The Focke-Wulf 190 has independently operated main landing wheels, each of which is carried by an arm pivoted at right angles to, and mounted on, the front main spar. It is held in landing position by a hinged link securing the wheel by a toggle action when down.
Upper half of the hinged link is secured to a rotatable member by a fork and pin joint, and the axis of this member is approximately parallel to the wheel arm pivot.
Rotatable member is the front end of the electric retracting gear. Each wheel retracting unit is a complete assembly held to the front face of the main spar by ten bolts with castellated nuts. Removal of the connecting pin and the ten nuts and breaking of two electrical connection plugs is all that is necessary to remove this unit from the fuselage.
Outstanding features of the retractor are its compactness, light weight, and unusual method of getting a very high gear reduction in a small space. The unit, including the motor, measures 13½ in in length and 8-5/8 in over the mounting flange. There are only three reductions needed to give an over-all of 10,500 to 1 from armature shaft to the rotating head. With a 10,000-rpm 27V motor this means a full rotation of the operating head in one minute. Since only a quarter revolution is enough to raise the landing wheel, retraction time is about 15 to 20 sec.
There is a 3.3-to-1 reduction in the motor head so the shaft extending from the motor runs about 3.200 rpm. On this are mounted two cast iron shoes, driven by a cross pin and held in place by a continuous coiled spring band. The latter is stiff enough to hold the shoes in place and to resist centrifugal force so as to allow the motor to attain considerable speed before the shoes engage a surrounding drum of steel. This gives an automatic clutch effect.
The drum is attached to the intermediate reduction mechanism. Giving a final over-all reduction of 3,180 to 1, the last two mechanisms are compacted into a space of 4 in axially and of 7½-in dia. Both reductions are also coaxial with the unit, and the casing head rotates in the final reduction. Apparently no standard spur gear train of equal ratio could be crowded into this small space.
The last two reductions, while of the same type, are attained without gears of the tooth type, instead being enabled through shallow scallops ground in the edge of hardened steel disks and engaging hardened steel sleeves that float on pins fixed in the surrounding housing.
Principle employed is similar to that sometimes used with toothed gears where an internal gear meshes with a spur gear having one or more teeth less than the internal gear, the spur gear being oscillated by an eccentric at its center, but prevented from rotation by a pin. The axis of the eccentric shaft must be coaxial with the internal gear. Then from each rotation of the eccentric shaft the internal gear will be advanced one tooth by the spur gear. This design was used at one time for operating variable pitch propellers.
Only difference between the above gear reduction and that of the FW-190 design is that in the latter the gear teeth are eliminated from both members. The aforementioned semi-circular scallops ground in the edge of the disks replace the spur gears, and the internal gear teeth are replaced by the pins on which the hardened steel sleeves are mounted.
In each reduction two disks are used so as to give a continuous drive at all times. The eccentrics are also arranged diametrically opposite one another so as to keep the two disks balanced when running.
Another unusual feature of the design is the operation giving the two reductions. For the intermediate one, the outer member is secured so that the oscillating disks rotate slowly, making one complete rotation every 53 oscillations.
Eight pins engage these oscillating disks and thus rotate with them. These pins are mounted in a flange on the end of the eccentric shaft to give the final reduction. The latter reduction mechanism is mounted on the flanged forging which supports the entire reduction gear and is bolted to the front main spar. On this member are eight heavy sleeved pins that pass through the final reduction-oscillating disks and prevent them from rotating. Therefore they force the internal gear or retracting gear head which encloses them to rotate one turn for every 60 oscillations of the disks.
The head is the member which is pinned to the arm that lifts or lowers the landing gear. To take the lifting load, the outside member is carried on two roller bearings located on each side of the internal gear pins. These bearings have 64 rollers, ¼ x ¼ in.
Outer member is held in place endwise by a plate secured by eight cap screws engaging the pins used in attaining the final gear reduction. A cover plate attached to the front of the rotating outer member encloses the entire mechanism.
Design of the mechanism eliminates gear cutting by enabling processing of the parts with a grinder, lathe, and drillpress.
This article was originally published in the March, 1945, issue of Aviation magazine, vol 44, no 3, pp 156-157.
This article is included in the FW-190 PDF. It includes 2 photos and 2 drawings:
- Cross-sectional assembly drawing of Focke-Wulf 190 landing wheel retracting gear. Note that ball bearings are used on intermediate reduction eccentric and roller bearings for final reduction. Details of centrifugal clutch between electric motor and reduction gearing are also shown. [drawing 1]
- Exploded view of operating mechanism of Focke-Wulf 190 landing wheel reduction mechanism. Housings have been omitted for clarity. [drawing 2]
- Parts of Focke-Wulf 190 retracting gear. At left is electric motor with half of centrifugal clutch and its retaining spring removed. Next is housing containing intermediate reduction mechanism. Clutch drum is clearly shown on this unit. In foreground is mounting plate with its stationary pins, which carry hardened steel sleeves, two of which have been removed. At right is final reduction head with operating fork. Retainer plate hides oscillating members. This plate, mounted on stationary pins, carries a ball bearing to support outer end of eccentric shaft. (AAF photo) [photo 1]
- Reduction mechanism of retracting gear. At left are oscillating disks of intermediate reduction; center, final reduction oscillating disks, with flanged eccentric shaft carrying pins which engage intermediate reduction disks; right, pin and sleeve internal gear mechanism that is attached to wheel lifting arm. (AAF photo) [photo 2]