GB2519998A

GB2519998A - Trailer-mounted hydraulic braking system

Info

Publication number: GB2519998A
Application number: GB1319483.2A
Authority: GB
Inventors: Robert Clive Browning Davis
Original assignee: Ifor Williams Trailers Ltd
Current assignee: Ifor Williams Trailers Ltd
Priority date: 2013-11-05
Filing date: 2013-11-05
Publication date: 2015-05-13
Anticipated expiration: 2033-11-05
Also published as: GB2519998B; GB201319483D0

Abstract

A trailer hydraulic braking system comprises a plurality of hydraulic actuators 10, 11, 12, 13 and a main hydraulic fluid conduit 16 connectable to a towing vehicle for actuation of the brakes. Distribution valves 14 distribute the fluid and braking effect to the trailer brakes. Load sensing valve (LSV) 15, connected to a load sensing mechanism of the trailer, modulates the hydraulic braking pressure. An anti-lock braking system (ABS) 20-24 is provided capable of reducing braking pressure to prevent wheel locking. In order to prevent wheel locking when ABS cannot operate (e.g. because no connection possible to tractor electrical system), a normally closed valve 30 is provided, e.g. between distribution valves 14 and LSV 15, along with a by-pass conduit 32 with a restriction valve 34, preferably a pressure limiting or pressure apportioning valve. Therefore, whenever there is no power supply to ABS, hydraulic flow is restricted via valve 34, reducing braking pressure; when there is power to the ABS the valve 30 opens.

Description

Trailer-mounted hydraulic braking system

Field of invention

[0001] The present invention concerns a hydraulic braking system for a trailer, particularly but not exclusively an agricultural trailer, that is to say a trailer which is typically towed by an agricultural tractor and used in an agricultural environment, including frequent use on rough and uneven ground and on unmade roads as well as off road, but also on normal highway.

BACKGROUND

[0002] The present invention has arisen in the context of expected legislative changes to increase the allowable speed limit for agricultural tractors and trailers from 20mph (32 km/h) to something in the region of 40mph (64 km/h) or thereabouts. An increase in the weight limit is also proposed. These increases will require a commensurate improvement in the braking systems for such vehicles.

[0003] Currently agricultural trailers need only provide 25% retardation providing they do not exceed 20 mph. Improved braking is required when a trailer is travelling at higher speeds commensurate with effective operation at such speeds. Similar improve braking will be required under the new legislation which will likely require an increase to a minimum of 45% retardation, as currently required tor road-going articulated vehicles. This higher minimum level of retardation would need to be proven during annual testing, thus in practice high speed' trailers would be specified to achieve no less than 50% retardation.

Such an increase in the level of retardation has significant implications for situations where the retardation exceeds the coefficient of friction between the tyres and the ground. When this arises the wheels will lock and the trailer will lose directional stability. By their very nature agricultural trailers will frequently be used in situations where coefficients of friction are lower than would be expected on the highway, so it is important that measures are in place to prevent wheel locking. High speed trailers will require an anti-lock braking system (ABS) as well as a load sensing mechanism to inhibit wheel locking when unladen or partially laden and in conditions of limited adhesion between the tyres and ground.

[0004] The required ABS system must be powered by way of a special electrical connector. The special electrical connector is currently defined by 1S07638. Such a connector would connect to a corresponding plug or socket connection on the towing vehicle, namely the tractor. Agricultural trailers that do not exceed the current legal maximum of 20 mph (32kph) are not required to have load sensing or ABS system because with only 25% retardation applied during braking they are unlikely to exceed the coefficient of friction anticipated during low speed use and the scope for lock-up (wheel locking) is consequently minimal.

[0005] For safe braking during high speed travel load sensing is necessary to reduce the retardation in situations where the trailer is unladen or partially laden as the specified 50% level of retardation corresponds to the maximum retardation required when the trailer is fully loaded. Maximum retardation is applied by the operator when maximum allowed force is applied to a brake pedal, resulting in maximum hydraulic pressure in the brake system.

The specified levels of retardation will be exceeded if maximum hydraulic pressures occur in the brake system when the trailer is less than fully laden. This could often result in the retardation exceeding the coefficient of friction, and hence in locking of the wheels. Load sensing, typically achieved by measurement of the trailer's suspension deflection, reduces the hydraulic pressure in the brake system proportionally with respect to reduction in actual load compared to maximum load.

[0006] It is desirable to provide low (25%) and high (50%) retardation on the same trailer so as to allow both low speed and high speed tractors to tow the trailer. One way to achieve this is to use hydraulic actuation for the 25%, namely for braking during low speed operation, and pneumatic actuation for the 50%, namely for braking during high speed operation. Depending on which system is connected, the braking system then meets legal requirements for either low or high speed travel, respectively.

[0007] Very few high speed legal tractors exist at present and clearly a high speed legal trailer without a high speed legal tractor to pull it is pointless. However with EC legislation for higher speed agricultural vehicles in preparation, most European tractor manufacturers are working on high speed legal developments. It is believed that many of these high speed tractors will still not have air brakes and will therefore not have an air supply for pneumatic brake actuation on the trailer. Therefore, it is desirable to develop a hydraulic-only high speed trailer braking system. Such a system is shown in Figure 1. This includes a load sensing mechanism to modulate braking pressure at low load conditions, as already mentioned, and an anti-lock braking system.

[0008] However, a potentially dangerous situation can occur when the trailer is towed by an older low speed tractor with no means of powering the ABS; there is nothing to differentiate between braking at high speed (where 50% retardation is required) and braking at low speed (where only 25% retardation is desirable). The load sensing function would still be present and is capable of reducing the level of braking pressure. However, where the trailer is partially laden, for example where the unladen weight is 15% of the fully laden weight, the load sensing system will not be able to reduce the brake retardation by the same amount. If the trailer is partially laden or unladen then the brake retardation can increase beyond the target 50% towards a much higher figure due to of the limited hydraulic pressure correction available from the load sensing system. Then, without ABS, wheel locking is likely to occur. Owing to the limited grip conditions inherent in farm trailer operation agricultural trailers are particularly vulnerable to wheel locking. The coefficient of friction (directly analogous to brake deceleration) might drop as low as 40%. With at least 50% braking, and possibly more being demanded, wheel locking becomes certain.

[0009] Without the ABS being powered the tractor is not legal above 20mph and it is desirable to automatically reduce the maximum braking to the 25% retardation required for such trailers in order to prevent wheel locking.

BRIEF SUMMARY OF THE DISCLOSURE

[0010] In accordance with the present inventions there is provided a means to differentiate between when it is appropriate to apply braking retardation appropriate to high speed and when it is appropriate to apply braking retardation appropriate to low speed depending upon whether a connection has been made to a towing vehicle's ABS in a trailer having a hydraulic braking system with ABS, therefore suitable for travelling at speeds above 20mph (32kph). Such a hydraulic braking system comprises a plurality of hydraulic actuators, one adjacent each brakeable wheel of the trailer to operate a respective brake associated with each said wheel and a hydraulic conduit connectable to a towing vehicle for receiving hydraulic fluid flow from the towing vehicle for actuation of the respective trailer brakes, the hydraulic conduit communicating with, and transmitting braking effect to, the actuators by way of first valve means, which distributes the fluid and the braking effect to the wheels at the right and left sides of the trailer, and second valve means, which is operatively connected to a load sensing mechanism of the trailer and modulates the hydraulic pressure, and hence the braking force applied at the wheels, dependent on the load status of the trailer, such that the hydraulic pressure is restricted and the braking force proportionately decreased as the weight of the load reduces from a fully laden condition to an unladen condition, the first valve means being disposed between the second valve means and the respective brake actuators; and having an anti-lock braking system (ABS) comprising respective wheel speed sensors operatively connected to a central processing unit (CPU) which is configured to control the hydraulic pressure distributable by the first valve means dependent on signals from the wheel speed sensors so that pressure is reduced by diverting fluid back to a hydraulic fluid reservoir on the towing vehicle, thus reducing braking pressure, whenever the wheel deceleration is above a pre-programmed threshold (which would otherwise lead to wheel locking), and a cable connectable to an electrical system of the towing vehicle, when present and compatible, for power supply to the CPU and sensors.

[0011] Said means to differentiate between when it is appropriate to apply braking retardation appropriate to high speed and when it is appropriate to apply braking retardation appropriate to low speed depending upon whether a connection has been made to a towing tractors ABS takes the form of providing a normally closed valve in the hydraulic conduit, but also providing a by-pass conduit provided with a restriction valve bridging (namely in parallel with) the normally closed valve, the normally closed valve being appropriately connected and configured so as to be opened whenever there is power supply to the CPU and the wheel speed sensors so that whenever the ABS cable is not connected and/or power is not supplied to operate the ABS. hydraulic flow will be via the restriction valve, which will automatically reduce the braking pressure applied.

[0012] The normally closed valve and its bypass conduit with restriction valve may suitably be located between the first and second valve means, namely between the load sensing valve and the hydraulic distribution valve unit.

[0013] Effectively, the resultant braking system is self-selecting as regards to suitable brake pressure, hence retardation, to be applied depending on whether ABS is available, so that trailers fitted with ABS can operate safely (i.e. obviating risks of wheel locking occurring) without same when their towing tractor does not allow for the ABS to function.

[0014] The restriction valve in the bypass conduit may be either a pressure limiting valve or a pressure apportioning valve.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be described further with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a hydraulic and electrical arrangement in the horizontal plane of a trailer showing the configuration of the components responsible for control of retardation with ABS. the hydraulic system being represented by a solid line and the electrical system by a chain dot line; Figure 2 is a diagram similar to Figure 1, but showing the configuration of the components responsible for control of retardation with ABS but modified in accordance with the present invention so as to be capable of self-selecting a reduced level of retardation suitable for low speed legal use only; and Figure 3 is chart showing the relationship between trailer weight, and braking force ( retardation) in trailers with and without the self-selecting system of the present invention.

DETAILED DESCRIPTION

[0016] As shown in Figure 1 a braking arrangement of a hydraulically braked trailer which is adapted for high speed travel (namely at speeds over 20mph (32kph)) comprises tour hydraulic brake actuators 10, 11, 12, 13, one at each wheel location, connected to a hydraulic pressure distribution unit comprising a pair of solenoid valves 14. The hydraulic pressure delivered to the solenoid valves 14 is controlled by a load sensing valve (LSV) 15, which typically takes the form of a variable pressure apportioning valve. The purpose of the LSV 15 is to reduce the brake retardation when the laden weight of the trailer is less than the trailer's maximum laden weight.

[0017] As shown, the aforesaid components 10-15 are connected by hydraulic fluid lines as part of a trailer hydraulic system, which is supplied with pressurised fluid from a towing vehicle (not shown), which in the context of the envisaged application of the invention will be an agricultural tractor. Thus, a main hydraulic fluid conduit 16 connects to the tractor hydraulic system and supplies braking signal by means of fluid flow to the distribution valves 14, by way of the modulating LSV. The LSV 15 is controlled by a mechanism for sensing spring deflection in the trailer's suspension system, which is proportional to the loading of the vehicle. This mechanism preferably includes means, such as a lever arrangement, for amplifying the deflection, so as to increase sensitivity of control of the LSV. Nevertheless, as outlined above, the practical reduction in fluid pressure which can be achieved in the low load or unladen condition cannot be sufficient to prevent wheel locking.

[0018] The hydraulic system includes a free return line 17 from the distribution valves 15 to a reservoir of hydraulic fluid on the tractor, the presence of which is relevant to the ABS.

[0019] The ABS is provided to overcome the risk of wheel locking when the trailer is operating in a situation where the required retardation exceeds the available grip, as outlined in the instruction hereto. The provision of an ABS is well known in general in respect of road vehicles. In the present trailer braking system it comprises respective wheel speed sensors 20, 21, 22, 23, operatively connected to a central processing unit (CPU) 24 which is configured to control the hydraulic pressure distributable by the solenoid valves 14 dependent on signals from the wheel speed sensors. Pressure is reduced by diverting some fluid back, via line 17, to the reservoir on the towing vehicle. In this way braking pressure is reduced appropriately whenever the wheel deceleration is above a pre-programmed threshold (which would otherwise lead to wheel locking).

[0020] The ABS must be powered by way of a cable 28 fitted with a dedicated electrical connector 26 connecting to a corresponding, specifically compatible, plug or socket connection (not indicated) on the towing vehicle, namely the tractor. The special electrical connector is currently defined by 1S07638. When the ABS is connected and operational, an ABS light 25 on the trailer is illuminated to indicate to the driver and others that the ABS is powered. An additional indicator light may be provided in the cab of the tractor, probably on the dashboard, for the same purpose, namely to indicate to the driver that the ABS is powered. As already explained in the introduction, very many existing agricultural tractors do not have an electrical system with such a compatible connector, so when the trailer provided with a braking system as in figure 1 is towed by such a tractor the ABS ot the trailer is not connected to a power supply and is not functional. Without the ABS functioning there is a risk of wheel locking.

[0021] Figure 2 illustrates a system in accordance with the invention which has been modified to overcome this problem. This system has all the same components and connections as in the Figure 1 embodiment, but additionally a normally closed solenoid valve 30 is inserted between the LSV 15 and the pair of solenoid valves 14. Also a by-pass conduit 32 loops around in parallel with the normally closed valve, and a restriction valve 34 is provided in this loop 32. When closed, the normally closed solenoid valve 30 directs hydraulic flow from the main hydraulic conduit 16 through the loop 32so that the valve 34 significantly reduces the pressure received by the pair of solenoid valves 14.

[0022] The normally closed solenoid valve 30 is powered from the same power supply cable 28 as the ABS system (by branch cable 29), and when powered it is opened, and the overall system operates exactly as the figure 1 embodiment. However, if the ABS system is not connected to a power supply/is not powered the solenoid valve 30 is in its normally closed condition. Brake pressure is then transmitted only through the loop 32 which bridges the valve 30, and the additional valve 34 in this loop 32 limits the output pressure.

Typically it may limit that pressure to 60% of the input pressure. The maximum deceleration of the wheels is then 60% of 50%, equalling 30% and, with the load sensing valve 15 still working to further reduce that when the trailer is less than fully laden, wheel locking should be avoided.

[0023] The restriction valve 34 in the loop 32 may comprise either a pressure limiting valve (PLy), which is essentially what is described above with reference to possible limit of 60% of the required 50% retardation, or a pressure apportioning valve (PAy). CA PAV is similar in principle to the type of valve used as the LSV 15 in that it will proportionally reduce the fluid flow and hence the pressure, such as to 60% of whatever is supplied from line 16, as mentioned above. A PLV merely applies a fixed limit to the hydraulic pressure, without proportionality in reaching that limit. So, a PLV may be a less costly alternative to a PAy.

[0024] Figure 3 depicts differing possible relationships between available retardation and suspension deflection (a measure of trailer load) in a brake system in accordance with the invention. When the ABS is powered (with the normally closed solenoid valve 30 being open) retardation varies proportionally between a value of retardation corresponding to no suspension deflection and the high speed retardation requirement (50%), at which point the suspension is at a predetermined maximum defection. Reduction in retardation in this instance, when ABS is powered, is caused by the LSV 15 alone unless the ABS is required to further reduce retardation as described above. When the ABS is not powered (with hydraulic flow directed though the restriction valve 34) retardation is depicted to vary in one of two ways. When the restriction valve 34 is a PAy, retardation varies by some proportion (50% is shown in Figure 3) relative to the retardation available with ABS power and as such will not exceed the low speed retardation requirement (25%, being 50% of 50%). When the restriction valve is a PLV retardation varies as in the case with ABS, varied only by the LSV 15 up to the low speed retardation requirement (25%) determined by the PLy, after which retardation will remain at the low speed retardation requirement.

[0025] In a modified version of the system, the LSV 15 may be connected to the fluid return line 17 so that it can be operated to allow brake pressure to be decreased again after an apparent increase in load (which would normally cause an increase in the modulated brake pressure) which is only temporary, even momentary, such as when the trailer rides over a bump.

[0026] The invention is not restricted to the details of the foregoing embodiments.

Variations in detail and application to trailers with other capabilities are possible as will be understood by a person skilled in this technical field. For example, in the case of a trailer equipped with air suspension (rather than mechanical spring suspension in the embodiment described above) the load sensing valve will suitably be controlled by a mechanism sensing the pressure in the air springs, which will be substantially proportional to the imposed load on the suspension and therefore to the trailer weight.

[0027] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other components. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0028] Features described in conjunction with a particular embodiment of the invention are to be understood to be applicable to any other embodiment described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims and drawings) may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Claims

CLAIMS1. A trailer having a hydraulic braking system comprising: a plurality of hydraulic actuators, one adjacent each brakeable wheel of the trailer to operate a respective brake associated with each said wheel; a hydraulic conduit connectable to a towing vehicle for receiving hydraulic fluid flow from the towing vehicle for actuation of the respective trailer brakes; the hydraulic conduit communicating with, and transmitting braking effect to, the actuators by way of first valve means, which distributes the fluid and the braking effect to the wheels at the right and left sides of the trailer, and second valve means, which is operatively connected to a load sensing mechanism of the trailer and modulates the hydraulic pressure, and hence the braking force applied at the wheels, dependent on the load status of the trailer, such that the hydraulic pressure is restricted and the braking force proportionately decreased as the weight of the load reduces from a fully laden condition to an unladen condition, the first valve means being disposed between the second valve means and the respective brake actuators; an anti-lock braking system (ABS) comprising respective wheel speed sensors operatively connected to a central processing unit (CPU) which is configured to control the hydraulic pressure distributable by the first valve means dependent on signals from the wheel speed sensors so that pressure is reduced by diverting fluid back to a hydraulic fluid reservoir on the towing vehicle, thus reducing braking pressure, whenever the wheel deceleration is above a pre-programmed threshold (which would otherwise lead to wheel locking), and a cable connectable to an electrical system of the towing vehicle, when present and compatible, for power supply to the CPU and sensors; characterised in that a normally closed valve is provided in the hydraulic conduit and a by-pass conduit provided with a restriction valve bridges the normally closed valve; the normally closed valve is connected and configured so as to be opened whenever there is power supply to the CPU and sensors so that whenever the ABS cable is not connected and/or power is not supplied to operate the abs, hydraulic flow will be via the restriction valve, which will automatically reduce the braking pressure applied.
2. A trailer having a braking system according to claim 1 wherein the normally closed valve is provided in the hydraulic conduit between the first and second valve means.
3. A trailer having a braking system according to claim 1 or 2 wherein the first valve means comprises solenoid valves.
4. A trailer having a braking system according to claim 1, 2 or 3 wherein the restriction valve in the bypass conduit is a pressure limiting valve.
5. A trailer having a braking system according to claim 1, 2 or 3 wherein the restriction valve in the bypass conduit is a pressure apportioning valve.
6. A trailer-mounted braking system substantially as hereinbefore described with reference to and as illustrated by figure 2 of the accompanying drawings.Amendments to the claims have been filed as foflowsCLAIMS1. A trailer having a hydraulic braking system comprising: a plurality of hydraulic actuators, one adjacent each brakeable wheel of the trailer to operate a respective brake associated with each said wheel; a hydraulic conduit connectable to a towing vehicle for receiving hydraulic fluid flow from the towing vehicle for actuation of the respective trailer brakes; the hydraulic conduit communicating with, and transmitting braking effect to, the actuators by way of first valve means, which distributes the fluid and the braking effect to the wheels at the right and left sides of the trailer, and second valve means, which is operatively connected to a load sensing mechanism of the trailer and modulates the hydraulic pressure, and hence the braking force applied at the wheels, dependent on the load status of the trailer, such that the hydraulic pressure is restricted and the braking force proportionately decreased as the weight of the load reduces from a fully laden condition to an unladen condition, the first valve means being disposed between the second valve means and the respective brake actuators; (0 an anti-lock braking system (ABS) comprising respective wheel speed sensors If) 20 operatively connected to a central processing unit (CPU) which is configured to control the hydraulic pressure distributable by the first valve means dependent on signals from the wheel speed sensors so that pressure is reduced by diverting fluid back to a hydraulic fluid reservoir on the towing vehicle, thus reducing braking pressure, whenever the wheel deceleration is above a pre-programmed threshold, which would otherwise lead to wheel locking, and a cable connectable to an electrical system of the towing vehicle, when present and compatible, for power supply to the CPU and sensors; characterised in that a normally closed valve is provided in the hydraulic conduit and a by-pass conduit provided with a restriction valve bridges the normally closed valve; the normally closed valve is connected and configured so as to be opened whenever there is power supply to the CPU and sensors so that whenever the ABS cable is not connected and/or power is not supplied to operate the ABS, hydraulic flow will be via the restriction valve, which will automatically reduce the braking pressure applied.2. A trailer having a braking system according to claim 1 wherein the normally closed valve is provided in the hydraulic conduit between the first and second valve means.3. A trailer having a braking system according to claim 1 or 2 wherein the first valve means comprises solenoid valves.4. A trailer having a braking system according to claim 1, 2 or 3 wherein the restriction valve in the bypass conduit is a pressure limiting valve.5. A trailer having a braking system according to claim 1, 2 or 3 wherein the restriction valve in the bypass conduit is a pressure apportioning valve.ct 6. A trailer-mounted braking system substantially as hereinbefore described with reference to and as illustrated by figure 2 of the accompanying drawings.CDLO