Manual Transmission/Transaxle Basics
A manual transmission requires operator input to shift gears. Manual transmissions are generally more reliable than any form of automatic because they have less moving parts, powerflow is simple compared to an automatic, they don’t require a module/electronics (the driver is the module) and everything is mechanical/no hydraulics involved (inside the transmission). Standard transmissions are starting to fade away in recent years because of a combination of advances in automatic transmission technology as well as general laziness. I personally believe that everyone with a drivers licence should be required to learn how to drive a manual. If you are given a licence to drive any passenger car on the road, then you should be able to drive every passenger car on the road.
Transmission vs Transaxle
“Transmission” is a term commonly used to describe a transmission or a transaxle. By definition, a transmission is responsible for gear shifting only while a transaxle shifts gears and houses the differential. A transmission is commonly found on rear-wheel drive vehicles with the engine mounted longitudinally, as long as the transmission is in the front of the vehicle, bolted to the engine. A transaxle is found on all front-wheel drive, transverse mounted engine setups, as well as many vehicles with mid or rear mounded engines. Some manufactures mount the engine in the front of the vehicle and the transaxle in the rear of the vehicle and connect the two with a drive shaft to provide better weight distribution. Four-wheel drive and All-wheel drive vehicles can use either. Some will use a transmission connected to a transfer case which directs power to the differentials. Others will use a transaxle that houses the middle and front differentials and sends rotational torque to the rear differential through a drive shaft.
Manual Transmission Shafts
A typical manual transmission will have 3 shafts; the input shaft, the counter shaft and the output shaft. The 1st, 2nd and 4th gears on the output shaft ride on the shaft but are not connected to the shaft. They may freewheel when the corresponding counter shaft gear drives them but do not transmit torque until its synchro engages the gear. The synchro’s hubs are the only pieces that are splined to the output shaft. (Synchro operation will be explained later in the article.)
The input shaft receives rotational torque from the clutch disk. It acts as the input for rotational torque for the transmission. The input shaft sits on the same plane as the output shaft and even with the transmission apart, it may look like they are one piece. They are two separate pieces that need to rotate at different speeds in all gears except 3rd. They will support each other using needle bearings that allow the two shafts to rotate at different speeds. The input shaft gear meshes with the counter shaft driven gear. Any time the input shaft rotates, the counter shaft rotates. The input shaft gear is smaller than the counter shaft driven gear. This causes a slight underdrive to increase torque and decrease RPM.
The counter shaft counter-rotates the other two shafts. It is one solid piece, when the input shaft gear drives the counter shaft driven gear all the counter shaft gears rotate. The counter shaft gears mesh with the all the output shaft gears except reverse. When the counter shaft gears rotate, the output shaft gears rotate.
The output shaft gears ride on the output shaft but they do not transmit torque to the shaft. The only thing that splines to the output shaft is the synchros hub. All the output shaft gears are driven by the counter shaft gears. Since they all produce a different ratio, only one can be engaged at a time. It is the synchro’s job to smoothly engage a gear based on the inputs of the operator. When a gear is engaged, rotational torque is transmitted from the output shaft gear, through the synchro at the ratio of the selected gear, to the output shaft. The output shaft can then transmit rotational torque to the next component in the driveline (drive shaft or transfer case).If two gears are engaged at the same time, the output shaft will try to be driven at two different speeds. This will cause the transmission to lock up.
The 1-2 synchro engages 1st gear by moving to the right. This locks the output shafts 1st gear to the output shaft. The clutch disk transfers rotational torque to the input shaft. The input shaft gear transfers torque to the counter shaft driven gear. The counter shaft gears rotate all the output shaft’s gears but only 1st gear is locked to the output shaft. The output shaft rotates at 1st gears ratio to the input shaft. A typical 1st gear ratio could be 2.5:1.
The 1-2 synchro engages 2nd gear by moving to the left. Power flow through 2nd gear is very similar too 1st gear except that the output shaft second gear is locked to the output shaft. So the output shaft rotates at the 2nd gears ratio. A typical 2nd gear ratio could be 1.4:1.
You may have noticed that there is no 3rd gear in our example. 3rd gear is achieved by the 3-4 synchro moving to the left. This locks the input and output shaft together. Rotational torque is transferred from the input shaft, to the 3-4 synchro, to the output shaft. There is no change in gear ratio so the ratio is 1:1 or direct drive. The counter shaft is still driven by the input shaft gear, but since none of the output shaft gears are engaged, they simply freewheel.
The 3-4 synchro engages engages 4th gear or overdrive gear by moving to the right. Power flow through 4th is similar to 1st and 2nd gear. The output shaft’s 4th gear is locked to the output shaft by the synchro. The output shaft turns at 4th gears ratio. 4th gear is an overdrive gear so the counter shaft 4th gear will be bigger than the output shaft 4th gear. A typical 4th gear ratio could be 0.8:1. The output shaft rotates faster than the input shaft.
More Forward Gears
Most manual transmissions on the road today are 5-speed or 6-speed manual transmissions. To turn our example into a 5-speed, it would just be a matter of adding a 5th gear counter shaft gear, a 5th gear output shaft gear and another synchro to engage 5th gear. To go from a 5-speed to a 6-speed, the transmission would already have enough synchros, it would simply be a matter of adding another counter shaft gear and another output shaft gear.
For reverse, the direction of rotation needs to be reversed. To do this a reverse idler gear is moved between a counter shaft gear and an output shaft gear that is splined to the shaft. A synchro with teeth along the outside is commonly used as the output shaft reverse gear. These teeth will be straight cut teeth and not helical cut. It is much easier to engage and disengage straight cut gears than it is helical cut gears. Straight cut gears make a whining noise when rotating quickly. This is why many people notice a whining noise in reverse only on manual transmission vehicles. This does not necessarily indicate a problem. It is important to come to a complete stop before engaging or disengaging reverse gear, even with the clutch pedal to the floor. Shifting in a and out of reverse with a manual transmission, without coming to a complete stop, will damage the teeth on the reverse idler gear as well as it’s driving and driven components. This damage can cause reverse gear to become even louder.
In neutral, no synchro is engaged. If the input shaft is rotating, the counter shaft is also rotating, which means that the output shaft gears are rotating. Since no synchro has locked a gear to the output shaft, that is the end of power flow. It is important to note that when the operator shifts gears, he/she must always go through neutral to get to the next gear. This is important for synchro operation. (explained later in this article)
A manual transaxle is similar to a manual transmission in many ways. The biggest differences are that there is no counter shaft which means that all gear reduction (from the gearbox) needs to be done in one gear set. The other difference is the internal differential and final drive. A manual transaxle must get a lot more done than a manual transmission in a smaller amount of space.
Manual Transaxle Shafts
Manual transaxles only have 2 shafts. Gear ratios must be achieved without the additional gear reduction that a transmission has from the input shaft gear and the counter shaft driven gear.
The input shaft receives rotational torque from the clutch disk. It acts as the input for rotational torque for the transaxle. In this example the input shaft is a solid piece that transfers rotational torque to all of the output shaft gears.
Just like in a transmission, the output shaft has all of the output shaft gears riding on it, but none of them transmit rotational torque to the shaft. The synchro’s hubs are the only pieces that are splined to the shaft. All the output shaft gears rotate when the input shaft rotates, but only the gear engaged by the synchro transmits rotational torque to the output shaft, at that gear’s ratio. The output shaft then transfers that rotational torque to the final drive pinion and differential assembly.
The 1-2 synchro engages 1st gear by moving to the left. This locks the output shafts 1st gear to the output shaft. The clutch disk transmits rotational torque to the input shaft which drives all the output shafts gears. Since only the output shaft 1st gear is locked to the output shaft, the output shaft is driven at 1st gears ratio. The output shaft then drives the final drive ring gear.
Powerflow through the other forward gears are similar to the 1st. The only difference is which gear is engaged by witch synchro, similar to the operation of 1st, 2nd and 4th gear of a manual transmission. Since a direct drive is impossible in a transaxle, 3rd gear must actually have an input shaft gear and an output shaft gear. It can still be a 1:1 ratio but most are a slight underdrive or slight overdrive.
More Forward Gears
Most manual transmissions on the road today are 5-speed or 6-speed manual transmissions. To turn our example into a 5-speed, it would just be a matter of adding a 5th gear input shaft gear, a 5th gear output shaft gear and another synchro to engage 5th gear. To go from a 5-speed to a 6-speed, the transmission would already have enough synchros, it would simply be a matter of adding another input shaft gear and another output shaft gear.
Just like in a manual transmission, the direction of rotation needs to be reversed to achieve a reverse gear. The reverse idler gear will be moved between the input and output shaft reverse gears to mesh with them. These gears will be splined to the shafts or a synchro with teeth along the outside will be used for reverse. These teeth will be straight cut teeth and not helical cut. It is much easier to engage and disengage straight cut gears than it is helical cut gears.
In neutral, no synchro is engaged, just like in a manual transmission. If the input shaft is rotating, it is driving the output shaft gears, but since neither synchro has engaged a gear, torque is not transmitted to the output shaft. It is important to note that when the operator shifts gears, he/she must always go through neutral to get to the next gear. This is important for synchro operation. (explained later in this article)
Manual Transmission and Transaxle Designs
Not all transmissions and transaxles have the synchros on the output shaft. Some transmissions have synchros on the counter shaft in a transmission or on the input shaft in a transaxle. Also, many transmission’s or transaxle’s gears are not in order. This means that there could be a 1-3 synchro and a 2-4 synchro for example. This could be done where space is an issue, or for another reason that the engineers decide on.
The Clutch Assembly
The clutch disconnects the engine from the transmission. This is necessary to engage gears as well as for proper synchro action when shifting gears. The clutch assembly must be able to fully disconnect the engine from the transmission when the clutch pedal is pressed, but also transmit full engine torque to the input shaft when the clutch pedal is out. The clutch also provides a way to gradually engage engine power and get the vehicle rolling slowly and/or smoothly instead of a sudden, jerky on/off type switch.
The flywheel is bolted to the crankshaft so it turns any time the engine is running. It is much heavier than a flexplate on an automatic transmission application. This is because the entire clutch assembly is much lighter than a torque converter. The engine needs the extra weight to provide momentum between powerstrokes. The flywheel also provides one of the two surfaces that transfer the engine’s rotational torque to the clutch disc. Many manufacturers now use a dual-mass flywheel. This type of flywheel is made up of two separate pieces that transfer torque to each other through one or several springs. This absorbs the firing pulses of the engine and reduces shock from sudden engine acceleration. These flywheels are much heavier than a single-mass flywheel but can save the transmission from damage.
The pressure plate is bolted to the flywheel, so it also rotates any time the engine is running. It provides the other surface that transfers the engine’s rotational torque to the clutch disk. It is also the component that disengages the clutch to disconnect the engine and transmission as well as the component that applies the pressure necessary to grab and hold on to the clutch disc. The diaphragm type of pressure plate is the most common. It has many fingers that provide the clamping force that grabs the clutch disc as well as provide a way to disengage the clutch using the fingers lever action.
The clutch disc is squished between the flywheel and pressure plate. It has friction material on each side of it that contacts and tries to grab on to either the flywheel or pressure plate. The clutch disc is splined to the transmission/transaxle input shaft which means that when the clutch disc is driven, so is the transmission input shaft. When the clutch pedal is out, the pressure plate, the flywheel and the clutch disc all turn as a unit. The clutch disc has 2 types of springs. The obvious springs located near the centre of the disc are the torsion coil springs. These absorb the firing pulses of the engine to protect the transmission/transaxle. The other type of spring is the wave spring. It is less obvious and is located in the friction material. It allows the friction material to squish a small amount when the clutch is engaged and also soften clutch engagement.
Clutch activation is how the system translates the movement of the clutch pedal to actual clutch disengagement. There are two common ways of doing this. The older way is by a cable. A clutch cable connects the clutch pedal to the clutch fork. When the clutch pedal is pressed the clutch fork and release bearing disengage the clutch. The modern way to disengage the clutch is with a hydraulic system. The clutch pedal is connected to the clutch master cylinder. Brake fluid will transmit pressure from the clutch master cylinder to the clutch slave cylinder. The clutch slave cylinder will use the fluid pressure to push the clutch fork and release bearing to disengage the clutch.
Clutch Fork and Release Bearing
The clutch fork operates the release bearing. It will pivot when the clutch pedal is pressed to move the release bearing. When the clutch pedal is pressed, the release bearing pushes the inner surface of the pressure plate fingers. Since the pressure plate will most likely be rotating, the release bearing allows the fingers remain in motion while the release bearing contacts the fingers. The action of the release bearing pushing on the inside of the pressure plate’s fingers cause the pressure plate to let go of the clutch disc. This disengages the engine from the transmission. When the clutch pedal is not in use, the release bearing sits just off of the pressure plates fingers. This way they don’t drive/wear out the release bearing when the release bearing is not in use.
Gear Shifting and Synchro Operation
For a smooth gear shift to take place, the gear being selected or locked to the shaft needs to be spinning at (close to) the same speed as the shaft. It is easier to think of a transaxle when learning synchro operation,. When the vehicle is stationary and the driver selects 1st gear, the wheels are not turning, which means the output shaft is not turning. If the clutch is engaged, the input shaft and the output shaft gears are being rotated by the engine. The clutch needs to be disengaged before 1st gear can be selected. When the driver disengages the clutch, the input shaft and output shaft gears are freewheeling. It is the synchro’s job to bring the output shaft gears to the same speed as the shaft, in this case stationary. Once 1st gear has been selected, the driver can let the clutch out slowly to get the vehicle rolling. When vehicle speed increases, it is time to shift into second gear. The driver will disengage the clutch and shift through neutral into 2nd gear. This causes a few things to happen inside the transmission. When the clutch pedal is pressed, the engine is disengaged from the transmission. When the shifter is going through the neutral position, once again the output shaft gears and the input shaft are freewheeling. As 2nd gear is engaged the synchro must slow down the output shaft 2nd gear to the speed of the output shaft and slow down the input shaft through the input shaft 2nd gear. Then the shifter and synchro will pop into 2nd gear position smoothly, and the driver can re-engage the clutch and continue driving in 2nd gear. This is how all up-shifts happen. The only difference for down-shifts is that the synchro will need to speed up the output shaft gear and input shaft instead of slow it down. This of coarse all happens very quickly. The only difference in a manual transmission compared to a transaxle is the action of the counter shaft between the input and output shaft.
A synchro is made up of a hub, a sleeve, inserts, a blocker ring, and one or multiple springs. They are controlled by the shift forks that receive input from the shifter inside the cabin through cables or linkages. The hub is the piece that is splined to the output shaft and always turns with the shaft. The sleeve has teeth that are splined to the hub, so it also rotates with the shaft. The blocker ring is usually a brass (softer metal) cone shaped piece with teeth on the outside. The blocker ring is the piece that contacts the output shaft gear to change its speed. If a synchro operates two gears, it needs two blocker rings. The gears that the synchro controls have a cone that can mate with the cone of the blocker ring and “dog teeth” that can mesh with the teeth of the sleeve. The inserts are held in place, in the hub, by a spring and push the blocker ring into the output shaft gear as well as rotate the blocker ring at the same speed as the output shaft. Lets assume that a vehicle is rolling down the road in 1st gear. When the driver disengages the clutch and shifts into second gear, the shifter fork will move the synchro sleeve towards the second gear position. As the sleeve begins to move towards the applied position, the sleeve pushes the inserts towards the gear. The inserts push the soft blocker ring into the gears cone and drive the blocker ring at the same speed as the output shaft. As the sleeve continues to apply pressure, more pressure is placed on the gears cone by the blocker ring, forcing the gear to turn at the same speed as the blocker ring and/or output shaft. Once the gear and the shaft/synchro assembly are rotating at the same speed, the sleeve can slide over the blocker ring so that its teeth mesh with the dog teeth of the output shaft gear. When this happens, the selected gear is locked to the shaft. The gears dog teeth are meshed with the sleeves teeth, which are meshed with the hub, which is splined to the shaft. If a synchro fails to change the speed of the gear, or the driver shifts to quickly and does not give the synchro enough time, a loud crunch or grinding noise can be heard. This is the sound of the sleeves teeth trying to mesh with the gears dog teeth while they are spinning at different speeds. This action causes damage to both sets of teeth.
The hub is splined to the output shaft and always turns with the shaft. It has small cut-out areas for the inserts to ride in. The outer edge of the hub has teeth that mesh with the sleeve.
The sleeve has teeth that always mesh with the outer teeth of the hub. These teeth can also mesh with the dog teeth on the drive gears to lock the gear to the shaft when the sleeve is moved by the shift fork. The sleeve also controls the insert’s pressure on the blocker rings.
Inserts and Spring
The inserts force the blocker rings to rotate with the hub and apply pressure to the blocker rings when a gear is being engaged. There are usually 3 of them per synchro and they are all held in place by the spring. When the sleeve meshed with the dog teeth on the drive gear, the inserts are pushed into the hub and out of the way of the sleeve.
The blocker rings are the synchro component that comes into contact with the drive gears cone to change its speed. The cone shape of the contact surface has a wedge action that grabs the gear. The blocker rings contact surface has sharp rings that are designed to cut through gear oil and cushion the grabbing of the gear.
The drive gears ride on the output shaft but is not splined to the shaft. They have the cone surface that the blocker ring contacts to change the speed of the gear. Around the cone surface is the dog teeth that the sleeve uses to lock the gear to the hub/shaft.
The shifter forks control the movement of the synchro sleeves. They ride on the shift rails and are controlled by the shift lever.
Shifting Without Using the Clutch
It is possible to shift gears without disengaging the engine from the transmission with the clutch. For the synchro’s sleeve to engage the dog teeth of the output shaft gear, they need to be rotating at the same speed. The output shaft gear is meshed with the input shaft, which is driven by the engine through the clutch assembly. Engine RPM can be used to get the gear to rotate at the correct speed. When up-shifting, the driver will release the accelerator pedal allow the engine to slow down. This should allow a brief moment where the shifter can be moved into neutral without grinding. Then when the engine slows down to the correct RPM to engage the next gear, the shifter should pop into place without much force. Down-shifting is also possible but it is harder to do. This involves popping the shifter into neutral and revving the engine up to the RPM that will allow the lesser gear to engage. This takes some practice and can cause severe and immediate damage to the transmission if done incorrectly. Also, some manual transmissions will do this easier than others. Even if you do get good at this, I do not recommend you do this all the time. Many people think that this “saves the clutch.” The clutch suffers the most wear when the clutch is slowly engaged as the driver gets the vehicle moving, especially on a steep hill. The clutch does not get worn down from a proper, smooth shift using the clutch. Shifting without the clutch will however wear out the synchros very quickly. As a gear is trying to be engaged, the blocker ring is trying to change the speed of the gear. Since the clutch is engaged, that gear is connected to the engine. The synchro is not strong enough to change the speed of the engine so the soft blocker ring gets worn down until it is useless. At this point the transmission will need to be replaced or rebuilt. This will cost more than replacing the clutch that you were not helping anyway. Many transport truck drivers shift without the clutch but their engines have a much more limited RPM range, their big diesel engines rev down much faster than our little gasoline and light duty diesel engines and they have much tougher manual transmissions. Not to mention the fact that they spend most of their work day behind the wheel of their trucks. In the end, this is a good way to impress other car guys but not something you should be doing in your every day driving.
Manual transmissions require lubrication to prevent metal to metal contact and wear. They do not have a pump like an automatic, they rely on the gears splashing oil alone. Most manual transmissions do not have a dipstick or any other way of easily checking fluid level or condition. Most drivers completely forget to check or change their manual transmission fluid. If you are going to service your own manual transmission, I would recommend that you find out where fluid is drained from and where fluid is added. Make sure that the fill plug can be opened before you drain the fluid. One of the worst things that can happen in servicing a manual transmission is draining the fluid with no way of getting new fluid back in. Look in your owners manual or online to find out what kind of gear oil your transmission requires. Most manual transmissions take high viscosity gear oil (eg. 80W90). They also have an GL rating that must be taken into consideration when selecting gear oil (eg. GL-4). Always use the viscosity and GL rating that the manufacturer recommends. The manufacturer may also specify a synthetic gear oil only. Using the wrong gear oil can interfere with the synchro action, leading to gear crunch or accelerated transmission wear.