The most popular transmissions are the friction, planetary and sliding gear. The friction and planetary, with few exceptions, are only used on the light vehicles, while the sliding gear type is extensively used on all sizes of vehicles.
There are two types of friction transmission in use at the present writing, the single disc and the double disc This single-disc type consists of a driving disc which is attached to the flywheel or an extension of the crank shaft, and always novates with it and a driven disc which can be slid along a cross shaft and brought into frictional engagement with the driving disc. By moving the driven disc out from the center of the driving disc the speed can be varied from nothing to maximum, and by slid-ing it to the opposite side the direction of motion is reversed. Before the wheel is slid in the direction of its axis it must be disengaged from the driving dise. This is accomplished by either moving the cross shaft and its bearings or by moving the driven dise. From this cross shaft the final drive may be through a single chain to the rear axle. or it may be through a single chair to the jack shaft and then through double side chains to the mad wheels. Fig. 76 illustrates a friction transmission of the single-disc type. The discs are shown in the high position while the lower speeds are obtained by moving the driven disc in towards the center, and reverse is obtained by mov-ing the driven disc toward the opposite side of center.
Fig. 76. Single-Disc Friction-Type Transmission.
The planetary transmission is somewhat cheaper to manufacture than the sliding gear type and also requires less skill in operation. There are two types of planetary gears, those comprising internal gears and those comprising only spur gears in their makeup. The latter is the most popular type and will be considered.
Fig. 77 depicts this type of transmission, and its principle of operation may be described as follows:
The driving shaft A carries the driving pinion B, which meshes with the planetary pinion C. The latter forms part of sets of three pinions which are formed integral. D is the low-speed planetary pinion meshing with the low-speed gear E which is secured to the driven shaft F. By applying the brake band G to the combined pinion carrier anil drum II, the planetary pinions are held stationary in space and act like a back gear. Pinion B rotating in a right-hand direction (see end diagram), turns pinions C and D on their pin M in a left-hand direction, and pinion D turns gear E and the driver shaft F in a right-hand direction; that is. in the same direction as the driving pinion B.
For reverse, band I is applied to the drum J, which has the reversing pinion K keyed to it. being thus held stationary when pinion B is rotated by the engine planetary pinion L. is forced to roll on K in a left-hand direction, carrying the pinion pin M and pinion driver H with it.
Direct drive is obtained by engaging the high-speed clutch N, which locks the reversing gear K to the driving shaft A, and since two equal gear B and K are now secured to the shaft A, the planetary pinions are locked against axial motion and the whole transmission revolves as a unit.
Fig. 77. Planetary Transmission with Spur Gears.
The sliding-gear type of transmission consists of two parallel shafts mounted on suitable bearings in a housing called the transmission case. The first of these shafts is known as the primary or main driving shaft. This shaft is divided into two parts, the forward or driving part and the rear or driven part, the latter being provided with a bearing at its forward end, inside the former. The second of these shafts is known as the secondary or countershaft. The driven part of the main shaft is either squared or provided with integral keys and carries the sliding gears, whose common hubs have squared holes or key-ways to coincide with the shaft to make a sliding fit upon it. The driving part of the main shaft is provided with a gear, which meshes with a gear on the countershaft and forms a drive for the latter. The countershaft has a number of gears fixed upon it, depending upon the number of speeds. The gears on both shafts are so spaced that by shifting the primary set corresponding gears on the two shafts can be brought into mesh successively without interference from the other gears. Shifting of the sliding set is accomplished by means of a hand lever located conveniently to the operator and a suitable connecting linkage. The shifter rod carries a fork, which is attached to the gears in such a manner as to permit them to rotate with their shaft.
There are two common arrangements of shafts. In some cases the countershaft is located below the main shaft, while in others the two shafts are located in a horizontal plane.
When the shafts are placed vertically the case is generally cast in one piece, with a large hole cover plate for inspection purposes. When the shafts are placed in a horizontal plane, the case may either be cast in one piece or in halves joined through the centers of the bearings.
There are three general methods of mounting the sliding gear transmission: combining them with the motor to form a unit power plant, individual mounting on a sub-frame or main frame cross members, and combining them in a unit with the jack shaft.
All of these mountings may be made with a more or less degree of flexibility. Three-point support is most generally resorted to, with the intention of relieving the case of the stresses set up by frame weaving.