Chainsaw safety features
From Wikipedia, the free encyclopedia
Chainsaw manufacturers have invented numerous design features to improve safety. Some of these features have become de facto standards, and some are legal requirements in particular jurisdictions. Chainsaw safety courses teach that a user should inspect the saw before starting work and only operate the saw if all the safety features are properly functional.
Additional safety features are a significant commercial advantage to chainsaw producers. Companies continue to develop new features over time.
In addition to the safety features built in to the chainsaw, users should also wear specific chainsaw safety clothing.
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[edit] On/Off Switch
The on/off switch must be clearly marked with the stop position. There must be a positive click action, so that there is no chance that the switch will change position accidentally, even while the saw is vibrating under heavy load.
If the switch were to move to the off position while the operator was in the middle of a critical cut while felling a tree, there would at least be a delay, which increases the chance that the tree might fall in an uncontrolled manner. Also, the saw might jam in the cut, requiring that the operator spend extra time under the unsafe tree freeing it.
If the operator is injured while using the saw, a bystander might have to move in to turn the saw off. The bystander may be unfamiliar with the saw and needs to be able to identify the on/off switch by its markings.
[edit] Front Hand Guard/Manual or Inertia Chain Brake
While operating the saw, a right-handed user always keeps the left hand on the front handle. If the user accidentally loses this grip, the hand guard in front helps stop their hand moving forward onto the cutting chain.
The hand guard also operates the chain brake. When the hand guard is pulled back towards the user, the brake is switched off. When the hand guard is pushed forward, the chain brake is switched on. The chain brake is designed to stop the chain dead, very quickly, even when the saw's engine is running at full power. Once the chain brake has been switched on by pushing the hand guard forward, it stays on, and the user has to deliberately pull the guard back to switch the brake off.
Kickback is likely to occur if the upper quadrant of the tip of the chainsaw is used for cutting. If kickback occurs, the tip of the chainsaw cutter bar moves up and back towards the user in a way that is very difficult to control. The hand guard is designed to hit the user's left wrist at the front handle on kickback. This pushes the hand guard forward and operates the chain brake. In this way, even if kickback causes the chain to hit the user's arm and head, the chain should have already stopped moving.
Some chainsaws incorporate an internal mechanism that switches the chain brake on using an inertial sensor. The sensor detects the rapid rotation characteristic of kickback, and triggers the chain brake by pushing the hand guard forward itself. It is not necessary for the guard to hit the user's wrist.
[edit] Hand/Eye/Ear Defender Symbols
In the EU at least, it is a legal requirement that chainsaws carry certain standardized warning labels.
[edit] Safety Throttle
The engine throttle is operated by the trigger under the rear handle of the 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.
The safety throttle prevents the chain from being driven if the trigger if accidentally pushed by an obstruction, such as a branch in undergrowth.
The pictured model has an enlarge lock-out or 'dead man' switch which, when released, also activates the chain break, thus instantly halting the running chain as well as disengaging the throttle.
[edit] Chain Catcher
The chain catcher helps prevent the chain being thrown back towards the user, if the chain breaks or becomes derailed.
[edit] Rear Hand Guard
The rear hand guard, when used in conjunction with the chain catcher and appropriate gloves, prevents injury to the hand from a derailed or broken chain. It is designed to take the impact of the chain whipping back onto the underside of the saw when such an event occurs as the chain is being pulled back into the drive sprocket.
[edit] Anti-Vibration System
Excessive vibration over long periods can cause the user to develop hand-arm vibration syndrome (HAVS), or white finger. This is a potentially permanent and debilitating industrial injury. To reduce vibration, saws are divided into two parts. One part is a rigid assembly of the cutter bar and engine. This part vibrates strongly when the chain is cutting. The other part is a rigid assembly of the handles and controls of the machine, the part the user holds. These two rigid assemblies are joined together by mounts which provide spring suspension and damping.
The connection between the two parts of the chainsaw has to be stiff enough that the user can control the movement of the cutter bar precisely, but flexible enough that engine and cutter vibrations are significantly attenuated.
Both metal springs and rubber bushes can be used to provide suspension. Metal springs are more robust and longer wearing, but rubber bushes provide damping in addition to a spring action.
[edit] Exhaust
The exhaust directs the hot and noxious gases coming from the engine away from the user. A faulty exhaust increases noise, decreases engine power, can expose the user to unsafe levels of exhaust gases, and can increase the chance that the user could accidentally touch extremely hot metal.
[edit] Scabbard
The cutter chain is sharp enough to cause injury even when it is not being driven. The scabbard covers the chain when the saw is in storage or being transported.
[edit] Chain
The chain has to be properly matched to the guide bar and the saw. Chainsaw manufacturers specify a selection of suitable chains for each model of saw.
Best safety requires that the chain is properly sharpened. One key sharpening parameter is the depth gauge setting. The depth gauge is the small steel protuberance in front of each cutting tooth. 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.
Some chains also have guard links in front of each cutter link. The guard link provides extra limits on cutting depth in situations likely to cause kickback.
