MANUAL TRANSMISSION PRINCIPLE

On all the wheel type vehicles such as automobiles and forklift trucks, engine output is transmitted to the wheels to drive the vehicle to run. When a vehicle comes to an uphill, a manipulation of the transmission in the vehicle is necessary to shift down the running speed of the vehicle, while giving an increased torque to each wheel. Similarly, on a bulldozer that is faced with a heavy load to move it, the transmission will be shifted down to the lower speed gear so the sprocket wheels are given with an increased torque enough to force the bulldozer against the heavy load.

The manipulation of a transmission is also required for reversing the direction of rotations of wheels or sprockets when a vehicle or a bulldozer is to be run backward. Basically, a transmission is composed of a combination of gears. Thus, it will be reasonable to start the description first from the mechanism of a couple of gears through which a drive power is transmitted.

When two gears, different from each other in their sizes, are engaged and rotated together, the large gear will rotate slower than the small one. The ratio in rotating speed between the two is inverse proportion to that in number of teeth between the two. While the torque obtained from the large gear rotating at some speed will be larger than that from the small gear engaged. The relation is in direct proportion to the ratio in the number of teeth between two gears.

When the drive power is transmitted from the small gear to the large gear, the larger the difference in the number of teeth between the two, the lower the rotating speed and the larger the value of torque will be produced on the large gear.

The use of two couples of gears, of which ratios in the number of teeth between two gears engaged are different from each other, will permit in two ways transmitting the rotating speed and the value of torque through these two couples of gears.

Similarly, necessary transmission of the drive power in three ways and in four ways can be available with three couples of gears and four couples of gears, respectively.

in the illustration below two gear shafts are arranged in parallel with each other. On the lower shaft three gears are fixedly installed, while on the upper splined shaft three gears are loosely installed so they are able to slide lengthwise on the shaft. ,

The drive power is transmitted from the lower shaft to the upper shaft through any couple of gears, the ratio in the number of teeth between two gears deciding the rotating speed of the upper shaft. Since any of three upper gears can be selectively slided to the position where the gear comes into engagement with its mate gear on the lower shaft, any of different three rotating speeds of the upper shaft can be available according to the selection of the upper gear to be brought in mesh. if four couples of gears are used in this structure, total four ways of selection of any couple of gears in ‘Mesh will be available.

A transmission having three ways of selection of gears in mesh is called the three-stage transmission; four or more stages of transmission are, therefore, so called depending upon the number of ways of gear-meshing they can select. Also, this type of transmission, in which each gear is slided lengthwise on the shaft to come into engagement, can be called the slide-mesh type transmission from its structural feature.

in the illustration above, output rotation of the upper shaft can be allowed to vary in its speed as previously mentioned, but not in direction of the rotation. Shifting from forward travel to reverse travel, and vice versa, of a vehicle, therefore, cannot be available with such a structure of the transmission.

To make a transmission possible in forward-to-reverse shifting; an additional gear called the `’reverse gear” may be used in place as shown in the next figure. Through this gear the drive power from the drive gear on the lower shaft is transmitted to the drive gear on the upper shaft and, as a result, the direction of the upper shaft rotation will be reversed.

in the illustration above, output rotation of the upper shaft can be allowed to vary in its speed as previously mentioned, but not in direction of the rotation. Shifting from forward travel to reverse travel, and vice versa, of a vehicle, therefore, cannot be available with such a structure of the transmission.

To make a transmission possible in forward-to-reverse shifting; an additional gear called the `’reverse gear” may be used in place as shown in the next figure. Through this gear the drive power from the drive gear on the lower shaft is transmitted to the drive gear on the upper shaft and, as a result, the direction of the upper shaft rotation will be reversed.

in the illustration above, output rotation of the upper shaft can be allowed to vary in its speed as previously mentioned, but not in direction of the rotation. Shifting from forward travel to reverse travel, and vice versa, of a vehicle, therefore, cannot be available with such a structure of the transmission.

To make a transmission possible in forward-to-reverse shifting; an additional gear called the “reverse gear” may be used in place as shown in the next figure. Through this gear the drive power from the drive gear on the lower shaft is transmitted to the drive gear on the upper shaft and, as a result, the direction of the upper shaft rotation will be reversed.

SPLINE-A number of grooves provided lengthwise around a shaft and in the bore of a mating gear so the gear fated to the shaft at their splined.surfaces can transmit its rotation to the shaft without slip. Splines are classified according to the shape of their section into two types : the square spline and the involute spline.

The teeth of gears are usually rounded or chamfered, as shown. This is necessary not only for providing easy meshing of gears, but also for preventing the gear teeth edges from breaking off which would be liable if sharp edges are remained without being rounded.

Sliding of a gear on the shaft is performed with a shifter fork of which a half-circle surface is in engagement with the groove in the gear. As the fork is always engaged with the gear and is subject to sliding friction from the gear whenever it rotates; contacting surfaces on both the shifter fork and the gear are hardened to make the surfaces hard.

Each shifter fork is combined with a gear or two to shift them. Two gears different in size from each other are arranged, in combination with a fork that can move to left and right, so that either one comes in mesh according to the shifting direction of the fork. A fork in combination with a single gear may be designed for its movement in one way or both ways depending upon the necessity to shift the gear.

Every movement of a shifter fork is controlled from the operator’s seat with a gearshift lever. Various types of gearshift levers are designed for this purpose. Some of them popular on many vehicles and bulldozers will be illustrated in the following description.

In the figure below three shafts are arranged, each of which having its stroke to move a shifter fork fixedly fitted to the shaft. To keep each shaft in any shifting position, the shaft is provided with notches to which a spring-loaded detent bail may be clicked. This is necessary to ensure the stationary positioning of the shifter fork without any possibility of slipping off that position, which will result in an accidental engagement or disengagement of gears that the shifter shaft concerns with.

 

The movement of the shifter forks are controlled by a gearshift lever located nearby the operator’s seat. Supported by a ball joint in the transmission case cover, the lever knob has its freedom of back-and-forth and right-and-left movement.

Turning the lever knob right allows the bottom end of the lever to engage with the left shift fork, thus making the fork ready for back-and-forth movement.

 

When the lever knob positioned right is lightly turned left, the lever will come into contact with a stopper that gives some restriction to the movement of the lever. At this position the lever bottom is engaged with the center shift fork to make it ready for back-and-forth movement.

  

The gearshift lever can be positioned in engagement with the tight shifter fork when the lever knob is forcedly turned all the way left against the resistance of the spring-locked stopper.

A shifting pattern of the gearshift lever knob will be conveniently indicated near the operator’s seat. In such a pattern diagram, numerals indicate the steps of travel speeds staring from the lowest first speed. Letters F, R and N represent Forward, Reverse and Neutral, respectively, which are indication of the direction of travel. Every combination of a letter and a numeral such as F1, F2 and F3 for the forward speed travel and R1 and R2 for the reverse travel will be noted on the pattern table. When the gearshift lever knob is maintained in the position N, no gear is in mesh to transmit the drive power.

The principal function of a transmission, would be obstructed if two or more couples of gears should come simultaneously into meshing. Such trouble can be avoided by the adoption of an interlock mechanism that permits the movement of only one shifter fork at a time, leaving the others kept immovably. The followings are provided to describe the function of an interlock mechanism.

An interlock mechanism below is formed by the grooves rounding three shifter fork shafts and three pins installed as shown in the figure.

  

When the center shaft is moved downward, the shaft will farce two pins, left and right on the shaft, to come into close engagement with the respective grooves in the left and right shaft. As a result the left and right shafts are kept stopped unless the center shaft returns to its original position.

When the left shaft is moved downward, a pin next to the shaft is forced to become close to the groove in the center shaft, thus making it immovable. At the same time, a pin inserted in the center shaft is pushed to the right and subsequently forces the next another pin to make it close to the groove in the right shaft so the. right shaft can be also kept stopped.

   

Another type of interlock mechanism is shown below. A gate, as shown, is moved right and left according to the movement of the gearshift lever. When the lever is engaged with any of three shifter fork shafts to move it, other shafts will be locked in their position by the plates projecting from the gate.

The following unique interlock mechanism is used for some bulldozers. This mechanism is interconnected to the main clutch. The plunger, as shown in the sketch, is kept pushed by the cam against the shifter fork shaft unless the main clutch is disengaged.

While, after the main clutch is once disengaged, the rotation of the cam is restricted as long as the plunger is pushed back from the shaft.

During the main clutch is in engagement, the cam keeps the plunger pushed into the groove in the shifter fork shaft, as a result the shaft being kept stopped. On the other hand, during the shifter fork shaft is on the course of movement to the next shifting position, the plunger pushed back by the shaft prevents the cam from rotating, thus keeping the main clutch unable to come into engagement until the completion of the gear-shifting.

The illustrated below is a four-speed forward, one-speed reverse, slide-mesh type transmission. The drive power is transmitted from the input shaft to the lower shaft (countershaft) through the gearing arranged at the utmost left. The second from the left on the countershaft is the countershaft 3rd-speed gear.

The succeeding gears are the countershaft 2nd-speed and 1st-speed gears, in that order. The countershaft reverse gear is located behind the 1st-speed gear.

All the main gears on the upper shaft (main shaft) and shifter forks concerned have their functions that we have already studied in the previous pages.

An additional mechanism we have not been mentioned is a gear clutch. When the main shaft 3rd-speed gear is shifted left, the gear clutch acts to make the gear connect direct to the input shaft gear, as shown in the detail drawing. Under this condition the drive power is directly transmitted to the main shaft, resulting in the 4th-speed output from the main shaft.

You are requested to examine this drawing about the various patterns of gear shifting with the relative gears to be in mesh and the path through which the power is to be transmitted.

 

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