mechanical linkage which connects each master cylinder with its corresponding brake pedal, connecting fluid lines, and a brake assembly in each main landing gear wheel (figure 9-25).
Each master cylinder is actuated by toe pressure on its related pedal. The master cylinder builds up pressure by the movement of a piston inside a sealed, fluid-filled cylinder. The resulting hydraulic pressure is transmitted to the fluid line connected to the brake assembly in the wheel. This results in the friction necessary to stop the wheel.
When the brake pedal is released, the master cylinder piston is returned to the off" position by a return spring. Fluid that was moved into the brake assembly is then pushed back to the master cylinder by a piston in the brake assembly. The brake assembly piston is returned to the "off" position by a return spring in the brake. Some light aircraft are equipped with a single master cylinder which is hand-lever operated and applies brake action to both main wheels simultaneously. Steering on this system is accomplished by nosewheel linkage.
The typical master cylinder has a compensating port or valve that permits fluid to flow from the brake chamber back to the reservoir when excessive pressure is developed in the brake line due to temperature changes. This ensures that the master cylinder won't lock or cause the brakes to drag.
Various manufacturers have designed master cy!- inders for use on aircraft. All are similar in operation, differing only in minor details and construction. Two well-known master cylinders--the Goodyear and the Warner-are described and illustrated in this section.
In the Goodyear master cylinder (figure 9-26) fluid is fed from an external reservoir by gravity to the master cylinder. The fluid enters through the cylinder inlet port and compensating port and fills the master cylinder casting ahead of the piston and the fluid line leading to the brake actuating cylinder.
Retur speing FIcURE 9-26. Goodyear master brake cylinder.
Application of the brake pedal, which is linked to the master cylinder piston rod, causes the piston rod to push the piston forward inside the master cylinder casting. A slight forward movement blocks the compensating port, and the buildup of pressure begins. This pressure is transmitted to the brake assembly.
When the brake pedal is released and returns to the "off" position, the piston return spring pushes the front piston seal and the piston back to full "off" position against the piston return stop. This again clears the compensating port. Fluid that was moved into the brake assembly and brake connecting line is then pushed back to the master cylinder by the brake piston which is returned to the "off" position by the pressure of the brake piston return springs. Any pressure or excess volume of fluid is relieved through the compensating port and passes back to the fluid reservoir. This prevents the master cylinder from locking or causing the brakes to drag If any fluid is lost back of the front piston seal due to leakage, it is automatically replaced from the fluid reservoir by gravity. Any fluid lost in front of the piston from leaks in the line or at the brake assembly is automatically replaced through the piston head ports, and around the lip of the front piston seal when the piston makes the return stroke to the full "off" position. The front piston seal functions as a seal only during the forward stroke.
These automatic fluid replacement arrangements always keep the master cylinder, brake connecting line, and brake assembly fully supplied with fluid as long as there is fluid in the reservoir.
The rear piston seal seals the rear end of the cylinder at all times to prevent leakage of fluid, and the flexible rubber boot serves only as a dust cover.
The brakes may be applied for parking by a ratchet-type lock built into the mechanical linkage' between the master cylinder and the foot pedal. Any change in the volume of fluid due to expansion while the parking brake is on is taken care of by a spring incorporated in the linkage. The brakes are unlocked by application of sufficient pressure on the brake pedals to unload the ratchet.
Brake systems employing the Goodyear master cylinder must be bled from the top down. In no case should bleeding be attempted from the bottom up, because it is impossible to remove the air behind the piston seal.
The Warner master cylinder (figure 9-27) incorporates a reservoir, pressure chamber, and compen- 361
sating device in a single housing. The reservoir is vented to the atmosphere through the filler plug, which also contains a check valve. A fluid level tube is located in the side of the reservoir housing.
have a parking feature which consists of a ratchet and spring arrangement. The ratchet locks the unit in the "on" position, and the spring compensates for expansion and contraction of fluid.
Power Brake Control Systems Power brake control valve systems (figure 9-28) are used on aircraft requiring a large volume of fuid to operate the brakes. As a general rule, this applies to many large aircraft. Because of their weight and size, large wheels and brakes are re- Filler quired. Larger brakes mean greater fluid displaceplug ment and higher pressures, and for this reason independent master cylinder systems are not practical Return on heavy aircraft.
spring Reservoir Fluid To main level system tube From main system Brake system Pilot's brake .Copilot's brake valves valves Compensating valve Seal - Pressure CODE chamber Pressure Return Brake pressure Actuating relief valve Emergency- Brake port Right brakes FIcURE 9-27. Warner master brake cylinder.
Toe pressure on the brake pedal is transferred to the cylinder piston by mechanical linkage. As the piston moves downward, the compensating valve closes and pressure is trapped in the pressure chamber. Further movement of the piston forces fluid into the brake assembly, creating the braking action. When toe pressure is removed from the brake pedal, the piston return spring returns the piston to the "off" position. The compensating device allows fluid to flow to and from the reservoir and pressure chamber when the brakes are in the "off" position and the entire system is under atmospheric pressure.
Certain models of the Warner master cylinder Brake shuttle valves From emergency brake valve FIGURE 9-28. Typical power brake control valve system.
In this system a line is tapped off from the main hydraulic system pressure line. The first unit in this line is a check valve which prevents loss of brake system pressure in case of main system failure.
The next unit is the accumulator which stores a reserve supply of fluid under pressure. When the brakes are applied and pressure drops in the accumulator, more fluid enters from the main system and is trapped by the check valve. The accumulator also acts as a surge chamber for excessive loads imposed upon the brake hydraulic system.
Following the accumulator are the pilot's and copilot's control valves. The control valves regulate and control the volume and pressure of the fluid which actuates the brakes.
Four check valves and two orifice check valves are installed in the pilot's and copilot's brake actuating lines. The check valves allow the flow of fluid in one direction only. The orifice check valves allow unrestricted flow of fluid in one direction from the pilot's brake control valve; flow in the opposite direction is restricted by an orifice in the poppet. Orifice check valves help prevent chatter.
The next unit in the brake actuating lines is the pressure relief valve. In this particular system, the pressure relief valve is preset to open at 825 p.s.i. to discharge fluid into the return line, and closes at 760 p.s.i. minimum.
Each brake actuating line incorporates a shuttle valve for the purpose of isolating the emergency brake system from the normal brake system. When brake actuating pressure enters the shuttle valve, the shuttle is automatically moved to the opposite end of the valve. This closes off the hydraulic brake system actuating line. Fluid returning from the brakes travels back into the system to which the shuttle was last open.
Pressure Ball-Check Brake Control Valve A pressure ball-check power brake control valve (figure 9-29) releases and regulates main system pressure to the brakes and relieves thermal expansion when the brakes are not being used. The main Cover - Pressure inlet Mounting flange.
Ball check Retum spring • Retainer Pilot pin To brake To reservoir Seal Cross pin - Slot •Fibre plug Bushing Mounting flange "Packings
- Primary piston Body
•Packing nut Tuning fork Link parts of the valve are the housing, piston assembly, and tuning fork. The housing contains three chambers and ports: pressure inlet, brake, and return.
When toe pressure is applied to the brake pedal, the motion is transmitted through linkage to the tuning fork. The tuning fork swivels, moving the piston upward in the cylinder. The first movement upward causes the piston head to contact a flange on the pilot pin, closing the return fluid passage.
Further movement upward unseats the ball-check valve allowing main system pressure to flow into the brake line. As the pressure increases in the brake actuating cylinder and line, the pressure also increases on the top side of the piston. When the total force on top of the piston is greater than the force applied at the brake pedal, the piston is forced downward against the bar spring tension. This allows the ball-check valve to seat, closing off system pressure. In this position, the pressure and return ports are both closed and the power brake valve is balanced. This balancing action cuts system pressure down to brake pressure by closing off the pressure from the main system when the desired brake pressure is attained. As long as the valve is balanced, fluid under pressure is trapped in the brake assembly and line.
Power Brake Control Valve (Sliding Spool Type) A sliding spool power brake control valve (figure 9-30) basically consists of a sleeve and spool installed in a housing. The spool moves inside the sleeve, opening or closing either the pressure or return port of the brake line. Two springs are provided. The large spring, referred to in figure 9-30 as the plunger spring, provides "feel" to the brake pedal. The small spring returns the spool to the "off" position.
When the plunger is depressed the large spring moves the spool closing the return port and opening the pressure port to the brake line. When the pressure enters the valve, fluid flows to the opposite end of the spool through a hole when the pressure pushes the spool back far enough toward the large spring to close the pressure port, but does not open the return port. The valve is then in the static Off adjusting condition. This movement partially compresses the
• screw Weld large spring, giving "feel" to the brake pedal. When Shackle the brake pedal is released the small spring moves High pressure clamp the spool back and opens the return port. This FICURE 9-29. Pressure ball-check power brake control allows fluid pressure in the brake line to flow out ralve. through the return port.
Pressure Brake Retur Spool return spring Plunger moving Fluid under pressure in this area Plunger acting on spool resists spring Plunger giving "feel" to brake pedal spring Spool Pressure Brake Retur Constant pressure (plunger stationary) Return Plunger moving FIGURE 9-30. Sliding spool power brake control valve.
Brake Debooster Cylinders In some power brake control valve systems, debooster cylinders are used in conjunction with the power brake control valves. Debooster units are generally used on aircraft equipped with a highpressure hydraulic system and low-pressure brakes.
Brake debooster cylinders reduce the pressure to the brake and increase the volume of fluid flow.
Figure 9-31 shows a typical debooster cylinder installation, mounted on the landing gear shock strut in the line between the control valve and the brake.
As shown in the schematic diagram of the unit, the cylinder housing contains a small chamber and a large chamber, a piston with a small head and a large head, a spring-loaded ball-check valve, and a piston return spring.
In the "of" position, the piston assembly is held at the inlet (or small) end of the debooster by the piston return spring. The ball-check valve is held on its seat in the small piston head by a light spring.
Fluid displaced by thermal expansion in the brake unit can easily push the ball-check valve off its seat to escape through the debooster back to the power control valve.
When the brakes are applied, fluid under pressure enters the inlet port to act on the small end of the piston. The ball check prevents the fluid from passing through the shaft. Force is transmitted through the small end of the piston to the large end of the piston. As the piston moves downward in the housing a new flow of fluid is created from the 364
21 large piston head, pressure at the outlet port is
- 23 - 22 24 reduced. At the same time, a greater volume of fluid is displaced by the large piston head than that used to move the small piston head.
25 Normally, the brakes will be fully applied before 26 the piston has reached the lower end of its travel.
However, if the piston fails to meet sufficient resistance to stop it (due to a loss of fluid from the 23 brake unit or connecting lines), the piston will continue to move downward until the riser unseats the ball-check valve in the hollow shaft. With the ball- 13 check valve unseated, fluid from the power control 14 Debooster valve will pass through the piston shaft to replace 15 cylinder the lost fluid. Since the fluid passing through the
- 16 piston shaft acts on the large piston head, the piston
- 17 will move up, allowing the ball-check valve to seat when pressure in the brake assembly becomes nor-
- 18 mal.
19 When the brake pedals are released pressure is removed from the inlet port, and the piston return spring moves the piston rapidly back to the top of 20 the debooster. The rapid movement causes a suction in the line to the brake assembly, resulting in faster release of the brakes.
Power Boost Brake Systems m As a general rule, power boost brake systems are used on aircraft that land too fast to employ the line sure ter independent brake system, but are too light in weight to require power brake control valves.
clamp In this type of system, a line is tapped off the
- main hydraulic system pressure line, but main hy.
5. draulic system pressure does not enter the brakes.
er valve Main system pressure is used only to assist the pedals through the use of power boost master cylin-
7. Piston ders.
10 25. Ball clamp
27. Barrel FIGURE 9-31, Brake debooster cylinder.
large end of the housing through the outlet port to the brakes. Because the force from the small piston head is distributed over the greater area of the A typical power boost brake system (figure 9-32) consists of a reservoir, two power boost master cylinders, two shuttle valves, and the brake assembly in each main landing wheel. A compressed air bottle with a gage and release valve is installed for emergency operation of the brakes. Main hy.
draulic system pressure is routed from the pressure manifold to the power master cylinders. When the brake pedals are depressed, fluid for actuating the brakes is routed from the power boost master cylinders through the shuttle valves to the brakes.
When the brake pedals are released, the main system pressure port in the master cylinder is closed. Fluid that was moved into the brake assem- 365
To reservoir
BRAKE ASSEMBLIES 10-0 Brake assemblies commonly used on aircraft are the single-disk, dual-disk, multiple-disk, segmented rotor, or expander tube types. The single- and dual-disk types are more commonly used on small Brake aircraft; the multiple-disk type is normally used on pedal medium-sized aircraft; and the segmented rotor and expander tube types are commonly found on heav.
ier aircraft.
Main pressure anD Brake line Single-Disk Brakes = Emer. air pressure mom Return With the single-disk brake, braking is accomplished by applying friction to both sides of a
1. Brake reservoir 6. Shuttle valve rotating disk which is keyed to the landing gear
2. Power boost 7. Air vent wheel. There are several variations of the singlemaster cylinder 8. Main system disk brake; however, all operate on the same prin-
3. Emergency pressure manifold ciple and differ mainly in the number of cylinders brake control 9. Emergency air and the type of brake housing. Brake housings may
4. Air release valve bottle be either the one-piece or divided type. Figure 9-33
5. Wheel brake 10. Emergency air gage shows a single-disk brake installed on an aircraft, FIGURE 9-32. Power boost master cylinder brake system. with the wheel removed. The brake housing is attached to the landing gear axle flange by mounting bly is forced out the return port by a piston in the bolts.
brake assembly, through the return line to the brake reservoir. The brake reservoir is connected to the main hydraulic system reservoir to assure an adequate supply of fluid to operate the brakes.
Nose Wheel Brakes Many transport type aircraft such as the B-727 have brakes installed on the nose wheel. Movement of either right or left brake pedal will actuate the corresponding right or left main gear brake metering valve.
Movement of both brake pedals together will apply both main gear brakes and the nose gear brakes after approximately half the pedal stroke.
Actuation of one brake pedal for directional control braking will not actuate the nose gear brakes until nearly the end of the pedal stroke. Nose gear brake application is controlled through the brake differential linkage.
When the brake pedals are depressed the differential directs the force through linkage to the main gear metering valve first. After this valve is opened, continued movement of the brake pedals is directed to the nose gear metering valve, opening it and activating the brakes. Nose wheel braking is available above 15 mph from straight ahead to approximately 6° of steering. At this point the nose wheel steering brake cutoff switch activates the anti skirt valve and shuts off nose wheel braking. There is no nose wheel braking below 15 mph.
FIGURE 9-33. Typical single-disk brake installation.
Figure 9-34 shows an exploded view of a typical single-disk brake assembly. This brake assembly has a three-cylinder, one-piece housing. Each cylinder 366
3 19 18 17 16 VerO | 21 20 10 14 13 12
1. Brake disk 13. Brake return spring
2. Lining puck 14. Adjusting pin
3. Adjusting pin nut 15. Bleeder screw
4. Cylinder head 16. Washer
5. O-ring gasket 17. Bleeder valve
6. O-ring packing 18. Bleeder adapter
7. Adjusting pin grip 19. Gasket
8. Washer 20. Fluid inlet bushing
9. O-ring packing 21. Gasket
10. Piston 22. Screw
11. Internal retainer ring 23. Washer
12. Spring guide 24. Brake housing FIGURE 9-34. Exploded view of single-disk brake аве.
in the housing contains a piston, a return spring, and an automatic adjusting pin.
There are six brake linings, three on the inboard side of the rotating disk and three on the outboard side of the rotating disk. These brake linings are often referred to as "pucks." The outboard lining 367
pucks are attached to the three pistons and move in and out of the three cylinders when the brakes are operated. The inboard lining pucks are mounted in recesses in the brake housing and are therefore stationary.
Hydraulic pressure from the brake control unit enters the brake cylinders and forces the pistons and their pucks against the rotating disk. The rotating disk is keyed to the landing gear wheel so that it is free to move laterally within the brake cavity of the wheel. Thus, the rotating disk is forced into contact with the inboard pucks mounted in the housing. The lateral movement of the rotating disk ensures equal braking action on both sides of the very similar to the single-disk type, except that two rotating disks instead of one are used.
Multiple-Disk Brakes Multiple-disk brakes are heavy-duty brakes, designed for use with power brake control valves or power boost master cylinders. Figure 9-35 is an exploded view of the complete multiple-disk brake assembly. The brake consists of a bearing carrier, four rotating disks called rotors, three stationary disks called stators, a circular actuating cylinder, an automatic adjuster, and various minor components.
Regulated hydraulic pressure is applied through the automatic adjuster to a chamber in the bearing carrier. The bearing carrier is bolted to the shock strut axle flange and serves as a housing for the When brake pressure is released, the return springs force the pistons back to provide a preset clearance between the pucks and the disk. The self- Bleeder valve (typical) adjusting feature of the brake will maintain the Automatic adjuster - desired puck-to-disk clearance, regardless of lining O-ring wear. Bearing carrier and sleeve When the brakes are applied, hydraulic pressure moves each piston and its puck against the disk. At Retractor spring ing pin (through the spring guide) and moves the the same time, the piston pushes against the adjust- assembly (typical 10 places) pin inboard against the friction of the adjusting pin Felt wiper ring grip. When pressure is relieved, the force of the Felt wiper ring - return spring is sufficient to move the piston away from the brake disk but not enough to move the O-ringadjusting pin, which is held by the friction of the Backup ring pin grip. The piston moves away from the disk until Annular piston it stops against the head of the adjusting pin. Thus, Retractor plate regardless of the amount of wear, the same travel of the piston will be required to apply the brake. Nut (typical 10 places) Maintenance of the single-disk brake may include Flinger ring bleeding, performing operational checks, checking Bearing retaining nut lining wear, checking disk wear, and replacing worn linings and disks. Snap ring A bleeder valve is provided on the brake housing Pressure plate and for bleeding the single-disk brake. Always bleed insulator brakes in accordance with the applicable manufac- Rotor turer's instructions. Stator Operational checks are made during taxiing. Rotor Braking action for each main landing gear wheel should be equal, with equal application of pedal Stator pressure and without any evidence of soft or fotor spongy action. When pedal pressure is released, the brakes should release without any evidence of drag. Backup plate Backup plate retainer Dual-Disk Brakes Dual-disk brakes are used on aircraft when more Locking piece braking friction is desired. The dual-disk brake is FIcURE 9-35. Multiple-disk brake.
annular actuating piston. Hydraulic pressure forces the annular piston to move outward, compressing the rotating disks, which are keyed to the landing wheel and compressing the stationary disks, which are keyed to the bearing carrier. The resulting friction causes a braking action on the wheel and tire assembly.
When the hydraulic pressure is relieved, the retracting springs force the actuating piston to retract into the housing chamber in the bearing carrier.
The hydraulic fluid in the chamber is forced out by the returning of the annular actuating piston, and is bled through the automatic adjuster to the return line. The automatic adjuster traps a predetermined amount of fluid in the brake, an amount just sufficient to give correct clearances between the rotating disks and stationary disks.
Maintenance of the multiple-disk brake may include bleeding, checking disks for wear, replacing disks, and performing operational checks.
Bleeder valves are provided, making it possible to bleed the brake in any position. Bleeding should be accomplished according to the instructions for the specific aircraft.
The disks are checked for wear using a gage equipped with a movable slide, a stop pin, and an anvil.
Segmented Rotor Brakes Segmented rotor brakes are heavy-duty brakes, especially adapted for use with high-pressure hydraulic systems. These brakes may be used with either power brake control valves or power boost master cylinders. Braking is accomplished by means of several sets of stationary, high-friction type brake linings making contact with rotating (rotor) segments. A cutaway view of the brake is shown in figure 9-36.
The segmented rotor brake is very similar to the multiple-disk type described previously. The brake Adjuster pin nut Tire Wheel Piston return spring Adjuster pin sleeve Piston Adjuster clamp
• Rotor Carrier assembly assembly
- Piston cup Brake assembly pin Piston Pressure plate Brake Stator Drive sleeve bolts Adjuster pin lining assembly disk
- Hydraulic line
- Rotor assembly sleeve Rotor assembly Stator plate - Brake lining disk (typical) - FICURE 9-36. Segmented rotor brake assembly unita.
assembly consists of a carrier, two pistons and pis ton cup seals, a pressure plate, an auxiliary stator plate, roto segments, stator plates, a compensating shim, automatic adjusters, and a backing plate.
The carrier assembly is the basic unit of thr brake. It is the part which is attached to the landing gear shock strut flange upon which the other components are assembled. Two grooves, or cylinders, are machined in the carrier to receive the piston cups and pistons. Hydraulic fluid is admitted to these cylinders through a line which is attached to a threaded boss on the outside of the carrier.
The automatic adjusters are threaded into equally spaced holes (figure 9-36) located in the face of the carrier. The adjusters compensate for lining wear by maintaining a fixed running clearance with the brake in the "off" position. Each automatic adjuster is composed of an adjuster pin, adjuster clamp, return spring, sleeve, nut, and clamp hold- ‹own assembly.
The pressure plate is a flat, circular, nonrotating plate, notched on the inside diameter to fit over the atator drive sleeves.
Following the pressure plate is the auxiliary stator plate. This is also a nonrotating plate, notched on the inside diameter. Brake lining is riveted to one side of the auxiliary stator plate.
The next unit in the assembly is the first of several rotor segments. Each rotor plate is notched on the outside circumference. This enables it to be keyed to the landing gear wheel and rotate with it.
This particular model of the segmented rotor brake has four sets of these rotor segments.
Mounted between each of the rotor segments is a stator plate (figure 9-37). The stator plates are non-rotating plates and have brake linings riveted to both sides. The linings are in the form of multiple blocks, separated to aid in the dissipation of heat.
Following the last rotor segment is the compensating shim. The compensating shim is provided so that all the brake lining available for wear can be used. Without the shim, only about one-half of the lining could be used due to the limited travel of the pistons. After approximately one-half of the brake lining has been used, the shim is removed. The adjuster clamp is then re-positioned on the automatic adjuster pin, restoring piston travel so that the remaining lining can be used.
The backing plate (figure 9-38) is the final unit in the assembly and is a non-rotating plate with brake linings riveted to one side. The backing plate receives the ultimate hydraulic force resulting from brake application.
Backing plate and lining assy, Drive sleeve bolt nuts Washer Clamp, Friction block Shoulder FIcURE 9-38. Backing plate installed.
Sleeve hold-down Clearance nut plate Adjusting rod Hydraulic pressure released from the brake con- Compensating trol unit enters the brake cylinders and acts on the shim piston cups and pistons forcing them outward from Torque the carrier. The pistons apply force against the plate pressure plate, which in turn pushes against the Backing auxiliary stator. The auxiliary stator contacts the Piston plate first rotor segment, which in turn engages the first cup Auxiliary stator Rotor assembly stator plate. Lateral movement continues until all Annular ring piston Pressure plate - Lining - Stator plate the braking surfaces are in contact. The auxiliary stator plate, the stator plates, and the backing plate are prevented from rotating by the stator drive FIGURE 9-37. Cross section of a segmented rotor brake. sleeves. Thus, the non-rotating linings are all forced 370
in contact with the rotors, creating enough friction to stop the wheel to which the rotors are keyed.
The function of the automatic adjuster is dependent upon the correct friction between the adjuster pin and the adjuster clamp. Adjustment of brake running clearance is governed by the distance obtained between the adjuster washer and the end of the adjuster nut when the brake is assembled.
During brake application; the pressure plate moves toward the rotors. The washer moves with the pressure plate, causing the spring to compress.
As piston travel increases and as the pressure plate moves farther, the lining then comes in contact with the rotor segments. As the linings wear, the pressure plate continues to move and eventually comes into direct contact with the adjuster sleeve through the adjuster washer. Thus, no further force is applied to the spring. Additional travel of the pressure plate caused by lining wear will force the adjuster pin to slide through the adjuster clamp.
When hydraulic pressure in the brake is released, the return spring forces the pressure plate to return until it bottoms on the shoulder of the adjuster pin.
As this cycle is repeated during brake application and release, the adjuster pin will advance through the adjuster clamp due to lining wear, but the running clearance will remain constant.
Brake block Frame Torque bar Torque bar bolt Brake frame bolt Adjuster Brake spider FIcURE 9-39. Assembled expander tube brake.
Expander Tube Brakes The expander tube brake (figure 9-39) is a lowpressure brake with 360° of braking surface. It is light in weight, has few moving parts, and may be used on large aircraft as well as on small aircraft.
An exploded view of the expander tube brake is shown in figure 9-40. The main parts of the brake are the frame, expander tube, brake blocks, return springs, and clearance adjuster.
The brake frame is the basic unit around which the expander tube brake is built. The main part of the frame is a casting which is bolted to the torque flange of the landing gear shock strut. Detachable metal sides form a groove around the outer circumference into which moving parts of the brake are fitted.
The expander tube is made of neoprene reinforced with fabric, and has a metal nozzle through which fluid enters and leaves the tube.
The brake blocks are made of special brake lining material, the actual braking surface being strengthened by a backing plate of metal. The brake blocks are held in place around the frame and are prevented from circumferential movement by the torque bars.
The brake return springs are semi-elliptical or half-moon in shape. One is fitted between each separation in the brake blocks. The ends of the return springs push outward against the torque bars, while the bowed center section pushes inward, retracting the brake blocks when the brakes are released.
When hydraulic fluid under pressure enters the expander tube the tube expands. Since the frame prevents the tube from expanding inward and to the sides, all movement is outward. This forces the brake blocks against the brake drum, creating friction. The tube shields prevent the expander tube from extruding out between the blocks, and the torque bars prevent the blocks from rotating with the drum. Friction created by the brake is directly proportional to brake line pressure.
The clearance adjuster (figure 9-40) consists of a spring-loaded piston acting behind a neoprene diaphragm. It closes off the fluid passage in the inlet manifold when the spring tension is greater than the fluid pressure in the passage. Tension on the spring may be increased or decreased by turning the adjustment screw. Some of the older models of the expander tube brake are not equipped with clearance adjusters.
For brakes equipped with adjusters, clearance between the brake blocks and drum is usually set to a 371
Expander tube - Torque bar Brake Return spring block Retur spring shield Torque bar bolt Tube shield - Adjuster bolt Frame Nozzle Torque flange Frame Adjuster Brake frame bolt FIcURE 9-40. Exploded view of expander tube brakes.
minimum of 0.002 to 0.015 in., the exact setting depending on the particular aircraft concerned. All brakes on the same aircraft should be set to the same clearance.
To decrease clearance, turn the adjuster knob clockwise; to increase clearance, turn the adjuster knob counterclockwise. It should be kept in mind, however, that turning the adjuster knob alone does not change the clearance. The brakes must be applied and released after each setting of the adjuster knob to change the pressure in the brake and thereby change the brake clearance.
INSPECTION AND MAINTENANCE OF BRAKE
SYSTEMS Proper functioning of the brake system is of the utmost importance. Hence, conduct inspections at frequent intervals, and perform needed maintenance promptly and carefully.
When checking for leaks, make sure the system is under operating pressure. However, tighten any loose fittings with the pressure off. Check all flexible hoses carefully for swelling, cracking, or sott spots, and replace if evidence of deterioration is noted.
Maintain the proper fluid level at all times to prevent brake failure or the introduction of air into the system. Air in the system is indicated by a spongy action of the brake pedals. If air is present in the system, remove it by bleeding the system.
There are two general methods of bleeding brake systems-bleeding from the top downward (gravity method) and bleeding from the bottom upward (pressure method). The method used generally depends on the type and design of the brake system to be bled. In some instances it may depend on the bleeding equipment available. A general description of each method follows.
Gravity Method of Bleeding Brakes In the gravity method, the air is expelled from the brake system through one of the bleeder valves provided on the brake assembly (figure 9-41).
A bleeder hose is attached to the bleeder valve, and the free end of the hose is placed in a recepta- External reservoir Whoel brake Bleed hose Air bubbles Brake shor Fluid cu FICURE 9-41. Gravity method of bleeding brakes.