Note: Descriptions are shown in the official language in which they were submitted.
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~ his invention relates to a control coupling which
includes a viscous shear coupling and a friction clutch,
the control coupling comprising inner and outer coupling
parts defining an annular space therebetween, and two
sets of annular plates disposed in such annular space,
each set comprising mutually interleaved plates with
alternate ones thereof rotationally fast with one of said
coupling parts and the remaining ones thereof
rotationally fast with the other coupling part, the
plates of one of said sets being spaced from one another
and a viscous liquid being accommodated therebetween to
form a viscous shear coupling, and the plates of the
other set being movable by a piston arrangement into
engagement with one another for torque transmission
between the coupling parts by friction between the
plates. Such a control coupling will hereafter be
referred to as a coupling of the kind specified.
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A control coupling of the kind specified is
disclosed in US PS 4058027. In that coupling, the
purpose of the plates which constitute the friction
clutch is to compensate for the drop in torque
transmissible by the viscous shear coupling when its
temperature~rises, and to assist the temperature of the
viscous coupling to fall by partly taking over the
transmission of torque. Expansion of the viscous liquid
as its temperature rises is used to cause the friction
clutch plates to be urged together, by way of a
differential piston arrangement.
One disadvantage of such a control coupling is that
even when torque is being transmitted between the
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coupling parts by the friction clutch plates, the viscous
shear coupling still participates to a considerable
extent in torque transmission. The apportionment of
torque between the viscous shear coupling and friction
clutch is highly temperature dependent, so that torque
transmitted by the control coupling as a whole is
relatiYely uncontrollable.
Yet a further disadvantage is that the plates
forming the viscous shear coupling are not separated from
the plates forming the friction clutch, so that both sets
of plates have to operate in the presence of the same
liquid. ~uch liquid will usually be selected for its
properties in relation to the viscous shear coupling, but
such a liquid may not be the optimum for the friction
clutch.
Yet another disadvantage is that the maximum torque
transmissible by the control coupling as a whole cannot
be clearly defined, as it depends upon the temperature of
the viscous shear coupling elements thereof.
It is the object of the present invention to provide
a control coupling in which these disadvantages are
overcome or reduced, i.e. its torque transmission
characteristics should be capable of being well defined
in relation to temperature and its maximum torque
transmission capability should be largely independent of
the transmission behaviour of the viscous shear coupling.
~urther, it should be possible to operate the viscous
shear coupling and friction clutch in the presence of
appropriate different media.
According to the present invention, we provide a
control coupli~ng of the kind specified wherein a fluid
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tight axially movable annular piston i8 disposed between
said sets of plates, said piston being arranged, by
movement in one direction upon expansion of the liquid in
said viscous shear coupling, to urge said plates forming
the friction clutch into torque transmitting engagement
with one another, the movement of said piston in said
direction being limited by stop means and there being
spring means interposed between said piston and said
friction clutch plates, the torque transmissible by said
viscous shear coupling being negligibly low compared with
that transmitted by said friction clutch when the latter
is transmitting torque.
une advantage of a control coupling according to the
invention is that uncontrollable temperature dependent
torque transmitting behaviour of the viscous shear
coupling cannot lead to uncontrollable torque
transmission by the control coupling as a whole, since
the torque is determined by the friction clutch and the
maximum torque is limited by contact of the piston with
said stop means, and the characteristics of the spring
means interposed between the piston and pla-tes forming
the friction clutch. Any further increase in pressure of
the liquid in the viscous shear coupling would not
increase the torque transmitted by the control coupling
as a whole.
A control coupling according to the invention may be
disposed in a drive line connecting two axles of a motor
vehicle. ~hen thus installed, the maximum torque capable
of being transmitted by the control coupling is
preferably slightly higher than the maximum torque which
ca~ be introduced by the vehicle's engine into such
inter-axle drive line. When we refer to "axles" of a
vehicle, it is to be understood that we do not
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necessarily mean live axles or, indeed, any particularsuspension and drive arrangement, but rather a pair of
drivable wheels, either at the front or rear, of the
vehicle.
In such an installation, the control coupling is
capable of automatically causing engagement of a second
drive axle when any wheel slip occurs at the main drive
axles~ ~imultaneously, excessive drive line torques are
avoided. In effect, the control coupling provides for
automatic engagement of four wheel dri~e in a vehicle
which normally has only two wheel drive, without
requiring an inter-axle differentialO
A control coupling according to the invention may
also be used in a differential unit, to limit slippage
between two outputs thereof whilst still being capable of
accommodating differences in output speeds due to
cornering.
A further feature of the present invention is
concerned with the ability rapidly to discontinue torque
transmission by the friction clutch of the control
coupling when requiredD This may be required, for
example, for safety reasons.
It has been proposed, in G~ P~ 414662, that torque
transferrable by a fluid clutch can be controlled by
changing the quantity of fluid therein, and if all fluid
is withdrawn from the clutch into storage chambers, no
torque can be transferred. ~uch changes in torque 9
however, are very slow and thus the arrangement is not
suitable where a rapid change into a free wheeling
condition is necessary.
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According to this further feature of the invention,
at the end of said set of plates which forms said
friction clutch of the control coupling, remote from said
piston, there may be provided an annular reaction member
which is spring biased towards said piston by a second
spring means which is stronger than the first said spring
means associated with said piston, said reaction member
being movable away from said piston against said second
spring means by an electro-magnet through a distance such
that when said piston contacts said stop means, said
first spring means is completely unloaded.
Thus, when the reaction member has been moved by the
electro-magnet, because the first spring means is
completely unloaded the friction clutch can transmit no
torque whatsoever. ~he change to such "freewheeling"
condition is instantaneous by operation of the
electro-magnet.
~ control coupling having this feature may be useful
in a motor vehicle where an automatically engaged further
drive axle may be required to be disengaged, e.g. when
the brakes of the vehicle are applied. This may be
necessary if the vehicle is equipped with an anti-lock
bra~ing system with the operation of which there might be
interference when the further axle is driven. The
electro-magnet of the control coupling can easily be
brought into operation by application of the vehicle's
brakes, so that the wheels of the further axle act as
undriven wheels during braking.
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This principle may also be applied to differential
units which achieve a "limited slip" effect by use
of a control coupling according to the invention,
- and whose effect is required to be eliminated during
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braking.
The invention will now be described by way of
example with reference to the accompanying drawings, of
which:-
~ igure l is a longitudinal section through part of acontrol coupling according to the invention;
Figure 2 is a schematic illustration of the
application of a control coupling according to the
invention in a motor vehicle drive line;
Figure ~ is a section showing the application of a
control coupling according to the invention to a
differential unit;
Figure 4 is a longitudinal section through part of a
further embodiment of control coupling according to the
invention.
Referring firstly to ~igure l of the drawings, the
control coupling of which part is illustrated comprises
an outer coupling part t, which is in the form of two
components having abutting flanges secured together by
bolts, and an inner coupling part 2 which also comprises
two components abutting one another. The inner coupling
part defines a through bore 16 which has axially
extending splines 15, and the two components of the inner
part 2 are maintained rotationally fast with one another
by virtue of their engagement, in use, with an externally
splined shaft fi-tting in the bore 16. ~he outer coupling
part l is provided with a flange 14 having screw-threaded
recesses 13 for a bolted connection to a driving flange
on a shaft element.
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Two sets of annular plates are disposed between the
coupling parts l 9 2, one set of plates constituting a
viscous shear coupling and the other set of plates
constituting a friction clutch. The viscous shear
coupling comprises a set of mutually interleaved plates
6, of which alternate plates are rotationally fast with
the outer coupling part l and the remaining plates are
rotationally fast with the inner coupling part 2. The
plates are spaced from one another by spacers 7, e.g.
wire rings, and are limited in their axial movement by
outer and inner axial fixing rings 18a, 18b.
'~he friction clutch 11 comprises a set of mutually
interleaved plates 12, of which again alternate ones are
rotationally fast with the outer coupling part l and the
other alternate plates fast with the inner coupling part
2. These plates, as for the plates 6, may have their
connection to the coupling parts established by splined
peripheral profiles.
Between the plates constituting the viscous shear
coupling and the plates constituting the friction clutch,
there is disposed an annular piston ~. Annular seals 4
render the piston fluid tight, while permitting axial
movement thereof. lhe chamber containing the plates 6 is
filled with an appropriate liquid of known type for use
in a viscous shear coupling, and the chamber containing
the friction clutch plates 12 either is filled with an
appropriate liquid for service in such conditions, or
else contains no liquid at all.
lmmediately to the right of the piston ~, with
reference to the drawing, there is disposed an annular
pressure plate 10, which is rotationally fast with the
outer coupling part 1 and movable axially relative
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thereto. ~he piston 3 has a plurality of
circumferentially spaced bores open towards the pressure
plate 10 and extending through bosses 9 on the piston,
and the pressure plate has projections 19 exten~ing into
the bores in bosses 9. Compression springs 8 are
provided in the bores in the piston 3, abutting the
closed ends of such bores and the ends of projections 19.
By virtue of the engagement of projections 19 in bosses
9, the piston 3 is, with the pressure plate 10,
rotationally fast with the outer coupling part 1.
Movement of piston towards the plates of friction
clutch 11 is limited by engagement of the piston with a
stop 17. Opposite the pressure plate 10, the end one of
the plates 12 abuts a reaction member 30 fixed to the
outer coupling par-t 1 and having sealing engagement with
the inner coupling part 2 by way of a seal 31.
In use, if due to slip between transmission
components to which the inner and outer coupling parts 1,
2 are connected there is relative angular movement
between these parts, the plates 6 forming the viscous
shear coupling 5 tran~mit torque between the coupling
parts by the shearing action on the viscous liquid there-
be-tween, in known manner. The shear forces which occur
in the liquid will cause a rise in the tempera-ture
thereof and consequent expansion, which will displace
piston 3 to the right with reference to the drawing, i.e.
torwards the friction clutch 11. Springs 8 are thus
compressed. Torque is transmitted between the coupling
parts 1, 2 by friction between the plates 12~ the
magnitude of such transmitted torque depending on the
strength of springs 8. Stop 17, limiting movement of
piston 3, ensures that the torque transmissible by the
friction clutch 11 is limited to a maximum value. ~he
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length of the springs 8 is preferably selected so that
there is no axial force whatsoever acting on the plates
of the friction clutch when there is no load on the
coupling, i.e. the piston 3 has not been displaced by any
expansion of the viscous liquid in the viscous shear
coupling 5.
It will be appreciated that springs ~, shown as
helical springs, may be replaced by springs of other
configurations. For example, spring washers may be
employed and it is possible to use such washers in a
manner such that any wear occuring in the friction plates
12 would cause the force exerted on the friction plates
to remain constant or nearly so, or even possibly
increase.
Referring now to ~igure 2 of the drawings, there is
shown in principle how a control coupling according to
the invention may be utilized in a motor vehicle. Ihe
illustrated vehicle has a forwardly mounted engine 20,
transmission 21, and driven front wheels by way of drive
shafts 22. A power take-off from the transmission 21 is
arranged to drive the rear axle 23 of the vehicle by way
of a control coupling 24 according to the invention, and
a longitudinally extending drive shaft 32. If the front
wheels slip, the relative rotation which occurs between
the parts of the control coupling cause torque to be
transmitted to the rear wheels of the vehicle.
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Referring now to ~igure 3 of the drawings, this
shows how a control coupling according to the invention
may be incorporated in a differential unit. ~he
differential unit comprises a casing 33, input pinion 34,
crown wheel 25, planet carrier 35 and planet bevels as 36
carried on pin 26. ~he bevels as 36 engage with side
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gears 37 splined -to stub shafts 38 provided with
respective outpu-t flanges 27. ~he inner coupling part 2
is connected to one of the stub shafts 38, and the outer
coupling part 1 is formed by part of the planet carrier
35. The viscous shear coupling and friction clutch parts
11, 5 of the control coupling according to the invention
are referenced in ~'igure 3, and piston 3 therebetween.
The control coupling ensures that if the wheel
connected to one output o~ the differential unit slips,
its maximum torque is available for the other driven
wheel.
Referring now to Figure 4 of the drawings, certain
parts of the control coupling there illustrated
correspond to parts in the embodiment of ~'igure 1, and
such parts have corresponding reference numerals to the
parts of ~igure 1 with the addition of 100. Thus, the
control coupling comprises an outer coupling part 101, a
two part inner coupling part 102 with a bore 116 having
splines 115~ A shaft with which the inner coupling part
engages is indicated at 132, The coupling comprises a
viscous shear coupling portion 105 with plates 106,
spaces 107 and fixing means 118a, 118b, and a friction
clutch arrangement 111 having plates 112, and a pressure
plate 110. There is a piston 103 with seals 104, and
projections 119 from pressure plate 110 extending into
bosses 109 on the piston with Gompression springs 108
therein. A stop 117 limits movement of the piston. Thus
far, the operation of the control coupling is as above
described in relation to Figure 1.
In this embodiment, however, the one of the plates
112 which is furthest from piston 103 abuts a reaction
member 128 which includes an outwardly extending flange
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portion. The reaction member 128 is spring biased
towards the piston 103 by a second spring means in the
form of a stack of belville washers 129, which engage the
reaction member and react against a stop such as a circlip
131 engaging the inner coupling part 102. The force of
the second spring means provided by washers 129 is
greater than that of the springs 108, so that the
reaction member normally adopts the position illustrated
in which it abuts the outer coupling part 101, and is not
moved from this position even when the piston 103 loads
the plates 112 into torque transmitting frictional
engagement. The reaction member 128 faces an electro-
magnet 130, spaced therefrom and supported by being fixed
to a transmission casing part 133, a seal 13~ being
provided between the casing part 133 and the shaft 132.
When the electro-magnet 130 is energised, it pulls the
reaction member 128 away from the piston 103, against the
spring washers 129. To minimize the force required to be
exerted by the electro-magnet to hold the reaction member
128, the spring washers 129 may be arranged to provide a
decreasing spring rate.
The distance by which the reaction member 128 is
moved by the electro-magnet 130 is selected so as to be
greater than the permissible movement of piston 103
before it meets stop 117. Thus, there is no longer any
possibility of any torque being transmitted by the plates
112 of the friction clutch 111, and a friction clutch is
instantly brought to a completely free-wheeling condition
when the electro-magnet 130 is energised.
To avoid any possibility of rubbing of the reaction
member 128 on the electro-magnet 130, travel of the
reaction member towards the magnet must be limited by
some form of stopO Conveniently, spring washers 129 may
be arrangea to be completely compressed to provide such
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limitation of movement of the reaction member.
A control coupling as shown in Figure 4 is useful in
the case where a vehicle has an anti-lock braking system
and torque transmission by the friction clutch section of
the control coupling is required to be discontinued so as
not to ~nterfere with the operation of such an anti-lock
system. it is easy to arrange for the electro-magnet of
the control system to be energized when the vehicle's
brakes are applied, e.g. by way of the switch which
operates the vehicle's brake lights.
A control coupling as shown in Figure 4 may also be
utilized in a differential unit~ if the limited slip
action of a differential is required to be disabled in
conjunction with the operation of an anti-lock braking
sy~tem.
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