Proper chain tension is critical to achieving acceptable service life, as excessive tension can cause accelerated wear or chain overload and excessive slack can cause rough chain operation and possibly result in the chain skipping a sprocket tooth, resulting in a catastrophic failure. For the majority of slow- and medium-speed drives, the total mid-span movement in the slack span of the chain should be approximately 4-6% of the drive’s center distance. For drives operating at high speeds, impulse, or reversing loads, the total mid-span movement should be reduced to 2-3% of the center distance. Drives with vertical centers should also be adjusted to the smaller percentage. If the drive is designed to incorporate shaft adjustment or an idler, the amount of movement or “take-up” should always allow for the removal of two pitches of chain.