End-to-End understanding of Tractor Transmission System
Table of Contents
What is a Transmission System and What are Its Components?
Transmission system is basically the drive line of a tractor which consists of components that are used to transmit the torque generated by the engine to the driving wheels or other moving parts of the tractor. It is responsible for transmitting the power for moving the tractor and agricultural implements. Some of the important functions that the transmission system performs are as follows:
- To disconnect the engine from the road wheels whenever required.
- To transfer the torque smoothly without any jerks.
- To desirably adjust the engine speed to deliver the required forward speed.
- To switch the engine torque and speed according to the required torque and speed for a task.
- To shift the axis of rotation of power to line it as per the adjustment of the drive wheels.
These functions are performed with the help of its components. The transmission system consists of clutch, gearbox, PTO drive, brakes, differential, and axle. Let’s look into each of its components in detail.
Clutch
Clutch is a component of the transmission system which is required to transfer the power of the engine to the gearbox and then to the drive wheels particularly at the time of gear selection and starting or moving the tractor from the rest position. Some of the clutch types are explained below.
Dry Clutch
A dry clutch functions without any lubrication and due to this there is less concern of it wearing out when exposed to air. Its drive shaft and clutch plate help in minimizing the power loss. However, due to no lubrication and friction its moving parts produce loud noises. Dry clutch is quite reliable and inexpensive but it often burns through more gasoline which has the potential to damage synchronization systems like chains or crankshaft linkages. Dry clutch requires frequent maintenance and has a shorter lifespan. In India, majority of the tractors come with dry clutch.
Wet Clutch
The wet clutch is covered or lubricated with engine oil for ensuring smooth operations. It also comes with a small reserve to provide extra lubrication for preventing overheating during each rotation. Wet clutches cost is on the higher side but it comes with great benefits such as it delivers higher torque, need lesser maintenance and has a longer lifespan.
Diaphragm Type Clutch
Diaphragm style clutch uses a diaphragm or conical spring to produce required pressure for engaging the clutch. It is easy to depress and is able to deliver quite large hold power. Also, as the diaphragm clutch wears its clamping power increases which enhances the clutch performance. Its pedal effort is also quite less.
Split Torque Clutch
Split torque clutch is a popular choice for higher HP tractors because it distributes torque across multiple paths. It's designed to efficiently distribute power which helps in reducing stress on individual components. They offer better performance and longevity. However, they are quite complex and high maintenance compared to other types of clutches.
Now that we have learned about the technology of clutches, let’s discuss the types of clutches based on operations below.
Single Clutch
It comes with only one clutch plate and clutch pedal through which you can control the gearbox and PTO shaft of the tractor. However, you can’t engage or disengage the PTO and gearbox separately in a single clutch which makes it less flexible. Many tractors under 50 HP are seen featuring a single clutch.
Dual Clutch
It comes with two clutch plates controlled with a clutch pedal. You can control both the gearbox and PTO shaft of the tractor in the dual clutch. You can disengage the tractor by pushing the half clutch and disengage the PTO by pushing the full clutch. It is more flexible than the single clutch as it provides two clutch plates to control the PTO and gearbox independently.
Double Clutch
Double clutch is similar to dual clutch as it also features two clutch plates. However, it comes with two separate clutches for controlling the PTO and gearbox independently. This allows more efficient and smoother transmission.
Gearbox
The requirement of torque keeps continuously varying according to the different agricultural operations. Hence, there is a constant need for changing the set of gears involved in the transmission of the engine power, torque, and speed to the wheels. A series of gears are combined together in an orderly manner in the gearbox to alter the torque being supplied to the wheels. The gearboxes are classified on the basis of the method in which the gears are meshed and speed ratios are selected. The transmission can be manual as well as automatic. However, In India, manual transmission is followed, so keeping that in mind let’s look at the types of tractor gearboxes below.
Sliding Mesh Gearbox
This oldest and simplest design features gears. However, this method requires higher efforts compared to constant or synchromesh gearbox. In order to shift the gears, the operator needs to ensure the speed & gear change is done in complete alignment otherwise you will hear a strong grinding noise. Also, carefully gear changes at low speeds, avoid any damage to the gears. This gearbox is simple in design, suitable for high torque applications, has low manufacturing costs and quite robust. Some of the disadvantages of sliding mesh gearbox are that it is harder to operate, noisy and can be difficult to shift smoothly.
Constant Mesh Gearbox
In a constant mesh gearbox, the gears are always engaged, when an operator shifts the gear, only the collar disengages and then gets engaged to the desired gear. The operator is required to select the desired pair and synchronize their speeds with the clutch for shifting gears. This offers quite smoother transmission when compared to the sliding mesh gearbox. A majority of tractors up to 50 HP today come with constant mesh gearbox, a much suitable gearbox for Indian farming. With time the combination of constant mesh and sliding mesh gearbox was also introduced and is known as partial constant mesh gearbox. This offered balance between cost, complexity, and performance.
Synchromesh Gearbox
This gearbox is an advancement to the constant mesh gearbox which comes with a synchronizing unit to assist in gear shifting. It uses the synchronizer rings to match the speeds of input and output shafts before engagement. Owing to this, the operator experiences a smooth gear shifting experience which is also less prone to wear and tear. However, its price lies on the higher end and is also high maintenance. Due to this partial synchromesh gearbox was introduced which combines synchromesh technology with constant mesh gears.
PTO
The power outlet provided at the rear of the tractors is known as PTO and this is used for operating PTO driven implements like rotavator, power harrow, baler, etc. Tractors are not only used for pulling the implements for tillage and sowing operations but also for power stationary operations like crop threshing, pumping water from tube wells which are performed by PTO. Also, the PTO driven implements need a certain HP to perform which is known as PTO HP. The PTO HP is basically the amount of engine power available to run various implements. It is always less than the tractor HP due to the friction losses in between. Furthermore, the PTO shaft gets the power from the gearbox.
Read More: Different Types of PTO’s
There are two standard PTO speeds i.e. 540 rpm and 1000 rpm. The standardization of the PTO speed was introduced so that the designing of implements can be done around the given speed. When it comes to splines, the shaft has 6 splines in case of 540 rpm and the shaft has 21 splines in case of 1000 rpm.
Brakes
Braking system is highly important in a transmission system of the tractor as it is responsible for slowing or stopping the moving tractor. Brakes are basically that changes the kinetic energy of the moving tractor into heat by using friction between a rotating and stationary component. Let’s discuss the commonly used braking systems in tractors.
Mechanical Brakes
Mechanical brakes work by using levers or linkages. The operator pushes the brake pedal and it engages the linkages to push the brake shoes against the brake drum for creating friction to slow or stop the tractor. Internal expanding shoe type, external contracting shoe type, and disc type are the three types of mechanical brakes. However, except for the disc type brakes, other two have become outdated technologies. Mechanical brakes are quite affordable and simple but they require frequent maintenance and adjustment.
Hydraulic Brakes
The hydraulic brake system is based on Pascal's law principle. It uses fluid pressure to operate. The brake fluid is basically a mixture of alcohol and glycerin which is filled in the master cylinder. The hydraulic fluid is forced into the cylinder at each wheel when the brake pedal is pushed. It then pushes the brake pads against the discs or drums. The hydraulic brakes are quite reliable and need less manual adjustment than mechanical brakes. They are also better at heat dissipation. However, they are complex, expensive, and high maintenance.
Differential
A different unit is a special arrangement of gears that allows the tractor's one rear wheel to rotate slower or faster than the other rear wheel. It usually works when the tractor takes a turn on a curved path. The inner wheel has to travel less distance and thus needs less power than the outer wheel. In this case, the differential unit allows one rear wheel to move faster than the other rear wheel at the turning point. You can learn in detail about differential by reading our blog Differential Lock in a Tractor: Its Working and How to Use it.
Axle
Axle is a shaft with connecting parts that transmits torque from the differential to the wheels. It also helps in supporting the weight of the tractor as well as the load carried by the tractor. The axle is classified as the front and rear axle.
Front Axle
The front axle is located in the front of the tractor and is responsible for assisting the tractor’s steering along with processing shocks on the uneven surface. There are four types of front axle.
Fixed Front Axle
This axle as the name suggests is rigid. The both front wheels are connected to a single structure which does not allow independent movement of any of the connected parts. They all move together as a unit. It is based on a simple design, is low maintenance and less expensive than other axles.
Adjustable Front Axle
This type of axle provides the flexibility of adjusting the width between the front wheels of the tractor. The operator can change the width according to its farming needs.
Bow Type Front Axle
Bow type front axle is designed in a curved shape which looks like a bow. This design provides more stability and better turning radius to the tractor and is suitable for farming operations like puddling.
Heavy Duty Front Axle
This type of front axle comes with a robust look. It can easily withstand heavy loads and is best suited for implements like dozer and loader applications.
Rear Axle
This component of the transmission system transfers power from the differential to the rear wheels of the tractor.
Let’s read about the four types of rear axle.
Straight Axle
A straight axle is also known as a solid axle or live axle. It is a common type of axle used in many tractors. It offers simplicity, durability, load distribution, and excellent traction, making it suitable for heavy-duty agricultural tasks and off-road operations.
Inboard Reduction
It consists of a set of gears and shafts that transmit power from the tractor's transmission to the rear wheels. The gears are called bull gears, which mesh with small pinions attached to the axle shafts. The bull gear absorbs the half of the shock and load coming to the crown pinion.
Hub Reduction
The hub reduction is quite similar to the inboard reduction. However, the gear reduction takes place at the hub of the wheel rather than along the length of the axle in this type of axle.
Epicyclic Reduction
Epicyclic reduction, also known as planetary reduction, is one of the strongest reductions in the industry. It comes with a set of planetary gears to transfer power from the transmission to the wheels. These gears are compact and provide a higher gear reduction. It results in increased torque and traction and is suitable for heavy implements and commercial operations.
You can learn in detail about all types and functions of rear axles in our blog Final Drive in a Tractor: Components, Types and Their Functions.
We hope with this blog you would have understood the transmission system and its components better. If you are heading to buy your next tractor, then this blog will prepare you thoroughly to make the right choice. Also, many brands offer options to choose between from different clutches and gearboxes according to your farming requirements & budget. We have also further detailed many other technical understandings on final drive, differential lock etc. We recommend you to go through these blogs as well to understand the working of tractor end-to-end. Finally, for any kind of further technical assistance you can reach out to our technical experts.
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