while the gear assembly comprised of members B, C and E are termed "planet" gears.

Q. Describe the construction of a typical planetary gearset.

A In the form shown at Fig. 158, the shaft K is attached to the flywheel of the engine by means of the flange machined integral with it. This serves as the main driving member for the gearset and the gear D and the high speed spider which carries the clutch plates J are keyed to this shaft so they must turn with it. The change speed gearing is contained in a casing which has bushings

Fig. 158.-Parts of All Spur Gear Two-Speed and Reverse Planetary

Gearset.

so that it can turn on the sleeves to which the sprocket A-1 and gear F are attached and these sleeves are in turn provided with an internal bearing so they may revolve independently of the driving shaft K if necessary. The planet gears, B, C, and E are joined together to form an assembly which turns on a suitable shaft parallel to the shaft and spaced on a circle whose radius is equal to the distance between the centers of the planet gear pin and the main

shaft. Gear A is keyed to the same sleeve to which the drive sprocket A-1 is attached, while gear F is keyed to a sleeve tɔ which the reverse drum is fastened. Suitable clutch plates serve to establish connection between the high speed clutch plate carrier which is keyed to the engine shaft and the reverse drum to which gear F is secured. Brake bands are provided, capable of arresting motion of either the low speed drum which also acts as the gear casing or the reverse drum to which the driven direct drive clutch plates are attached. These bands are not shown in the illustration.

To obtain a low speed the slow speed drum is kept from revolving by clutching it and the drive is from the gear D to the planet gears C which turn in the opposite direction and from the gear B of the planet gear assembly to the gear A which is keyed to and must turn with the drive sprocket A-1. As there is a difference in size between gears D and C, gear C turns slower than gear D, and as gear B has but half the number of teeth of the gear A the sprocket A-1 is turned at half the speed of gear B and a little less than half the speed of gear D and in the same direction. If the reverse drum is kept from rotating the gear F attached to it is also held stationary. The result is that the gear D drives the gear C just as when the slow speed drum was held, but as gear E cannot turn without running around the fixed gear F the motion of gear A which is attached to the sprocket A-1 is reverse to that of the central shaft.

If the two brakes or clutches are held out of contact with the slow speed and reverse drums and the high speed clutch plates are pushed together by the action of the clutch dogs and toggle the reverse drum is firmly locked to shaft K. As gear D cannot produce movement of the planetary gear assembly B, C and E without causing the reverse drum to rotate the entire assembly is locked together as a unit and the sprocket A-1 turns at the same speed as the shaft K and in the same direction.

Obviously, only one clutch can be engaged at a time. When the slow speed drum is held from rotation, the reverse drum must be free to turn and vice versa. When the high speed clutch is engaged both slow speed and reverse drums must be capable of turning. Another form of planetary gearset has received some appli

cation in which internal spur gears are used and a separate set of pinions employed for low or reverse ratios.

Q. How many speeds are provided in the usual form of planetary gear?

reverse.

A. Planetary gears, as a rule, have two forward speeds and a It is possible to make a planetary gear that will provide three forward speeds but these are usually so complicated and require so much gearing that they are not as practical as the less costly sliding gear forms.

Fig. 159.-Outlining Action of Planetary Gearing When Internal Gear is Employed.

Q. Describe action of slow speed gearing when an internal gear is used in the planetary system.

A. In order to show clearly the method of operation of the other form of planetary gearing a section through the slow speed gearset is shown at Fig. 159. In this the main driving gear A is keyed to the engine shaft extension E and must turn with it. The planetary pinions C are carried on studs attached to the plate D, which is

free to rotate around the shaft E. The internal gear B is in mesh with the planetary pinions C. This form of gear assembly can be used for either low or reverse speed depending upon whether the plate D or the internal gear B is kept from rotating. If the internal gear B is kept from turning the drive is taken from the plate D. If the plate D is kept from turning the driving sprocket is attached to internal gear B.

Considering first the action of this assembly to provide a slow speed ratio, let us assume that the internal gear B is provided with a brake band to hold it stationary and the plate D is in rigid connection with the drive sprocket or shaft which turns the rear wheels. If the engine shaft E is turning in the direction of the arrow, gear A must turn in the same direction. If the internal gear B is kept from rotating the planetary pinions C must not only turn around on their supporting stud in a direction opposite to that of the drive gear A but they must roll around the internal periphery of the gear B and carry the disc or plate D forward at a slower speed than driving shaft A but in the same direction as indicated by arrow F. As the drive sprocket is attached to plate B it turns at a slower speed than the engine shaft E.

Q. Describe action of reverse gearing.

A. In order to obtain a reverse motion it is necessary to provide a brake band for plate D and attach the driving sprocket to internal gear B. When the plate D is kept from rotating the planetary pinions C turn around on their studs and rotate the internal gear B in a direction opposite to engine rotation, as indicated by the arrow H, and at a slower speed than that of the drive gear A. Q. How is direct drive obtained?

A. Direct drive is obtained with any form of planetary gearset by some form of friction clutch which is adapted to lock all parts of the transmission into one rigid unit.

Q. How does the gearset act when on direct drive?

A. When all parts of the planetary gear assembly are locked together the gears are still in mesh but are not turning and the revolving mass acts as a flywheel member.