Lithium electric chain saw provided with cam gear having logarithmic spiral for controlling brushed motor to brake

Provided is a lithium electric chain saw provided with a cam gear having a logarithmic spiral for controlling a brushed motor to brake which is innovated with the purpose of solving long-standing problems so that prevention of chain falling and safety brake are realized. When an accident happens, a front security baffle plate is enabled to pushed forwards spontaneously due to machine-human inertia and arms of an operator and therefore a real pushover curve (11-1) of the logarithmic spiral arranged on a brake shaft of a front baffle plate (11) takes effect. The real pushover curve has equal characteristics at all pressure angles. In addition, a curve brake head (11-7) of a control rod of a brake machine (11-6) and a brake cam provided with the logarithmic spiral, equal to pressure angles at all positions, are enabled to brake so that accident injuries are avoidable. An output shaft (26) is closely attached to the brake cam, in sleeve joint with a reinforcing packed spiral restoring spring, of a chain saw gearwheel and is additionally provided with a chain wheel spring (30) and an anti-escaping chain wheel (36). As for the additionally-arranged chain wheel spring (30), one end of a steel wire ring is ground flush and a steel wire head which is not ground smooth at the other end is in contact with and grabs the side surface of an anti-escaping chain wheel so that the steel wire head repels along an axial direction. Therefore, the anti-escaping chain wheel repels the chain wheel along an axial direction so that the effect of preventing from escape is achieved.