When an excavator's steel tracks start looking a little loose, the common reaction on the job site is to grab the grease gun and pump the track tensioner until the track is tight as a guitar string. Operators hate loose tracks because they derail, and they assume tight tracks are secure tracks. This logic is destroying final drives and track chains at an alarming rate.
The track assembly is a heavy steel chain looping around the idler, the carrier rollers, and the drive sprocket. The only thing keeping it tensioned is a grease cylinder pushing the front idler forward. If you over-grease the cylinder, you remove all the slack. But steel expands when it gets hot, and dirt packs into the sprocket pockets under load.
An over-tensioned track has zero room for expansion or debris. The extreme tension pulls the drive sprocket teeth hard against the chain links, rapidly wearing the sprocket into sharp points ("shark fins"). It also puts massive radial loads on the final drive bearings, causing them to overheat and fail prematurely. The correct way to check track sag is to place a straight edge across the top of the track from the front idler to the first carrier roller. The mid-point of the track should droop below the straight edge by a specific amount-usually 1 to 2 inches depending on the machine size. If the track is touching the straight edge, it's too tight. A little sag is by design; a tight track is a slow death for the undercarriage.
