Abstract:
A parking brake system for motor vehicle air brakes provides automatic engagement of the parking brakes when a gearshift lever is moved to park, but requires manual disengagement of the parking brakes. The effect is achieved in part by controlling the discharge conditions of two piloted latch valves connected to feed air to a cab mounted push pull double check valve.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to a parking brake lock-in key switch system for a truck or bus air brake system. 
   2. Description of the Problem 
   In prior art air brake systems used on truck and bus chassis manufactured by International Truck and Engine Corp., separate air lines run from the primary air tank and the secondary air tank to a parking brake actuator. The parking brake actuator is a push pull double check (PPDC) valve, which aligns the tank with the higher pressure to supply air to the brake chambers for individual wheel parking brakes. When pulled out to an ‘Engaged’ position, the push pull portion of the actuator closes off the air supply passageway from the air tanks and vents a single downstream air line. The single downstream air line from the PPDC valve runs from the valve through an inversion (relay) valve and a quick release valve to spring brake chambers which act in a parking brake mode. The spring or park brake chambers are part of the individual brake assemblies for each wheel subject to park braking. Typically, the vehicle&#39;s rear wheels have the brake assemblies which act as parking brakes in addition to having a service brake function. The springs in the spring brake chambers act to lock the individual wheels when there is no air applied to counter the spring force. Operation of the push pull knob of the parking brake actuator with its resulting venting of the down stream air piping to the brake chambers allows the spring brakes to lock the individual wheels associated with the park braking. In normal operation the spring brake chambers lock the individual wheels only so long as the parking brake actuator is in the ‘Engaged’ or pulled out position. 
   Vehicles with hydraulically operated service brakes have had air assisted drive line parking brakes in the prior art. While the hydraulic service brakes act directly on the wheels of the vehicle, the drive line parking brakes act to lock the drive line prop shaft of the vehicle. An air brake cylinder is engaged to the drum in the driveline which locks the shaft. The air brake cylinder allows the shaft to rotate when air is supplied to the cylinder. The air is provided from a single air reservoir through a parking spring brake control through a one-way check valve to the air brake cylinder. When the driver operates the parking spring brake control to the engaged position, the air supply to the cylinder is stopped and the downstream piping is vented. Upon the venting of the piping between the spring brake central and the air brake cylinder, the air brake cylinder will cause the drum brake to lock the driveline prop shaft. Similar to the above described air brake system, the drum brake will only continue to lock the prop shaft so long as the parking spring brake control is in the ‘Engaged’ position. 
   Automatic parking brake systems exist in the prior art which automatically vent downstream air lines supplying brake chambers or cylinders associated with park braking upon the operator turning the ignition key to an ‘off’ position. Automatic engagement of the parking brakes follows. A problem with such systems is that a driver or passenger may inadvertently engage the parking brakes while the vehicle is operating at highway speeds by bumping the ignition key to a non-operating or ‘off’ position. Involuntary braking could result. In addition, bus engines are not typically shut off at bus stops, with the result that the system does not operate when needed. 
   U.S. Pat. No. 6,234,586 describes vehicles such as school buses equipped with air brake systems and having a driver operated parking brake actuator. Following manual engagement of the parking brake actuator, the parking brake lock-in key switch system will lock the parking brake in the engaged position upon the driver turning the ignition key to the ‘off’ position. This effectively disables the parking brake actuator thereby preventing inadvertent release of the parking brake. This system does not allow inadvertent initiation of the parking brake should the ignition key be moved to the ‘off’ position while the vehicle is in motion. The driver must first consciously operate the parking brake actuator for the parking brake lock-in key switch system. In some vehicles with automatic transmissions an automatic apply-automatic release parking brake system operates in parallel with the parking brake lock-in key switch system. The automatic apply-automatic release parking brake system has a valve that acts to operate and engage the parking brake when the automatic transmission is placed in the _Park_ position. The valve of the automatic apply-automatic release parking brake system in these systems deactivates and releases the parking brake when the transmission shifter is moved out of the _Park_ position and the engine of the vehicle is running. With automatic application of the parking brakes when the vehicle transmission is placed in park, but a requirement for service brake application, key on and deliberate brake release prior to release of the parking brakes, there is little chance of a bus passenger releasing the brake, even if the driver is not in his seat. 
   Air brake vehicles are available with a key and service brake interlock function which requires a driver to turn the ignition key to the ON position and to apply the service brake prior to releasing the park brake. It is desirable to provide a system which automatically applies the park brakes when the vehicle automatic transmission is shifted into the Park position. It is also desirable that the driver would then have to reapply the service brake prior to disengaging the parking brakes. 
   SUMMARY OF THE INVENTION 
   The invention provides for these and other purposes and is robust, easily installable and maintained. In as the preferred embodiment is intended as an add on or optional feature usable with standard air brake systems, all additional components are adapted to fit into the cab or cowl area to ease installation as an accessory. 
   The invention provides a parking brake system for an air brake system which automatically applies the parking brakes upon an operator moving an automatic transmission shift lever to park and which requires the driver to reapply the service brake prior to disengaging the parking brakes. The parking brake system of the present invention has primary and secondary sources of compressed air, primary circuit and secondary circuit lock in valves, a push pull double check valve connected to the primary and secondary sources of compressed air with the primary and secondary circuit lock in valves located between the primary and secondary sources and the inputs to the push pull double check valve. The primary and secondary circuit lock in valves are pilot valves. Exhaust of the primary and secondary circuit lock in valves is controlled using exhaust control valves. An air signal source responsive to the position of an ignition switch provides a pilot signal for the actuation of the primary and secondary lock in valves. An air signal line extends from the push pull double check valve. Transmission status and the pressure in the air line connecting the push pull double check valve to the spring chambers of the parking brakes effect control of the parking brakes. 
   Additional effects, features and advantages will be apparent in the written description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic illustration of an air brake system built pursuant to the teaching of the present invention. 
       FIG. 2  is a detailed schematic illustration of particular control arrangements for the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and in particular to  FIG. 1 , an air brake system  10  for a vehicle such as a school bus is illustrated. Air brake system  10  is illustrated as configured for a vehicle having two front wheels and two rear wheels (not shown). Associated with the front wheels and the rear wheels are individual wheel mounted service brakes  104 . The rear wheel brake assemblies  106  include a park or spring brake chamber  105  in addition to the service brake  104 . Thus the rear brake assemblies  106  provide both service braking and park braking. The dual function of the rear brake assemblies  106  is accomplished by having two separate air ports  111   a  and  111 b on the service brake chambers  104  and the spring brake chambers  105 , respectively. The service braking air port  111   a  allows air to be directed to the service brake chamber  104  to move brake pads (not shown) to stop the rear wheels. The park braking port  111   b  allows air to be directed to the spring brake chambers  105  to act counter internal springs which normally urge application of the brake pads. When the parking brake is disengaged, compressed air holds the park brakes off and free movement of the rear wheels is allowed. 
   The automatic parking brake system of the invention operates in cooperation with the vehicle ignition system. An ignition  109  for starting a vehicle engine is provided having four states, off, accessory, run and start. A driver of the vehicle inserts a key  110  into the ignition  109  and turns the key to the start position to start the engine. After the engine starts and the driver releases the key, the key returns to a run position which allows the engine to continue operating. The driver will turn the key  110  to an ‘off’ position to stop the engine after parking the vehicle. The ignition  109  also has an ‘accessory’ position to allow operation of vehicle utility equipment  119  such as a radio. 
   Components in the air brake system  10  are supplied with compressed air via air lines  19   a–j . An air compressor  22  supplies air via air lines  19   a  to, and though, an air dryer  23  to a wet tank  24 . The wet tank  24  acts as a supply reservoir for both the primary air tank  20  and the secondary air tank  21 , which in turn directly supply the service and parking brake systems. Air lines  19   b  and  19   c , respectively, deliver air from the wet tank  24  to the primary tank  20  and the secondary tank  21 . Check valves  25  are incorporated into air lines  19   b  and  19   c  allowing air to flow out from the wet tank  24  but not back into the wet tank. 
   Primary air tank  20  and a secondary air tank  21  are the direct sources of supply of pressurized air for a redundant air brake system  10 . In a bus or truck where most of the weight can be over the rear wheels, the primary air tank  20  supplies air for service braking for the rear wheels. The secondary air tank  21  supplies air for service braking for the front wheels. Since independent sources of air are used for the service brakes for the rear and front wheels the service brake system is considered to be redundant. Air is routed from primary air tank  20  via air line  19 d through a foot actuated double valve  26  upon depression of foot pedal  26   a . For a brake system predating anti-lock braking systems (ABS), such as illustrated, air is then routed to a rear wheel service brake quick release valve (QRV)  27 . On ABS equipped vehicles QRVs are used only for rear parking brake functions. ABS modulators perform the QRV functions. A relay valve  430  uses air from the food pedal  26  as a pneumatic signal for applying air to QRV  27  directly from primary tank  20 . Air from secondary air tank  21  is coupled to the service brake chambers  104  for the front wheels for service braking via air line  19   e  through the double valve  26  upon depression of foot pedal  26   a . Again a front wheel service brake QRV  28  is illustrated, although in vehicles equipped with ABS, no QRV is present and the functionality of the QRV is carried out by an ABS modulator. The operation of the quick release valves, or ABS modulators, is conventional and well known in the art. 
   The park brakes are held in a disengaged state by the application of compressed air to park braking ports  111   b . The park brake function occurs when air is vented from park braking port  111   b  of the spring or park brake chamber  105  through QRV  31 . Air is coupled from the primary air tank  20  and the secondary air tank  21  to the spring brake chambers  105  for holding the springs open and controlling the engagement of park braking. Air lines  19   f ,  19   g  and  19   h  are directly involved in the routing of air to and from the ports  111   b.    
   The parking brake system makes use of the redundant compressed air sources to avoid unintended engagement of the parking brake system should one compressed air source fail. Air lines  19   f  and  19   g  supply air from the primary and secondary tanks  20 ,  21  through the double valve  26  to a push pull double check (PPDC) valve  29 . The air enters the double valve  26  into tees, past the double check valves and into the IN ports of the pilot valves  32 ,  33 . When the pilot valves  32 ,  33  open air is supplied to the primary and secondary inputs of the PPDC  29 . From the pilot valves  32 ,  33  the air is introduced to the push pull double check valve  29  from which a single air line  19   h  emerges. Air line  19   h  extends from PPDC valve  29  to an inversion valve  30 . Air is applied through to parking brake QRV  31  from inversion valve  30  in response to pneumatic input signals on air lines  19   f  and  19   h.    
   Application of the park brakes occurs automatically or manually, as described below. Automatic engagement can be initiated when the appropriate combination of signals appear for actuating control valve  34 , which in turn applies air actuation signals to the pilot valves  32 ,  33  which in turn supply air to the inputs of the PPDC  29 . When solenoid control valve  34  and pilot valves  32 ,  33  close, the air signals to the PPDC  29  cannot escape so long as exhaust control valves  234  and  434  remain closed. The PPDC  29  air supply is in effect latched and the parking brakes prevented from actuating. With pilot valves  32 ,  33  normally closed due to lack of brake application and thus lack of an interlock signals, the exhaust control valves  234 ,  434  control automatic actuation of the parking brakes. Latched pressure in the PPDC valve  29  lines escapes with opening of exhaust control valves  234 ,  434 . 
   To manually engage the parking brake the driver of a vehicle  101  moves a parking brake actuator or knob  29   a  to an engaged or pulled out position which operates the push pull double check valve  29 . Push pull double check valve  29  operates to shut off the air supply from air lines  19   f  and  19   g  from the primary air tank  20  and the secondary air tank  21  and to vent air line  19   h  to the external atmosphere. Lack of pressure in air line  19   h  allows QRV  31  to vent air from ports  111   b  to the atmosphere. The springs in the spring brake chambers  105  then act to Lock the two rear wheels. If the parking brake actuator or knob  29   a  is moved out of the engaged position (i.e. pushed in), air will be supplied through the push pull double check valve  29  to the spring brake chambers  105  through air line  1   9   h  to release the rear wheels. The control signal for solenoid valve  34  comes from transmission shifter  300 , which can be set to implement an interlock limiting operation of the parking brake to when the transmission is in park. Operation of PPDC valve  29  is readily automated using a transmission state signal. 
   The air brake system  10  of the present invention includes a park brake lock-in based on key switch position. Lock-in is, in essence, the latching of the state of certain air pressure signals. A primary parking brake lock-in or pilot valve  32  and a secondary parking brake lock-in or pilot valve  33  provide air signal state latching. The primary parking brake pilot valve  32  is located in air line  19   g  (downstream from air line  19   d ) between the primary air tank  20  and the push pull double check valve  29 . Closure of the primary parking brake pilot valve  32  will stop air from flowing from the primary air tank  20  to the push pull double check valve  29 . The secondary parking brake pilot valve  33  is located in air line  19   f  between the secondary air tank  21  (via air line  19   e ) and the push pull double check valve  29 . Closure of the secondary parking brake lock-in/pilot valve  33  will stop air from flowing from the secondary air tank  21  to the push pull double check valve  29 . Closure of both the primary parking brake lock-in valve  32  and the secondary parking brake lock-in valve  33  stops air flow through push-pull double check valve  29  to the parking brake port  111   b  of the spring brake chambers  105 . The primary parking brake pilot valve  32  and a secondary parking brake pilot valve  33  are of a design such that downstream air line piping to the spring brake chambers  105  through the push pull double check valve  29  is not vented when the parking brake pilot valves  32  and  33  are closed, absent loss of pressure in the primary and secondary air tanks  20 ,  21 . Air trapped in air lines  19   h  and  19   g , by the parking brake pilot valves  32  and  33  maintains a counter force against the springs of the spring brake chambers  105  to hold the parking brakes off. 
   The parking brakes should engage in case of total pressure failure in the air brake system. Accordingly, a failsafe venting system for air lines  19   f  and  19   g  is provided. Check valves  332 ,  333  couple the sections of air lines  19   f ,  19   g  downstream the parking brake pilot valves  32  and  33 , but upstream from PPDC valve  29 , back to air lines  19   f  and  19   g , respectively, but upstream from the parking brake pilot valves. Loss of air pressure anywhere in air lines  19   e  and  19   d  results in sections of air lines  19   f    19   g  subject to latching being vented through the check valves  332 ,  333 . This results in PPDC  29  operating normally, exhausting air from air line  19   h  and thereby applying the parking brakes. 
   The parking brake system and ignition key switch work in cooperation with one another. Turning key  110  in the ignition switch  109  to the “off” position operates to close parking brake pilot valves  32  and  33  via solenoid control valve  34 . The mechanism linking ignition switch  109  position with pilot valve  32 ,  33  operation is indirect. An intermediate solenoid pilot valve  34  controls valving of compressed air from primary tank  20  along an air line  19   j  to the control ports of parking brake pilot valves  32 ,  33 . In other words, pilot valves  32 ,  33  are actuated by an air signal controlled by solenoid valve  34 , the state of which depends on the position of the ignition switch  109 . Solenoid valve  34  thus enables an interlock based on ignition switch  109  position. 
   Solenoid valve  34  is supplied with air from primary tank  20  by air line  19   i . When solenoid  34  delivers air to the pilot ports of the pilot valves  32 ,  33 , the pilot valves open and allow air to reach PPDC valve  29  through airlines  19   f ,  19   g . This in turn allows the parking brake to be held in a released state as already described. When solenoid valve  34  returns to its normal state, air exhausts from the pilot valve  32 ,  33  control lines (including the branched portions of air line  19   j ) through the exhaust port for the solenoid pilot valve  34 . Solenoid exhaust control valves  234 ,  434  on the exhaust ports from pilot valves  32 ,  33 , respectively, and prevent air from escaping from the PPDC valve  29  however, thus latching the PPDC valve  29  open under certain circumstances. The source  448  of control signals for solenoid exhaust control valves  234 ,  434  is described below. 
   The presently disclosed system readily accommodates various interlocks. As illustrated, the ignition switch  109  position signal reaches solenoid valve  34  through an AND gate  377 . AND gate  377  represents a variety of possible logic arrangements which may be implemented in hardware or through software control arrangements to define with particularity the conditions under the parking brake system operates. Interlock signals limiting operation of the park brake system may readily be based upon the state of not just the ignition switch  109 , but also the position of the transmission gear selector  300 , an engine running status signal  108 , or the position of the brake pedal  26   a.    
   Ignition switch  109  position is one interlock always used. So long as the parking brake actuator  29   a  is not engaged, moving the key  110  to the ‘off’ position will not result in release of air from air lines  19   h, f  and  g  since the operation of moving the key closes solenoid controlled valve  34 . The rear wheels of the vehicle thus remain free to rotate. Any danger from accidental movement of the key  110  or intermittent faults in the ignition circuit are reduced. 
   The interaction of the ignition switch interlock and the pilot valves  32 ,  33  is now described. If the driver moves the parking brake actuator  29   a  to a parking brake engaged or pulled out position, the PPDC valve  29  closes off air to air line  19   h  and vents air line  19   h  as already described. Subsequent turning of the key  110  to an ‘off’ position closes solenoid controlled valve  34 , closing the parking brake piloted valves  32 ,  33  which locks the park brake status by preventing new air to flow from the primary air tank  20  or secondary air tank  21  to the PPDC valve  29 . If the parking brake actuator  29   a  is subsequently moved to its disengaged position (i.e. pushed in), the remaining air trapped in air line  19   h  between the primary parking brake lock-in valve  32  and the secondary parking brake lock-in valve  33  on the one hand and the relay valve  301  would be the small volume was trapped in air lines  19   g  and  19   f  between the parking brake lock-in valves  32  and  33  and the push pull double check valve  29 . This small volume of air would be insufficient even to counter to internal springs (not shown) in the push pull double check valve  29  to open the push pull double check valve, much less hold the parking brakes off. In this manner, the turning of the key  110  of the ignition  109  to the ‘off’ position effectively disables the parking brake actuator  29   a  and maintains the rear wheels in a locked condition. 
   The state of solenoid exhaust control valves  234  and  434  is controlled by the air pressure level in air line  19   h  and the transmission gear selector  300 . Operation of this aspect of the invention is best understood by reference to  FIG. 2 . With pilot valves  32 ,  33  closed (due to lack of the appropriate interlock signal) and solenoid exhaust valves  234 ,  434  opened, any trapped air pressure in  19   f  and  19   g  will exhaust back through pilot valves  32 ,  33  to the atmosphere, resulting in the parking brakes coming on. This occurs in response to moving the transmission gear selector  300  to park. In addition, a NO pressure switch  250  operates to trip relay  260 . This signal is passed to the transmission gear selector  300 , the signal from which is passed by transmission gear position switch  300 , when in the Park position, to exhaust control solenoid valves  234 ,  434 . 
   The automatic transmission is shifted using a shifter  300 . The shifter  300  at a minimum has _Park_, _Neutral_, _Reverse_, and _Drive_ positions. When a driver depresses foot pedal  26   a  and moves transmission shifter  300  to the park position, an automatic parking brake application signal is generated. With an brake foot pedal  26   a  depressed and ignition  109  in the “run” state, the interlock requirements required for opening supply valve  34  are present. This opens pilot valves  32 ,  33  which keeps the parking brakes disengaged, even though transmission shifter  300  may be supplying an automatic parking brake application signal to exhaust solenoid valves  234 ,  434  opening the valves. However, as soon as service braking is discontinued, and the interlock signal to supply valve  34  interrupted, air escapes from the exhaust ports for pilot valves  32 ,  33 , and the parking brakes are applied as already described. Dual exhaust control valves  234 ,  434  provide against single valve failure. That is, if one of the pilot valves  33 ,  32  failed, the PPDC valve  29  would still have air supply charge from the other pilot valve and the park brakes would remain disengaged. 
   Referring again to  FIG. 2 , the configuration of the electrical signal source for exhaust valves  234 ,  434  is illustrated and the layout of components relative to the cab firewall  500  is shown. Double valve  26 , exhaust control valves  434 ,  234 , push pull double check valve  29  and pilot valves  32 ,  33  are all located inside of the cab. 
   Disengagement of the parking brakes is required to be manually executed. A source  448  of a control signal for the operation of exhaust valves  234 ,  434  is illustrated in  FIG. 1 . In  FIG. 2  this source  448  is developed as including a no pressure switch  250 , a relay  260  and transmission switch  300 . No pressure switch  250  exposed to spring brake signal air line  19   h  and is normally open. A relay  260  is controlled by switch  250  and provides for passing the signal to the transmission selector  300 . Thus, if ignition switch  109  is moved to the off position before the transmission selector  300  is moved to the Park position, and before the driver manually engages the parking brakes, the NO air pressure switch  250  would result in relay  260  supplying power for the circuit, and the exhaust valves  234 ,  434  would be held open in order to vent air trapped by the closed pilot valves  32 ,  33 . The NO pressure switch  250  is located on the delivery side of PPDC valve  29 . A trigger level for NO air pressure switch, that is the pressure level at which power is made available to the circuit is low enough to assure that the spring brakes release before the switch  250  opens and continuity is broken. 
   Many of the major elements of the system are installed inside the cab environment  290 , which means they can be installed off the main assembly line for a vehicle. The use of electrical signals and logic make the system for flexible in terms of selecting interlocks. Automatic application is retained, but deliberate disengagement of the parking brakes makes the system less vulnerable to human error. This is particularly important in school bus applications, where passengers may manipulate the controls without an appreciation for the consequences of their actions or the ability to respond to those consequences. 
   While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.