Chainsaw Safety Features - Chain Brake

Chain Brake

Chain brakes prevent movement of the saw's cutting chain by applying a steel brake band around the driven clutch drum. Clamping force for the brake band is provided by a powerful spring. The chain brake has two purposes. First, it can be used to secure the chain when changing position, moving between cuts or starting a cold saw, which requires a partly open throttle. This would otherwise lead to uncontrolled chain movement, a major hazard in older saws. Secondly, the chainbrake can activate under kickback conditions to prevent the operator from being struck by a running chain. Of course being struck by even a static chain may cause serious injury, but anything that can be done to mitigate the usually dreadful injuries caused by contact with a moving chain is of obvious benefit to operators. Kickback injuries usually occur to the head, face, neck and shoulders; when a running chain is involved, such injuries are usually very serious, often disfiguring and sometimes fatal.

The chain brake is principally operated by the top-hand guard being pushed forward to engage the brake, and pulled back to disengage. The spring-loaded action allows powerful braking under emergency conditions and can halt a chain under full power in a fraction of a second. Correspondingly, it may require considerable force to reset.

The chain brake may be activated deliberately by the operator, or automatically by the force of a kickback event. In the former, the operator usually rotates his left wrist and knocks the top hand guard forward with the back of his hand, re-setting it by reaching forward with his fingers to pull the top hand guard backwards. In the case of a kickback event the operator's left hand may be violently dislodged from the handle and the top hand guard will be thrown onto his hand, forcing activation of the chainbrake. Husqvarna models also incorporate a link between the top handle and the chain brake trigger, applying the brake if the saw's bar is forced suddenly upwards. This is known as an "inertia" chainbrake and will allow activation of the chainbrake even if the operator's left hand is not removed from the handle.

The chain brake may also be of use when sharpening a chain on a bar, as it allows robust filing to take place without the chain slipping about.

"Wrap-around" top-hand guards have made an appearance in recent years, but are unpopular. They restrict movement, and make no allowance for the fact that when the handle is gripped on the lower section of the handle (left side of the saw from the operator's viewpoint) it is usually for making lateral, right-to-left cuts. In this situation the operator's head, neck and shoulders are out of the cutting plane of the saw and will not be struck even if a kickback does occur.