Chainsaw safety features
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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. Most older saws have few or none of these features, and extra care should be taken in their use.
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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] Hand-/Inertia-Actuated (Kickback-Safe) Chainsaw Chain Brake
While using a chainsaw, the saw operator (whether right-handed or left-handed) always grasps the saw's front handle with his/her left hand. If, while making a cut, a chain saw (chain guide bar) kickback occurs, instantaneous and "automatic" operation of the chain brake (i.e., clenching of the saw's chain drive sprocket binder) occurs in either of two ways to prevent or limit injury — injury otherwise inflicted by a powered, moving saw chain — to the saw operator: passively by operator impetus or reactively via spring-powered chain brake linkage. Involuntary, automatic (i.e., kickback-induced) — as opposed to voluntary, manual (i.e., "normal) — actuation of the chain brake is accomplished by means of special modifications to brake actuation linkage within the chain brake assembly enclosure; the chain brake is "normally" operated manually by pushing or pulling the chain brake handle hand grip (often called by the misnomer: "hand guard") in order, respectively, to (1) manually prevent saw chain movement (as while idling saw engine between cuts &or while carrying saw with uncertain footing) by applying the brake, or (2) to manually release (to un-brake) the saw chain for the purpose(s) of: (A) performing (chainsaw-engine-OFF) adjustments and maintenance to saw chain and saw; or (B) enabling the chain saw for performance of cutting, in addition to "arming" the brake's internal actuation linkage (its kickback-safe mode) for comparatively "safe" outcome in the event of kickback.
Chain brake (and kickback-safe-mode) function essentials — General
On chain saws equipped with "kickback-safe" chain brakes, either (or both) of two means of "auto-actuating" brake application in the event of kickback is provided. The original implementation of kickback-impelled saw chain braking — this feature is incorporated on all recent IC-engine-powered, kickback-safe chainsaws — provides for the back of chainsaw operator's left hand, when that hand is dislodged from saw's front-handle grip by a kickback, to strike and press forward against the saw's chain-brake-actuation handle grip which is situated (cocked in similar fashion as a pistol hammer's thumb grip/tang) just forward of the saw's front handle grip (as well as above and to rear of the forward-traveling portion of the saw chain). In effect, this "action" reproduces the normal, voluntary (non-passive) action taken by saw operator to inactivate — to seize — to "safe" — the saw chain. Because the "action" (the "kicking" of chain guide bar tip and saw chain towards saw operator's upper body and head) of a kickback happens so rapidly — faster than saw operator's ability to react — it will sometimes be the case that, both: the saw's front handle grip will rotate within operator's grasp; and saw operator's hand will pronate at the wrist...driving the wrist and forearm forward, ahead of the left-hand fingers, and into the brake handle grip. In this wise, it is possible for emergency brake actuation to occur without the left hand's being fully dislodged from and leaving the saw's front handle. In either event, the inherent limitation of (the chainsaw manufacturer's primary design constraint with) this mode of kickback-induced brake actuation is that the operator's left-hand purchase on the saw's front handle must be restricted to a location (typically, only at top of the saw) where it will invariably contact the chain brake handle hand grip in the event of kickback. The "work-around" for this limitation came with the advent of "inertially induced" chain braking on saw's of more recent manufacture.
All (especially IC-engine powered) chain saws manufactured in recent decades incorporate the above-described mode of chain brake auto-actuation as a safeguard against kick back injury. With the (more recent) incorporation of (supplemental) inertially-impelled chain braking capability on chainsaws, the incorporation of "wrap-around" chainsaw front handles also became feasible in that, it was no longer necessary to restrict operator's left hand placement so as to always contact the chain-brake-handle hand grip during a kickback event. (With a wrap-around front handle it is more readily possible to operate a chainsaw over a range of [saw chain guide bar] tilt angles...from vertical [in the case of firewood cutting or limbing], to oblique, to horizontal [in the case of tree felling]. It also enabled chainsaw operators, both as a convenience and as a safety consideration, to approach various cutting angle requirements without undue bodily physical contortion and with lessened risk of losing stance equilibrium.)
Generally speaking, the implementation of "inertial" chain braking (or, more precisely, of chain-drive sprocket braking) comes about during a sudden (accelerated)change of chain saw inertia: from static (relatively speaking...as between saw operator's hands and chainsaw's chain-guide-bar tip) to suddenly and rapidly "upward" rotational motion (...relative to operator's purchase on the saw's rear handle grip). During such events the chain brake handle, with handgrip situated at its "upper" extreme (after the fashion of a bob on a pendulum), being free to move (and having a free-play range within which to move) independently of saw or operator, works in conjunction with a "cocked" linkage mechanism and a powerful "brake" spring, both situated internally within the chain brake assembly, to bring about (in like manner as with back-hand-impelled or voluntary manual) chain braking by the clenching of a corralling binder strap onto the chain drive sprocket--as a result of which the chain is seized from further motion and the engine is killed. Because the chain brake handle is free to move during a kickback, it is also free to impart "triggering" action to the cocked, drive-sprocket binder's actuation linkage. This mechanical "action" within the chain brake assembly is not dissimilar — in fact is remarkably analogous — to the firing movement in hand pistols popularly said to have "hair" triggers: in both, a relatively slight pressure on trigger (on chain saw brake handle) or a sudden change of triggering mechanism inertia (as with kick back...or as with the dropping of a pistol) can be sufficient to trigger the intended action; in both, cocking is required (whether by pulling back the chainsaw's brake handle or the pistol's hammer); in both, a spring acts as prime mover to bring about the intended end result (whether that be the uncontrolled or controlled [as in, "uncocking"] movement of firing hammer towards pistol cartridge, or the uncontrolled or controlled [as in, manual brake application] clenching of brake binder onto chain drive sprocket). In contrast, the action of chainsaw chain brakes (albeit that intuition would seem to dictate otherwise) is anti-analogous respecting brakes used in cars: whereas drivers apply force to pedal or handle to apply a vehicle's wheel brakes, force applied to a chain saw's chain brake handle serves to release the saw's chain brake; and whereas the brake springs in a vehicle's wheel brake assembly serve to quickly retract brake shoes or discs and permit the wheel to turn, a chainsaw's brake spring serves to quickly apply the brake binder and prevent further saw chain drive sprocket rotation.
Chain brake (and kickback-safe-mode) function essentials — Detailed
[Update issue & publication pending]
brake springs (and brake movements) 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.
[edit] Chain saw kickback explained—
Dangerous kickback is likely to occurs when, if during a cutting operation (or otherwise by incidental contact), the revolving chain at the tip — in particular, the upper, forward quadrant of the (imaginary) circle which circumscribes the tip — of the chainsaw's chain guide bar makes contact with a substantially immovable object (such as a tree or firewood log) and is propelled rapidly and forcibly along an arc which intersects the saw operator's upper body and head. The phenomenon is best understood by first considering the cutting function of the saw chain during "normal" (linear) cutting along the upper or lower straight edge of the saw chain guide bar; and then evaluating the change of dynamics that occurs when the saw chain "attempts to cut" as it passes upward and around the chain-guide bar's tip.
As each cutting link of a powered, rapidly moving saw chain, in turn, contacts the object to be severed, it both "stabs" into and chisels away a small amount of material, after which it propels the removed material in the direction of saw chain movement at the point of contact (rearward for downward cuts, forward for upward cuts); the effect of rapid material removal in order to effectuate a rapid overall cut is accomplished by the very high speed at which each successive, trailing cutting link arrives at, bites into, removes, and ejects additional material. During this process, and depending on sharpness of the cutting links' cutting edges, there will be a tendency for the saw itself to be pulled forward (if cutting upwardly with top edge of chain guide bar...pushed backward), as if being tugged from operator's grasp — the force of such tugging being proportional to the degree to which a saw chain's cutting links are able to "bite" into (i.e., to grab), but are less able to sever and eject material from, the object being cut: a well-sharpened saw chain tending less — a dulled saw chain tending more — to need restraining from chain-induced (linear) motion by the operator's grasp on the chain saw handles, as well as by his/her much greater mass (weight) relative to that of the saw. (Hence the adage...that "a dull saw chain is a dangerous saw chain"; an adage having even greater criticality in the context of a kickback event. Likewise hence, that a well-maintained saw chain with very sharp cutters can, itself, be considered to be a built-in safety feature of any chain saw.) So with normal cutting, and in the sense that the saw would otherwise "tractor" across the cutting object but not cut the object, it could be said that the saw operator's grasp (and weight) are an essential, and positive component of the saw's success at cutting. But, in the event of contact between an object and the "kickback" zone (as described above) on a running saw's chain-guide bar, the same, positive aspect suddenly "turns against" the operator, forcing his/her grasp on the saw to become an "essential dynamic" (no longer to control the saw but, rather) in the ensuing kickback event — saw operator becomes, in effect, a "participant" in the kickback event.
The occurrence of a kickback event consists in two basic, simultaneously-acting sets of dynamics: (1) those imparted by the chain saw and (2) those imparted by the saw's operator.
(1) As a running, cutting saw chain makes contact with an object within the chain-guide bar's kickback zone, its "effort" to cut (its functionality in cutting) the object will be no different than as described above for "normal" cutting along the bar's upper or lower, straight edge. However, since the saw chain (along with saw chain cutting links), is continuously changing direction as it rounds the saw chain guide bar's tip, the (yet continuing, tractoring) tendency of the saw chain to impart motion to chain guide bar (and, thence, to chain saw) likewise changes, forcing the bar to move not only forward and then upward (as the chain "tractors" through the kickback zone), but also — and precipitously — backward as the immobile contacted object (figuratively speaking) approaches the chain guide bar's extreme tip; the moving saw chain must also accelerate as a function of angular motion about the circular bar tip relative to its linear motion along the bar's top or bottom edge (a factor that ultimately serves to multiply the velocity and force of any kickback reaction). In addition to these, an additional dynamic will, to greater or lesser degree, be at play as the tractored object (again, figuratively speaking) "attempts to move" about the circular bar tip, namely: to the degree to which the cutting links bite into and become (themselves) objects of embedment within the contacted object, and are unable to simultaneously cut themselves free of the contacted object (such as with a dulled chain), to that same degree there will be a tendency for the saw chain to "fling away" the object due to centrifugal force.
These "saw dynamics," in and of themselves, would, in all likelihood, be of little or no critical significance (i.e., the saw's chain guide bar tip and the contacted object would merely be flung apart from each other), but for the presence of two exacerbating factors: the resistance to being moved of the contacted object (owing either to its being a fixed object [such as a tree] or its being a "strongly inert" object [such as a heavy or constrained firewood log]; and...
(2) ...the simultaneous presence of the chainsaw's operator whose grasp on the saw's handle, where it had been serving to stabilize the saw, now suddenly serves instead to prevent the saw's being forced (and expelled) backward and away from the object...prevented, that is, until the kickback response has already commenced — after which point any effective operator control of the saw has already been effectively lost. In addition to being a (necessary) factor in precipitating a kickback, the operator's grasp of the saw plays an even more insidious role in "ensuring" that injury to him/her self will be a likely ultimate outcome! Because of the operator's grasp on the chainsaw's rear handle, and because the operator's grasp on that handle is likely to be tightened reactively as the impending kickback is first sensed, the saw's rear handle (in combination in some degree with the saw's front handle) will effectively become a virtually fixed point of rotation, at roughly arm's length from the operator's head, about which the saw, and in particular the saw's chain guide bar and chain, will be forced to travel as it is flung clear from the point of contact with the kickback-inducing object. And, the degree to which the operator "tenses" and increases grasping pressure on the saw as result of normal, involuntary reflex, such will be the degree to which it becomes likely that the saw's chain guide bar, and saw chain, will "find" the operator's body at the end of its kickback path. As against any prospect that the operator might be able to impart control using the chainsaw's front handle, yet another, physio-motory, dynamic militates against the possibility. When, as was mentioned earlier, the force imparted by the kickback causes pronation (the flexing towards the elbow) of operator's left hand at the wrist joint, that hand's ability to regain effective control of the saw (before the kickback event has run its course) will have been greatly diminished or eliminated.
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 is 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 brake, 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 protect the users right hand if the chain was to snap
[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.
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.
