gear teeth or serrations to provide engagement
- engaged at 300 RPM
Friction Clutches
- smooth transfer of power
- depend on coefficient of friction between the clutch surfaces and a pressure plate
- engage at any speed
Clutch Plates
- dampening devices (springs) to reduce shock and torsional vibration
- solid or discs of steel or bronze
- sintered bronze bonded to a steel backing
Radial Clutches
- expanding shoe
- contracting shoe
Expanding Shoe
- a set of shoes connected to one shaft. Expand outward to engage the inner section of a drum connected to another shaft
- engaged by external linkage that can be over centred
- receives extra force on the shoes from centrifugal force
- activated mechanically
Contracting Shoe
-Disengagement is assisted by centrifugal force pushing the pads away from the drum (opposite of expanding shoe)
Axial Clutches
- single disc
- multidisc
- cone
over centering
-locking it in. (will show a handle being pushed forward)
Axial Friction Clutches
-use a soft friction disc sandwiched between harder pressure plates
Types of Axial Friction Clutches
-single, multi friction and cone
Single Friction
- clutch plate is sandwiched between the pressure plate and the flywheel when engaged
- actuation is spring applied and mechanical release
- a release bearing or sleeve is used to push on fingers of the pressure plate to disengage the clutch
Multi-Disc
- the more discs the more power
- increase power without adding to disc diameter
Electromagnetically Activated
-uses magnetic field to pull a moveable arm with a friction disc mounted to its face
Cone Clutches
- the cone wedges provide a mechanical advantage due to the wedge in the drum
- they have a tendency to grab
Hydraulic clutch
- similar to a fluid coupling except its fluid can be controlled
- speed depends on speed of impeller and the amount of oil in the coupling
Scoop Tube
-
Electric and Magnetic
- Torque transmitted is proportional to the coil current or strength of the magnetic field
- Hysteresis
- magnetic particle
- eddy current
Hysteresis
- electronically activated
- electromagnetic coil on the input rotor and a hardened steel drag cup on the output shaft
- can slip
Eddy Current
- two drums running very close to each other on soft iron rotors
- transmit high torque with moderate speeds
- can slip
Magnetic Particle
- output housing contains magnetic metal particles
- when coil is energized the metal particles form a link between the input and output
- can slip
Torque Limiting Clutches
- driven member is held between friction plates under a predetermined amount of pressure
- main adjustment is a torque bolt with fine adjustments with cap screws
One way (overrunning) clutches
- roller ramp
- cam type
- wrap spring
Roller Ramp
-compression springs and rollers transmit the power
Cam Type (Sprag)
-if the inner race attempts to rotate clockwise at a higher rpm than the outer race, the cam will lock and power from the inner race will be transferred to the outer.
Wrap Spring (very basic clutch)
-used on extremely low power applications and moderately large torque
One way or over running
- simple overrunning
- indexing
- holdbacks/backstops
Over running
Indexing
- position contact in one direction
- move, fill, move
Dynamic Brakes
-the brakes need to slow or stop the inertia and mass of the load, as well as all of the moving and rotating parts
Disc Brakes compared to drum brakes
-disc brakes are stronger because they have a larger sweep area.
Hydraulic Brakes
- similar to fluid couplings but the turbine or runner is anchored
- to activate fluid is pumped into the retarder
One way clutch
-to move freely in one direction but to lock up in the opposite direction
