Liquid friction coupling

A liquid friction coupling comprises a housing that contains a viscous liquid, a shaft protruding into said housing, alternating and interdigitating inner and outer blades, which are axially slidably and non-rotatably connected to the shaft and to the housing, respectively, annular expanding springs, which extend between adjacent inner blades or outer blades, and an annular piston, which is sealed in the housing and axially displaceable therein and is operable from the outside and arranged to act on the blades in order to change the blade spacing. In order to increase the range in which the blade spacing can be controlled and possibly to permit the coupling to be used as a clutch coupling, the volume of the viscous liquid which is contained in the housing equals the total volume of the free spaces in the housing outside the outer periphery of the inner blades when the piston is in the position which corresponds to the largest blade spacing.

BACKGROUND OF THE INVENTION 
1Field of the Invention 
This invention relates to a liquid friction coupling, which comprises a 
housing that contains a viscous liquid, a shaft protruding into said 
housing, alternating and interdigitating inner and outer blades which are 
axially slidably and non-rotatably connected to the shaft and to the 
housing, respectively, annular expanding springs, which extend between 
adjacent inner blades or outer blades, and an annular piston, which is 
sealed in the housing and axially therein and is operable from the outside 
and arranged to act on the blades in order to change the blade spacing. 
2. Description of the Prior Art 
Such a liquid friction coupling is already known from Austrian Patent 
Specification No. 384,086 and the corresponding U.S. Pat. No. 4,844,219, 
the disclosure of which is incorporated herein by reference. In that known 
coupling the width of the gaps between adjacent inner and outer blades can 
be changed to control the slip of the speed of the driven part of the 
coupling because the coupling effect and/or the torque being transmitted 
is approximately inversely proportional to the width of the gap. But it 
has been found that the control range is relatively small in practice 
particularly because it has not been taken into account in the known 
proposal that the free space within the housing should be minimized. 
Besides, even when the blade spacing is large the gaps between adjacent 
inner and outer blades will always contain liquid so that torque will 
still be transmitted. For this reason a motor vehicle which has two driven 
axles and is provided with such a coupling often requires additional means 
for interrupting the power train extending between the axles if an 
improved handling of the vehicle is to be ensured during braking. 
SUMMARY OF THE INVENTION 
For this reason it is an object of the invention so to improve the liquid 
friction coupling described first hereinbefore that its control range can 
considerably be increased and even an operation involving no liquid 
friction and no torque transmission at all will be enabled and this can be 
accomplished with very simple means. 
This object is accomplished in accordance with the invention in that the 
volume of the viscous liquid which is contained in the housing equals the 
total volume of the free spaces in the housing outside the outer periphery 
of the inner blades when the piston is in the position which corresponds 
to the largest blade spacing. 
The invention is based on the recognition that during a rotation of the 
liquid friction coupling the centrifugal force acting on the liquid will 
effect a segregation of liquid and air and will cause said liquid to form 
an annular body of liquid, which adjoins the inside peripheral surface of 
the housing. If the total volume of those free spaces which lie in the 
housing outside the outer periphery of the inner blades has the stated 
magnitude relative to the amount of liquid, no liquid will be disposed 
between the inner blades when the piston is in the position which 
corresponds to the largest blade spacing so that no torque can then be 
transmitted. In that case it will obviously be required for a satisfactory 
function that during an inward movement of the piston in order to reduce 
the width of the gaps adjacent inner and outer blades the liquid will be 
compelled to escape into said gaps so that the torque being transmitted 
will increase. In dependence on the dimensions the torque being 
transmitted can be increased to such a magnitude that the two parts of the 
coupling are non-rotatably connected to each other. In order to avoid an 
existance of spaces into which the liquid could escape during an inward 
movement of the annular piston, that piston has at its end facing the 
blades a continuous planar end face which is normal to the axis of the 
coupling. 
If the liquid friction coupling is incorporated in a motor vehicle it will 
be required that the torque being transmitted by the coupling can be 
decreased quickly so that the two driven axles can quickly be disconnected 
in order to ensure a stable handling of the vehicle during a braking 
thereof. 
In the present coupling that effect is ensured during a quick outward 
movement of the piston in the torque-decreasing direction because various 
results are produced by the features of the invention. For instance, the 
fact that the annular body of liquid is free of air will have the result 
that a certain suction will be exerted if the piston is moved as fast as 
possible in the outward direction so that part of the liquid will be 
sucked out of the gaps between adjacent inner and outer blades. At the 
same time the highly compressed air inside the liquid friction coupling 
will force the liquid out of said gaps so that cushions of air will be 
formed between adjacent inner and outer blades. It will be understood that 
the centrifugal force acting on the liquid constitutes an additional 
radially outwardly directed force, by which a complete escape of the 
liquid out of said gaps will be ensured. In that case the widening of said 
gaps will facilitate the flow of liquid out of said gaps and will more 
than proportionately reduce the average local fill factor i.e., the 
quotient of the volume of liquid contained in the region in which adjacent 
inner and outer blades which define a given gap overlap, and the entire 
volume. There is no need to specify the required viscosity of the liquid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The liquid friction coupling comprises a housing 1, which is adapted to be 
non-rotatably connected by splines 2 to a shaft, not shown. The coupling 
also comprises a tubular shaft 3, which protrudes into the housing 1 and 
is provided with splines for a connection to an external shaft that is to 
be coupled. The housing 1 contains a viscous liquid and also contains 
interdigitating inner blades 5 and outer blades 6, which are arranged in 
alternation and are axially slidably and non-rotatably connected to the 
tubular shaft 3 and the housing 1, respectively. Disk springs 7 are 
provided between adjacent inner and outer blades 5 and 6 and urge said 
adjacent blades apart. An annular piston 8 is disposed in the housing 1 
and at the end which faces the blades 5, 6 has a perfectly planar end 
face. That piston can be displaced from the outside of the housing 1 by 
means of an adjusting ring 9 in order to change the spacing of the blades 
5, 6. 
During a rotation of the coupling, a centrifugal force will be generated 
which causes the viscous liquid contained in the coupling to form an 
annular body of liquid 10. When the piston 8 is in the position which is 
shown in FIG. 1 and corresponds to the largest blade spacing, the volume 
of liquid contained in the housing equals the volume of the free spaces 
which exist in the housing 1 outside the outer periphery of the inner 
blades 5. The disk springs 7 always ensure that the gaps between adjacent 
inner and outer blades will have a uniform width within the entire set of 
blades regardless of the position of the annular piston 8. 
The annular piston 8 can be moved inwardly to the position which is shown 
in FIG. 2 to reduce the width of the gaps to a minimum. That position may 
be defined by a stop or in that the springs 7 are entirely flattened. 
During such inward movement of the piston 8 the viscous liquid is forced 
to flow from the annular body of liquid 10 into gaps remaining between 
adjacent inner and outer blades 5 and 6. 
From the foregoing description and the drawing it is apparent that the 
coupling comprises a first set of axially spaced apart inner blades 5, 
which are axially slidably mounted on and non-rotatably connected to the 
tubular shaft 3 and have outer peripheries which are radially inwardly 
spaced from the housing 1 and define with said housing an annular space. A 
second set of axially spaced apart outer blades 6 are provided, which 
alternate and interdigitate with the inner blades 5 and are axially 
slidably mounted in and non-rotatably connected to said housing 1 and 
extend radially through said annular space. A plurality of annular 
expanding springs 7 are provided, each of which is disposed between 
adjacent inner blades 5 and urges said adjacent blades apart. An annular 
piston 8 is axially slidably mounted and sealed in said housing and is 
axially operable from the outside of said housing to act on the blades 5, 
6 in order to control the axial spacing of adjacent ones of the blades 5, 
6. The piston 8 is axially movable to and from a predetermined position, 
which is shown in FIG. 1 and which determines a largest possible axial 
spacing of the blades 5, 6 and in which said annular space inasmuch as it 
is not occupied by said outer blades 6 has a predetermined volume. The 
viscous liquid 10 which is contained in the housing 1 has in a gas-free 
state a volume which is not in excess of said predetermined volume.