US Patent 7316299 Brake device for chain saw

A brake device for a chain saw is provided, wherein a driving shaft, which passes through a brake block, a torsion spring, a driving block and a chain wheel, is assembled with a driven gear, and two ends of the torsion spring are respectively fixed to the brake block and the driving block. The driven gear is loosely fit to the brake block, and the chain wheel is fixed to the driving shaft. When the driven gear is driven to rotate, the brake block, the driving shaft and the chain wheel are brought to rotate synchronously. When a force is applied to the brake block via a brake pin, the torsion spring will deform and separate from the driving block so that the driving block fails to transmit a driving force to the brake block, and thereby the braking effect is achieved.