LESSON TWENTY-SEVEN

METHODS OF SPEED CHANGING OUTLINED

Q. Name the principal types of speed changing systems. A. Speed changes may be effected by means of friction, planetary, sliding gear, and individual clutch gearing.

Q. Which are the simplest types to control?

A. The friction and planetary gearing are the easiest operated by the novice, though the individual clutch and selective sliding gear are handled just as easily by the more experienced motorist. Q. What is the feature of the planetary gear set?

A. The important feature of an epicyclic or planetary gear set is that the speed reduction gears are always in mesh and that any speed may be obtained instantly by a simple movement which tightens a friction band or clutch acting only for the speed selected. As the gears are always in mesh and as the operation of obtaining a speed and applying a clutch is simultaneous, there is no danger of stripping gears or of doing other damage to the transmission. Q. Name principal planetary gear car and describe controlling system.

A. Practically the only planetary gear car now on the market, but one that is sold in very large quantities, is the Ford Model T. The control system of this is shown at Fig. 251. The spark and throttle control are on the steering post under the wheel which is located at the left side of the car. The lever at the outside is for the spark advance while that on the right is the throttle lever controlling the gas supply. A single hand lever, which combines the function of an emergency brake actuator and high speed clutch release, is provided on the left side of the car. Three pedals are used, the one at the extreme left engaging the low speed when pushed forward and allowing the direct drive clutch which is spring engaged

High And Low Speed

Fig. 251.-Control System of Ford Model T Car Which Uses Planetary Change Speed Gearset.

to come into action if allowed to drop to its rearmost position. When the emergency brake lever is applied the high and low speed pedal is in a neutral position. The middle pedal engages the reverse ratio when pressed forward, while that at the extreme right is a foot brake acting on the gearset.

Q. Describe simplest form of planetary control system.

A. The simplest form of planetary gear control system is that by which a single lever convenient to the operator's hand is used to engage all speeds. The lever has five positions, three of which

bring the gearset into operation, while the other two are neutral points in order to prevent passing from one speed to another too quickly. As generally arranged, if one brings the lever to its extreme rear position the reverse motion will be secured. Just forward of this point is a neutral position from which the lever may be brought into slow speed by pushing forward. If the motion of the lever is continued the second neutral point is passed and when the extreme forward position is reached the high speed is engaged. It will be seen that there are two neutral points, one between the low and reverse, the other between the low and high speeds. The lever may be placed in either neutral point when stopping the car, though it is preferable to place it between the low and reverse instead of between the high and low when stopping, as the car may then be moved either forward or backward, as conditions dictate.

Q. How are low speed and reverse ratios obtained in planetary gearset?

A. The low speed and reverse ratios are secured by tightening brake bands around the drum or case in which the reduction gears are carried.

Q. How is direct drive obtained?

A. Direct drive is obtained by means of a friction clutch of either the cone or multiple disc type, which locks the entire gearset assembly together so that the gearing does not revolve on its own axis but around the driving shaft center line so that the drive is obtained without any reduction in ratio except that existing between the bevel pinion and ring gear at the rear axle.

Q. How are speed changes obtained with friction drive? A. Speed changes of friction drive cars are obtained by sliding the driven disk across the face of the driving member by means of a suitable handle or disk shift lever at the side of the car.

Q. Describe typical friction drive car control system. A. The control system of a friction driven automobile is simple as it consists of the usual spark and throttle control levers on the steering post, a hand lever to obtain the varying speed ratios and

two pedals. One of the pedals is used to release and apply the power by moving the friction members from or toward each other. This pedal is provided with a ratchet lock so it may be retained in position desired by the operator. The other pedal is a brake applying member. Before the hand lever can be moved to shift the friction disk it is necessary to break the driving connection between them by releasing the pressure at the pedal. Changes of speed are obtained progressively, the usual arrangement being to engage the discs for reverse motion when the lever is toward the rear of the car and to obtain high forward drive ratios as the lever is pushed toward the front of the car. The degree of pressure to

be applied at the friction pedal depends upon the character of the road surfaces. On good roads but little pressure is needed to secure positive driving, but as road conditions become more severe a greater degree of frictional adhesion is desired so more pressure must be applied by the pedal.

Q. What two methods of sliding gearset control are used? A. Sliding gearsets may be controlled either by the progressive or selective system.

Q. What is a progressive gearshift, and what are its disadvantages?

A. The progressive gearshift is the type, where the hand lever must be pushed from one extreme of a segment to the other to obtain varying speed ratios which are progressive in their application. This sector is usually provided with five notches if used on a three speed and reverse gearset. The rearmost notch is the reverse position. The next one working toward the front of the car is a neutral point and then the low speed, intermediate and direct drive

positions follow in the order named. The disadvantage of this system is that if the lever is in the reverse position and it is desired to engage the intermediate or high speed, it is necessary to first pass through the low speed before the higher ratios can be secured. This gear control is not very widely used as it is slow in action and requires careful gear shifting in order to avoid injuring the gear teeth.