This Washer-Lock 5/16 is a silver, metal part that is sold individually and comes direct from the manufacturer. With a 5/16 Inch Interior Diameter and 1/2 Inch Outside Diameter, it is used to prevent the nut from backing off and becoming loose on the pulley shaft, and is attached to the drive belt pulley assembly. You will know if you need this because your owners manual will specify it, or the model number of your machine will correspond with this part. If the part is broken due to fatigue, the shafts will free spin when they should be locked together, and must be replaced.