Manual Transmission Fundamentals Chapter 71

Objectives Describe the relationship between gears and torque Understand the basic types of gears Calculate gear ratios Trace the power flow through three-, four-, and five-speed transmissions Name all of the transmission parts

Introduction Manual transmission Used with clutch Transmission Shifted between gears manually Transmission Used in rear-wheel-drive cars Transaxle Used in front-wheel-drive cars

Purpose of a Transmission Provide a means of changing torque to fit engine operating requirements Low gear Crankshaft turns three times to one turn of transmission output shaft Small gear drives a larger gear Gears provide leverage

Using Gears to Increase Torque and Gear Ratio Driving gear smaller than driven gear Output speed decreases Output torque increases Gear radius Distance from center of a gear to its outside edge Gear ratio Number of teeth on driven gear divided by number of teeth on driving gear

Transmission Gear Ranges and Overdrive Transmissions in cars and light trucks Three-six forward gear ranges Overdrive Opposite of gear reduction Output shaft turns faster than input shaft Ratio a step beyond 1:1 ratio of high gear Planetary gears Automatic transmission with lock-up torque converter

Final Drive Ratio and Gear Types and Operation Ratio between transmission output shaft and differential ring gear Gear tooth shape Allows teeth to roll into and out of mesh with minimum friction Contact pattern: where teeth of two gears meet Pitch diameter: diameter of meshed gear Manual transmission uses two types of gear Spur and helical gear

Spur Gears Simple gears with straight-cut teeth Backlash One tooth carrying the load at time No end thrust Transmission will not attempt to pop out of gear Backlash Clearance between meshing gear teeth Clicking sound results as one gear rolls out of contact and new one rolls in As backlash noise gains speed, it turns into gear whine

Helical Gears and Idler Gears Helical gears replaced spur gears Quieter Continuous flow of power across gear teeth Minimum backlash Greater gear strength More area of tooth contact Cause end thrust under load Idler gears Used between two other gears Changes output rotation direction

Transmission Parts Power flows from clutch disc to input shaft Each forward gear has a synchronizer Keeps two meshing gears from clashing Shift linkage acts on shift forks within transmission to select gear range Power flows from input shaft to countergear Then to mainshaft or output shaft Parts are housed in transmission case Has drain and fill plugs for adding and draining oil

Transmission Lubrication and Transmission Bearings Transmission parts Separated by oil at all times Splash lubrication Oil moved throughout case by rotating gears Bearings support ends of almost all rotating parts Allow parts to rotate with very little friction Reverse idler shafts and gears Supported by bushings

Transmission Gears and Shafts Countergear Single part made of a series of gears that mesh with various gears on mainshaft Mainshaft Includes all transmission gears and synchronizers Manual transmission Forward gears in constant mesh Reverse idler gear is only gear that moves into mesh with another gear

Synchronizer Assembly Helps two gears spinning at different speeds mesh without clashing Blocking ring synchronizers Shift collar fits around hub outside Gears are in constant mesh Rotate freely on bearing areas Splines on outside of hub become meshed with gear teeth Synchro assembly Locks input shaft gear to output shaft gear

Gear Shift Mechanisms and Shift Patterns Major components: Shift forks: fit into grooves cut in outside of synchro collar Shift linkage: internal shift rail or external rod Detent mechanism: holds transmission in gear Spring tension: holds detent balls into detent notches in shift rail Interlock mechanism: prevents selection of two gears at once Shift patterns: various patterns for different transmissions

Transmission Power Flow Modern transmissions are constant mesh Synchro collar: only thing that moves All manual transmissions operate in a similar fashion Whether there are three speeds or six speeds Five-speed transmissions: most common today Most have direct power in fourth gear Fifth gear provides an overdrive

Four-Speed Transmission Power Flow Four-speed transmission without overdrive Neutral: synchro sleeves centered and do no mesh with clutch teeth of any gear High gear: power runs straight through the transmission from input to output shaft Third gear: power enters through input shaft Second gear: rear synchro sleeve engages engage the second-gear clutch teeth First-gear: rear synchro sleeve is moved toward rear to engage first-gear clutch teeth Reverse: synchro sleeves are in neutral position

Five-Speed Transmission Gear flow in five-speed Same as in first four speeds Extra gears in extension housing Fifth gear: synchro sleeves in transmission case in neutral position Power flow through end of countergear to gear at the end of countergear Some have both reverse and fifth gear in extension housing or rear section of case

Speedometer Drive Some cars use electric speedometers Receive signal from vehicle speed sensor (VSS) Vehicles with VSS If the tire diameter is changed: computer can often be programmed with new tire information Speedometer will be accurate Speed inputs to the computer will be meaningful

Switches and Sensors Computer technology Provided several electronic features to transmissions VSS on late-model transmissions Shift blocking solenoids Reverse lockout Gear range selector switch Sensors are not prone to wear like a switch that has electrical contacts