The bar has a groove around the outer edger and the chain has teeth that run inside the groove to keep the chain moving straight. If either of these are damaged, the chain may come loose. Remove the chain cover, chain, and bar to be sure the groove on the bar is clean and in good shape. If it is worn down and shallow, you should replace the bar immediately. Inspect the chain for any worn, bent, or broken teeth and replace it if necessary. Making sure your chain oiler is working properly can also help lengthen the life of your bar.
If your chain is tensioned too loose, it can come free from the guide bar during use, that is bad news. The chain should never sag from the bar. If you can pull more than a half inch off the guide bar, it is too loose. Before running your chainsaw, check the tension of the chain and be sure to double check every 20 minutes for the first hour you are cutting. If you have it set correctly but it loses tension, your tensioner may be broken.
Remove the chain cover and the tensioner itself inspecting the screw and tension gear. If you notice any general wear or stripping of the screw itself, replace them immediately.
This blog is a step-by-step instruction on how to replace the clutch on chainsaws. The most common reason for replacing the clutch is when the chainsaw...