Fundamental Friction Clutch
Friction clutch; input signals are static friction torque and sliding friction torque.
Description
Block Fundamental Friction Clutch is a mechanism that transmits torque by friction. The clutch contains two friction discs, each of which is rigidly connected to a shaft and comes into contact for engagement. Once in contact, the discs take up a frictional torque that allows power to be transferred between the shafts.
The clutch can be bidirectional or unidirectional:
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A bidirectional coupling can slip in the positive and negative directions.
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A unidirectional coupling can only slip in the positive direction.
The slip direction is positive when the driven shaft rotates faster than the main shaft and negative when it rotates slower. The unit defines the slip rate as the difference:
,
where
-
- is the slip rate;
-
- is the angular velocity of the driven shaft;
-
- angular velocity of the drive shaft.
Clutch states
The clutch can be in three states:
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Locked - the actual state of the coupling in which the friction discs rotate as a unit. A locked clutch has one rotational degree of freedom. There is no power loss due to friction.
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Unlocked - the actual state of the clutch in which the friction discs slip relative to each other. An unlocked clutch has two rotational degrees of freedom. Power loss is equal to the product of slip velocity and sliding friction torque.
-
Waiting is a virtual state of the clutch in which the motion of the previous state is preserved during locking and unlocking checks. The clutch degrees of freedom and power loss depend on the previous state of the clutch.
The diagram shows the conditions under which the coupling locks and unlocks. The clutch is normally locked if the torque it transmits is within the static friction torque and the slip rate is less than the allowable slip rate. Otherwise the clutch is unlocked.
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- torque transmitted between the clutch discs.
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and are the static friction torque limits.
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- sliding friction moment between the coupling discs.
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- tolerance of coupling slip velocity.
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- slip speed between the coupling discs.
The clutch status is displayed via the physical signalling port M using the values -1, 0 and +1.
The table shows the correspondence between the states and the output values.
| State | Value |
|---|---|
Unlocked (forward stroke) or pending (forward stroke) |
+1 |
Unlocked (reverse stroke) or pending (reverse stroke) |
-1 |
Locked or unlocked initial state |
0 |
Transitions between states
At the start of the simulation, the coupling is in one of two states - locked or unlocked. The initial unlocked state is unique in that it has no direction of motion. The coupling remains in this state until the coupling slip rate becomes non-zero. The clutch then moves to the appropriate state - unlocked (reverse) or unlocked (forward) - according to the diagram.
During the simulation, the coupling is tested under different dynamic conditions to determine the appropriate state transitions, if any. The diagrams show the possible transitions, their dynamic conditions and the resulting states. If the coupling is unidirectional, the diagram is reduced to the right half.
Transition diagram for a unidirectional coupling.
Transition diagram for bidirectional coupling.
Modelling of thermal effects
Block Fundamental Friction Clutch simulates the thermal effects of frictional power loss when you turn on the thermal port.
To simulate thermal effects, you must connect the port to another unit with the thermal port enabled.
To enable the thermal port, select the Enable thermal Port checkbox for the Enable thermal Port parameters.
Ports
Input
tK - sliding friction moment, N*m
`scalar
Input port that sets the values of sliding friction torque.
t+ - upper limit of static friction torque, N*m
scalar
Input port specifying the upper boundary of the static friction torque.
t- - lower boundary of static friction torque, N*m
scalar
Input port specifying the lower bound of the static friction torque.
Output
S - clutch slip rate, rad/s
scalar
Output port related to the clutch slip rate.
M - clutch state
scalar
Output port associated with the coupling state.
Non-directional
B - drive shaft
`rotational mechanics
A mechanical rotary port associated with the drive shaft.
F - driven shaft
`rotational mechanics
A mechanical rotary port associated with a driven shaft.
H - heat flux
`heat
Heat port associated with heat flow.
Heat ports allow modelling the heat flow between the unit and the connected network.
Dependencies
To enable this port, set the Enable thermal port parameters to `enabled'.
Parameters
Main
Directionality - allowable directions of sliding between discs
Bidirectional (by default) | Unidirectional
The directions of slip of the clutch between its discs.
A bidirectional clutch allows positive and negative slip rates.
A unidirectional clutch allows only positive slip.
A unidirectional clutch is equivalent to a friction clutch connected in parallel with a unidirectional clutch, which disengages only when the slip rate becomes positive. To simulate a unidirectional clutch with negative slip, swap the base and slave port connections.
Enable thermal port - Enable thermal port
Enable (by default) | Enable
Modelling of thermal effects.
To enable thermal effects modelling, set the parameters checkbox to enabled.
Thermal mass - thermal mass associated with the thermal port
25 kJ/K (by default)| scalar.
The thermal mass associated with the heat port is H. It represents the energy required to raise the temperature of the thermal port by one degree. The greater the thermal mass, the more resistant the component is to temperature change.
Dependencies
To enable this parameter, set the Enable thermal port parameter to `enable'.
Clutch velocity tolerance - sliding velocity tolerance limit
0.001 rad/s (By default) | scalar.
The slip speed below which the clutch may lock.
The clutch will lock if, after falling below the permissible clutch speed, the sliding friction torque is not zero and the transmitted torque is between the static friction torque limits.
Initial state - initial state of the coupling
Unlocked (by default) | Locked.
State of the coupling at the beginning of the simulation. The coupling can be in one of two states - locked and unlocked.
A locked clutch makes the main and driven shafts rotate at the same speed, i.e. as a single unit. To do this, set the parameters to Locked.
An unlocked clutch allows the two shafts to rotate at different speeds, resulting in slippage between the clutch discs. To do this, set the parameters to Unlocked.
The clutch in the initial unlocked state has no slip direction. For this reason, after checking that the clutch has entered the unlocked state, the appropriate slip direction is automatically determined based on the transmission dynamics. Depending on the direction of travel, the clutch switches to the unlocked forward or reverse state.
Temperature - Clutch temperature
300 K (by default) | scalar.
Coupling temperature, is related to the thermal port H.
Dependencies
To enable this parameter, set the Enable thermal port parameters to `enable'.