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 straight across the belt width.
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 or increasing the number of chords in the belt.
- The pulley profile is inaccurately formed.
- The pulley surface finish is rough or has burrs left from the machining or molding processes.
- 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 break is characterized by a belt that has a clean break across the width; the reinforcement does not stick out beyond the urethane. This break usually occurs with glass reinforcements.
Possible Root Causes:
- Damage during shipping/handling. The belts could be severely bent, causing a crease in the belt.
- Damaged during installation. The belt should not be severely bent the belt during installation.
Flex breaks are 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: Some wear is normal and usually occurs on the working surface(s) of the belt. Excessive belt wear is a problem and may be characterized by dusting and changes in physical dimensions.
Possible Root Causes:
- Incorrect selection of elastomer type.
- Incorrect installed tension.
- Too much torque transmission.
- Incorrect pitch length or center distance.
- Pulley has poor surface finish and/or burrs.
- Pulley outside diameter incorrect.
- Debris and contamination buildup between belt and pulley.
Belt edge wear, is the wear that occurs on one or both edges of the belt.
Possible Root Causes:
- Poor belt tracking caused by the twist and wind characteristics of the belt reinforcement.
- Poor belt tracking due to misaligned pulleys.
- Pulley face width too narrow for the belt used. Pulley face width should be at least 1mm wider than the belt width.
- Pulley taper. This is common on plastic pulleys as they can shrink unevenly after molding. Alterations to the mold are often required to correct a tapered pulley.
- Tooth and edge wear can result from environmental factors including extremes of temperature, humidity and chemical contamination.
Shear Failure
Fenner Precision manufactures integrally molded multi-v belts. This allows special features to be added to the belts including lugs. Integrally molded lugs are typically more robust and longer lasting than competitor designs in which lugs are stuck to the belt using a secondary bonding operation. However, even integrally molded lugs will fail if overloaded.
Possible Root Causes:
- Problems with the product design, drive design, material selection or belt attachments.
- Product design issues include too much reinforcement density because the turns per inch are too high, inappropriate elastomer selection, incorrect torque rating or inappropriate tooth profile.
- Drive design problems include poorly-formed pulley profiles, high accelerating forces.
- Environmental problems include extremes of temperature and humidity.
Slip Failure can occur from problems with low installed tension, excess torque transmission or drive design.
Possible Root Causes:
- Low tension can be caused when the belt length is too long. Sometimes this can occur when the drive tolerances are too small or when environmental factors like temperature and humidity cause belt stretch or shrinkage.
- Belt slip can sometimes be traced to problems with pulley outside diameters, surface finish, or the incorrect placement of idler pulleys.
- If the peak torque exceeds the maximum rated belt torque, slip can occur. Increasing the belt width and tension, pulley diameter, or changing belt material are ways to resolve the problem.
- Slip can also occur if the drive acceleration forces are too high or if the transport mass is too high.
