TM 1-410. Technical Manual, Airplane Structures 1941: Section 10 - Wheels

SECTION X: WHEELS

35. General.-a. Most wheels used on main and auxiliary landing gear of military airplanes are cast from aluminum or magnesium alloys. This construction combines strength with light weight but permits very little repair to the wheel itself. These alloys corrode readily, consequently all surfaces except bearing and friction surfaces must be well covered with protective coatings. The rim contour, tire well, bearing cups, and brake drum liner surfaces are machined for accuracy and uniformity. All steel parts except brake surfaces and those parts fabricated from stainless steel are cadmium plated. Aluminum and aluminum alloy parts are either anodically treated, or protected with one spray coat of oil base primer and two coats of aluminum enamel or aluminum lacquer. The interior of the wheel between the inner and outer bearings is treated as described above, or covered with a light coating of bearing lubricant.

FIGURE 39.-Wheel and axle assembly.

b. Wheels are not stocked at depots with tires and tubes assembled, but each must be ordered separately as required. Brakes are also a separate item and are not issued as a part of the wheel. A typical wheel assembly is shown in figure 39. Airplane wheel bearings are of the tapered roller type consisting of bearing cone, rollers with retaining cage and bearing cup, as shown in figure 40 (2). Each wheel has the bearing cup pressed into place and is furnished with a hub cap to prevent foreign matter from entering the outside bearing. Suitable retainers are provided inboard of the inner bearing to prevent grease from reaching the brake lining. Seals are provided on amphibian airplanes to exclude water. Wheel side contour is obtained by a disk formed as a structural part of the wheel or by an auxiliary fairing disk. In either case the contour of the wheel is maintained smooth and free f rom imperfections as far as possible.

FIGURE 40-Wheel bearing assembly.

c. Wheels used by the Air Corps on main and auxiliary landing gear include a number of varieties and types, which may be classified according to features of their construction and types of casings used.

(1) Main landing gear.-(a) Fixed flange, drop center rim for streamline casings and for smooth contour casings.

(b) Removable flange, drop center rim for low pressure casings.

(c) Removable flange, flat base rim for low pressure casings.

(d) Air wheels, for air wheel casings.

(2) Auxiliary landing gear.-(a) Removable flange, drop center rim for streamline casings and for smooth contour casings.

(b) Split wheel, flat base for low pressure casings.

(c) Air wheels, for air wheel casings.

36. Main landing gear.-a. Wheels for the main landing gear having a fixed flange at the present time are all of drop center rim construction. The principal difference between those used for streamline casings and those used for smooth contour casings is that the latter are wider between the rim flanges. Figure 41 shows a typical fixed flange wheel with drop center rim and shows the outboard radial ribs which give added support to the rim at the outboard bead seat, as well as a portion of the internal ribs which help to support the rim at the well bottom. The outboard view of the wheel shows the tapered roller bearing cups and the threaded holes for the outer fairing attaching screws while the inboard view shows the brake drum side and the method of attaching the brake drum liner to the wheel. It will be noted that the brake drum liner is held in place by steel bolts projecting through the castings with elastic stop nuts on the inner side which can be tightened readily through the spokes of the wheel. On some of the larger sizes the wheels have a hub cast between the two bearing hubs which eliminates use of the two felt seals on the inside of the tapered roller bearing.

b. The main landing gear wheels of the removable flange type are now used with low pressure casings and may have either a drop center rim or a flat base rim. Figure 42 shows examples of both, (1) shows the flat base rim which may be withdrawn readily from the tire by disconnecting the bolts which hold the halves of the two-piece removable flange in place, and lifting the flange from its seat in the rim. A brake drum of conventional type is installed on each side of the wheel providing a dual brake assembly. The wheel shown in figure 42 (2) is the drop center rim construction and uses a one-piece removable flange which is held in place by means of a retainer snap ring. Provision is made for a multiple disk brake as shown.

c. Air wheels are designed for the low pressure air wheel casings for use on main or auxiliary landing gear. They are essentially hubs with usual wheel structure eliminated. The removable flange of the air wheel generally screws onto the main hub which is of the flat base type. The cross sectional area of the assembled air wheel is much greater than that of the usual wheel and tire combination, but probably due to its shape, tests indicate that its air resistance is approximately the same. An installation on the main landing gear showing use of the multiple disk brake is shown in figure 43.

FIGURE 41-Fixed, flange wheel with drop center rim.

FIGURE 42.-Removable flange wheel.

37. Auxiliary landing gear.-a. Tail or nose wheels may be classified generally as removable flange or split wheel type both of which are shown in figure 44. The principal feature of the split wheel type shown in figure 44 (1) is that the wheel is made in two halves held together by bolts and nuts, which permits easy removal or installation of tire and tube. In mounting the tire on the wheel the nuts are removed from tie bolts, one-half of wheel removed, tire installed over remaining half of wheel, removed half replaced, and two sections of wheel bolted together. Care should be taken to see that the inner

FIGURE 43.-Air wheel installation

tube is not pinched between the halves and the valve stem is not binding in its mounting hole. If the, tube is slightly inflated before mounting, no difficulty should be encountered. When mounted on a tail wheel fork, proper spacers should be placed on the axle which will allow the wheel to fit without excessive end play. The axle nuts then should be tightened until the wheel turns freely without lost play in the bearings. The nuts are then safetied with cotter pins.

b. On the removable-flange wheels with drop center rim shown in figure 44 (2), the flange forms a joint with the wheel-rim well, which is smooth and continuous to prevent chafing or pinching of the inner tube. This flange is held in position by a snap ring making removal of a tire an easy operation. To install this type of wheel, the fairing screws which hold the fairing in place are first removed; the hub cap which is held in place by the fairing is then taken off, and the wheel is then placed on the axle. There is no difference between the smooth contour and streamline types except that the former is wider to accommodate the smooth contour tire.

FIGURE 44.-Auxiliary landing-gear wheels.

c. All types of tail or nose wheels have grease retainers on the outside of each bearing, as shown in figure 45, to retain grease in bearings and to exclude dust and water. Closures are provided where needed which rotate with the wheel and act as bearing retainers when the wheel is removed from the axle. The outer fairing makes a tight joint with the rim, providing a smooth contour, and is of sufficient stiffness to press down and retain the rubber valve stem.

38. Maintenance.-a. At the time wheels are removed for inspection. the entire surface of the wheel, both inside and out, is cleaned thoroughly. Grease retainers and bearings are removed. These bearing cavities and any other surfaces where grease may accumulate are cleaned with unleaded gasoline. An air blast should be used to dry the grease seals and the bearing assembly and remove foreign matter, but care should be taken that the bearing is not permitted to spin in the air blast as very high speeds can be obtained which may damage the bearing, or the bearing may disintegrate from centrifugal force with resultant injury to personnel. The air-

FIGURE 45.-Cross section auxiliary landing-gear wheel.

line used for this purpose should have a separator to remove water. The wheel should be thoroughly dry before the tire and tube are remounted.

b. During the cleaning operation all surf aces of wheels are inspected carefully for defects, including nicked hubs, flanges, webs, or rims, and scored or loose brake-drum liners. The wheel is replaced with a new one if a loose liner cannot be corrected by tightening or replacement of screws, wheel is cracked or corroded, or rims are bent or distorted. It is important that surface of rim between bead seats be free from defects that would be injurious to casing or tube. Chipped off, cracked, or otherwise damaged protective coatings should be corrected whenever found as corrosion will take place rapidly at such places. The same material as the original coating should be used for this purpose.

c. When bearings are free from all dirt and grit and thoroughly dry if defects are found such as roughness of rollers, pitted or corroded rollers, cones and cups out of round, or distorted or broken cage, the bearing is replaced with a serviceable assembly.

FIGURE 46-Lubrication of wheel bearing.

d. Bearing cups may be replaced locally if facilities are available. Cups are removed by an arbor press or if done carefully with a hammer and metal bar (preferably brass). Use light hammer blows and move driving bar about one-third the circumference of the bearing cup between blows. To install new cups an arbor press must be used. The smaller size wheels have a felt grease seal behind the bearing cup and these seals will be damaged beyond repair when the bearing cup is driven out. Therefore in these cases when reassembling, new grease seals are always installed behind the bearing cups. Other than brake drum liner screws and bearing cups, no replacement of wheel parts is made except at repair depots.

e. It is very important that proper lubricant be used in wheel bearings. The latest technical order pertaining to the particular type of equipment should be consulted in this respect. The proper method of applying lubricant to the roller bearing assembly, shown in figure 46. is to place a small amount of the proper lubricant in palm of left hand. Grasp cone of bearing assembly with thumb and first two fingers of right hand, keeping cage side of bearing assembly next to palm of left hand. Move bearing assembly across palm toward the thumb, forcing lubricant into opening between the rollers, turning or rotating assembly with each stroke until openings are filled with lubricant. Remove surplus lubricant from cone and outside of cage and then install assembly in wheel.

f. Installation of wheels on the axle is simple. However, adjustment of roller bearings must be made caref ully as this adjustment may affect operation of wheel and brake. Using an axle nut wrench, tighten axle nut until a bearing drag is noticed when spinning the wheel by hand. Back off the nut to the next castellation and lock it in position with proper cotter pin. Brake drag must not be confused with bearing tightness when making bearing adjustment and it is well to make sure that brake adjustment is backed off so that there is plenty of clearance between brake band and brake drum.