About Manual Transmissions
A manual transmission, also known as a manual gearbox or standard transmission (informally, a "manual", "straight shift", "stick", or "straight drive") is a type of transmission used in motor vehicle applications. It generally uses a driver-operated clutch, typically operated by a pedal or lever, for regulating torque transfer from the internal combustion engine to the transmission, and a gear stick, either operated by hand (as in a car) or by foot (as on a motorcycle). A conventional manual transmission is frequently the base equipment in a car, with an option of an automatic transmission, semi-automatic transmission, or the continuously variable transmission (CVT). If you've ever driven a vehicle with a manual transmission, you know how to depress the clutch, select a gear, and release the clutch while applying power to get the car to move. In its simplest form, the clutch allows engine power to be applied gradually when a vehicle is starting out and interrupts power to avoid gear crunching when shifting. Engaging the clutch allows power to transfer from the engine to the transmission and drive wheels. Disengaging the clutch stops the power transfer and allows the engine to continue turning without force to the drive wheels.
Basic Clutch Components:
- Flywheel: A large steel or aluminum "disc," the flywheel is bolted to the crankshaft of the engine. The flywheel does many things - acts as balancer for the engine, dampens engine vibrations caused by the firing of each cylinder, and provides a smooth-machined "friction" surface that the clutch can contact. But its main function is to transfer engine torque from the engine to the transmission. The flywheel also has teeth along the circumference, allowing the starter motor to contact when turning the engine over.
- Clutch: disc is basically a steel plate, covered with a frictional material that goes between the flywheel and the pressure plate. In the center of the disc is the hub, which is designed to fit over the spines of the input shaft of the transmission. When the clutch is engaged, the disc is "squeezed" between the flywheel and pressure plate, and power from the engine is transmitted by the disc's hub to the input shaft of the transmission.
- Pressure Plate: is a spring-loaded "clamp," which is bolted to the flywheel. It includes a sheetmetal cover, heavy release springs, a metal pressure ring that provides a friction surface for the clutch disc, a thrust ring or fingers for the release bearing, and release levers. The release levers lighten the holding force of the springs when the clutch is disengaged. The springs used in most pressure plates are of a diaphragm-type, however a few use multiple coil springs. Some high-performance pressure plates are "semi-centrifugal," meaning they use small weights on the tips of the diaphragm springs to increase the clamping force as engine revolutions increase.
- Throw-out bearing: is the heart of clutch operation. When the clutch pedal is depressed, the throw-out bearing moves toward the flywheel, pushing in the pressure plate's release fingers and moving the pressure plate fingers or levers against pressure plate spring force. This action moves the pressure plate away from the clutch disc, thus interrupting power flow.
- Linkage: mounted on an iron casting called a hub, the throw-out bearing slides on a hollow shaft at the front of the transmission housing. The clutch fork and connecting linkage convert the movement of the clutch pedal to the back and forth movement of the clutch throw-out bearing. To disengage the clutch, the release bearing is moved toward the flywheel by the clutch fork. As the bearing contacts the pressure plate's release fingers, it begins to rotate with the pressure plate assembly. The release bearing continues to move forward and pressure on the release levers or fingers causes the force of the pressure plate's spring to move away from the clutch disc. To engage the clutch, the clutch pedal is released and the release bearing moves away from the pressure plate. This action allows the pressure plate's springs to force against the clutch disc, engaging the clutch to the flywheel. Once the clutch is fully engaged, the release bearing is normally stationary and does not rotate with the pressure plate.
Diagnosing transmission problems:
- Pay Attention to the Details: The way to diagnose transmission problems is to first take note of when the problem is present and under what conditions. If a manual transmission makes noises just when the clutch pedal is all the way up or engaged and a grinding or rattling noise is heard, the throw-out bearing for the clutch is bad. If the transmission makes a rattling noise that is loudest in neutral, although it is always there, and the noise changes when it is put into gear, but shifts well, the input shaft bearing is bad. This is the shaft in the front of the transmission; the clutch is attached to it by the splines on the shaft.
- Listen for the Noise: If the transmission makes a loud whining or grinding noise continuously while running, check the oil in the transmission. If it is not full, fill it with the approved fluid and test drive it again. If the transmission makes noise when it has the correct fluid, the bearings are faulty and the transmission requires a complete rebuild. It would very likely be more cost effective to buy a used or new transmission.
- How Does It Feel: If the transmission sounds and works well, but is hard to shift, the linkage is faulty and not a difficult repair in most circumstances. If the transmission is a top loader, which is the type where the gear shift enters the top of the transmission, the shifting fork or the ball at the end of the gear shift is the problem. When the gear shift handle is sloppy, it is just a matter of the ball end being loose in its socket. A top-loader transmission uses a gear shift handle that has a spring under the plate, and it pushes down on a ball at the bottom of the shifter. This ball fits into the shift forks in the center of the gearshift hole into a round pocket. When the plate is bolted down, the spring forces the ball into its socket in the shift fork, which moves the gear bank to change gears.
- Physically Difficult to Shift: If the transmission is running quiet, but is difficult to get into gear without grinding, the synchronizers are bad. They are the same circumference as the gears, but are narrow and in place of teeth they employ triangular-shaped short teeth designed to slow the main gear, making it easier to mesh the gear bank. When both gears to be meshed are moving at drastically different speeds, they will not mesh without grinding badly. The synchronizers bring the speeds closer together for a smoother shift. This type of problem requires a complete rebuild as well.
If you're currently experiencing problems with your vehicle's transmission or if you believe there is a problem with your transmission or clutch, do not hesitate to contact us for your Free Diagnostic Analysis.