Now that we have mentioned the different types of sprocket (rim & spline, and spur), and also seen how to remove or replace the sprockets, it's time to take a closer look at how they wear. Just how do you decide when to replace them? More after the jump.
Drive sprockets have a hard life - they're constantly pushing the chain around the bar, they have the centrifugal clutch weights working on them and they have the chain brake band clamping the outside edge of the sprocket. All in all, they get it tough and it's no surprise that they don't last forever - in fact, you should be looking to replace your drive sprocket every 2 to 3 chains.
There are other ways of telling when the sprocket should be replaced. The teeth will wear down as they transfer drive to the chain by pushing the drive links along, the result is a groove that runs down the face of the tooth and wear on the top edge of the tooth caused by the tie-straps - when this wear reaches 0.5mm it's time for the sprocket to go (as shown in the image to the right).
The action of the clutch weights spinning out and contacting the inside of the sprocket will wear a grove in to metal, and it's not long before a slight lip appears on the inside. This is not normally a problem, but I have found that on Stihl saws, if you don't take the sprocket off for cleaning every now and then, the build of gunk and grime caused by sawdust and chain oil, can actually make removal of the sprocket very difficult.
Whilst mentioning Stihl, the other thing to watch out for are hairline cracks. I've noticed this on just a couple of sprockets in the past and they're caused by that little notch on the edge of the sprocket that drives the oil pump. I guess it's a weak point with the constant heating up and cooling down causing a stress fracture... eventually (if you don't catch it in time) it will break, as you can see from these photographs. Click on the images to see them full-size).
The chain brake operates on the outer edge of the sprocket and when used correctly created very little wear. However, many chainsaw users get in to the habit of revving the saw up, making their cut and then slamming the chain brake on - without waiting for the chain to come to a halt first. As well as greatly increasing the wear on the sprocket, the stresses involved in stopping a saw running at 12,000rpm to a dead halt in a matter of milliseconds does the chainsaw no good at all. Please wait for the chain to stop before putting the chain brake on! The wear can be seen as a series of grooves around the outer edge.
In fact, we'll take a closer look at the chain brake mechanism in a later post. But for now, you know what to look for on your drive sprocket.