Chainsaw safety features

The difference in height between the leading cutting edge and the depth gauge determines the thickness of the wood chip taken by the cutter.

If the depth gauge is too low, the cutter takes too deep a bite from the wood, the saw becomes difficult to control and the chances of kickback increase.

The guard link reduces the tendency of the leading edge of the depth gauge to snag on small-diameter objects such as small branches, or to engage with ordinary timber if forced into contact with it.

They find a useful application in pole pruners and one-handed battery-powered saws that are used for trimming, hedge laying etc.

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.

In this situation the operator's head, neck and shoulders are out of the cutting plane of the saw and will generally not be struck even if a kickback does occur.

Chainsaw kickback can occur when the tip of the bar comes into contact with a relatively massive or immovable object with the chain under power.

In a fraction of a second the chain may jam hard into the wood and hurl the bar upwards towards the operator, often causing very severe injury or death.

The violence of a full kickback event is such that no evasive action is possible, and if the operator's head, neck or shoulders are in line with the plane of the bar, they will certainly be struck by it.

At this point, the operator must rely on the chain brake and protective clothing to save them from injury or worse.

Another form of kickback may occur where the top of the bar is used for cutting and becomes suddenly pinched by the wood moving.

By keeping their head and body out of the cutting plane of the bar, injury may be prevented if kickback occurs.

A useful tip is that if the operator can't read the logo on the side of the bar, they are too close to the cutting plane and should lean left to ensure safety.

Since these present a solid leading edge at the depth gauge should an object be suddenly forced into the chain, they can easily be made to engage fully with the foreign body and launch into a full kickback.

Careful filing of the depth gauges, making a radius down to the leading edge, reduces the risk somewhat.

Newer chains with ramped drive links fill in the gap ahead of the depth gauge, reducing the tendency to grab still further.

Full safety (bumpered) chains are often fitted as original equipment to domestic and entry level saws.

Professional bars designed for logging and felling may have much larger tips as they are often used for boring cuts to free trapped timber or fell difficult trees, and a tapered, small-tipped bar will wedge easily when boring, stalling the chain.

While kickback prevention is a great concern in the context of chainsaw use, it is not the only means by which an operator can be injured by a saw.

Unless the lock-out switch above the rear handle is also pressed, the throttle cannot move from the idle position, and the chain will not be driven.

[1] The safety throttle prevents the chain from being driven if the trigger is accidentally pushed by an obstruction, such as a branch in undergrowth.

It also prevents throttle activation when hot-starting a saw on the ground with one boot inside the rear handle.

The safety throttle is an additional layer of protection in this case, since the chain brake should be applied before starting a saw in any context.

In a gas-powered chainsaw, the on/off switch stops the engine running by preventing the ignition coil from firing.

Also, the saw might jam in the cut, requiring that the operator spend extra time under the unsafe tree freeing it.

A switch failing "on" would also present a hazard, since the saw may then start inappropriately, such as when testing compression or assessing starter function.

The Stihl model shown combines the choke, throttle start setting and ignition switch into one unified control lever.

[4] Many rear-handled chainsaws are used in a state, owing to poor maintenance, where there is some clutch drag and so the chain does move slowly at idle.

[9] Owing to the nature of work at height, and the difficulty of continually restarting the saw, the engine will often still be running when it is not used for cutting at that moment.

For this reason it is particularly important that the centrifugal clutch is operating correctly so that the chain doesn't rotate when the engine is idling.