IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:
SLIDING GEAR TRANSMISSIONS
SLIDING KEY TRANSMISSIONS
COLLAR SHIFT TRANSMISSIONS
GARDEN TRACTOR TRANSMISSION SYSTEMS
SLIDING GEAR TRANSMISSIONS
Basic gear type transmission is sliding gear transmission. Consists of input shaft and output shaft parallel to each other inside housing or case. Third shaft called idler shaft reverses power to output shaft. Idler shaft parallel to input shaft. One pair gears provides each forward speed. In each gear pair, one gear located on input shaft, other on output shaft. Gears on input shaft solidly attached to shaft by splines or keys. Gears on output shaft slide so can move to engage mating gear on input shaft. Reverse occurs when sliding gear engages idler gear.
Most basic gear type transmission is sliding gear transmission. Common in farm and industrial machines, as well as compact equipment. These transmissions simple to operate and repair and provide variety speeds.
Sliding gear transmissions contain simple arrangements spur gears and shafts. Usually contain input shaft and output shaft held parallel to each other in housing or case. Third shaft called idler shaft reverses power to output shaft. Idler shaft held parallel to input shaft.
Sliding spur gears arranged to mesh to provide changes in speed or direction. One pair gears provides each forward speed. In each gear pair, one gear on input shaft, the other on output shaft. Gears on input shaft solidly attached to shaft by splines or keys. Gears on output shaft slide to engage mating gear on input shaft. Reverse occurs when sliding gear (usually first gear) engages idler gear.
Consider three speed sliding gear transmission. Has three forward speeds, one reverse. Driver engages, disengages various gears with shift linkage. This transmission contains four shafts: clutch shaft, countershaft, output shaft, and idler shaft. Also contains eight gears: low speed drive gear, second speed gear, transmission drive gear, countershaft drive gear, countershaft low gear, countershaft second gear, reverse idler gear, and countershaft reverse gear. When lever moved by operator, causes sliding gears to move back and forth on output shaft.
In first gear, input shaft runs two and a half to three times as fast as output shaft. Thus torque on output shaft increased and speed reduced. In first gear, transmission drive gear drives countershaft drive gear. Rotating motion of clutch shaft transferred to countershaft. Countershaft low gear then drives low speed gear. This transfers rotating motion of countershaft to output shaft.
When transmission in second gear, different pair gears meshed. Shift lever slides low speed gear out of mesh with countershaft low gear, slides second speed gear into mesh with countershaft second gear. Second speed gear smaller than low speed gear. Countershaft second speed gear larger than countershaft low gear. So overall speed reduction in second gear less. Typical transmission gear ratio for second gear 2:1.
When transmission shifted into third gear, shift lever slides second speed gear out of mesh with countershaft second gear. Then shift lever slides second speed over to mesh with transmission drive gear on end clutch shaft. External teeth on transmission drive gear mesh with internal teeth on second speed gear. This causes transmission output shaft to lock together with clutch shaft. So clutch shaft and output shaft rotate together. Since output shaft now rotates at same speed as engine crankshaft, gear ratio in third gear direct drive or 1:1.
Transmission also has neutral and reverse. In neutral, transmission drive gear and countershaft drive gear mesh. Power flows into transmission through clutch shaft and countershaft turns, but none gears on countershaft in mesh with gears on output shaft. As result, output shaft doesn’t turn and no power flows out transmission to drive components.
To provide reverse, output shaft must rotate in opposite direction from clutch shaft. To accomplish, shift lever moves low speed gear over to mesh with reverse idler gear. Reverse idler gear changes direction of rotation between countershaft reverse gear and low speed gear. As result, output shaft rotates in opposite direction and reverses machine.
SLIDING KEY TRANSMISSIONS
Some garden tractors contain sliding key transmissions. Like sliding gear, sliding key contains input and output shafts parallel to each other, mounted inside housing. Gears on input shaft and mating gears on output shaft in constant mesh. Gears on input shaft keyed or splined to shaft. Gears on output shaft spin independently on it.
Output shaft has two opposing keyways cut into it. Key slides in each keyways, each key engages one gear at time. Grooved sliding collar moves two keys simultaneously along two keyways and engages chosen gear. Gears mounted on output shaft have keyways cut into them so one gear at time may be engaged by sliding key. This allows power to flow from input shaft gear through mating gear on output shaft.
Sliding key transmission moves into reverse when output shaft’s sliding key engages a sprocket. Sprocket connected by roller chain loop to sprocket splined to input shaft. With sliding key transmission, operator can shift through forward gears without stopping and disengaging clutch. Tractor must be stopped and clutch disengaged before transmission shifted into reverse.
Some outdoor power equipment uses sliding key transmissions. Like sliding gear transmission, sliding key transmission contains input and output shafts parallel to each other, mounted inside housing. However, gears on input shaft and mating gears on output shaft held in constant mesh. Mesh all the time. Gears on input shaft keyed or splined to shaft; gears on output shaft spin independently on it.
Output shaft has keyways that hold shifter keys cut into it. A key slides in each keyway, and each key engages only one gear at time. Keys moved simultaneously in keyways by shift fork. A grooved sliding collar moves keys along in keyways and engages chosen gear. Each key shaped to engage just one gear at time. Gears on output shaft have keyways cut into them so one gear at time may be engaged by sliding key. This allows power to flow from input shaft gear through mating gear on output shaft, then out transmission to drive components.
In neutral, shift fork places keys outside gears. Therefore, no power transmitted to shifter shaft. Sliding key transmission moves into reverse when output shaft’s sliding key engages sprocket. Sprocket connected by roller chain loop to a sprocket splined to input shaft. With sliding key transmission, operator can shift through forward gears without stopping and disengaging clutch. Tractor must be stopped and clutch disengaged before transmission can be shifted into reverse.
COLLAR SHIFT TRANSMISSIONS
Collar shift transmission contains parallel shafts fitted with constant mesh gears. When transmission in neutral, gears turn freely on their shafts. When transmission engaged, shift fork slides collar device over to engage selected gear. Sliding collar locks gear to shaft, and power transferred to output shaft. Collar slid in opposite direction to disengage gear. By varying gear connections, number of speeds can be selected.
Synchromesh transmission is type collar shift transmission equipped with devices called synchronizers. Structure synchromesh transmission similar to collar shift transmission, with parallel shafts and gears in constant mesh. Single synchronizer mechanism then installed between each pair gears.
Synchronizer device used to equalize speed of shaft and gear before two components meshed. Synchronizer makes two components rotate at same speed before they mesh. This eliminates gear clashing, allows gears to shift smoothly while machine moving.
Synchromesh transmissions primarily used in automobiles, but may see in very large power equipment. These gaining popularity in larger garden tractors, are sometimes called “shift on the go” transmissions. Operation synchromesh transmission basically same as collar shift, except for addition of synchronizer mechanisms between gear pairs.
When transmission shifted into first gear, first gear synchronizer moves toward first speed gear. Synchronizer ring presses against first gear cone, causing friction. Friction causes synchronizer ring to turn with hub. When first gear and hub turning at same speed, synchronizer guide splines engage first gear teeth, so synchronizer and first gear lock.
Basic operation gears in synchromesh virtually identical to operation gears in regular collar shift transmission. Only difference is collar shift transmission won’t contain synchronizer devices.
Consider operation typical three speed synchromesh transmission in neutral. When transmission in neutral and clutch engaged, engine power flows through clutch to clutch input shaft and transmission drive gear. As clutch shaft revolves, sets countershaft gears in motion. Since first-reverse synchronizer and second-third synchronizer both in neutral, no power transferred to output shaft.
When transmission shifted from neutral into first gear, first-reverse synchronizer moved forward by shift mechanism. Synchronizer guide (which revolves with main shaft) couples synchronizer to first gear. This allows power to flow through transmission. Engine power delivered to input shaft and gear by clutch puts countershaft in motion. Countershaft gear drives first speed gear, and power delivered to main shaft.
When transmission shifted from first to second, clutch depressed to release power from gear train. As shift lever moved, shift fork assembly returns first-reverse synchronizer to neutral position. Synchronizer held in place while other shift lever moves second-third synchronizer toward second gear. Second-third synchronizer, which carried on main shaft, couples main shaft to second speed gear.
In three speed transmission, third gear or high gear also direct drive. To shift from second to third, clutch depressed and shift lever slides second-third synchronizer from rearward position. This releases second speed gear to forward position where engages with sprocket like section of transmission drive gear.
Since clutch and main shaft coupled together and turn as one, no gear reduction occurs. Now consider operation of transmission in reverse. Transmission should be shifted into reverse only after machine fully stopped and clutch pedal depressed for few seconds. When transmission shifted into reverse, first-reverse synchronizer moved rearward so reverse gear held to main shaft.
Many manual transmissions don’t use synchronizer in reverse gear. Instead reverse attained by moving first-reverse gear to the right and into mesh with reverse idler gear, causing power to flow through this gear from the countershaft. This type transmission uses synchronizers only for forward gears, and a sliding gear for reverse.
When transmission in forward gear (or in reverse), only one synchronizer device engages gear at any one time; other synchronizers held in neutral positions. If all synchronizers in neutral, no power transferred. Remember this when troubleshooting manual shift transmission. If two synchronizers were engaging gears at same time, transmission would lock up, and no power would be transmitted through it.
Many outdoor power equipment applications use special type transmission called a transaxle. Transaxle is device that combines transmission and differential in one unit. This helps save space in machine, makes transmission lighter in weight, and device easier to maintain. For these reasons, transaxles good choice for machines like garden tractors.
In transaxle assembly, transmission, differential, and connections for drive axles all located in a lightweight housing. Many transaxles use constant mesh helical gears for all forward speeds, and spur gears for reverse. Transaxle usually has two separate shafts: an input shaft and an output shaft. The input shaft usually located above and parallel to the output shaft. The input shaft’s gears drive the output shaft’s gears directly. Engine torque applied to input shaft, and revised torque rotates output shaft.
A pinion gear is usually machined onto the end of transaxle’s output shaft. This pinion gear in constant mesh with the differential’s ring gear. When output shaft rotates, the pinion gear causes the differential ring gear to rotate. Resulting torque rotates other differential gears, which in turn rotate machine’s drive axles and wheels.
Transaxles can be used with variable speed belt drives or hydrostatic transmissions. May also be coupled directly to engine through a drive belt (with an idler tensioning clutch), or through a friction disc clutch and drive shaft. Transaxle may provide from two to five forward speeds, and one or more speeds in reverse, depending on transaxle design and machine operating conditions.
GARDEN TRACTOR TRANSMISSION SYSTEMS
Suppose you have a garden tractor with a 10 hp motor. By the time the engine’s horsepower was used to turn the large blade under the cutting deck, there would be little horsepower or torque left to turn the rear wheels. For this reason, many pieces of outdoor power equipment use transmissions to increase the horsepower of the engine.
A transmission is a gearing arrangement that transfers turning force from an engine into increased horsepower. Transmissions work on a simple principle. If the speed of the output shaft of the engine is decreased through one or more sets of gears, the horsepower and torque at the output of the transmission is increased.
For example, consider the transmission gearing arrangements: if the engine is directly coupled to the output in a one to one (1:1) ratio, if the engine produces 10 hp and is running at 4,000 rpm, the output of the transmission will also be 10 hp and 4,000 rpm. This transmission produces no change in either horsepower or rpm.
Suppose the input shaft turns two revolutions for every one revolution of the transmission shaft. Thus, the ratio is two to one (2:1). In this case, the output horsepower will be twice as much as the horsepower produced by the engine. So, if the engine produces 10 hp and is running at 4,000 rpm, the output horsepower will be 20 hp (2 X 10hp = 20hp) and the output speed will be 2,000 rpm (4,000 rpm ÷ 2 = 2,000 rpm). The output of the engine will be twice as powerful at half the speed.
The same applies for any gear ratio. For example, assume a gear set with a 4:1 ratio, the input shaft turns four revolutions for every one revolution of the transmission shaft. Thus, the ratio is four to one (4:1). In this case, the output horsepower will be four times as much as the horsepower produced by the engine. If the engine produces 10 hp and is running at 4,000 rpm, the output horsepower will be 40 hp (4 X 10 hp = 40hp) and the output speed will be 1,000 rpm (4,000 rpm ÷ 4 = 1,000 rpm).
A gearing arrangement like this would be found in the three speed transmission on a garden tractor, automobile, or any similar machine. In the 4:1 transmission position, the gears would produce first (low) gear. In the 2:1 position the gears would produce second gear. Finally, in the 1:1 position, the gears would produce high gear.
First gear would produce the most horsepower at the rear wheels; however, the speed of rotation would be very slow. In high gear, the least amount of horsepower is at the rear wheels, but the speed is at its highest.
The transmission is shown as a set of sprockets connected by chains. The diameter of the sprockets set the gear ratio of the system. In an actual transmission, a series of gears turns on two or more shafts within the transmission. The gear sets are engaged or disengaged by shifting or moving a series of shifting forks within the transmission. The gears inside the transmission contain varying numbers of teeth to provide the ratio needed.