A manual transmission helps a vehicle achieve a desired speed through the use of gears, a clutch, and a shifter. It works like this: when the driver wants to shift gears, he or she pushes in the clutch pedal while letting off the gas. This disengages the clutch within the transmission, allowing the driver to shift into a higher or lower gear.
The reverse gear represents another option utilized when the vehicle is not in motion. Connected at the engine through the clutch, the transmission turns at the same amount of RPMs as the engine, due mainly to the flywheel. The pressure plate pushes the clutch plate against the flywheel, locking the engine to the transmission, ensuring that they remain at the same speed.
So what about the benefits? A few of the benefits of a manual transmission over an automatic version include improved fuel economy, durability, and cost. Some of the drawbacks include a more complex learning curve, slower shifting speeds, and more focus needed to drive, especially when operating the vehicle in hilly terrain.
The Five-Speed Transmission
Also called a Continuously Variable Transmission (CVT), the concept behind a manual transmission is to keep the engine's RPMs at a low enough level to keep it from burning up, while also maintaining the speed desired. So just how does the transmission accomplish this? It controls the speed of the vehicle through the use of gears in relation to each other. Called gear ratios, each gear on the shift lever has a corresponding gear ratio, as well as an RPM range.
Think about it this way: Using the example of a five-speed transmission, the engine revs higher as the vehicle picks up speed. Once the RPMs hit a certain level, drivers then press in the clutch pedal while releasing the gas, shifting the gear shift as the clutch disengages. This, in turn, slips the transmission into the next higher gear, reducing the RPMs produced, and starting the process again as the engine winds up to the next gear.
Most often drivers accomplish downshifting by pressing the clutch pedal and shifting to the desired gear, which does not have to be in exact reverse order. For example, a driver can shift from third to first if desired with not too many undesired effects, though they should avoid doing so.
The Gears and RPMs
The gears in most manual transmission vehicles include first, second, third, fourth, and reverse, with some high-performance vehicles going all the way up to fifth or sixth gear. When in Neutral, the clutch is not engaged, requiring the driver to put the parking brake on to keep the vehicle from rolling while parked. As the driver shifts through the gears, the engine puts out an increasing amount of RPMs, also known as Revolutions Per Minute.
But how important are RPMs to a vehicle’s optimum performance? The RPMs in a vehicle engine represent the number of times a crankshaft turns in the span of one minute. The higher the RPMs, the faster the vehicle goes and the harder the engine works. If the engine ran at the maximum RPMs for any great length of time, the engine would soon wear out due to the heat and stress produced. The gears in the transmission along with the clutch slow the RPMs down upon each upshift so that the engine does not constantly run at high RPMs.
Drivers must to shift each gear as it reaches the upper RPMs of its range. After driving for a while, drivers can usually tell when they need to shift. The higher the gear, the higher its max RPMs, with first gear hovering around 1200 to 1300 RPMs and fifth gear maxing out at over 3,000 RPMs.
The shifter lever controls the three rods that push the three forks that engage the various gears. As the driver moves the shifter left and right, the forks moves the collars that engage each gear. While in between gears, the shifter lever remains in neutral and the clutch remains disengaged within the transmission.
The Idler Gear and Synchronizers
Certain basic driving maneuvers, like shifting the vehicle into reverse, require vehicles with a manual transmissions to use a small gear called the idler gear. When shifted into reverse the fork pushes the collar, moving a larger gear into contact with the gear for the upper gear and the idler gear, allowing the car to back up. To prevent the vehicle from going into reverse while the car is moving forward, the reverse gear turns in the opposite direction of the other gears. This ensures that the dog teeth do not engage when doing so, eliminating the possibility of damaging the transmission.
In the past, vehicle drivers followed a practice called double clutching. This maneuver included placing the transmission into neutral and using the engine brake to match the speed of the engine. The driver then engaged the clutch again to shift into the desired gear. Modern vehicles use synchronizers to eliminate the need for this practice. The synchronizer allows the collar to make frictional contact with the dog teeth on the gear. The collar and gear become synchronized in this way. This gives a smoother shift without having to put the transmission into neutral beforehand.
The Crankshaft and Clutch
The effective operation of a manual transmission includes parts like the crankshaft and clutch. The crankshaft sits within the engine of a vehicle and turns according to the firing of the chambers. The more chambers, or cylinders, a vehicle engine contains, the more power in the form of torque it produces. The crankshaft transfers this torque, or rotational force, to the transmission, which connects to the engine at the bell housing. The transmission, in turn, transfers this torque to the drive shaft and on to the differential. The differential contains a simple planetary gear train, which helps to gradually apply the power generated by the engine to the wheels, giving the vehicle forward motion.
The clutch plays an important role in this whole process. When the driver places the vehicle in gear and lifts their foot off of the clutch pedal, the vehicle begins to move forward. When drivers need to stop, they push the clutch pedal along with the brake, while letting off the gas. This disengages the clutch and stops the transmission from spinning, allowing the vehicle to come to a stop.
How Gear Ratios Work
The gears in a manual transmission are a vital part of the vehicle’s operation. The gears turn in time with the engine. The clutch pad working in sync with the flywheel, make sure that the engine and transmission remain locked together to prevent the gears from slipping. The gears used depend primarily on the particular gear the vehicle is currently in. Gears range from first through fourth and fifth, and sometimes sixth gear on high-performance cars.
The gears in the transmission matchup to produce the amount of RPMs they need for that particular gear and speed. The gear ratio is the difference in the size of the various gears to each other and the number of teeth each gear contains.
Synchromesh vs. Non-synchromesh Transmissions
To get an even more in-depth look at how manual transmissions work, let’s take a look at synchromesh and non-synchromesh transmissions. While many smaller vehicles contain a synchromesh transmission, heavy-duty trucks and other machinery do not. Instead they contain a non-synchromesh transmission.
A synchromesh transmission uses synchronizers to match the speed of the gear to that of the engine by rubbing the collar against a small brass clutch on the gear. An unsynchronized transmission does not have this ability, requiring drivers to double clutch to bring the engine down in speed to match that of the higher gear drivers are shifting into. Non-synchromesh transmissions do not suffer from as much wear as the synchromesh versions, and the shifting action of the non-synchromesh versions work a lot faster.
Taking care of a manual transmission, including changing out the fluid according to the maintenance schedule ensures that it remains in good repair for many years. Over time, a clutch normally suffers wear and tear and eventually needs to be replaced. When this happens, have a mechanic perform the necessary repairs and maintenance, and get back on the road as soon as possible.
Common Issues and Symptoms of Manual Transmission Problems
When determining problems with a manual transmission, keep these common issues and symptoms in mind:
- A dragging clutch signifies a clutch plate that does not disengage from the flywheel. When this happens, the transmission and clutch continue to rotate at the same speed, making it difficult or impossible to change gears. A mechanic will inspect and make recommendations to fix the transmission in this case.
- Slipping gears means the transmission slips in and out of gear. You can attribute this to worn or broken linkage that holds the gear in place.
- A vehicle that grinds and shakes may have a faulty transmission. The cause ranges from a bad clutch to worn or damaged gears and synchronizers.
- A lit Check Engine Light can also indicate a problem with a manual transmission. Have a mechanic run a diagnostic to first determine the problem; then have them fix it if necessary.
This article originally appeared on YourMechanic.com as How a Manual Transmission Works and was authored by Cheryl Knight.