Introduction Clutches are widely used in various vehicle powertrains system. Unlike cars, tractors use a dual clutch system for smooth operation of the external power takes off shaft. The dual clutch system allows the operator, while working with an implement, to change gears without affecting on the operation of implement. When the pedal of clutch is fully pushing, the drive was be interrupted in transmission system and PTO shaft. Because the torque of the tractor’s engine is more than the car’s engine, the extra force is be need to hold the tractor’s clutch.
The nature of tasks on a tractor, necessitates a number of actions to be performed by the operator, which puts varying physiological demands on his body. Examples of these tasks are: steering of the tractor, looking backward to observe and control the machine/implement, and force required to operate the clutch, brake, and hydraulic control system.
This tasks and workplace, was determines the postures and load distribution on the body structures of the operator.
Pedals are one of the most important controllers used in vehicles. Dupuis (1995), reported that a driver pushes the pedal of clutch 230 times per hour on average. Lehmann (1958), pointed out, that the force is exerted on the pedal generally is far greater than the required force. If the force exerted on the pedal is high, the driver cannot detect the moment that the clutch is actually released. Forasmuch as, the clutch is operated by a leg force, and it is mounted on the left side of the tractor driver. Then it’s more important to use a mechanism to amplify the force that applied on the clutch by the driver. use a booster is an appropriate way to amplify the force.
 
Materials and Methods First of all, before installing the booster, the force required for pushing the clutch was calculated. Keeping in mind that no change should occur on the tractor body and pedal position. Then, the booster was installed between the clutch pedal and clutch plug lever. Since the force from clutch to clutch plug lever is tensional, the booster was initially in compression, the direction of the force attached to the clutch was changed using a lever. The direction of resulting force from booster was changed using another lever that to be applied to the clutch plug lever.
 
 In order to, providing vacuum for booster, the booster’s vacuum chamber was connected to the engine’s input manifold by a plastic tube. Forty experiments were conducted to calculate the requirement force for pushing the clutch. These experiments were carried out for 5 engine speeds (800,1100, 1400, 1700 and 200 rpm), and 8 inlet diameters of manifold (15, 20, 25, 30, 35, 40, 45 and 50 mm). Experiment was repeated 3 times. Finally, the mean results from three experiments was considered as the true value for each experiment. A load cell (ZEMIC (H3)) was used to calculate the force required for pushing the clutch and data of each experiment was saved in computer distinctively. For measuring the suction in connecting tube that placed between the booster and air manifold, the water column upwelling method was used.
 
Results and Discussion Results of analysis of variance shows that the interaction effect of inlet diameter × engine speed and force, as well as, interaction effect of inlet diameter × engine speed and suction was significant (P <1%). This means that by increasing the engine speed and decreasing the inlet diameter of manifold, more air is sucked in, and therefore, the pressure inside the manifold was be reduce. by decreasing the pressure in the manifold, the difference pressure on both sides of the booster diaphragm was increased, and cause increasing the auxiliary force of booster, thereby, the force required to press the pedal was reduced. Maximum reduction on the force required to push the pedal by using this system is about 170 N, that occur in 15 mm inlet manifold diameter and 2000 rpm engine speed.
 Conclusion In this paper, a brake booster was installed on the MF285 clutch to reduce the force required to push the clutch pedal. It was observed that this force was about 33.6 kg in the normal state while by using of this system, the required force was reduced to 26 kg.