Usage:
Air And Braking System Schematic
DJB Serial Numbers: D25C: 6766-6784, 6811-6910, 6959-6968, 7111-7433 D350C: 6551-6979, 7104-7426 D400: 6490, 6537-7388
Air And Braking System Schematic
1. Tire Inflator Connector
2. Safety Valve
3. Air Dryer
4. System Protection Valve
5. Manifold
6. Parking Brake Reservoir
7. Service Brake Reservoir
8. Service Brake Reservoir
9. Service Brake Reservoir
10. Horn Valve
11. Horn
12. Differential Lock Valve
13. Parking Brake Valve
14. Dual Brake Valve
15. Pressure Gauges
16. Low Pressure Switch
17. Low Pressure Switch
18. Spring Brake Actuator
19. Double Check Valve
20. Quick Release Valve
21. Spring Brake Actuator
22. Low Pressure Switch
23. Relay Valve
24. Suspension Control Valve
25. Suspension Rotary Valve
26. Compressor
27. Air Filter
28. Governor
29. Differential Lock Cylinder
30. Quick Release Valve
31. Quick Release Valve
32. Spring Brake Actuator
33. Quick Release Valve
34. Spring Brake Actuator
35. Spring Brake Actuator
36. Spring Brake Actuator
ARRANGEMENT FOR THREE AXLE MACHINES
The air and braking systems are illustrated schematically in figures 1, 2 and 3.
An engine driven compressor (26) supplies air to dryer (3) which contains an internal non-return valve. The pressure in the air dryer is fed back to the governor (28) which operates an unloader mechanism on the compressor to maintain the correct working pressure. A purge line (line "d") feeds unloader pressure back to the dryer again so that each time the compressor is unloaded the dryer is subjected to an internal reverse flow of air which regenerates the dryer element. The dryer has a safety valve (2) installed to protect against failure of the governor cut-out action.
When the pressure in the air dryer reaches a certain value, system protection valve (4) opens and air flows through the four outlets to parking brake reservoir (6), manifold (5) and the front and rear service brake reservoirs (7, 8 and 9).
Air passing through manifold (5) flows to the tire inflator connector (1), filter (27), horn valve (10) and differential lock valve (12).
The tire inflator is included in the truck tool kit. The filter is provided to clean the air being supplied to the suspension rotary valve (25).
The differential lock valve controls the supply of air to the inter-axle differential actuating cylinder (29).
Parking Brakes
The parking brake reservoir (6) is charged with air through one element of the system protection valve. This air is used to supply the relay valve (23) and parking brake valve (13).
When the parking brake valve is in the OFF position it directs air from line (b) to line (a) and through double check valve (19) to the pilot port on the relay valve. This pilots the relay valve open and air flows through the valve to the spring chambers of the axle brake actuators. The actuator power springs are compressed and the brakes are released. Air flowing to the front axle brake actuators passes through quick release valve (20). Air flowing to the rear frame brake actuators passes through quick release valves (31 and 33). A low pressure switch (22) is screwed into the relay valve. The switch closes with decreasing air pressure and is therefore open when the park brakes are off.
When the parking brake valve is in the PARK position air in line (a) is exhausted through line (c) and the relay valve closes. Air in the spring chambers of the front axle brake actuators is exhausted through quick release valve (20) while air in the rear axle brake actuators is exhausted through quick release valves (31 and 33). The actuator power springs then take over to apply all wheel brakes.
When the pressure is released from the relay valve, low pressure switch (22) closes and a warning light illuminates to indicate that the park brake is applied.
Service Brakes
The service brake reservoirs are charged with air from system protection valve (4). The service brake reservoirs supply air to the dual brake valve (14) which is operated by the service brake footpedal. When the footpedal is pressed air flows from the dual brake valve to the service chambers of the spring brake actuators. The spring brake actuators then apply the brakes on all wheels. Low pressure switches (16 & 17) cause a warning to sound when the air pressure in the front or rear systems falls below a certain value.
When the service brake is released the air supply is blocked at the dual brake valve. Air pressure in the service chambers of the spring brake actuators is exhausted and the brakes are released. Air pressure in the front axle actuators is exhausted at the dual brake valve (14). Air pressure in the rear frame actuators is exhausted through quick release valve (30).
Normally the parking brakes are off when the service brakes are applied so that air from the relay valve will be holding the actuator power springs compressed. If, for any reason, the parking brakes are on when the service brakes are applied air then flows from the dual brake valve, through double check valve (19) to pilot the relay valve (23) open. The relay valve then allows air to flow to the spring chambers of the brake actuators to compress the power springs. This makes the operation of the actuators completely independent of the parking brake valve and thus avoids overloading of the brake and actuator components.
Tractor Air And Braking System Schematic
DJB SERIAL NUMBERS: D25C: 6970-7088, 7439-7550 D350C: 7008-7070, 7435-7602 D400: 7512-7611
CATERPILLAR PIN (9YC1-9YC283, 8XC1-8XC130, 1MD1-1MD78)
Tractor Air And Braking System Schematic
1. Filter
2. Park Brake Valve
3. Spool Valve
4. Spool Valve
5. Relay Valve
6. Hydraulic Control Valve
7. Suspension Rotary Valve
8. Quick Release Valves
9. Brake Actuators
10. Air Line
11. Air Line
The air system on later machines was modified to incorporate two spool valves. The purpose of these valves is to cut-off the air supply to the suspension rotary valve when the park or service brakes are applied, to prevent the suspension from self levelling. The air supply to the rotary valve (7) from the filter (1) passes through both valves, (3) and (4). The valves are identical but opposite ports are used for inlet and exhaust. Valve (3) operates in conjunction with the service brakes. When the brakes are released the valve spool is offset by spring pressure. Inlet port (C) is connected to delivery port (B), and exhaust port (A) is blocked. When the service brakes are applied, air pressure in line (10) offsets the spool against the return spring. The rotary valve air supply is blocked at port (C), and the delivery line from port (B) is exhausted to atmosphere through port (A). Valve (4) operates in conjunction with the parking brakes. When the brakes are released the valve spool is offset by air pressure in line (11). Inlet port (A) is connected to delivery port (B) and exhaust port (C) is blocked.
When the brakes are applied, line (11) is exhausted through park brake valve (2), and the valve spool is offset by spring pressure. The rotary valve air supply is blocked at port (A) and the delivery line from port (B) is exhausted to atmosphere through port (C).
D400 Tractor Air And Braking System Schematic - 1MD79-Up
D400 Tractor Air And Braking System Schematic - 1MD79-Up
1. Manifold
2. Pressure Switch
3. Pressure Switch
4. Suspension Control Valve
These machines incorporate electrically controlled suspension levelling. The suspension air filter, spool valves and rotary valve are not fitted. Instead the air supply from manifold (1) is fed directly to the suspension control valve (4). This change is associated with the introduction of electronic suspension.
Pressure switch (3) prevents the suspension from levelling when the parking brake is applied while pressure switch (2) prevents the suspension from levelling while the service brake is applied.
Air Compressor
1. Crankshaft.
2. Piston head
3. Inlet valve
4. Spring
5. Discharge line
6. Spring
7. Discharge valve
8. Plunger
9. Saddle spring
10. Unloader piston
Air Compressor
The two cylinder, single stage, reciprocating compressor is driven through a power take off on the engine.
During the piston down stroke a vacuum is created above the piston head (2) which causes the inlet valve (3) to lift off its seat. This allows air to flow from the engine inlet manifold, past the inlet valve to the compressor cylinder. When the piston reaches the end of its down stroke spring (4) forces the inlet valve back against its seat.
During the piston up stroke the air drawn into the cylinder is being compressed. Discharge valve (7) is forced off its seat and compressed air flows into the air dryer. When the piston reaches the end of its up stroke the pressure in the discharge line (5) together with the effect of spring (6) forces discharge valve (7) against its seat.
When the pressure in the air dryer reaches the governor cut-out setting, the governor allows air to flow from the dryer to a cavity below the unloader pistons (10). This lifts the unloader pistons and plungers (8). The plungers hold the inlet valves off their seats so that air flows between the two cylinders. The compressor is then off load.
When the air dryer pressure falls to the governor cut-in setting, the air holding the unloader plungers is exhausted. The unloader saddle spring (9) forces the plungers and pistons down, and the inlet valves can then return to their seats. The compressor is then on load again.
Governor
1. Conical Spring
2. Piston
3. Disc Valve
4. Chamber
5. Wire Mesh Filter
6. Inlet Port
7. Passage
8. Unloader Port
9. Wire Mesh Filter
10. Passage
11. Exhaust Stem
12. Purge Port
13. Exhaust Port
14. Control Spring
Governor
Air from the dryer enters the governor through inlet port (6) and flows through wire mesh filter (5) to chamber (4). Pressure in chamber (4) acts against the underside of piston (2). As the pressure in the air dryer rises the piston rises until disc valve (3) comes into contact with the spring loaded exhaust stem (11). A further increase in pressure results in further upward movement of the piston and the exhaust stem pushes disc valve (3) off its seat. Air then flows from chamber (4) past the disc valve and into passage (7). Passage (7) is connected to passage (10) at this point so that air flows through wire mesh filter (9) to the compressor unloader port (8) and the air dryer purge port (12). The unloader port directs air to the compressor unloader mechanism and the compressor cuts out. The purge port directs air back to the dryer to initiate the purge cycle.
As the dryer pressure falls the piston (2) moves downward under the effect of control spring (14) allowing disc valve (3) to return to its seat. This isolates chamber (4) from passage (10). As the exhaust stem (11) lifts clear of the disc valve air pressure in passage (10) is exhausted through the hollow exhaust stem and port (13). The compressor then cuts in to recharge the air dryer.
Air Dryer
1. Check Valve
2. Outlet Port
3. Purge Chamber
4. Check Valve
5. Safety Valve Port
6. Filter
7. Heater
8. Silencer
9. Purge Valve
10. Spring
11. Purge Line Port
12. Piston
13. Dryer Body
14. Desiccant Cartridge
15. Inlet Port
16. Purge Vent
Air Dryer
The air dryer cools, filters and dries the pressurised air delivered by the compressor.
Compressed air entering the dryer will always have a very high water vapour content. Air enters through inlet port (15) and flows downward between the dryer body (13) and cartridge (14). As this air is cooled condensate forms and collects in the bottom of the dryer. Most of the dirt in the air settles out at this stage. The air then passes up through the filter (6) which removes most of the oil and water droplets remaining in suspension in the air. Any remaining particles of dirt are also trapped by the filter.
Cartridge (14) is filled with thousands of tiny pellets. As the air passes through the cartridge, water vapour is removed by a process known as adsorbtion. This moisture collects in the pores of the micro-crystalline structure of the pellets (called desiccant pellets).
After passing through the desiccant the dry air passes through check valve (4) and enters the purge chamber (3). From the purge chamber air passes through check valve (1) and outlet port (2) to the system protection valve and reservoirs.
When the pressure in the reservoirs and purge chamber is high enough the governor operates to unload the compressor. At the same time the governor feeds pressure to port (11). This pressure forces piston (12) downward against the effect of spring (10). The opening of the valve allows the existing presure in the dryer to force accumulated condensate and dirt out through the vent. The air in the purge chamber expands back through purge vent (16) and the desiccant cartridge to dry it out. In escaping past the open purge valve (9) the air carries this moisture with it. The desiccant material is regenerated in this way each time the compressor is taken of load, thus preventing it from becoming totally saturated with moisture. Check valve (1) prevents reservoir pressure from blowing back through the dryer.
Silencer (8) reduces noise during the purge cycle and a thermostatically controlled heater (7) prevents condensate from freezing.
Safety Valve
1. Spring
2. Ball
3. Seat
4. Exhaust port
Safety Valve
Air pressure in the dryer acts against the underside of ball (2). If the governor cut-out action fails the pressure will rise until the effect of spring (1) is overcome and ball (2) lifts off its seat (3) to exhaust air through port (4).
System Protection Valve
1. Chamber
2. Passage
3. Outlet Port
4. Check Valve
5. Spring
6. Outlet Port
7. Check Valve (Seat)
8. Chamber
9. Valve Element (Auxiliaries Manifold)
10. Outlet Port
11. Valve Element (Parking Reservoir)
12. Chamber
13. Outlet Port
14. Chamber
15. Inlet Port
16. Valve Element (Service Brakes)
17. Valve Element (Service Brakes)
System Protection Valve
Air from the dryer enters the valve through port (15) and passes to chambers (14) and (1) in the valve body. This pressure acts against pistons (30) which rise against the effect of springs (25). Air then flows past the valve seats (19) to the front and rear service brake reservoirs through ports (3) and (13). Air also flows past check valves (4) and (7) to chamber (8).
Once the opening pressure of pistons (30) has been reached and they lift off their seats, inlet pressure acts against the full face of the pistons, keeping them raised against their springs. Normally, the pistons will remain raised and the service brake reservoirs will be interconnected. This permits the equalisation of air pressure in the service brake reservoirs and the air dryer and enables the governor to control service brake pressure directly.
18. Spring Retainer
19. Valve Seat
20. Valve Seat
21. Piston Insert
22. Hole
23. Breather Cap
24. Adjusting Screw
25. Spring
26. Spring
27. Piston
28. Check Valve
29. Piston Insert
30. Piston
System Protection Valve
Air in chamber (8) is fed to chambers (12) - one for each valve (9) and (11) - by internal passages. This air acts against check valves (28) which lift off their seats (20). Once the check valves lift, air pressure acts against the faces of pistons (27), which lift against springs (25). Air then flows through outlet port (10) to the parking brake reservoir and through port (6) to the manifold which supplies the horn valve, differential lock valve, tire inflator connector and suspension filter. The pistons (27) are kept raised against their springs by inlet pressure. When the inlet air supply is interrupted, such as when the governor cuts out the compressor, check valves (28) close against their seats (20) to isolate ports (6) and (10) from each other and from the other two ports (3) and (13). Check valves (28) then have inlet pressure acting against their front faces and outlet pressure acting against their back faces as well as springs (26). If the outlet pressure falls the check valves will lift to recharge the system.
If there is a failure in one of the service brake circuits the reservoir pressure will fall and pistons (30) will fall until they are fully closed. The pressure at which they are fully closed is lower than their initial opening pressure because inlet air acts against the full face area when the pistons are open and against reduced area when they are closed. Once fully closed the inlet pressure will rise again to the value required to initially lift pistons (30) from their seats. The unaffected service brake circuit is thus assured of a continued supply of air at not less than this initial opening pressure. The check valve (4) or (7) between the affected valve piston and chamber (8) will also close, preventing loss of air from the chamber.
If there is a failure in one of the circuits fed from port (6) or (10) the piston (27) will fall and close off that circuit. The remaining circuits will continue to receive air at the initial opening pressure of the pistons.
The initial opening pressure of the pistons is controlled by adjusting screws (24) which determine the pre-load in springs (25). The spring chambers are vented to atmosphere through holes (22) and breather caps (23).
Relay Valve
1. Cover
2. Pilot port
3. Piston
4. Body
5. Exhaust seat
6. Pressure passage
7. Spring
8. Inlet seat
9. Outlet chamber
10. Spring
Relay Valve
Pilot pressure enters the cover (1) through port (2). System pressure from the parking brake reservoir enters the body (4) through passage (6). Passage (6) has two inlet ports connected to it. One carries inlet pressure from the reservoir and the other carries a low pressure switch.
Pilot pressure acting against piston (3) moves the piston downward until exhaust seat (5) is covered. Further downward movement compresses spring (7) and uncovers the inlet seat (8). Air from the reservoir can then flow into outlet chamber (9). The pressure in outlet chamber (9) is felt against the under-side of piston (3). This, combined with the effect of spring (10) moves the piston upward until the relay valve is in a state of balance. Exhaust seat (5) remains covered and the effect of spring (7) causes inlet seat (8) to be covered. If the pilot pressure is increased (due to further movement of the parking brake valve) the relay valve will re-open and pass more air to outlet chamber (9). As the pressure in the outlet chamber increases the relay valve will move back into a state of balance. This results in a progressive release of the wheel brakes as the parking brake valve is moved from the "PARK" position to the "OFF" position.
As the parking brake valve is moved away from the OFF position pilot pressure in port (2) is reduced and spring (10) moves piston (3) upward, opening exhaust seat (5). Air pressure in outlet chamber (9) is reduced as air flows to exhaust until spring (7) moves inlet seat (8) back into contact with exhaust seat (5). The valve is then back in a state of balance with reduced pressure in outlet chamber (9). This action results in a progressive application of the wheel brakes as the parking brake valve is moved from "OFF" to "PARK".
Outlet chamber (9) has four ports connected to it. One is connected to the front axle brake actuators, one to the rear brake actuators and the other two are plugged.
Dual Brake Valve
When the service brake footpedal is pressed force is transmitted through spindle (1), cam (24) and plunger (2) to spring seat (22) and rubber spring (23). This force moves piston (20) downward and exhaust seat (19) closes against valve (17). Further downward movement pushes valve (17) away from inlet seat (18) allowing air to flow from inlet port (4) to outlet port (16).
Air passing through outlet port (16) also flows through transfer holes (15) and acts against the relay piston (12). The downward movement of piston (20) and valve (17) results in an additional force being applied to piston (12) by spring (14). Piston (12) moves downward and exhaust valve seat (11) closes against valve (8). Valve (8) moves away from inlet seat (10) allowing air to flow from inlet port (5) to outlet port (9).
If the brake footpedal is held in one position the air pressure under piston (20) will increase until it combines with the upward forces exerted by springs (21) and (3) to balance the force exerted on the footpedal. The piston (20) and valve (17) then lift sufficiently to allow inlet seat (18) to contact the valve. At this point exhaust seat (19) and inlet seat (18) are both covered and the valve element (17) is in a state of balance. Similarly, increasing pressure under piston (12) combined with the effect of spring (7) overcomes the downward force exerted by spring (14) and air pressure acting above piston (12). The piston then lifts sufficiently to allow valve (8) to close against inlet seat (10) while keeping exhaust seat (11) covered. The lower valve element (8) is then also in a state of balance.
If the brake application is increased the downward force exerted on piston (20) increases. Valve (17) is pushed off inlet seat (18) allowing more air to flow from inlet port (4) to outlet port (16). The outlet pressure then increases to give increased braking effect. Valve (8) also moves away from inlet seat (10) in the manner described previously. Thus the pressure in outlet port (9) also increases until both elements again move into a state of balance.
1. Spindle
2. Plunger
3. Spring
4. Inlet Port
5. Inlet Port
6. Rubber Flap
7. Spring
8. Valve Element
9. Outlet Port
10. Inlet Seat
11. Exhaust Seat
12. Piston
13. Passage
14. Spring
15. Hole
16. Outlet Port
17. Valve Element
18. Inlet Seat
19. Exhaust Seat
20. Piston
21. Spring
22. Spring Seat
23. Rubber Spring
24. Cam
25. Footpedal
Dual Brake Valve
When the brake application is reduced the downward force exerted on piston (20) is reduced and the piston moves upward under the influence of pressure in port (16) and spring (21). Exhaust seat (19) moves away from valve (17) allowing air to exhaust through passage (13) and past the rubber flap (6). This action continues until the pressure in port (16) falls sufficiently to restore the balance of forces on piston (20). As the pressure in port (16) falls the force exerted on the upper face of piston (12) also falls allowing the piston to rise. Exhaust seat (11) moves away from valve (8) allowing air to exhaust from port (9) through passage (13). The pressure in port (9) then falls until the forces on piston (12) balance again.
The self balancing action of the valve controls the outlet pressures in ports (9) and (16) at levels proportional to the force exerted on the footpedal.
Double Check Valve
1. Inlet port
2. Shuttle valve
3. Inlet port
4. Outlet port
Double Check Valve
Air enters the double check valve through either of the inlet ports (1 or 3). Shuttle valve (2) moves either to the left or right and allows air to flow through outlet port (4). If air is available at both inlet ports simultaneously the shuttle valve (2) will respond to and deliver air from the port containing the higher pressure.
Spring Brake Actuator
1. Spring Housing
2. Snap Ring
3. Power Spring
4. Piston
5. Spring Brake Chamber
6. Service Chamber
7. Clamp Ring
8. Diaphragm
9. Spring
10. Pushrod
11. Mounting Studs
12. Breather Tube
13. Seals
14. Piston Rod
15. Cap
16. Boss
17. Wind Off Bolt
18. Anti-explosion Washer
Spring Brake Actuator
When the parking brake valve is in the "OFF" position and the service brake footpedal is released air enters the spring brake chamber (5). Air pressure acting against the piston (4) moves it to the left against the effect of the power spring (3). At the same time spring (9) moves pushrod (10) to the left, thus releasing the brakes.
When the service brake footpedal is applied air flows from the dual brake valve into the service chamber (6). Air pressure acting against diaphragm (8) moves pushrod (10) to the right against the effect of spring (9). Thus the brakes are applied. As long as the parking brake valve remains in the "OFF" position air pressure in the spring brake chamber (5) will hold piston (4) left.
As the parking brake valve is moved toward the "PARK" position air pressure in the spring brake chamber will be progressively reduced due to air being exhausted at the quick release valves. The power spring (3) moves piston (4) to the right by an amount depending on the pressure remaining in the spring brake chamber, thus providing a gradual application of the brake. When the parking brake valve is in the "PARK" position all air pressure is exhausted from the spring brake chamber and the brake is fully applied.
If there is a failure of the air supply the power spring (3) will apply the brake. The brake can be manually released by removing cap (15) and turning the wind off bolt (17). The wind off bolt then draws the piston (4) and rod (14) to the left, releasing the brake.
Quick Release Valve
1. Inlet Port
2. Inlet Seat
3. Outlet Port
4. Exhaust Seat
5. Exhaust Port
6. Diaphragm
7. Outlet Port
Quick Release Valve
Air enters the valve through inlet port (1) and forces diaphragm (6) downward, opening inlet seat (2) and closing exhaust seat (4). The air then flows through outlet ports (3) and (7) to the brake chambers. When inlet pressure is removed from port (1) - due to the service brake pedal being released or the park brake valve being moved to the "PARK" position - pressure in the outlet ports (3) and (7) moves the diaphragm up to open exhaust seat (4) and close inlet seat (2). Air in the actuators is exhausted through port (5).
Differential Lock Valve
1. Lever
2. Cam
3. Spring
4. Valve
5. Spring
6. Inlet Port
7. Exhaust Seat
8. Inlet Seat
9. Outlet Port
10. Plunger
11. Exhaust Outlet
Differential Lock Valve
When the valve is in the disengaged position the cam (2) on the bottom of the lever (1) depresses plunger (10). The plunger moves downward closing exhaust seat (7) against valve (4) and lifting the valve from the inlet seat (8). Air then passes through inlet port (6) to outlet port (9) and on to the differential actuating cylinder.
When the lever (1) is moved to the engaged position, spring (3) moves plunger (10) upward. This opens exhaust seat (7) and allows spring (5) to move valve (4) upward against the inlet seat, thus closing off the air supply. Air already in the actuating cylinder flows past exhaust seat (7) to atmosphere through outlet (11).
Differential Lock Cylinder
1. Breather
2. Piston
3. Inlet Port
4. Rod
When the "disengaged" position is selected air enters the cylinder through inlet port (3). Piston (2) and rod (4) move down. The rod (4) is attached to the dog clutch selector shaft. The dog clutch is then disgengaged and the inter-axle differential operates normally.
When the "engaged" position is selected port (3) is connected to exhaust. Springs fitted to the dog clutch selector shaft then pull rod (4) and piston (2) up. The dog clutch is then engaged and the inter-axle differential is locked.
Wheel Brakes
The wheel brakes are air operated drum type brakes. The brake drum (5) is fastened to the wheel assembly. When the brake pedal is pressed air under pressure is directed to the spring brake actuators. The actuator push rods operate the brake cylinders (9) which in turn operate the brake shoes (6) which pivot about pins (4) and are forced into contact with the drum.
The brake actuator push rod (11) acts against follower (21) and piston (12). As the piston moves downward it pushes rollers (15) downward which bear against the tapered edges of plungers (19). The plungers then force the adjusting screws (8) out of the cylinder which force the brake shoe assemblies (6) into contact with the drum.
1. Cylinder
2. Spring
3. Spider
4. Pivot Pin
5. Brake Drum
6. Brake Shoe Assembly
Brake Assembly
2. Spring
3. Spider
4. Pivot Pin
6. Brake Shoe Assembly
7. Brake Lining
8. Adjusting Screws
9. Brake Cylinder
Brake Assembly
1. Cylinder
8. Adjusting Screw
10. Brake Actuator
11. Pushrod
12. Piston
13. Seal
14. Plug
15. Rollers
16. Springs
17. Pawls
18. Actuators
19. Plungers
20. Spring
21. Follower
Brake Cylinder Assembly
When the brake is released spring (20) forces piston (12) and rollers (15) upward again. At the same time spring (2) pulls the brake shoes away from the drum and forces the adjusting screws (8) and plungers (19) back into the cylinders.
Adjusting screws (8) are threaded into actuators (18) which are a free fit in plungers (19). Plugs (14), springs (16) and pawls (17) act as guide assemblies for the plungers (19). The ends of the pawls are toothed and engaged with helical teeth on the outside diameter of actuators (18). As the actuators move out of the slave cylinder during a brake application the helical teeth on the actuators lift the pawls (17) against the effect of springs (16). When the brake is released the actuators (18) and pawls (17) return to their original position.
As the brake linings (7) wear the stroke of the actuators (18) and hence the lift of the pawls (17) gradually increases. Eventually the pawls (17) climb over the tooth in the actuators and drop into the next tooth space. When the brakes are subsequently released and the plungers move back into the cylinder, the teeth on the pawl cause the actuator to turn and move outward. Thus the adjusting screw moves outward and reduces the clearance between the brake lining and drum.
Horn Control Valve
1. Button
2. Plunger
3. Inlet Port
4. Seat
5. Valve
6. Ball
7. Spring
8. Strainer
9. Outlet port
Horn Control Valve
Air enters the valve through inlet port (3). When the button (1) is depressed plunger (2) unseats valve (5) from seat (4) and air flows through outlet port (9) to the horn. When the button is released the spring (7) pushes valve (5) back into contact with seat (4).
Low Pressure Switch
1. Contact Disc
2. Piston
3. Passage
4. Diaphragm
5. Fixed Contact
6. Control Spring
Low Pressure Switch
Air enters the switch through inlet passage (3) and acts against diaphragm (4). The piston (2) and contact disc (1) move upward against the effect of the control spring (6), thus breaking the electrical circuit. This is the normal running condition.
If the supply pressure falls below the setting pressure the control spring moves the contact disc down against the fixed contact (5) to complete the circuit.
Two switches are installed in the service brake circuits (one front, one rear) which sound a warning buzzer if the circuit is completed.
One switch is installed in the park brake circuit which illuminates a warning light. This light illuminates if the park brake is applied.
Stop Light Switch
1. Spring
2. Piston
3. Diaphragm/Seal
4. Inlet Port
5. Contact Strip
6. Terminal
Stop Light Switch
Whenever the service brake valve is operated air pressure is applied to inlet port (4). This acts against the combined diaphragm/seal (3) which moves piston (2) upward against the effect of spring (1). Contact strip (5) then bridges the gap between the terminals (6) to complete an electrical circuit and illuminate the stop lights.
The switch contacts close at approximately 35 kPa (5 p.s.i.), thus ensuring that the stop lights illuminate immediately a brake application is made.
Parking Brake Valve
1. Delivery Port
2. Inlet Port
3. Exhaust Port
4. Adjusting Screw
5. Lockwasher
6. Graduating Spring
7. Piston - lower half
8. Inlet/Exhaust Valve
9. Exhaust Passage
10. Spring
11. Piston - upper half
12. Annular Chamber
13. Hole
14. Chamber
15. Inlet Valve Seat
16. Plunger
17. Spring
18. Chamber
19. Pin
20. Guide Slot
21. Cam
22. Torsion Spring
23. Cam Pivot
24. Pin
25. Handle
26. Detent
27. Collar
28. Knob
Parking Brake Valve
Air from the parking brake reservoir enters the valve through inlet port (2) and flows into annular chamber (12). This air flows through holes (13) to chamber (14).
When the operating handle is in the brakes "OFF" position (as illustrated) cam (21) holds cam follower (19) and thus valve plunger (16) down against the effect of spring (17) to close exhaust passage (9). The existing delivery pressure in chamber (18) pushes down piston assembly (11) against graduating spring (6) to close inlet seat (15) on inlet/exhaust valve (8). With both the inlet seat and exhaust passage closed a constant pressure is being maintained in chamber (18) and in the relay valve pilot line. The relay valve is held open thus ensuring the truck spring brakes are held off by the maintenance of air pressure in the spring brake actuators.
When the operating handle is moved against the effect of torsion spring (22) towards the "PARK" position cam (21) allows spring (17) to lift valve plunger (16) thus uncovering exhaust passage (9). Since inlet/exhaust valve (8) remains seated against piston assembly (11) the air pressure in chamber (18) begins to exhaust to atmosphere through exhaust passage (9) and port (3). The relay valve begins to close.
If the handle movement is stopped at some point between the brakes "OFF" and "PARK" positions, the reducing pressure in chamber (18) allows graduating spring (6) to lift piston assembly (11) until it seats against valve plunger (16) thus closing exhaust passage (9). Since inlet/exhaust valve (8) remains seated against piston assembly (11) the reduced pressure is held constant in chamber (18) and the relay valve is in a state of balance. In this way the wheel brakes can be applied progressively by controlled movement of the parking brake operating handle.
In the "PARK" position, valve plunger (16) is fully up thus uncovering exhaust passage (9). The air pressure in chamber (18) and thus the pilot line to the relay valve is completely exhausted. Graduating spring (6) lifts piston assembly (11) to the top of its stroke. However, exhaust passage (9) remains open and as inlet/exhaust valve (8) remains seated against piston assembly (11) no supply air can enter chamber (18). The truck brakes are held on by the power springs in the actuators.
When the handle is moved from the "PARK" position towards brakes "OFF", cam (21) depresses cam follower (19) and valve plunger (16) until exhaust passage (9) is closed. Inlet/exhaust valve (8) is unseated from piston assembly (11). Air pressure is then passed to chamber (18) and on through delivery port (1) to pilot the relay valve open.
A detent (26) locks the valve handle in the "PARK" position.