The balance shaft 1 is generally installed at the front of the engine or behind the crankshaft at the rear of the engine. The balance shaft of the old motorcycle engine in China is relatively easy to identify, and there will be an obvious protrusion of 1 in front of the engine, while some new engine protrusions of 1 are actually not obvious or basically invisible, so it is easier to identify familiar engine models.
When the engine is in working condition, the movement speed of the piston is fast and uneven. When the piston is at the top and bottom dead center, its speed is zero, but it reaches the highest when it is in the middle of the top and bottom dead center. Because the piston makes repeated high-speed linear motion in the cylinder, it will inevitably produce greater inertia force on the piston, piston pin and connecting rod.
Although the counterweight on the connecting rod can effectively balance these inertial forces, only a part of the moving mass participates in linear motion and the other part participates in rotation. Therefore, except for the position of top and bottom dead center, other inertial forces can't completely reach the equilibrium state, and the engine will vibrate at this time.
In order to eliminate this vibration, designers have adopted many methods, such as using light pistons to reduce the mass of moving parts, improving the rigidity of crankshaft, and adopting the "V" arrangement of engines with an included angle of 60 degrees.
Adding a balance axis is also one of these methods. Simply put, the balance shaft is actually a shaft with eccentric weight, which rotates synchronously with the crankshaft. Using the reverse vibration force generated by eccentric weight, the engine can obtain good balance effect and reduce engine vibration.
Extended data
Balance shaft can be divided into single balance shaft and double balance shaft. As the name implies, the single balance shaft adopts a single balance shaft and works through gear transmission. The crankshaft rotates to drive the fixed balance shaft driving gear, balance shaft driven gear and balance shaft. The single balance shaft can balance the first-order vibration which accounts for a considerable proportion of the whole vibration, and the vibration of the engine is obviously improved.
Single balance shaft is widely used in single cylinder small displacement engine because of its simple structure and small space occupation. The double balance shafts use chain drive to drive two balance shafts to rotate, and one of them has the same speed as the engine, which can eliminate the first-order vibration of the engine.
The rotation speed of the other balance shaft is twice that of the engine, which can eliminate the second-order vibration of the engine, thus achieving a more ideal shock absorption effect. Due to the complex structure, high cost and relatively large space occupied by the engine, large-displacement vehicles generally adopt double balance shafts.
In addition, there is a double balance shaft arrangement mode, that is, two balance shafts are symmetrically arranged at an angle with the center line of the cylinder, with opposite rotation directions and the same rotation speed as the crankshaft, so as to balance the first-order reciprocating inertia force of the engine.
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