Various types of these controls were illustraed in Chapter XIV (Steering Gears And Fundamental Principles Of Steering Mechanisms. Certain Principles That Must Be Understood In Designing Them), describing the steering gear. The general practice is to incorporate these in the steering gear, while the foot throttle or accelerator consists of a small pedal mounted on the dash or foot board and connected with the hand throttle in such manner that it can be operated without changing the position of the throttle lever on the wheel. This is accomplished by a slip joint, as shown in Fig. 236. the accelerator is hinged to the steering column and connected to the carburetor throttle lever by a rod which carries a slip joint. This joint has an extension to which the hand throttle is connected. The accelerator is normally held in the off position by a coiled spring.
Another type of accelerator was illustrated in Fig. 17G, Chapter XIV (Steering Gears And Fundamental Principles Of Steering Mechanisms. Certain Principles That Must Be Understood In Designing Them), showing the Reo steering gear.
The advantage of the foot throttle is that it permits the operator to control the speed of the engine with his right foot, thus leaving his right hand free to change gears, and the left to steer the vehicle. The advantage of quick gear-shifting is not to be denied, as anything which tends to reduce engine racing, gear clashing, etc., is quite desirable. However, motor trucks operate on solid tires and the floor boards are constantly vibrating, and all of the minor shocks which the vehicle springs do not absorb are transmitted to the cab. This vibration makes it quite difficult for the operator to keep his foot steady, as the slightest movement of his foot acts directly upon the throttle. Sudden acceleration is another disadvantage which is to be avoided. The hand throttle of course eliminates this, and it is possible to hold it stationary on the quadrant. Disadvantages of the hand throttle, besides the inconvenience in changing gears, includes the danger of shifting gears without throttling down the engine.
There are two general types of pedals, the straight and the bent type, both of which are illustrated. These pedals have to pass through the floor boards and their shape is dependent upon the room available. They may either be drop forgings or steel castings, and vary from 10 to 16 inches in length, depending upon the required leverage.
Brake and change-gear levers are generally drop forged of I-beam section, and in most cases pivot from a common pivot axis. The change-gear lever of selective type change gears moves in an H segment or gate and does not require a latch to hold it in position. However, a lock is sometimes provided to obviate the possibility of accidentally engaging the reverse gear. A latch lever must always be used with a progressive gear control, and the emergency brake lever must also be provided with a latch. There arc two general types of selective gear controls which are termed the sliding shaft and swinging lever types. While all controls may be classified under these two heads, there are numerous variations in detail.
Fig. 236. Pierce Throttle Control.
The Pierce five-ton control set (Fig. 237) is of this type and mounted on the right side rail of the frame, as the controls are arranged for right side drive. This selective gear control comprises a sliding shaft to one end of which the control lever is rigidly secured, and which at its inner end carries a downwardly extending arm which is arranged to engage with a semi-circular slot in one or the other of the sliding shafts of the transmission which carry shifting forks. The sliding shafts are provided with ball locks, which help to find the correct mesh, and also prevent shifting of both shafts together. The H plate or quadrant, which guides the lever in shifting to the various speeds, is placed slightly forward of the emergency brake lever and has a lock controlled by thumb latch on the lever handle for locking out the reverse gears. The gear shifting mechanism also has a lock which prevents .shifting gears until the clutch has been disengaged. This consists of a semi-circular cam or segment keyed to the sliding shaft of the control set and a plunger connected with the clutch pedal, which, while the clutch is engaged rests in holes in the cam surface. To shift gears the clutch must first be disengaged which also disengages the plunger.
The emergency lever is of the spoon latch type which releases a lock fitting into the ratchet teeth of the brake quadrant.
Another type of sliding shaft control is shown in Fig. 238. This is furnished by the Brown Lipe Co. with their transmissions for left side control, and is used on a number of commercial cars. This differs from the one above in that the control lever is hinged to the sliding shaft, and pivots in a rectangular shaped quadrant. Instead of the lever sliding with the shaft, it pivots to either side in the quadrant and moves the shaft in the opposite direction to which the lever is moved. The emergency brake lever has a spoon latch which operates on the conventional ratchet quadrant, and is pivoted from the same center as the gear lever, but its shaft extends to opposite side of the housing, which encloses the entire control. This type of control is intended for unit power plants where the transmission usually is located under the foot boards.
Fig. 237. Pierce Selective Type Control.
Fig. 238. Brown-Lipe Center Control Unit for Unit Power Plant Transmissions.