Pro Saw Sprocket Systems
There are two main types of drive sprockets used on chainsaws. One is the "spur" style (marked A in the image) and the other, the floating sprocket system (marked B).
The floating style is the best drive system for pro saws for several reasons:
With a floating sprocket system, the rim sprocket aligns perfectly with the groove in the bar. It self-aligns by "floating" on a splined hub.
Spur sprockets damage the chain's drive links when the chain derails. A floating sprocket system usually does not. On a floating sprocket system, a derailed chain usually drops into the space next to the rim sprocket. There, the shallow splines don't cause the kind of damage a spur sprocket will.
Floating sprocket systems don't cost much more than a spur sprocket. A significant economical advantage occurs when the sprocket wears or breaks. On a floating sprocket system, the rim sprocket is a separate part that is inexpensive and easily replaced. On saws with internal clutches, this is also very convenient. Sprockets are held on by an "E"clip and washer (marked C). This makes them easy to change in the field with minimal tools.
Sprocket Size & Gearing
The pitch of your sprocket, the pitch of your saw chain, and the pitch of your bar tip must all match. On most pro saws this will usually be: .325", 3/8", or .404". When replacing a sprocket, it is also important to know the amount of "teeth." For example, on a 3/8" pitch sprocket, both 3/8" X 7 and 3/8" X 8 sprockets are common.
The amount of teeth your sprocket has affects the "gearing" of your saw. Basically, smaller sprockets (seven tooth) give your saw the most cutting power. Larger sprockets (eight tooth) will produce the most chain speed.
Your chain's pitch also affects the diameter of its sprocket. A 3/8" X 8 sprocket is very close to the diameter of a .404" X 7 sprocket. This occurs because the 3/8" is a smaller pitch, so in spite of its extra "tooth," it is about the same size. If you are switching to 3/8" chain, from .404," use the 3/8" X 8 sprocket if you want about the same "gearing" as you had with the .404" X 7.
3/8" X 7 sprockets work well on small pro saws. These saws not only benefit from 3/8 chain (which is lighter and more flexible than 404), but the 3/8 X 7 sprocket is the smallest diameter of all standard seven spline sprockets. This gives these small pro saws low "gearing," which enables them to run a long bar with success.
For more information on sprocket size and gearing, see the section on Sprocket Tuning.
Sprocket Size & Chain "Throwing"
Most pro saw users know from experience that derailing or "throwing" a chain occurs more often when running large diameter sprockets. Most attribute this to the higher chain speeds these sprockets produce, but this is not the whole story. Large diameter sprockets also increase the distance a chain travels out of the bar on its heel. This unguided portion usually extends from the entry point on the heel of the bar to the contact points on the sprocket. The greater this distance, the more unstable the chain is.
If you are having trouble throwing chains, before you switch to a smaller sprocket, first check to see if the chain can be shortened. Some pro users wait until their chain adjuster is at its end before they remove a link from their saw chain. This is a bad practice. It is better to shorten the chain as soon as possible --always running the bar's heel as close to the sprocket as possible. This helps minimize derailures and helps extend the life of the bar heel.
A worn bar heel can also cause derailing problems. The reason is, as the bar wears, its heel becomes more narrow This also increases the distance the chain travels out of the bar rails. So, select bars that have a wider heel when running large sprockets. This usually means new bars will work better than bars with worn heels. Also, run chains as short as possible. This will increase the chain's stability.
When A Sprocket Should Be Replaced
A good general rule is: one sprocket will last for the complete life of two chains. If you are running an Oregon sprocket, another way of determining wear is to look at the wear bars. Oregon sprockets have small recesses cast into their surfaces. When the chain wears down to them, it is time to replace the sprocket. If the sprocket you have doesn't have wear bars, we recommend replacing it when it has 1/64" of wear on its surface.
From what we see in our shop, most sprockets are not replaced often enough. Too many pro saw operators apply the "If it ain't broke, don't fix it" rule instead. This is like running the tires on your truck until they won't hold air.
Sprockets need to be replaced because they get out of pitch. Sure they will eventually break if they are run long enough, but this usually doesn't occur until they are extremely worn. When a sprocket wears to a smaller diameter, it gets "out of pitch."Out of pitch conditions cause the chain to bind as it goes around the sprocket. This causes the chain to wear prematurely and affects its cutting performance. This usually occurs long before the sprocket is so worn that it breaks.