Principal stress, Principal plane & Mohr’s circle analysis
Manual transmission, or simply a gearbox, has been serving automobiles well for many decades. Even today it’s the most popular form of transmission. Globally manual transmission accounts for 52% of market share as per 2013 data. In this video, we’ll give a conceptual introduction on the workings of an actual manual transmission with a reverse gear.
A detailed webpage version of the video is given below.
Why the Transmission is Required?
The basic question, is why transmission is required in an automobile? The power generated by the engine flows through the transmission before it reaches the drive wheels.The basic function of the transmission is to control the speed and torque available to the drive wheels for different driving conditions.
For example, if you want to climb a hill, you need more torque. By reducing the speed at the transmission, we will be able to achieve higher torque for the same power input. This is simply conservation of energy. Power transmission through a shaft is torque times angular velocity of the shaft. When you reduce the speed of the shaft, it will automatically result in increase in the torque transmission. Conversely, if the torque demand is low , we can increase the transmission speed. These 2 cases are depicted in Fig.2.
The Basic Working Principle
Now let’s look at its inner workings. Manual transmissions work on the simple principle of gear ratio. As shown in Fig.3 a different output speed can be achieved by meshing gears of different size. The speed ratio is given by the simple equation shown in the figure (N represents speed, T represents number of teeth).
Sliding Mesh Transmission
Sliding mesh is the one of the earlier type of manual transmission technology, and the one which is easiest to understand. The most basic slidngmesh transmission mechanism is shown in the Fig.4. Here the input and output shafts are connected through a counter shaft.
This mechanism can operate under 2 different configurations. In the firs configuration, the output shaft will turn at a slower speed than the input. Just by sliding output gear and connecting the output shaft with the input will result in the second configuration. It is clear that, here the input and out will turn at the same speed. Direction of the power flow is represented as red dotted lines in the Fig.4.
A 3-speed mechanism will look as shown in the Fig.5. For the gear meshing shown in the figure, the output shaft will rotate at its slowest speed (1st Gear).It is clear that just by sliding the gears we can achieve different transmission ratios, such as 2nd and 3rd gears.
The sliding mesh transmission is good for controlling the speed, but they have an inherent disadvantage. It’s quite tricky to slide from one gear and engage with another gear. A technology known as double clutching should be used for achieving a smooth slide of gears. The driver should possess a good skill to do an effective double clutching. Maintenance associated with the double clutch transmissions are quite frequent too.
Solving the Sliding Problem – Synchromesh Transmission
The synchro mesh transmission permanently solves this problem. Here the gears are always in mesh, but with a major difference. Here the output gears are loosely connected to the shaft. You can see from Fig.6 that there is a small clearance between the output gears and shaft.
If we connect only one gear to the shaft at a time, the shaft will have the speed of the connected gear.
Understanding the basis using a Hypothetical connector.
We will first use a hypothetical connector to illustrate how different gear ratios work in the sycnhromesh transmission. Later on we will move to the actual technology. With the help of the hypothetical connector, different gear ratios are illustrated in Fig.5. It is interesting to note that in 4th gear the input and output shafts are directly connected. This means the output and input shaft will have the same speed in 4th gear.
The art of locking a loosely held gear to the shaft effectively and smoothly lies at the heart of the manual transmission. Let’s see how this is done in actual practice.
Synchronizer Cone-Teeth Arrangement
First of all, the main shaft gears have a synchronizer cone-teeth arrangement as illustrated in Fig.8.
A hub is fixed to the shaft. A sleeve that is free to slide over the hub is also used in this system.
It is clear that, if the sleeve gets connected with the teeth of the synchronizer cone, the gear and shaft will turn together, or the desired locking action will be achieved. But during the gearbox operation, the shaft and gear will be rotating at different speeds. So such a locking action is not an easy task.
Use of Synchronizer ring
A synchronizer ring helps to match the speed of the gear with that of the shaft. The synchronizer ring is capable of rotating along with the hub, but is free to slide axially. Before moving the sleeve, the clutch pedal is pressed. This way power flow to the gear is discontinued.
Fig.10 A synchronizer cone is placed between a hub and synchronzier cone
When we move the sleeve, the sleeve will press the synchronizer ring against the cone. Due to the high frictional force between the synchronizer ring and cone, the speed of the gear will become the same as the shaft. At this time, the sleeve can be slid in further, and it will get locked with the gear. Thus, the gear gets locked with the shaft in an efficient and smooth way.
Different Gear Ratios
What we have seen in last section was the technology behind the 2nd gear. In the same way the other gear ratios are also achieved. The details are described in this session.
Under Drive – 1st, 2nd and 3rd
In under drive the output shaft turns at a lower speed than the input. For the manual transmission technology we are explaining 1st , 2nd and 3rd gear ratios fall under the under drive category. The following figure depicts the sleeve motion required for 1st and 3rd gear.
As the name suggests in direct drive, the output and input shafts turn at the speed. For this purpose the output and input shafts are directly coupled using the synchronizer cone-sleeve mechanism. The hub is fixed to the output shaft, when the sleeve gets connected with the synchronizer teeth of the input shaft, they get coupled together. During the direct drive, the sleeve at the third gear position (2nd part Fig.12) should move to left side.
A 5th gear is used to turn the output shaft at a higher speed than the input shaft. You can note here that unlike the other gear pairs, in 5th gear the output shaft gear is smaller than the counter shaft gear. This generates the overdrive scenario.
You can note more difference in the 5th drive configuration, the output gear is fixed to the shaft and the counter shaft gear is loosely connected. As a result synchroizer ring – sleeve mechanism is arranged on the counter shaft. The sole purpose of such an arrangement is to accommodate the reverse gear mechanism. We will see that in next session.
The sleeve motion is controlled by a shift stick . You can also see the mechanism used for controlling the sleeve with the shift stick. You can note that using this mechanism, not more than one sleeve will be engaged with the output gears. That is important, since engaging 2 sleeves at a time will lead to an impossible turning condition.
The Reverse Gear
Now let’s see how the reverse gear works? The reverse gear uses a 3-gear arrangement, as shown. Out of those, one is the idle gear.
It is clear that addition of one more gear will turn the output shaft gear in the reverse direction. For engaging the reverse gear the idle gear is pushed and connected to the other 2 gears. Thus the required output shaft rotation in the reverse direction can be achieved. Please note here that the reverse gear does not have a synchronizer ring mechanism. This means that, the gearbox rotation has to stop completely before applying the reverse gear.
You might have noticed that in reverse gear, your vehicle moves in a very low speed. As you can see from the figure the three gear arrangement gives speed reduction in 2 stages. This results in very low output speed (high torque). Generally the reverse has a gear ratio of 4:1 (input speed : output speed).