The following are the methods to determine whether replacement is needed when checking the chain:

　　1、 Observe the appearance

　　(1) Wear and tear condition

　　Measure the size of the chain links using a caliper and compare the measured value with the original size of the chain. Generally speaking, if the wear of the chain link exceeds 10% -15% of the original diameter or thickness, it is necessary to consider replacing the chain. For example, if the original diameter of a chain link is 10mm, and the diameter is less than 8.5-9mm after wear, the chain may have been excessively worn.

　　(2) Crack inspection

　　Carefully inspect each link and connection of the chain for visible cracks. Pay special attention to stress concentration areas such as bends and welding points (if any) of chain links. Even minor cracks may gradually expand during use, leading to chain breakage.

　　(3) Deformation judgment

　　Check if there are any deformations such as bending or twisting in the chain. A normal chain should be straight. When the chain shows obvious bending (such as a bending angle exceeding 5 ° -10 °) or twisting, it will affect its meshing with the sprocket, reduce transmission efficiency, and may cause safety issues.

　　2、 Check the flexibility of the chain

　　(1) Rotation test

　　Manually rotate the chain and feel its flexibility in rotation. If you feel obvious jamming or unevenness during the rotation process, or require a large amount of force to rotate, it may be due to severe wear or damage to the internal components such as the pins and rollers of the chain. Meanwhile, pay attention to whether there are any abnormal sounds, such as creaking friction sounds.

　　(2) Gap between chain links

　　Check the gap between adjacent chain links. Under normal circumstances, the gap should be uniform and small. If it is found that the gap between certain links is significantly larger than other links, or the gap size is uneven, it may indicate that the connecting components between the links have worn out, affecting the normal operation of the chain.

　　3、 Measure the elongation of the chain

　　Marking measurement method

　　Select a section on the chain (such as 10 or 20 links) and mark both ends. Then measure the initial length of this chain with a ruler. After a period of use, measure the length of this chain again. If the elongation exceeds 3% -5% of the original length, the chain may have been excessively stretched and needs to be replaced. For example, a chain with an initial length of 1000mm should be considered for replacement when its length increases to 1030-1050mm.