GB2160941A - Emergency brake - Google Patents

Abstract

A train emergency braking system provides a braking force proportional to the weight of the train and also takes account of variation (caused by wet, leaves, etc) in the coefficient of friction between brake pad and rail. A downward force, which is a fraction of the weight, is imposed on a framework between a brake pad at one end and a roller at the other (leading) end. Brake friction tends to drag the framework back against a spring until the spring force equals the braking force. A stable condition is thus achieved. A lever or ramp system may replace the spring arrangement. <IMAGE>

Description

SPECIFICATION Vehicle braking arrangement This invention relates to vehicle braking arrangements and particularly to emergency braking arrangements. It is also primarily but not exclusively concerned with track vehicles such as railway trains.

Track vehicle systems frequently have two levels of braking; service and emergency. Service brakes are to stop vehicles in normal operation. To achieve smooth operation, closed loop control of these brakes, to achieve a particular deceleration rate, is frequently employed. These brakes may also convert the kinetic energy of the vehicle into a form where it can be re-used e.g. by regeneration into an electric supply.

Emergency brakes are the means employed to stop a vehicle, and ensure passenger safety in the event of a failure of the service brakes.

Emergency brakes must be fail-safe. This tends to imply that they must be open-loop, e.g. by release of air allowing springs to apply pressure to brake shoes.

Service braking rates are well controlled by closed-loop systems, and therefore deceleration rates very near the limit for passenger comfort and safety may be used. This increases transport system capacity by mini is ing headways between vehicles.

In the event of a failure of the service brakes, there will be a detection response time, i.e., delay, before it will be apparent that the failure has occurred, and the emergency brakes must then be applied, to maintain vehicle safety. It is desirable that the emergency braking rate is higher than the service braking rate, in order to stop the vehicle short of its normal stopping point.

Referring to Fig. 1 of the accompanying drawings, this shows a braking characteristic of velocity against distance. As illustrated, the vehicle proceeds initially at a velocity V to point A at which point service braking is applied. The velocity would then normally fall, along the curve ABC, to zero at point C. At point B however, it is assumed that the service brake fails. BD represents the detection response time, or delay, before the emergency braking is imposed at point D. The velocity then falls, at a greater rate than for service braking, to zero at point E which is short of the normal stopping point C.

It is sometimes not possible to design an emergency brake which will reliably provide a deceleration rate high enough to stop the vehicle safely, and yet not so high that standing passengers are injured due to falls resulting from the emergency brake application.

The factors which affect the deceleration rate of a vehicle with open-loop brakes, may include: Laden/unladen weight of vehicle.

Brake shoe/wheel friction coefficient.

Wheel/track friction coefficient (including the effect of oil/leaves/ice on track).

It is therefore an object of the present invention to provide a braking arrangement in which some at least of the above difficulties can be alleviated.

According to the present invention, a vehicle braking arrangement comprises relatively high and low friction bearing members arranged to engage braking surfaces which, when the vehicle is in motion, move relative to the vehicle, means for sharing an applied braking force between the bearing members in controlled proportions, and closed loop means for balancing the frictional braking force exerted on the bearing members against force reference means, any out-of-balance force being arranged to control the proportions to tend to maintain the balance and thus determine the frictional braking force.

The proportions of the applied braking force may be controlled by the position of application of the applied braking force, this position being urged toward the low friction bearing member by the frictional braking force and toward the high friction bearing member by the force reference means.

Preferably there are included means for deriving the applied braking force or the force reference means from the vehicle weight so that the frictional braking force is a function of the vehicle weight.

The applied braking force may be derived from the vehicle weight and the force reference means may comprise spring means. The spring means preferably has a non-linear force-displacement characteristic such as to tend to suppress the effect on the frictional braking force of variation of the coefficient of friction between the bearing members and the braking surfaces.

In an alternative arrangement according to the invention, the bearing members may be mounted on a support structure which incorporates an inclined surface, the applied braking force being applied to this inclined surface and the force reference means being provided by a component of the applied braking force determined by the inclination of the inclined surface.

The force reference means may be constituted by means for deriving a force which is a function of the vehicle weight comprising a lever system coupled to a support structure for the bearing members.

Two forms of vehicle braking arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 already described, shows a velocity-distance braking characteristic for an emergency braking situation; Figure 2 shows, diagrammatically, one embodiment of a braking arrangement according to the invention; Figure 3 is a force/applied-position diagram from which a stable condition can be obtained for the arrangement of Fig. 2; Figure 4 is a deceleration diagram contrasting the effect of different coefficients of friction in conventional arrangements and arrangements in accordance with the invention; and Figures 5 and 6 are diagrams of modifications or alternative embodiments to that of Fig. 2.

Having described Fig. 1, which is relevant to both the prior art and the present invention, reference is now made to Fig. 2.

The vehicle in question is assumed to be a railway train running on a track, one rail (1) of which is shown. Mounted on the framework of a carriage (or of each carriage) is a member 3 which, in an emergency braking situation is arranged, by means not shown, to carry a proportion of the weight of the carriage, for example by release of compressed air normally holding off the weight. The member 3 carries a bar 5, there being only lateral restraint between the two. The bar 5 can therefore move freely through the member 3 subject to a compression spring 7 and any other longitudinal force on the bar 5. The latter carries a frame 9 which has an upper horizontal bearing surface 11 for a roller mounted at the lower end of the member 3.

In addition to the spring 7, free movement of the frame 9 on the member 3 is limited by a damper mechanism 10 which prevents instability and oscillation of the assembly through a balance condition.

When the member 3 is loaded in emergency braking conditions, the weight of the carriage (or rather a proportion of it) is transferred by the roller 13 to the frame 9. The point of application of this applied braking force N on the surface 11 is determined as will be explained.

Carried by the frame 9 is a brake block 1 5 at one end A and a roller 1 7 at the other end B. These two bearing members share the applied braking force N in inverse proportion to the distances 'a' and 'b' to the pivot points A 8 B of the brake block 15 and roller 17.

The brake block and the roller bear on braking surfaces which in this embodiment are constituted by the common surface of the rail 1.

The brake assembly is arranged so that the brake block 1 5 trails the roller 1 7 in the direction of travel.

The brake block 1 5 is, of course, chosen to provide a high coefficient of friction with the rail surface while the roller 1 7 clearly provides a very low or negligible coefficient of friction.

It will be seen, however, that the principle of the invention is still retained even if the roller 1 7 were replaced by a friction block of low coefficient of friction relative to that of the brake block 15.

It may be seen that the greater the frictional braking force between the bearing members 1 5 and 1 7 and the rail surface the greater is the tendency for the frame 9 to be dragged back against the spring 7. When the vehicle is stationary in normal operation there is no frictional braking force, the spring 7 encounters minimum reaction and the member 3 is at the end A of the frame 9.If, with the vehicle in motion, the emergency brake is engaged in this condition of the spring 7, the point of application of the applied braking force N is at the point A, the brake block 1 5 receives 100% of the applied braking force and the resulting frictional braking force is a maximum, equal to jbN, where jt is the coefficient of friction between the brake block 1 5 and the rail 1. At this minimum compression of the spring 7 the frame 9 is dragged backwards by the high rail friction until the frictional braking force is balanced by the spring compression. This situation occurs at a point of application of the applied braking force which is a distance 'a' from point A and 'b' from point B.The frictional braking force is then: b ,uN a+b This is also, therefore, the compression force in the spring 7 in this stable condition.

Fig. 3 illustrates this balance situation. The straight lines sloping downward from left to right indicate the frictional force magnitude for the whole range of positions of the applied braking force N. The three different lines indicate relatively high, medium and low values of the coefficient of friction 1l.

The full line sloping downward from right to left indicates the spring compression force and assumes a linear force/displacement characteristic. A stable situation is achieved where the frictional braking line intersects the spring compression line. It can be seen that the stable position (a:b) varies with the value of y, but more importantly, the frictional brak ing force varies with the value of ju. The range of frictional braking force for the values of 1 illustrated is seen to be R,.

As explained above, variation of braking force, and consequent uncertainty of deceleration, is a disadvantage to be avoided if pos sible, despite unpredictable variation of ju.

Such variation can be reduced significantly by employing a non-linear spring such that, for example the force/displacement characteristic takes the form of the curved broken line in Fig. 3. It may be seen that the range of frictional braking force obtained with such a non-linear spring is reduced to R2. While a continuous non-linear characteristic may be difficult to achieve, a two section character istic, each section being linear, can be ob tained quite readily, for example by employing a two-part compression spring, a first unstressed stiff section in series with a second prestressed less stiff section. The characteristic will then 'flatten out' to that of the less stiff section at the prestress value.

This effect is illustrated in Fig. 4 which is a graph of deceleration on braking coefficient of friction IL in given conditions of applied braking force and of vehicle mass.

The straight line F shows the increase of braking deceleration for a conventional emergency brake. Deceleration in excess of about 0.3g is considered to be potentially dangerous to passengers and it can be seen that, in the particular circumstances, if the coefficient of friction should suddenly 'improve' beyond about 0.3 this danger zone will be entered.

By modifying the spring as proposed above, its characteristic may have a low slope, as indicated by the portion GH, over the probably range of values, say 0.15 to 0.5. The danger zone will then not be entered for any value of IL that is likely to arise.

Clearly, by reducing the stiffness of the less stiff section the slope of the operative portion can be reduced and the variation of braking force with coefficient of friction reduced accordingly. However, the prestressing to any significant value of a low stiffness spring would present difficulties and a comprise is required.

In an alternative arrangement, the stiffer spring may be removed and the less-stiff spring prestressed. The roller 1 3 in Fig. 2 will then only depart from the brake pad end when the braking force exceeds this prestress value. This arrangement has the merit of simplicity while maintaining the low sensitivity to variations of IL In the embodiment of Fig. 2, the applied braking force N was assumed to be a proportion of the weight of the vehicle (railway carriage). It may in some circumstances be more practical to make the applied braking force N a predetermined value and maintain the dependence of frictional braking force on vehicle weight by making the force reference means, i.e., the spring 7 in Fig. 2, dependent upon the vehicle weight.It has been explained that in Fig. 2 the effective frictional braking force is equal to the compression force in the spring. This spring can therefore be replaced by a lever system which imposes a horizontal force proportional to the vehicle weight and the frictional force will still be weight dependent.

Fig. 5 illustrates such an arrangement in which the bar 5 and frame 9 etc are as in Fig.

2 but the spring is replaced by a lever system comprising a link 21, bell crank 23 pivoted at 27 and a thrust member 25 subject to a proportion of the vehicle weight. The member 3 is, as in Fig. 2, constrained to horizontal movement on the bar 5.

The vehicle weight may be disposed on the member 25 permanently so that a force kW (where k is a constant and W is the vehicle weight) is applied horizontally to the frame 9.

In this case, in the absence of an applied braking force 'N' the frame 9 would be driven forward on the member 3 (until 'a' is zero) so that the frictional braking force is initially at a maximum value on first applying the emergency brake. The frame 9 would then be dragged back against the damper 10 by a reducing frictional braking force until this latter equalled the force kW.

Alternatively, both the applied braking force and the vehicle-weight 'reference force' could be held off until braking is required. The frame 9 could then be allowed to take up an arbitrary position under the control of the damper 10 only or could be biased into some intermediate or end position in readiness for the application of the brake.

Fig. 6 shows, very diagrammatically, a further alternative arrangement in which the appliedd braking force and the balancing, weight-dependent force are provided by components of a single applied force.

The support structure 33 for the brake block 1 5 and roller 1 7 incorporates an inclined surface 29 on which bears a roller 31 carried by a member 32. This in turn carries the weight of the vehicle 34 or a proportion of it.

The roller 31 is assumed frictionless so that the applied force on the inclined surface is perpendicular to the surface, and taking account of the inclination, is assumed to be kW.

If the angle of inclination is 0, a horizontal component kW sin 8 acts in a forward direction on the structure 33, this component replacing the reference force kW on the link 21 in Fig. 5.

The vertical component kW cos a acts as the applied braking force of the previous arrangements. If the position of the roller 31 is defined by 'a' and 'b' as before, the frictional braking force is then: b IL kWcosH a+b The structure 33 will then move horizontally, in a braking situation, with the roller 31 riding up or down the incline, until this frictional force equals the fixed component kW sin 8 in a balance condition. A damper 10 acts as before to prevent instability.

A similar effect to that of the inclined surface may also be achieved by the use of a swinging link or arrangement of levers.

In the above embodiments the friction brake block has always had to be the trailing member of the assembly, to ensure that greater braking force causes the load point to move towards the roller. To accommodate two-way vehicles the brake assembly may be duplicated, the two assemblies being mutually reversed and one or other activated according to the direction of travel. Alternatively, in Fig.

2 for example, the frame member 9 could have a roller 1 7 at each end and the brake block 1 5 positioned in the centre.

The frame member 9 is then angled downwardly at the centre point so that only the leading roller and the brake block are in contact with the rail. A spring 7 is placed on each side of the strut 3. The strut 3 then moves toward one roller or the other according to the direction of travel.

Other methods of accommodating two-way travel will be apparent.

Claims (11)

1. A vehicle braking arrangement comprising relatively high and low friction bearing members arranged to engage braking surfaces which, when the vehicle is in motion, move relative to the vehicle, means for sharing an applied braking force between said bearing members in controlled proportions, and closed loop means for balancing the frictional braking force exerted on said bearing members against force reference means, any out-ofbalance force being arranged to control said proportiions to tend to maintain the balance and thus determine the frictional braking force.

2. A vehicle braking arrangement according to Claim 1, wherein said proportions of the applied braking force are controlled by the position of application of the applied braking force.

3. A vehicle braking arrangement according to Claim 2, wherein said position of application is urged toward said low friction bearing member by the frictional braking force and toward said high friction bearing member by said force reference means.

4. A vehicle braking arrangement according to any preceding claim, including means for deriving said applied braking force or said force reference means from the vehicle weight so that the frictional braking force is a function of the vehicle weight.

5. A vehicle braking arrangement according to Claim 4, wherein said applied braking force is derived from the vehicle weight and said force reference means comprises spring means.

6. A vehicle braking arrangement according to Claim 5, wherein said spring means has a non-linear force-displacement characteristic such as to tend to suppress the effect on the frictional braking force of variation of the coefficient of friction between said bearing members and said braking surfaces.

7. A vehicle braking arrangement according to Claim 5, wherein said spring means has a linear force-displacement characteristic and is prestressed to provide maximum braking force up to the pre-stress in said spring means and thereafter a relatively low stiffness value to suppress the effect of variation of the coefficient of friction between said bearing members and said braking surfaces.

8. A vehicle braking arrangement according to Claim 2 or Claim 3, wherein said bearing members are mounted on a support structure which incorporates an inclined surface, the applied braking force being applied to this inclined surface and said force reference means being provided by a component of the applied braking force determined by the inclination of said inclined surface.

9. A vehicle braking arrangement according to Claim 3, including means for imposing an applied braking force of magnitude independent of vehicle weight at said position of application, said force reference means including means for deriving a force which is a function of the vehicle weight.

10. A vehicle braking arrangement according to any of Claims 2, 3 and 8, including damping means arranged to limit the rate of displacement of said position of application in achieving said balance.

11. A vehicle braking arrangement according to Claim 8, wherein said means for deriving a force which is a function of the vehicle weight comprises a lever system coupled to a support structure for said bearing members.

1 2. A vehicle braking arrangement according to any preceding claim, wherein said low friction bearing member comprises a roller.

1 3. A vehicle braking arrangement according to Claim 11, wherein said bearing members are arranged to engage a load bearing rail of a vehicle track.

1 4. A vehicle braking arrangement substantially as hereinbefore described with reference to Figs. 1, 3, 4 and any one of Figs. 2, 5 8 6.