The chain on your chainsaw often revolves around the bar at 50 feet per second. These incredibly high speeds cause enormous friction and heat on the chain, which will wear chains out quickly if not cared for properly. Chains are an extremely important part of the chainsaw and if they aren't working properly, they can cause serious damage to your saw and even injure or kill you.
Broken Cutting Teeth
The angled tip on the teeth is like a razor blade with a fine tip. Hitting dirt, duff, or rocks will dull and even break off teeth rapidly, forcing you to replace the chain. You may notice the entire tops of teeth missing if you hit the chain hard against rocks or metal. Whenever you sharpen the chain, pay close attention to the damage to the teeth. If you notice several teeth missing, it's time to replace the chain.
Teeth Too Small
Most chainsaw manuals suggest you sharpen your chainsaw during refueling stops and after each use. This heavy filing will keep your chain sharpened properly, but it will also file down the teeth quicker. When your teeth start to get too small to sharpen, you need to replace the chain. If these teeth are too small and you hit a hard surface, these small tips can break off and fly back, hitting you, which can cause serious injury.
Cutters Too Small
Each tooth has a rectangular piece of metal in front of it called a cutter. These cutters score the wood before the tooth slices through it. As you sharpen the chain over time, these cutters will get ground down, and, if you've been following the sharpening recommendations, they will wear about the same time as the teeth get too small to sharpen. You need to replace the chain when these cutters get small because the chain won't cut properly without them. You may notice there's either too much pull in the chain or not enough.
Tension, Other Problems
Chains stay tensioned against the bar via a bar tensioner screw and they stay held together via tie straps that connect two teeth. The tension on the chain takes a lot of the cutting force and distributes it over these straps. When the chain won't stay tight against the bar, these straps have lost their ability to keep the teeth connected properly. These straps are crucial for proper operation; if you notice the chain sagging during cutting or you can't tighten it, replace the chain immediately. These straps, if they rip apart, can snap a chain in half, which at full speed can fly back and hit you in the legs.