Q: What if my belt breaks?
A: A broken belt may be caused by many factors in a drive system. Often the belt is only the symptom rather than the cause of the failure. The belt fails because it is the weakest link. To determine the root cause it is useful to analyze the type of break. There are three common types of belt break: tensile, crimp and flex.
Tensile break is characterized by a belt that has a jagged edge with the reinforcement extending beyond the urethane. The break is usually within a tooth.
Possible Root Causes:
- The belt is under sized for the torque it is transmitting. The problem may be solved by increasing the belt width or by changing to a stronger reinforcement.
- The pulley profile is inaccurately formed or when the pulley surface finish is rough.
- Pulley is tapered. A tapered pulley will cause the reinforcement in the belt to be unevenly loaded.
- The belt tension is too high. The tension could be set too high or the belt pitch length may be too short. High tension can also result when the coefficient of thermal expansion of the drive mechanism is higher than that of the belt.
- Chemical degradation of the reinforcement can also create a tensile break.
Crimp failure is characterized by a belt that has a clean break within a tooth, the reinforcement does not stick out beyond the urethane. Usually occurs with glass reinforcements.
Possible Root Causes:
- Damage during shipping/handling. If the belts are mishandled and receive severe bending.
- Damaged during installation. Care is to be taken not to severely bend the belt during installation. Crimp damage can occur with as little as 15 pounds of force if the belt is bent upon itself.
Flex break is characterized by either a jagged break, straight break, or no visible break. Most often, the flex break looks like a tensile break (jagged), but the applied load needed to cause the failure is much lower than the design load. A crimp break (straight break) is usually caused when the belt is kinked over a small diameter pulley. Non-visible breaks are caused when the load needed to break the belt reinforcement is lower than the ultimate tensile strength of the urethane belt material. The belt will remain intact, but the broken section will become "stretchy".
Possible Root Causes:
- Pulley diameter is too small for the reinforcement and belt selected. During operation, the yarn fibers become damaged running over small diameter pulleys, causing yarn strength to reduce and finally break. Increasing pulley diameters will improve flex strength
Q: What if my belt wears?
A: Timing belt wear normally occurs on the belt teeth or the edges.
Tooth wear is wear that has occurred on the tooth surface. On unidirectional drives the wear will be on the working surface of the belt. On reciprocating drives, the wear will occur on both sides of the teeth and may be harder to detect until to starts to expose the reinforcement or through the appearance of dust build-up.
Possible Root Causes:
- Incorrect selection of elastomer type.
- Incorrect installed tension.
- Too much torque transmission.
- Incorrect pitch length or center distance.
- Pulley tooth wear has poor surface finish and/or burrs.
- Pulley outside diameter incorrect or tooth shape out of tolerance on pulley.
- Meshing problems due to debris and contamination buildup between belt and pulley.
Belt edge wear, is the wear that occurs on either or both edges of the belt.
Possible Root Causes:
- Poor belt tracking caused by the twist and wind characteristics of the reinforcement in the belt.
- Poor belt tracking due to misaligned pulleys.
- Pulley face width too narrow for the belt used. Pulley face width should be 1mm wider than the belt width.
- Taper in the pulley. This is common on plastic pulleys as they can shrink unevenly after de-molding. If this is not compensated for in the design of the pulley a taper may occur that can cause poor belt tracking.
- Tooth and edge wear can result from environmental factors including extremes of temperature, humidity and chemical contamination.
Dusting is the most common symptom of wear.
