Rotary flywheel skid sensing means for vehicle hydraulic braking systems

A rotary flywheel mechanism comprising a flywheel driven from a wheel driven shaft through a thrust member and angularly spaced balls located in complementary recesses in adjacent faces of the flywheel and the thrust member is disclosed. A locking mechanism is incorporated to prevent operation of the flywheel mechanism until the vehicle has attained a predetermined minimum road speed. The locking mechanism comprises a centrifugally-operable weight carried by the flywheel and which is normally urged inwardly by a spring and into engagement with the thrust member to lock the flywheel and the thrust member against relative movement. The centrifugal force generated at speeds above the minimum speed urges the weight outwardly to unlock the flywheel and the thrust member. The weight may comprise a plunger guided for radial movement in a radial bore in the flywheel, or the weight may comprise a mass carried by a lever which is angularly movable about a pivot parallel to the axis of the shaft.

This invention relates to rotary flywheel skid sensing means for a vehicle 
hydraulic braking system of the kind comprising a flywheel mechanism 
rotatable with a shaft adapted to be driven from a wheel to be braked and 
including a flywheel member and a thrust member together with camming 
means responsive to relative angular movement between the two said members 
to cause one of the said members to move axially relative to the other 
between a first angular position in which the two members are in a 
predetermined angular alignment and a second angular position in which the 
said one axially movable member co-operates with means to actuate brake 
pressure modulating means. 
In skid sensing means of the kind set forth problems may arise when a 
vehicle is travelling at low speeds, for example in traffic and, in 
response to a panic brake application, the brakes can be relieved 
prematurely, albeit in response to a genuine skid signal. Of course this 
will depend upon the sensitivity of the skid sensing means but it is 
desirable to prevent the skid sensing means from operating until the 
vehicle is travelling at a predetermined minimum speed, thereby obviating 
the danger of the vehicle "rolling-onto" a vehicle in front should that 
vehicle stop quickly. 
We are aware of GB No. 1 241 798 which discloses skid sensing means, 
particularly for heavy vehicles, in which inhibit means are incorporated 
for preventing operation of the skid sensing means at low vehicle speeds. 
The skid sensing means of GB No. 1 241 798 comprises a first 
speed-responsive governor rotatable synchronously with a wheel-driven 
rotatable member and adapted to effect brake release by displacing a 
control element axially with respect to the rotatable member when the 
speed of rotation of the rotatable member is at or below a critical speed 
at which the first speed-responsive governor mechanism will collapse. The 
inhibit means comprises a second speed-responsive governor mechanism which 
has a critical speed greater than that of the first governor mechanism 
and, in a collapsed position at low vehicle speeds, co-operates with the 
control element to prevent it from moving axially with respect to the 
rotatable member, thereby preventing the first governor mechanism from 
collapsing to release the brake. Each governor mechanism comprises a set 
of weights arranged circumferentially around the rotatable member, and 
each weight is secured to the outer end of an arm which, in turn, is 
pivotally mounted for angular movement about an axis lying in a plane 
normal to the axis of the rotatable member so that the arms and their 
respective weights move through arcs lying in planes containing the axis 
of the rotatable member. 
According to our invention in a skid sensing means of the kind set forth 
centrifugally-operable locking means are provided for locking the two 
members together in the said first angular position against relative 
movement, and the locking means are automatically releasable in response 
to a centrifugal force of a magnitude which corresponds to a predetermined 
minimum road speed for the vehicle, the locking means comprising at least 
one weight which is movable in a plane normal to the axis of the shaft, 
the weight being normally urged inwardly by resilient biassing means into 
a locking position to lock the two members together, and the weight being 
movable in an outward direction and into a disengaged position when the 
centrifugal force to which it is subjected attains a value greater than 
that corresponding to the said minimum road speed, whereafter the two 
members are free to move relative to each other. 
The weight may comprise a plunger which is guided in a radial bore in one 
of the members and is resiliently biassed into engagement at its inner end 
in a complementary notch in the other member. Alternatively the weight may 
be pivotally mounted on a pivot on one of the members for movement about 
an axis parallel to the axis of the shaft, with a spring provided normally 
on a pivot for urging a portion of the weight into engagement with a 
complementary notch in the other member. 
The notch may be of substantial circumferential length and may comprise a 
continuous groove. The engagement of the weight in the groove prevents 
relative axial movement between the two members. Alternatively the notch 
may comprise an axial slot, the engagement of the weight in which prevents 
relative angular movement between the two members. 
When the locking means comprises a plunger, two such plungers are provided 
and are preferably diametrically arranged with respect to each other. 
In one construction the flywheel member is journalled for rotation on the 
shaft and is fixed axially, and the thrust member is movable axially and 
of annular outline, being received, at least partially, in an annular 
recess in one end of the flywheel. In such a construction the locking 
means are carried by the flywheel member and the notch is provided in the 
adjacent outer face of the thrust member.

The sensing means illustrated in FIGS. 1 and 2 of the drawings comprises a 
housing 1 incorporating an hydraulic pump 2 and a combined pressure dump 
valve and modulator assembly 3. A longitudinally extending shaft 4 
projecting at opposite ends from the housing 1 is coupled at one end to 
the wheel to be braked and at the other end carries skid sensing means 5 
in the form of a flywheel assembly which is enclosed within a cylindrical 
radial rim portion 6 of the housing 1 of which the open, outer, end is 
closed by a detachable cover 7. The rim portion 6 protects the mechanism 
from radially directed flying debris and other objects. 
The flywheel assembly 5 comprises a flywheel 10 which is rotatable on 
spaced bearings 11 and 12 adjacent to one end of the shaft 4, and an 
annular thrust member 13 which is at least partially accommodated within 
an annular recess 14 in the end of the flywheel 10 which faces the outer 
end of the rim portion 6. A clearance is provided between the outer edge 
of the thrust member 13 and the outer wall of the recess 14, and between 
the inner edge of the thrust member 13 and the inner wall of the recess 
14. 
Three angularly spaced balls 15 are located in complementary recesses 16, 
17 in adjacent faces of the flywheel 10 and the thrust member 13. 
The thrust member 13 is carried from the free end of the shaft 4 by 
mounting means 18. The mounting means comprises a radial driving flange 19 
which is fast with the free end of the shaft 4, and is provided at its 
peripheral edge with a pair of diametrically arranged parallel-sided lugs 
20 which project axially towards the outer end of the rim portion 6, and a 
clutch plate 21 of dished outline which engages frictionally at its outer 
peripheral edge with a radial clutch face 22 on the thrust member 13 and 
in which is provided a pair of diametrically spaced notches 23 which 
slidably receive the lugs 20. The dished form in the clutch plate 21 
provides a clearance with respect to the free end of the shaft 4. 
The thrust member 13 is urged towards the flywheel 10 by a force from a 
bias spring (not shown) and this force is transmitted to the thrust member 
13 through the clutch plate 21 and an angular lever 24 mounted in the 
housing 1. The lever 24 acts on the clutch plate 21 in a position in axial 
alignment with the rotational axis of the shaft 4 through an adjustable 
thrust-transmitting member 25 which is screwed through the lever 24 so 
that the biassing force is applied centrally to the mounting means 18. 
Since the thrust member 13 can articulate and move radially with respect 
to the flywheel 10, the load from the bias spring is shared substantially 
equally between the three balls 15. 
A pair of diametrically opposed bob-weights 30 in the form of plungers are 
located in radial bores 31 in the outer wall or rim 39 of the flywheel 10 
which defines the outer wall of the annular recess 14. The radial bores 31 
lie on a plane normal to the axis of the shaft 4. Each plunger 30 has an 
inner end portion 32 of reduced diameter which is normally urged by a 
compression spring 33 in the respective bore 31 into engagement with a 
circumferentially extending groove 34 in the adjacent outer face of the 
thrust member 13. 
Normally, therefore, the plungers 30 are urged inwardly by the springs 33 
to engage in the groove 34. This locks the flywheel 10 and the thrust 
member 13 against relative axial movement and, because of the presence of 
the balls 15 housed in the recesses 16 and 17, also against relative 
angular movement. 
The weight of the plungers 30 is chosen such that at vehicle road speeds 
above-predetermined minimum speed, the centrifugal force so generated is 
sufficient to urge the plungers 30 in a radially outwards direction, 
withdrawing them from the groove 34 in order to unlock the flywheel 10 and 
the thrust member 13. In fact the weight of the plungers 30 is chosen such 
that the plungers 30 will not release the skid sensing means below a 
vehicle minimum road speed of for example, 10 m.p.h. 
After the plungers 30 have been released, in further operation relative 
angular movement between the flywheel 10 and the thrust member 13, due to 
excessive deceleration of the wheel and the shaft, with the flywheel 10 
running on due to its inertia, causes the balls 15 to tend to ride out of 
the recesses 16 and 17 thereby urging the thrust member 13 and the clutch 
plate 21 axially away from the flywheel 10. This moves the lever 24 
angularly, in turn to operate the combined modulator and dump valve 
assembly. The clutch plate 21 slides on the lugs 20. Thereafter the 
flywheel 10 and the thrust member 13 run on against the frictional force 
of engagement between the clutch plate 21 and the clutch face 22 of which 
the threshold is determined by the loading in the bias spring. This 
threshold can be readily adjusted or otherwise set by adjusting the axial 
position of the member 25 relative to the lever 24, after removal of the 
cover 7. 
In modification the circumferential groove 34 is replaced by an axial slot. 
In another modification the bores 31 and the compression springs 33 are 
omitted and each of the pair of diameterically opposed bob-weights 30 is 
carried by one end of a strip of resilient material, suitably an elongate 
blade spring or leaf spring, which extends in a generally circumferential 
direction and of which the other end co-operates with the flywheel, 
suitably by means of a rigid connection or other anchorage. The blade or 
leaf springs act to bias the bob-weights 30 substantially in the same 
directions as they are biassed by the compression springs 33. The 
construction and operation of this modified sensing means is otherwise the 
same as that of FIGS. 1 and 2. 
In the construction illustrated in FIGS. 3 and 4 of the drawings, a weight 
40 is pivotally mounted at one end for angular movement through an arc in 
a plane normal to the axis of the shaft. The weight 40 is movable about a 
pivot 41 in the end face of the flywheel 10 adjacent to the recess 14 and 
the pivot 41 is parallel to the axis of the shaft 4. The weight 40 
comprises lever 42 extending circumferentially from the pivot 41, and a 
mass 43 carried by the radially outermost end of the lever 42 and disposed 
relatively close to the rim 39 of the flywheel 10. The mass 43 is of 
arcuate outline and projects circumferentially beyond the outer end of the 
lever 42 which is remote from the pivot 41. An inwardly directed 
projection 44 at the outer end of the lever 42 is engageable in an axially 
extending slot 45 in the outer face of the thrust member 13. 
A tension spring 46 is attached at opposite ends between a slight recess 47 
between the lever 42 and the end of the mass 43 adjacent to the pivot 41, 
and an anchorage 48 in the end face of the flywheel 10 and spaced from the 
outer ends of the lever 42 and the mass 43. The recess 47 is displaced 
radially outwards from a chord 49 passing through the pivot 41 and the 
anchorage 48 so that the spring 46 applies a resultant force to the lever 
42 in a direction normally to urge the projection into registry in the 
slot 45. 
A lightening hole 50 is provided in the end wall of the flywheel 10 to 
compensate for the weight of the weight 40. 
By providing the majority of the weight in the mass 43 and arranging it 
close to the rim 39 of the flywheel 10 increases the centrifugal effect 
obtained for a given speed in comparison with that obtained in the 
embodiment of FIGS. 1 and 2. This is of advantage in providing a more 
sudden action to enable the projection 44 to snap into and out of 
engagement in the slot 45. 
This is achieved by holding the distance ".chi." between the line of action 
passing through the chord 49, and the line of action 51 passing through 
the spring 46 to a relatively small dimension. 
It follows, as shown in FIG. 4, that taking moments about 41 gives 
EQU Fr.times.L=Fs.times..chi., 
where Fs is the force in the spring 46; and Fr is the centrifugal force 
acting at the lever radius "L". 
Thus the centrifugal force Fr can rapidly overcome the action of the spring 
force Fs beyond the amount by which the spring 46 extends, due to movement 
of the lever 42, which is relatively small. 
The construction and operation of the skid sensing means of FIGS. 3 and 4 
are otherwise the same as FIGS. 1 and 2, and corresponding reference 
numerals have been applied to corresponding parts. 
The skid sensing means described above are suitable for use with motor 
cycles or other light motor vehicles, such as light passenger cars.