TM 1-410. Technical Manual, Airplane Structures 1941: Section 11 - Brakes

SECTION XI: BRAKES

39. General.-The three general types of brakes on airplane wheels are internal expanding shoe, multiple disk, and expander tube. The internal expanding type employs either mechanical or hydraulic control system and the multiple disk and expander tube types either hydraulic or pneumatic control systems.

40. Internal expanding shoe.-a. Internal expanding shoe brakes may be classified as one-way (single-servo) (fig. 47), or twoway (reversible) (fig. 48). The term "servo" as applied to brakes refers to utilization of forward rotary motion of the brake drum to expand further or apply the brake shoes. Single-servo action operates in one direction of rotation only, while reversible operates in either. Both these brakes are furnished in single-shoe and two-shoe type construction and all types may be operated mechanically or hydraulically. The brakes are attached to the brake flange by bolts, the torque arm of the brake usually having twelve equally spaced holes with bolts installed only in alternate holes. The extra holes are provided to permit a greater number of trial positions when installing the brake the first time.

FIGURE 47--Single-servo brake assembly.

b. The internal expanding shoe brakes of the one-way or single-servo type are marked either "right hand" or "left hand" by the manufacturer and care must be taken to see that the brakes are installed on the correct side. Figure 49 shows a dual brake installation and indicates location of right-hand and left-hand brakes. (It should be noted that RH and LH as applied to brakes refers to direction of rotation of brake drum over the shoes as could be seen through the wheel if it were transparent. RH and LH as applied to the wheels indicate their position on the airplane with respect to right or left side of pilot.) In all cases brakes marked "right hand" are installed on the left side (as determined from pilot's cockpit) of installed wheels, and brakes marked "left hand" are installed on the right side of installed wheels. Proper location may be assured also by installing assembly so that direction of rotation of brake drum follows from anchor pin toward toe of primary shoe.

FIGURE 48.-Reversible brake assembly.

c. The single-servo brakes (Hayes) use a one-piece cast aluminum alloy brake band of internal expanding type and are operated either mechanically or hydraulically as shown in figure 50. The mechanical brakes are made in sizes from 9-inch to 13-inch diameter for use in 21-inch to 31-inch wheels and the hydraulic brakes in sizes from 10-inch to 20-inch diameter for use in all sizes of wheels from 24-inch to 45-inch.

Figure 50 shows the one-piece brake band construction and three bell cranks which control radial clearance between band and drum, and which also serve to hold the band in alinement with the drum. In both cases the brake torque is transmitted to the axle torque flange. The brake mechanism is carried on the torque plate (A) as is the actuating means which in the mechanical brake is the operating shaft (B), and the operation lever (C), and in the hydraulic brake is the cylinder assembly (D). On the mechanical brake, the brake band is held in the released position by the spring (E), which is connected directly between the brake band locating plate (J) and the anchor pin (F) of the brake band. On the hydraulic brake, the spring is connected from the torque plate to the crank (G) which holds the operating end of the brake band in its released position. When force is transmitted to the end of the band, either through the operating shaft or the hydraulic

FIGURE 49.-Dual brake installation. (Brakes are stamped "right hand" and "left hand."
When mounting on wheels having dual brakes place the brakes as shown above.)

piston, the band is applied to the drum near the heel end first, and as the brake is applied further, the entire surface comes into contact. The only adjustments necessary are to maintain proper radial clearance between brake drum and brake lining. This is done through the three adjusting bell cranks (H) which work against the three hardened steel plates (J). The latter are fastened to the brake band at the three points, as shown in the figure.

d. The single-servo brakes (Bendix) are the two-shoe internal expanding type. Some of the smaller brakes up to and including the 13-inch are operated mechanically (by means of a cam and lever) and the brakes larger than 13-inch are usually operated by hydraulic pressure, both of which are shown in figure 51. It should be noted in both cases that the torque arm (O) of the brake is the basic unit and that no built-up or attached members are added for its reinforcement. On the mechanical brake the torque arm is designed to permit installation of a forged steel operating shaft, one end being forged to provide an internal lever (A) which is attached to the primary brake shoe through steel links (B). The brake, is applied by rotating

(1) Mechanically operated.
FIGURE 50-Single-servo brake, one-shoe construction.

the control lever (K) which causes the internal lever (A) to force the shoe outward. On the hydraulic brake, the torque arm is designed to incorporate a cylinder (A), fitted with a thin sleeve (S). When hydraulic pressure is admitted to the cylinder behind piston (P), the force of application is transmitted through connecting link (B) to primary shoe (C) forcing it against the drum. In both the mechanical and hydraulic brakes, the primary and secondary shoes are

(2) Hydraulically operated.
FIGURE 50.-Single-servo brake, one-shoe construction-Continued.

connected by means of a star adjusting screw (D) , which has a right- hand thread on one end and a left-hand thread on the other, thus permitting spreading or contracting the shoes. An eccentric cam (F) is provided for adjusting the clearance of the secondary shoe (E),

FIGURE 51-Single-servo brake, two-shoe construction-

which is held against the eccentric by means of a spring (H). As the brake shoes are anchored to the torque arm (O) at only one place (G), it is evident that any variation in the secondary shoe clearance will change the primary shoe clearance. Therefore, the secondary shoe must always be adjusted for clearance by eccentric (F) before the primary shoe is adjusted. After the secondary shoe has been set to the correct clearance, the primary shoe may be adjusted by means of the star wheel (D). As the brake drum rotates in the direction indicated by the arrow, the friction between the primary shoe and drum with brakes applied causes the shoe to follow the drum. In so doing, the primary shoe operating through adjusting screw (D) forces secondary shoe (E) in contact with the brake drum. At (M) and (N) steady rests are provided for primary and secondary shoes respectively, their purpose being to keep the shoes in correct alinement with the brake drum.

e. The two-way or reversible brakes of the internal expanding type are similar to the single-servo in most features except that instead of one end of the brake shoe assembly being anchored by a stationary anchor pin, it is connected to a second piston in the actuating cylinder. Thus when hydraulic pressure is admitted to the cylinder, the application of force is transmitted in opposing directions through connecting links to the two ends of the brake shoe assembly. These brakes may be of either single- or two-shoe type and are interchangeable as either right-hand or left-hand brakes by reversing the return springs. For information on installation and adjustments of these brakes, reference is made to the Air Corps Technical Order pertaining to the particular equipment on which the brake is installed.

41. Multiple disk.-The multiple disk brake may be pneumatically or hydraulically operated. It is used in only a few models of airplanes where structural and operating limitations require a relatively large braking area within a small diameter. As shown in figures 42 and 43, the assembly is composed of a series of alternately stationary steel disks between others which engage in recesses of the wheel hub and revolve with it.

42. Expander tube.-a. The expander tube brake, shown in cross-section in figure 52 (1) consists of three main parts, brake frame, expander tube, and brake blocks. It is furnished in two styles, single which has one row of brake blocks around the circumference, and duplex which has two rows. An inner fairing or shield fits between the torque flange on the axle and the brake frame to protect the frame against water. The brake expander tube is a flat tube 1/4 inch thick of rubber compound and fabric which is stretched over the brake frame between the side flanges. It is equipped with a nozzle which is connected with the brake fluid connection through suitable packing means. The brake blocks are made of special material and the number used corresponds to the number of inches of brake diameter, for example, 20-inch diameter brakes have 20 blocks. These blocks have notches on the sides to engage with bosses on the brake frame and have slots

FIGURE 52.-Expander tube brake.

across their ends into which flat springs are inserted. The ends of these springs fit into slots in the side flanges of the brake frame which hold the brake blocks against the expander tube and keep them from dragging when brake is released. The correct method for removing and replacing these springs is shown in figure 52 (2).

b. This type brake is operated by brake fluid and may be used with one of the conventional hydraulic brake systems. Applying brake pedal forces the brake fluid into the expander tube which is restrained from movement inwardly and to the sides. The fluid in the expander tube forces the individual brake blocks out radially against the brake drum. They are held from rotating by the lugs on the brake frame fitting into notches in the sides of the brake blocks. As soon as the pressure is released the springs in the ends of the brake blocks tend to force the fluid out of the expander tube. This action is assisted by the tube itself which is molded slightly smaller in diameter than the brake frame and which tends to retract itself without the aid of the springs in the brake block. As each block is independent in its action, there is no build-up of servo action and therefore no tendency for the brake to grab.

c. These brakes have a radial clearance between the brake lining and the brake drum which varies with the size of brake. There is no way of changing this amount except by grinding off the face of the blocks. As the lining wears, fluid displacement of the brake increases and when pedal movement becomes too great, it is necessary to reline the brake by installation of new blocks.

43. Actuating systems.-a. A typical mechanically operated system is shown in figure 53. These systems which employ control cable pulleys and in some instances bell cranks vary widely in detail with different airplanes.

b. A typical hydraulic brake system is shown in figure 54. There are two separate systems for any airplane, one for each brake, although on some installations the reservoir is common to both. The principal units are

(1) Reservoir.-The purpose. of this unit is to compensate for any slight leaks in connecting lines, to prevent admittance of air into the system, and to keep piston of master cylinder completely submerged at all times.

(2) Master cylinder.-This unit when operated builds up fluid pressure between master cylinder and brake cylinder, causing brake cylinder piston to move and in so doing to operate the brakes.

(3) Wheel cylinder.-This is the unit located in the brake assembly which through the piston actuates the brake shoe.

FIGURE 53.-Mechanical brake system.

(4) Fluid lines.-These lines transmit pressure from master cylinder to brake cylinder.

c. The major parts of the air-operated system are air storage tank and compressor, metering valves at toe pedals, pressure gage, parking brake lever and wheel and brake unit. The air compressor is automatic and when in operation should maintain a predetermined pressure, depending upon the particular airplane.

FIGURE 54-Hydraulic brake system and bleeding equipment.

About 15 minutes is required to develop the tank pressure which should always be checked before starting the engines or landing the airplane. The metering valves are operated by either the pilot's or copilot's brake pedals admitting air to the brake on the forward stroke and releasing it on the backward stroke. Rapid deceleration of airplanes of large size requires absorption of a tremendous amount of energy by the brakes with the consequent development of extremely high temperatures in the metal disks. The effect of the comparatively small contact area (tire to ground) is noticeable in the ease with which the wheels slide. For these reasons it is recommended that all braking be done with caution, and overuse of brakes avoided. Locking the brakes with parking brake immediately after extended taxying when the disks are hot should be avoided since the combination of heat and pressure may cause sticking.

44. Maintenance.-a. During inspection of brakes, the assembly is renewed if any of the following are found:

(1) Corroded, broken, or cracked parts.

(2) Distorted brake shoes.

(3) Loose rivets.

(4) Brake linings badly scored, excessively worn, or impregnated with oil.

b. Some maintenance instructions pertaining to brakes in general are given below:

(1) Distorted brake shoes will not allow full contact of the lining with the brake drum. Greasy brake lining may cause "grabby" or ineffective brakes, depending on amount of grease. Brake lining does not absorb grease rapidly and in cases where the condition is found early enough the grease can be washed off the lining with unleaded gasoline. Remove all worn particles of lining from recesses in the lining over the rivet heads. Wipe the lining clean with a dry cloth or with a fine wire brush.

(2) The backing plate is a dust cover made of thin gage aluminum stock and may be reformed readily in case interference is noted between the backing plate and wheel. Care should be exercised in doing this, however, because if the plate is pulled out sufficiently for air to get between the plate and wheel when the airplane is in flight, the thin cover plate will buckle.

(3) All return springs should be inspected carefully to see that they have good initial tension and that the hooks at each end of the spring have not been straightened out. In such cases the spring must be either reformed to insure security, or be replaced.

(4) Wipe all dirt and foreign particles from the brake mechanism. See that the felt retaining washers are in good condition (not saturated with grease). This is necessary to prevent lubricant from working out of the bearing and into brake drum and lining. Clean brake drum cavity of the wheel and bearing retainer with a cloth moistened with cleaning fluid obtained by mixing equal parts of naphtha and carbontetrachloride. Wipe dry with a clean cloth.

c. Before the brakes are adjusted it is essential that the hook-tip be checked over thoroughly. Before attempting to adjust the brake, jack up the wheel and apply the brake several times to be sure. the brake releases properly and promptly. The hook-up must be gone over carefully, frayed cables replaced, all interference with the cable or control removed, and all pulleys and bell crank bearings thoroughly lubricated. If these bearings are of the sealed type, under no circumstances will a grease solvent be used in cleaning them. The hook-up is tested to ascertain that brakes release rapidly and to the full "off" position.

FIGURE 55.-Adjustment of single-shoe single-servo brake (Hayes).

d. Brake adjustment procedure for the single-shoe single-servo internal expanding type brake (Hayes) is as follows (see fig. 55) :

(1) First open the three inspection holes (P), then with a screwdriver turn the adjusting screw (A) clockwise until the clearance specified in the airplane handbook is obtained. If the clearance is not given in the handbook adjust to approximately 0.010 inch between the brake shoe and drum. This can be measured by a feeler gage of this thickness through the inspection holes (P). The adjusting screws (B) and (C) are then tightened in turn until there is 0.010 inch clearance between the brake lining and brake drum at each of these two points. Return to the first adjusting screw (A) nearest the toe of the brake shoe and reduce the clearance to 0.010 inch. After this, it may be necessary to go over the adjustment again in the same order and slightly change each one to give required clearance. While adjusting brakes extreme care must be taken to see that the brakes are not applied while the wheels are removed as this will cause the onepiece shoe to be distorted and rendered unfit for further service. If mechanically operated, the cable is adjusted until the released position of the brake pedal is in the proper position in the cockpit before making adjustments at the brake shoe.

(2) As the above is the only adjustment, it will be noted that as the lining wears the operating arm (D) will change its angular position and it may be necessary to remove the arm and relocate it on the operating shaft so as to keep the cable pull at approximately 90° to the arm at midpoint between the "Off" and "On" position of the brake.

(3) After the brakes have been adjusted, the hook-up should be gone over carefully and the rods, foot pedals, and bell cranks set so that they will be working in the most advantageous position. The brake pedals are adjusted so that with pedals fully extended and the rudder full "On" the pedals will be unhindered in operation. Brake pedals are adjusted so as to allow some small movement of the pedals toward the "On" or braking position before the brakes actually take hold in order to avoid accidental application of the brakes by the pilot.

e. Brake adjustment procedure for the two-shoe single-servo internal expanding brake (Bendix) is as follows (see fig. 56) :

(1) Loosen eccentric lock nut and turn eccentric in direction of wheel rotation until wheel is locked. Back off eccentric until wheel just rotates freely and with a close-fitting wrench hold eccentric in this position and tighten lock nut. Uncover star wheel adjusting screw hole by rotating cover plate. With a screwdriver turn star wheel of adjusting screw away from axle until a brake drag is noticed when turning the wheel by band, and then back off star wheel until there is no brake drag. On brakes using star adjusting screw there is a positive stop provided for actuating end of primary shoe which definitely sets the "Off " position of this shoe. Therefore no adjustment is necessary to the brake control lever other than outlined below.

(2) On the mechanically operated brakes the angle between the actuating rod or cable and the brake control lever should not be over 80° when the brake is fully applied. This angle should be checked and corrected if necessary, but need not be changed when once corrected, as the star wheel adjustment entirely compensates for lining wear. Due to cable stretch it may be necessary to adjust the operating cable sufficiently to bring the brake pedals into a position convenient for the pilot to operate. In all cases the basic handbook for the particular airplane must be consulted.

FIGURE 56.-Adjustment of two-shoe single-servo brake (Bendix).

f. Adjustment of the Hayes reversible single-shoe brake is similar to the single-servo except the former has four adjusting bell cranks instead of three. The only adjustment of the reversible Bendix two-shoe brake is the star wheel adjustment described above.

g. Instructions for the multiple-disk brake may be found in the technical publications covering the particular airplane on which they are, installed and the expander tube brakes require no adjustment.

h. The following instructions apply specifically to hydraulic systems used for operating brakes and should not be considered as complete instructions for hydraulic systems in general:

(1) Whenever the hydraulic line connecting master cylinder to brake cylinder is disconnected, air will be admitted to the system. The same condition may develop if the fluid reservoir becomes empty. Air in the system is indicated by spongy action of brake pedals when applying brakes. When this occurs the system must be bled to remove the air.

(2) It will be noted in figure 54 that there are two fittings in the brake actuating cylinder, that is, the inlet fitting and the bleeder. The bleeder is a needle valve with a cap or dust cover. To bleed properly a brake hydraulic system, the special equipment illustrated in figure 54 is required. The free end of the filler can hose is attached to the filler plug opening of the master cylinder reservoir. The can is then partially filled with the proper hydraulic fluid. The hose (B) is attached to the bleeder valve and the free end submerged in hydraulic fluid in the glass jar (C). The brake pedal is then operated back and forth slowly. During this operation the bleeder valve is kept open one to one and one-half turns. The fluid coming out of the hose submerged in the glass jar will contain air bubbles. Operation of the pedal is continued until air bubbles no longer appear. This will require pumping at least, one pint of fluid through the system. The bleeder valve is then closed tightly and the hose removed. During the bleeding operation it is essential that the fluid level in the filler can be watched so that the supply of fluid is not exhausted, otherwise air will again be introduced into the system.

(3) Maintenance of the hydraulic brake system consists chiefly of keeping fluid in the reservoir. The lines unless properly supported to prevent vibration may leak at the fittings. These leaks must be corrected at once. Gaskets which have been scored or damaged must be replaced. If leaks are noticed at the brake cylinder, the piston and piston cup should be removed and inspected. Replacement of the rubber cup should correct such leaks unless the cylinder sleeve is scored. If the rubber cup is not soft. and pliable, or if it has been cut by the piston, it should be replaced. Care should be used in replacing these parts that no fluid is allowed to contact the brake lining. The master cylinder may be disassembled for replacement of piston cups. After long periods of service the rubber hose connections may show signs of deterioration such as swelling or cracking. In such cases these connections should be replaced. When checking lines and connections for leaks brake pedals must be depressed, thus placing the system under pressure so that leaks which otherwise would not be apparent may be found. When flexible lines are found bulged, kinked, or swollen due to oil saturation, they are replaced. All hydraulic brake lines are marked with paint bands of blue-yellow-blue.

i. In checking operation of brakes, they should-

(1) Hold wheels from turning against full ground throttle.

(2) Hold evenly and not grab.

(3) Be equally effective for a similar movement of brake pedals.

(4) Not drag when off. Listen for rubbing sounds indicating high spots on the lining, distorted brake drum, or distorted backing plates.