Principle of Tractor Operation and Influencing Factors

Driving Principle
The ability to drive relies on the power of the internal combustion engine transmitted through the transmission system, enabling the drive wheels to obtain the driving torque Mk. The drive wheels with driving torque then exert a small, backward horizontal force (tangential force) on the ground through the tire tread and tire surface. The ground exerts an equal magnitude but opposite direction horizontal reaction force Pk on the driving force. This reaction force Pk is the driving force that propels the tractor forward (also called thrust). When the driving force Pk is sufficient to overcome the rolling resistance of the front and rear wheels and the traction resistance of the attached implements, the tractor moves forward. If the drive wheels are lifted off the ground, making the driving force Pk zero, the drive wheels can only spin in place, and the tractor cannot move; if the sum of rolling resistance and traction resistance exceeds the driving force Pk, the tractor also cannot move. Therefore, it can be seen that the movement of a wheeled tractor is achieved through the interaction between the driving torque-driven wheels and the ground, and the driving force must be greater than the sum of rolling resistance and traction resistance.
Influencing Factors
1. Rolling Resistance. The rolling resistance of a tractor mainly arises from the deformation of the tires and soil. Under the tractor's weight, the tires are compressed and the soil is compacted. During the rolling process, the tire is flattened and deformed at various contact points along the circumference with the ground, and the soil in front of the wheel is pressed down causing soil deformation and forming ruts, which generate rolling resistance that hinders the wheel's forward movement. Many factors affect rolling resistance, mainly related to the firmness and moisture of the ground and the magnitude of the vertical load. For the same tractor, rolling resistance varies with ground conditions; for example, rolling resistance is low on asphalt, cement, or hard dry ground, resulting in greater tractor traction. Under the same usage conditions, the greater the weight on the tires, the greater the vertical soil deformation and rolling resistance. Generally, reducing tire deformation and vertical soil deformation helps reduce rolling resistance. When operating on soft ground, using low-pressure tires to increase the tire contact area can reduce vertical soil deformation and rolling resistance, thereby improving traction. Since tractors are mainly used in fields and often operate on soft ground, low-pressure tires are generally used to reduce vertical soil deformation, and wider tires are also used for the same reason. In practice, attention should be paid to the differences in use between low-pressure tires, wide tires, and high-pressure tires.
2. Traction Resistance. Traction resistance is the resistance that the tractor must overcome to pull agricultural implements. It equals the traction force transmitted to the implements through the connecting device. Since traction force equals driving force minus rolling resistance, increasing driving force and reducing rolling resistance are effective measures to improve traction.
3. Driving Force. It is the horizontal reaction force exerted by the ground on the drive wheels. Therefore, the magnitude of the driving torque Mk transmitted from the internal combustion engine through the transmission system to the drive wheels indicates the size of the tractor's driving force Pk. However, since Mk is determined by the engine power, Pk is also limited by the engine power. At the same time, Pk is limited by soil conditions and cannot increase indefinitely because when the soil's reaction force, i.e., driving force Pk, reaches a certain level, the soil is damaged, the drive wheels slip severely, and the driving force Pk cannot increase further. The maximum reaction force the soil can exert on the drive wheels is called "adhesion." Therefore, the maximum value of driving force Pk is limited not only by engine power but also by soil adhesion and cannot increase indefinitely.
Adhesion reflects the ability to generate the maximum driving force between the drive wheels and the soil. Many factors affect adhesion, mainly related to ground conditions, tire pressure, size, tread pattern, and the magnitude of the vertical load on the tires. For tractors, under certain soil conditions, reducing tire pressure within a certain range, increasing the tire contact area, improving the wheel's grip on the soil, and increasing the wheel's adhesion weight all help improve tractor adhesion. Low-pressure tires are commonly used on tractors; some tractors also use wider tires, high-tread tires, and add ballast weights to the drive wheels to increase adhesion and improve traction. However, it should be noted that although adding ballast weights to the drive wheels increases adhesion, it also increases vertical soil deformation and rolling resistance. Therefore, whether to add ballast weights depends on specific usage conditions and requires weighing the overall effect.
The maximum adhesion capacity and slip resistance generated between the tractor's drive wheels and the ground are called the tractor's adhesion performance. If adhesion performance is good and slip is light, the driving torque can be fully utilized, and the engine's power can be fully exerted, making the tractor appear powerful during operation. If adhesion performance is poor and slip is severe, the driving torque cannot be fully utilized, and the engine's power cannot be fully exerted, making the tractor seem weak or lacking power during operation. Severe drive wheel slip reduces the tractor's travel speed, decreases productivity and economic efficiency, accelerates tire wear on the drive wheels, and also damages soil structure.