GB2069642A - Improvements in rotary flywheel anti-skid braking systems - Google Patents

Abstract

In an anti-skid braking system for a vehicle lost motion is adjustable between a flywheel mechanism (3), and a dump valve (5) operable by the flywheel mechanism (3) to control a brake-applying fluid-pressure. The flywheel mechanism (3) comprises a flywheel (6) which is rotatably mounted on a shaft (2) driven from a braked wheel, a thrust member (8) mounted on, and keyed against rotation with respect to, the shaft (2), and camming means (11) disposed between the flywheel (6) and the thrust member (8) through which rotation of the shaft (2) is imparted to the flywheel (6) from the thrust member (8). Excessive deceleration of the shaft (2) causes relative rotation and relative axial movement between the flywheel (6) and the thrust member (8) to operate the dump valve (5). A degree of lost motion (30) between the flywheel mechanism (3) and the dump valve (5) is provided and a seating in the dump valve (5) is movable to adjust the size of a gap (30) such that a greater degree of lost motion is provided on a dry, high friction surface. <IMAGE>

Description

SPECIFICATION Improvements in anti-skid braking systems This invention relates to anti-skid braking systems for vehicles of the kind incorporating a flywheel mechanism and pressure-regulating means operable by the flywheel mechanism to control a brake-applying fluid-pressure, the flywheel mechanism comprising a flywheel which is rotatably mounted on a shaft adapted to be driven from a braked wheel, a thrust member mounted on, and keyed against rotation with respect to, the shaft, and camming means disposed between the flywheel and the thrust member and through which rotation of the shaft is imparted to the flywheel from the thrust member, the camming means being so constructed and arranged that relative rotation between the flywheel and the thrust member as a result of excessive deceleration of the shaft is accompanied by relative movement between the flywheel and the thrust member in an axial direction to operate the pressureregulating means.

To ignore the initial tyre "wind up" a degree of lost-motion is normally provided between the flywheel mechanism and the pressure-regulating means and this lost-motion must be taken up before the pressureregulating means can be operated. The degree of lost-motion corresponds to a free angular travel of the flywheel relative to the shaft equivalent to a loss in wheel speed which can occur before the brake-applying pressure is relieved to release the brakes of the braked wheel. However, if best results are to be obtained, we have found that different degrees of lost-motion are required, depending upon prevailing road conditions.For example, with a 3:1 step up drive between the road wheel and the flywheel, the degree of lostmotion is equivalent to an angular movement of the flywheel of substantially 45' on a dry surface, of substantially 30 on a wet surface, and of substantially 15 on an icy surface.

These figures correspond roughly to the amount of initial tyre wind up for optimum retardation when brakes are applied on the various surfaces.

According to our invention in an anti-skid braking system for a vehicle of the kind set forth a degree of lost motion is provided between the flywheel mechanism and the pressure regulating means and means is provided for adjusting the degree of lost-motion between the flywheel mechanism and the pressure-regulating means in proportion to the conditions of the surface over which the vehicle is travelling, to the deceleration of the vehicle, or to the magnitude of the brakeapplying fluid-pressure permitted by the flywheel mechanism.

Preferably the degree of lost-motion between the flywheel mechanism and the pres sure-regulating means is adjusted by means responsive to the magnitude of a control pressure acting an the pressure-regulating means, or to the brake-applying fluid pressure itself.

When the pressure-regulating means comprises a normally closed dump valve which is opened in response to a skid signal to relieve the brake-applying fluid-pressure by placing an expander chamber exposed to the control pressure in communication with a reservoir for fluid, and the flywheel mechanism acts through a lever on a valve member of the dump valve to permit it to move relatively a seating, the seating is itself bodily movable by the control pressure against the loading in a spring in order to increase the size of a gap between the lever and the flywheel mechanism, thereby automatically increasing the degree of lost-motion, with the spring acting to reduce the gap, and hence the degree of lostmotion, upon reduction in the control pressure in response-to the skid signal.

Conveniently the spring comprises a Belleville or other spring washer acting between a member incorporating the seating and a relatively stationary housing.

One embodiment of our invention is illustrated in the accompanying drawings in which: Figure 1 is a longitudinal section through a flywheel mechanism and a dump valve for a fluid-pressure operated anti-skid braking system; Figure 2 is a graph of flywheel speed and shaft speed against time during a period of wheel skid on a high friction surface, and of the corresponding brake-applying pressure during the same period; Figure 3 is a graph similar to Fig. 2, but during a period of wheel skid on a low friction surface; and Figure 4 is a graph of flywheel speed, shaft speed and vehicle speed plotted against time for decelerations of the vehicle on high and low friction surfaces.

The flywheel mechanism illustrated in Fig.

1 of the accompanying drawings comprises a housing 1 from opposite ends of which project opposite ends of a shaft 2. At one end the shaft 2 is coupled to a wheel to be braked and at the opposite end provides a mounting for a flywheel assembly 3. At an intermediate point in its length the shaft 2 is provided with an eccentric cam 4 forming the drive mechanism for a pump of a modulator and pump assembly (not shown) for controlling a brakeapplying fluid pressure in response to operation of a dump valve 5. The modulator and pump assembly forms the subject of our copending G.B. Patent Applications Nos.

8005304 and 8005305, and need not be described further herein.

The flywheel assembly 3 comprises a flywheel 6 which is journalled for rotation on a plain bearing 7 on the shaft 2, a radial thrust member 8 in the form of plate which an outwardly directed axial flange 9 through which the thrust member 8 is keyed against relative rotation and axial movement with respect to the end of the shaft 2 by means of a diametrical pin 10, and a ball and ramp mechanism 11 clamped between the thrust member 8 and plain face 1 2 at the base of an annular recess in the adjacent end of the flywheel.

The ball and ramp mechanism 11 comprises an annular reaction plate 1 3 which encircles, with a clearance, the hub 14 of the flywheel 6 and the plate 1 3 is coupled to the flywheel 6 by means of a one-way drive 1 5 of the pawl and ratchet type. The reaction plate 1 3 engages at one end with the face 1 2 to form a friction overrun clutch 1 6 biassed by a compression spring 17. and at the other end has a plain face in which is located a series of angularly spaced recesses 18 corresponding with, and superimposed upon, complementary recess 1 9 in the adjacent face of the thrust member 8. Balls 20 are disposed in corresponding pairs of the recesses 1 8 and 19.

The reaction plate 1 3 is urged towards the thrust member 8 by the compression spring 17.

The dump valve 5 is normally closed but is adapted to be opened by a lever 21 which is pivotably mounted at an intermediate point in its length on the housing 1. A spring 22 acts on the lever 21 to urge it at its outer end in a direction to hold the dump valve 5 closed, and the inner end of the lever 21 is normally spaced from a plain thrust bearing 23 upon which the adjacent inner face of the flywheel 6 acts. The compression spring 1 7 acts between the thrust bearing 23 and a sleeve 24 which is pressed onto the shaft 2 and is slotted at 25 to receive projections 26 which co-operate with the cage 27 of the thrust bearing 23.

The lever 21 is bifurcated at its inner end to straddle the sleeve 24 and the spring 1 7 and each limb of the fork has a nose 28 of substantially arcuate outline which is normally spaced by a gap 30 from the thrust bearing 23. The gap 30 determines the degree of lostmotion between the flywheel assembly 3 and the dump valve 5 which relates to the loss in wheel speed permitted before the brakes are released The dump valve 5 comprises a body 31 of stepped outline which is slidably received in a stepped bore 32 in the housing 1 and is urged inwardly of the bore 32 into a retracted position defined by a stop face 33 by means of a Belleville or other spring washer 34. The lever 21 engages with the outer end of a valve member compnsing a rod 35 which is slidably received in a bore in the body 31.

Normally the lever 21 is operative to urge the rod 35 into engagement with a seating 36 in the body 31 to cut-off communication between a convenient de-boost chamber 37 communicating with a brake line and an annular passage 38 which is defined between the bore 32 and the body 31 and which is connected to a reservoir for fluid. The chamber 37 may comprise an expansion chamber of the modulator and pump assembly.

The thrust member 8 rotates with the shaft 2 at all times. When the wheel is rotating in a "brakes-off" condition, or when the brake is applied normally in a non-skid condition, the flywheel 6 is driven with the shaft 2 at the same speed through the ball and ramp mechanism 11, and then to the one-way drive 1 5 and the overrun clutch 16.

When the brakes are applied normally the brake-applying pressure is increased and the control pressure in the de-boost chamber 37 also rises. This pressure acts on the inner end of the body 31 which moves relatively outwardly of the bore 32 against the loading in the Belleville washer 34. This movement of the body 31 is transmitted to the lever 21 through the valve member 35 to move the lever 21 angularly against the loading in the spring 22 in order to increase the size of the gap and hence the degree of lost-motion.

If the deceleration of the wheel is excessive, the speed of rotation of the shaft 2 is reduced suddenly. However, due to its inertia, the flywheel 6 runs on and thus causes relative rotation to take place between the thrust member 8 and the reaction plate 1 3. The balls 20 ride up ramps comprising end walls of the recesses 1 8 and 1 9 so that simultaneously the reaction plate 1 3 and the flywheel 6 move axially relatively away from the thrust member 8. After the flywheel 6 has moved axially through the gap 30 to take up the lostmotion, the flywheel moves the lever 21 angularly to permit the dump valve 5 to open and affect relief of the brake-applying fluid pressure as described in our co-pending G.B.

Patent Application Nows. 8005304 and 8005305, referred to above, by placing the de-boost chamber 37 in communication with the reservoir through the annular passage 38.

The brake-applying pressure and the control pressure both fall and the Belleville washer 34 is then operative to urge the body 31 towards its retracted position in order to reduce the size of the gap 30.

Once the dump valve 5 has been opened fully, the reaction plate 1 3 rotates with the thrust member 8 to maintain the ball and ramp mechanism 11 in the expanded condition, but the flywheel 6 is permitted to rotate relative to the reaction plate 1 3 by slipping the overrun clutch 16 and the one-way drive 1 5. Thus the flywheel 6 decelerates at a rate determined by the torque sustained by the overrun clutch 1 6 and the rate of deceleration of the shaft 2.

As the brake pressure is reduced, the wheel re-accelerates and, when speed of the shaft 2 reaches the speed of the flywheel 6, the ball and ramp mechanism 11 contracts and the dump valve 5 closes. The pump becomes operative and the pressure in the de-boost chamber 37 increases to re-establish the brake-applying pressure and to urge the body 31 outwardly of its bore 32 against the loading in the Belleville washer 34. When the next skid signal is received from the flywheel assembly 3, the gap 30 is correct for the magnitude of brake pressure that will cause a skid.

In the construction described above the body 31 carrying the seating 36 of the dump valve 5 is pressure sensitive and it is spring loaded into a position in which the gap 30 is a minimum. Thus, at least up to a predetermined threshold value corresponding to that braking pressure which provides optimum vehicle retardation when the brakes are applied with the vehicle travelling over a high ju sur- face, the lost-motion defined by the gap 30 will increase with increase in the magnitude of the brake-applying pressure to provide a greater degree of lost-motion, substantially 45 angular movement of the flywheel 6, at higher pressure on dry road surfaces. Similarly the size of gap 30, and hence the degree of lost-motion is smaller at lower pressure on road surfaces at poorer adhesion.

These effects are illustrated in the graphs of Figs. 2 and 3.

In Fig. 2, the vehicle speed, the flywheel speed, the shaft speed and the brake-applying pressure are plotted against time during a period of wheel skid on a high friction surface. The brake pressure rises to a high level before a skid takes place at E. When the skid occurs the shaft decelerates rapidly and the flywheel expands the ball and ramp mechanism to take up the clearance 30 at F. The shaded area A, represents the revolution of the flywheel to take up this clearance and the height of area A, is the speed lost. The time interval from E to F is the time taken for the valve to be first operated. When the clearance is taken up at F the brake applying pressure is reduced and the flywheel runs on to the end of its ramps, the run-on being indicated by the area B,.At G the flywheel runs on against its clutch at a slightly higher deceleration and, by this time also, the reduction in brake pressure enables the wheel to accelerate.

When the wheel speed equals the flywheel speed at H, the flywheel moves back to close the valve as indicated by area C1. However, the brake pressure release has caused the clearance 30 to be reduced so that Area C, is greater than Area B, and subsequent revolution of the flywheel to take up this clearance, area D, is smaller than area A1. The valve closes at J to allow the brake pressure to be re-applied.

Fig. 3 shows the same operation during a period of wheel skid on a low friction surface.

In this case, the pressure causing the skid, and the clearance 30 will both be smaller.

The "brakes off" signal at F will occur more quickly and with less loss of speed as the area of A2, which represents revolutions of the flywheel, has been reduced with respect to area A, of Fig. 2. Again B2 represents the flywheel run-on and will be of greater area than B, in Fig. 2. However A, + B1 = A2 + B2; A1 + B1 = C1 + D1 and A, + B, = C2 + D2 When the wheel accelerates the brakes are applied at J and the area C2 is again larger than area B2 In Fig. 4, lines X and Y respectively correspond to decelerations of the vehicle on high friction and low friction surfaces. Typical values of deceleration on such surfaces are 1.0g and 0.5g respectively.

As mentioned above the areas A, and A2 represent the revolution of the flywheel to take up the lost motion clearance on high friction and low friction surfaces respectively.

For the above values of vehicle deceleration, A, is equivalent to an angular movement of the flywheel of 45 , and A2 is equivalent to an angular movement of 22so. Also the height of area A2 is smaller than that of area A, indicating that less speed is lost on a low friction surface before the lost motion clearance is taken up.

Area A3 indicates the revolution of the flywheel to take up a fixed lost motion clearance on a low friction surface, in connection with a known device. For a fixed clearance corresponding to clearance 30 on a high friction surface, area A3 will be identical to area A,.

From the foregoing it will be appreciated that the present invention provides a quicker response time, and maintains the wheel in a higher slip band, than with a known fixed lost motion device, for example as indicated by area A2.

Claims (7)

1. An anti-skid braking system for a vehicle of the kind set forth in which a degree of lost-motion is provided between the flywheel mechanism and the pressure-regulating means, and means is provided for adjusting the degree of lost-motion between the flywheel mechanism and the pressure-regulating means in proportion to the conditions of the surface over which the vehicle is travelling, to the deceleration of the vehicle, or to the magnitude of the brake-applying fluidpressure permitted by the flywheel mechanism.

2. An anti-skid braking system as claimed in Claim 1, in which the means for adjusting the degree of lost-motion between the flywheel mechanism and the pressure-regulating means is responsive to the magnitude of a control pressure acting on the pressure-regu lating means.

3. An anti-skid braking system as claimed in Claim 1, in which the means for adjusting the degree of lost-motion between the flywheel mechanism and the pressure-regulating means is responsive to the brake-applying fluid-pressure itself.

4. An anti-skid braking system as claimed in Claim 2, in which the pressure-regulating means comprises a normally closed dump valve which is opened in response to a skid signal to relieve the brake-applying fluid-pressure by placing an expander chamber exposed to the control pressure in communication with a reservoir for fluid, and the flywheel mechanism acts through a lever on a valve member of the dump valve to permit the valve member to move relatively away from a seating, the seating being itself bodily movable by the control pressure against the loading in a spring in order to increase the size of a gap between the lever and the flywheel mechanism, thereby increasing the degree of lostmotion.

5. An anti-skid braking system as claimed in Claim 4, in which the spring comprises a Belleville or other spring washer acting between a member incorporating the seating and a relatively stationary housing for the dump valve.

6. An anti-skid braking system as claimed in Claim 5, in which the member incorporating the seating comprises a body slidably received in a bore in the housing, and the valve member comprises a rod slidably received in a bore in the body.

7. An anti-skid braking system for a vehicle substantially as herein described with reference to Figs. 1 to 4 of the accompanying drawings.