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Chainsaw Construction

- May 14, 2018 -

A chainsaw consists of several parts:

Engine

Chainsaw engines are traditionally either a two-stroke gasoline (petrol) internal combustion engine (usually with a cylinder volume of 30 to 120 cm3) or an electric motor driven by a battery or electric power cord. Combustion engines today (2016) are supplied through a traditional carburetor or an electronically adjustable carburetor.

The traditional carburetor needs to be adjusted, i. e. when operating in high or low altitudes, or their fuel oil-to-gasoline ratios must be adjusted to run properly. Electrically influenced carburetors make all adjustments automatically. These systems are provided by most large chain saw producers. Husqvarna calls its "Autotune," and it is commonly standard on most saws of the 5XX saw series.

To reduce user fatigue problems, traditional carburetors can be de-vibrated (protected from vibrations) or they can be heated as well. Many saws offer a Winter and Summer mode of operation. Winter mode applies in temperatures below 0 °C / 32 °F where inside the cover a hole is opened leaving warm air to the air filter and carburetor to prevent icing. In warmer environment the hole is closed and both units are not ventilated with warm air.

To ensure clean air supply to the carburetor, chainsaw producers offer different filters with fine or less fine mesh. In clean surrounding air a less fine filter can be used, in dusty environment the other. The fine filter keeps the air clean to its optimum (i.e. 44 µm) but has the tendency to clog. This leads to the engine dying.

The engines are designed so that they may be operated in different positions, upside-down or tilted 90 degrees. Early engines died when tilting (two man saw from Dolmar, Germany from 1930 to 1937).

Drive mechanism

Typically a centrifugal clutch and sprocket. The centrifugal clutch expands with raising spinning speed towards a drum. On this drum sits either a fixed sprocket or an exchangeable one. The clutch has three jobs to do: When the saw runs idle (typically 2500-2700 rpm) the chain does not move. When the clutch is engaged and the chain stops in the wood or another reason, it protects the engine. Most important it protects the operator in case of a kickback. Here the chain brake stops the drum and the clutch releases immediately.

Clutches and drums can be in two positions: either turned outside (Husqvarna) or inside (Stihl).

Guide bar

An elongated bar with a round end of wear-resistant alloy steel typically 40 to 90 cm (16 to 36 in) in length. An edge slot guides the cutting chain. Specialized loop-style bars, called bow bars, were also used at one time for bucking logs and clearing brush, although they are now rarely encountered due to increased hazards of operation.

All guide bars have some elements for operation:

Gauge

The lower part of the chain runs in the gauge. Here the lubrication oil is pulled by the chain to the nose. This is a very important mechanism.

Oil holes

At the end of the saw power head there are two oil holes, one on each side. These holes must match with the outlet of the oil pump. The pump pumps the oil through the hole in the lower part of the gauge. (See also below)

Saw bar producers provide a large variety of bars matching different saws.

Grease holes at bar nose

Through this hole grease is pumped, typically each tank filling to keep the nose sprocket well lubricated.

Guide slot

Here one or two bolts from the saw run through. The clutch cover is put on top of the bar and it is secured though this/these bolts. It depends on the size of the saw if one or two bolts are installed.

Bar types

There are different bar types available:

Laminated bars

These bars consist of different layers to reduce the weight of the bar.

Solid bars

These bars are solid steel bars intended for professional use. They have commonly an exchangeable nose since the sprocket at the bar nose wears out faster than the bar.

Safety bars

These bars are laminated bars with a small sprocket at the nose. The small nose reduces the kickback effect. Such bars are used on consumer saws.

Cutting chain

Main article: Saw chain

Usually each segment in this chain (which is constructed from riveted metal sections similar to a bicycle chain, but without rollers) features small sharp cutting teeth. Each tooth takes the form of a folded tab of chromium-plated steel with a sharp angular or curved corner and two beveled cutting edges, one on the top plate and one on the side plate. Left-handed and right-handed teeth are alternated in the chain. Chains come in varying pitch and gauge; the pitch of a chain is defined as half of the length spanned by any three consecutive rivets (e.g., 8 mm, 0.325 inch), while the gauge is the thickness of the drive link where it fits into the guide bar (e.g., 1.5 mm, 0.05 inch). Conventional "full complement" chain has one tooth for every two drive links. "Full skip" chain has one tooth for every three drive links. Built into each tooth is a depth gauge or "raker" which rides ahead of the tooth and limits the depth of cut, typically to around 0.5 mm (0.025"). Depth gauges are critical to safe chain operation. If left too high they will cause very slow cutting, if filed too low the chain will become more prone to kick back. Low depth gauges will also cause the saw to vibrate excessively. Vibration is not only uncomfortable for the operator but is also detrimental to the saw.

Tensioning mechanism

Main article: Tensioner

Some way to adjust the tension in the cutting chain so that it neither binds on nor comes loose from the guide bar. The tensioner is either operated by turning a screw or a manual wheel. The tensioner is either in a lateral position underneath the exhaust or integrated in the clutch cover.

The lateral tensioner has the advantage that the clutch cover is easier to mount but the disadvantage that it is more difficult to reach nearby the bar. Tensioners through the clutch cover are easier to operate, but the clutch cover is more difficult to attach.

When turning the screw, a hook in a bar hole moves the bar either out (tensioning) or in, making the chain loose. Tension is right when it can be moved easily by hand and not hanging loose from the bar. When tensioning, hold the bar nose up and pull the bar nuts tight. Otherwise the chain might derail.

The underside of each link features a small metal finger called a "drive link" (also DL) which locates the chain on the bar, helps to carry lubricating oil around the bar, and engages with the engine's drive sprocket inside the body of the saw. The engine drives the chain around the track by a centrifugal clutch, engaging the chain as engine speed increases under power, but allowing it to stop as the engine speed slows to idle speed.

Dramatic improvements, chainsaw safety devices and overall design have taken place since the chainsaw's invention, saving many lives and preventing countless serious injuries. These include chainbrake systems, better chain design and anti-vibration systems.

As chainsaw carving has become more popular, chainsaw manufacturers are making special short, narrow-tipped bars for carving. These are called "quarter tipped," "nickel tipped" or "dime tipped" bars, based on the size of the round tip. Chainsaw manufacturer Echo sponsors a carving series,as well as carvers such as former Runaways singer Cherie Currie. Some chainsaws such as the RedMax G3200 CV are built specifically for carving applications.

 

Safety features

A chainsaw used to trim the 2016 Capitol Christmas tree

Main article: Chainsaw safety features

Today's chainsaws show all a number of safety features to protect the operator. All these features are not a 100% guarantee that the operator will not be harmed. The best protection, even still, is experience.

Chain brake

The chain brake is located in the clutch cover. Here a band tensions around the Clutch drum stopping the chain within milliseconds. The chain brake is released by the upper handle with the hand or wrist. The brake is intended to be used in kick-back moments.

Chain catcher

The chain catcher is located between the saw body and the clutch cover. In most cases it looks like a hook made in aluminum. It is used to stop the chain when it derails from the bar and shortens the length of the chain. When derailing the chain swings from underneath the saw towards the operator. The shorting prevents hitting the operator, but it hits the rear handle guard.

Rear handle guard

The rear handle guard protects the hand of the operator when the chain derails.

Chain

Some chains show safety features as safety links as on micro chisel saws. These links keep the saw close to the gap between two cutting links and lift the chain when the space at the safety link is full with saw chips. This lifts the chain and lets it cut slower.


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