In a gear box, the counter shaft is mashed to the clutch with a use of a couple of gear. So, the counter shaft is always in running condition. When the counter shaft is bringing in contact with the main shaft by use of meshing gears, the main shaft starts to rotate according to the gear ratio. When want to change the gear ratio, simply press the clutch pedal which disconnect the counter shaft with engine and change connect the main shaft with counter shaft by another gear ratio by use of gearshift lever. In a gear box, the gear teeth and other moving metal must not touch. They must be continuously separated by a thin film of lubricant. This prevents excessive wear and early failure. Therefor a gearbox runs partially filled with lubricant oil.