MECHENG236 Clutches Flashcards
What is a clutch?
Mechanical device to transmit power
Conncet and disconnect power source to output when needed
Prevent relative motion between shafts when engaged. Brakes look similar but brakes prevent all motion!)
Why and where to use a clutch?
Power source - low to no load often required at start up. Prevent damage from output driving the power source or excessive load
Safety: isolate output from operating prime mover
Clutch operational considerations for selection
Default operating state
Space limitations
Operating environment
Types of clutches
Mechanical contact
-Positive contact, friction, overrunning, centrifugal
Magnetic
Fluid couplings
Positive contact clutches
Axial teeth engage with their opposite
Rigid mechanical connection, no slip possible
Larger teeth = more torque transmission
Two types of positive contact clutches
Square jaw
Multiple serration
Positive contact clutch adv/disadv
- Advantages
- Physical barrier to slip – high load capacity
- Small diameter vs friction (high load per tooth, material-limited) * Simple, low-cost
- Long life (if used correctly!)
- Disadvantages
- Require precise tooth alignment
- More teeth reduces this, but increases cost!
- Do not permit running engagement
- Wear on teeth reduces load capacity,
increases failure risk
Overrunning clutches
One-way clutches
Allow rotation only one direction
Overrunning clutches - 3 types of operation
- Overrunning - allows driven component to spin faster than the driving shaft (save pedalling downhill on bike)
- Indexing operation allows an intermittent, one-way rotation (ratchet)
- Backstopping - allows rotation one way only - effectively can be a form of emergency brake
Overrunning clutches types
Sprag clutch (peanut shape) - high torque
Roller clutch - smaller diameter, less prone to jamming/wear. Roller held by springs
Centrifugal clutches
Engage only above a pre-determined speed
Internal friction elements fling out, lock, as speed increases. Springs set speed required to engage
Allow low start loads, idle without driving output. e.g. chainsaw
Types of non-contact clutches
Magnetic
Fluid couplings
Property of non-contact clutches
Do not rely on direct mechanical contact (teeth, friction) to transmit torque
Magnetic clutches properties
Drag output via magnetic field
Smooth, quiet engage/disengage
Long life, no wear (except bearings), no mech actuation
High cost, heat, torque-limited
Applications: medical
Fluid coupling clutches properties
Use hydrodynamics to connect vane arrays
Momentum added to fluid at input
Smooth engagement, disengagement. Allows slip
Leaks, torque-limited
