A few days after the customer installed the timing belt, he called, “The belt was broken. How bad is the quality!” ! ! In fact, the failure mode of the timing belt has many reasons, and sometimes the belt failure mode is still difficult to determine. The purpose of this paper is to define, exemplify and determine the failure modes of normal timing belts so that we can take appropriate precautions and corrective actions.
Normal wear and failure of the belt
After the belt is operated for 2 to 3 years, when the core wire reaches the fatigue life, the belt failure is normal. After a long period of operation, the belt fails due to the fatigue life of the core, which is an ideal belt failure mode. Figure 1 is a jagged, broken 45-degree angle belt, which is typical of a belt with normal fatigue life.
The teeth of the timing belt also fail, but this is not an ideal belt failure mode. In the long-term operation, although the belt can maintain the initial size and shape, the wear of the belt teeth may occur. The exposed fibers of the belt canvas can make the belt teeth appear rough and frizzy, as shown in Figure 2.
Belts after 2 to 3 years of operation do not require any further improvement. The belt life will vary greatly due to different applications and various objective factors. The factors that affect the life of the belt include the transmission power level, the environment, the belt installation tension, the matching of the belt and the wheel, the quality level of the pulley, and even how to cut, package, transport and install the belt.
Figure 1 Normal breakage Figure 2 Canvas wear
Belt flexing failure
The belt bending failure mode usually shows that the fracture surface cores are arranged in a straight line. As shown in Figure 3.
This failure mode occurs when the belt core is bent to a very small diameter. Sharp bends can bend and damage the fibers of the belt cores under great pressure, thereby decreasing the tensile strength of the belts. Belt bending failure is the most common failure mode and is usually related to improper belt operation, low installation tension, small diameter of the pulley, and foreign matter in the pulley.
Figure 3 Flex failure
Causes of belt flexing due to improper handling include improper storage, improper packaging, and improper handling before and during belt installation. When the belt is running under too low tension, it may continue to jump until it reaches an acceptable tension. This phenomenon is called automatic tensioning.
The automatic tensioning can be most clearly observed where the loose edge of the belt or the toothing of the belt enters the wheel slot of the driven wheel. When the belt is automatically tensioned, the belt teeth will jump out of the pulley wheel groove until the tight tension of the belt tight edge forces the belt teeth into the pulley groove. When the belt is forced back into the pulley wheel groove, it often leads to violent and instantaneous bending of the contact point between the belt and the pulley. This kind of bending can cause damage to the belt core. This part of the core damage is called flexing. If the tight edge tension does not allow the belt teeth to enter the pulley groove, the belt will produce skipping teeth, which will also result in flexing failure or belt tooth damage.
When the belt enters a pulley with a too small diameter, it also causes the belt core to be damaged or flexed to fail. Pulleys and back idlers below the specified minimum, idlers between belts and pulleys, and even bending the belt at a small acute angle by hand can cause belt flexing to fail.
The entry of foreign bodies into the transmission system will also cause the belts to buckle. They will form a sharp angle between the belt and the pulleys, causing the belt cores to buckle at this point. Using a tool to force the belt onto the pulley also damages the belt. If the belt is damaged due to improper use of tools (such as a screwdriver) when entering or installing a foreign object, the belt will not fail immediately, but the overall belt life will be reduced.
When the intermittent or periodic torque load required by the driven equipment is greater than the normal level, the impact load exceeds the force that the belt itself can withstand, and the impact load in the transmission system appears. It often exacerbates belt failure. The common V-belt can be used to slow the impact load with an instant slip, but the timing belt must transmit all the load.
Severe shock loads can cause the belt core to break in a rough, uneven pattern, as shown in Figure 4. The teeth of the belt can develop tooth root cracking and/or tooth shedding after being subjected to an immediate impact load in the pulley. If the shock load only occurs once, or if the cycle repeats at the fixed position of the belt, the remaining teeth of the belt may still look normal. Figure 5 illustrates how the root cracking propagates within the tooth. Cracks formed at the root of the tooth sometimes spread to the tip of the tooth. When too many cracks accumulate, the teeth cut, leaving only part of the teeth.
Figure 4 Impact load Figure 5 Impact load
The impact load generated by the driven equipment may be an inherent part of the operation of the drive train, or it may be from an accidental, demanding situation such as a blockage. If the impact load of the transmission system is unavoidable, the core strength of the belt needs to be increased, or the belt drive can be replaced by a triangular belt that can be intermittently slipped.
Belt installation tension is too high
High belt tension in the timing belt can cause the belt teeth to shear or break. Many high-tension belts clearly leave traces of gear wear on the tooth surfaces of the belt. Figure 6 shows an example of a cracked surface area and root cracking of the belt. Root cracking usually extends through the core to its adjacent crack, and the individual belt teeth slowly fall off. Figure 7 shows traces of excessive tensioning of the belt on large pulleys. Excessive belt surface pressure can cause belt wear over a large area, eventually causing the belt core to be exposed. In order to prevent such wear problems, proper belt installation tension values must be accurately set.
Figure 6 Tension is too high Figure 7 Tension is too high
Belt installation tension is too low
Too low installation tension in a moderate to highly loaded driveline can also lead to premature failure of the belt. The belt failure mode, which is usually caused by too low tension, is represented by jumping. Belt skipping means that the belt's teeth climb out of their corresponding wheel slots and their roots no longer carry loads. The transmission load further acts on the side of the belt to bend the belt teeth and then beat. Rolling of the teeth can cause the rubber to tear from the tooth root along the core. As the rubber tear spreads, the belt teeth begin to strip off the belt, as shown in Figure 8. The failure due to excessive skipping may appear as if the viscosity of the rubber and string is not sufficient. However, due to the failure caused by insufficient viscosity, the exposed cores in the belt are usually kept clean and different from the hobbing failure modes.
When the belt tooth part climbs out of the pulley wheel groove and is automatically tensioned, the belt is prone to jumping before the rubber tears and the tooth parts fall off. Damage to the belt core caused by tooth skipping tends to lead to premature failure of the belt, and the damage is similar to that of a broken core wire (a neat fracture) is also similar to the impact load fracture (sawtooth and angled). If the belt does not jump and continues to run even when it is self-tensioning, excessive wear of the belt teeth often occurs. This kind of tooth wear is called a hoop wear because the belt tooth does not match the pulley, as shown in Fig.9. Hook wear is caused by insufficient belt mounting tension and an unstable centering system at low tension.
Figure 8 Tension is too low Figure 9 Tension is too low
Increasing the belt installation tension generally prevents premature tooth skipping and hook wear. If you can't prevent the belt from failing after increasing the installation tension, it may be that the drive system structure is not strong enough to prevent deviation. In order to improve the belt use performance, it is very necessary to increase the supporting force of the transmission structure. If it is not practical to increase the installation tension, increasing the diameter of the pulley can make the belt transmit a higher load under low tension. Appropriate mounting tension values can be obtained from the design manual, calculated through the design manual or sought by the support company's sales staff.
Pulleys are not parallel
When the belt is in operation, the pulley shaft is angled, or the pulley tooth shape has a taper problem during processing. Due to the uneven load applied to the belt, uneven pressing may occur between the belt teeth. The belt failure often starts from the cracking of the tooth root or from the side of the belt with the greatest load-carrying tension and extends to the entire belt width, resulting in shearing of the belt teeth. Due to the relatively large tension in the fibers, the belt may have significant wear on the severely squeezed side, and the belt may climb out or roll onto the pulley rim. Figure 10 shows that high fiber tension causes severe wear on one side of the belt.
Figure 10 Pulleys are not parallel
When the belt is run on a pulley that is not parallel to the rib, if the belt is squeezed in two opposite ribs, it will cause serious wear on both sides of the belt. In this case, the belt will tear from the root of the tooth or from both sides. This tear eventually spreads over the entire belt, causing the belt teeth to shear.
When the belt runs on both sides of a pulley with ribs and when there is no rib on both sides of the pulley, if the two pulleys are not parallel, then the belt will partially move toward the pulley without ribs, without the rib pulley. Some belts will bear the full load and may produce a concentrated wear area after a period of operation. Figure 11 shows the concentrated wear of most tooth surfaces and no wear elsewhere. There may be root cracks under the worn area. Belt strength or tooth surface fatigue can eventually lead to premature belt failure.
Figure 11 Pulleys are not parallel
It is often difficult to identify premature failure of the belt, whether it is due to non-compliant production of the pulley or worn out of the standard. This is partly because when the belt fails, few people go to check the pulley carefully, and it is usually considered to be the belt's own problem. When the belt is running on the pulley with the problem size, the side of the tooth will be highly worn, and the canvas on the side of the belt will be fuzzy fuzzy or fluffy. As shown in Figure 12.
Figure 12 Pulley failed
Running an arc-toothed belt (HTD) in a pulley system with an undersized diameter results in a large area splitting of the belt and a breakage of the belt tension, as shown in Fig.13. Trapezoid teeth (XL, L, H) Belts are usually tooth root cracks or toothed shears, however, belts with tensile fractures are not common.
Figure 13 The pulley is too small Figure 14 pulley wear
Excessive tension may result in more wear on the pulley. Belts or tarpaulins of long running belts are sometimes completely worn away, which indicates that pulley wear has also occurred. The wear of the belt causes the core wire to come in contact with the pulley, causing the peripheral groove of the pulley to wear. The crown on the tooth top of the pulley is a sign of wear on the pulley, as shown in Figure 14. Note: The worn surface is very sharp. It is best to feel with a screwdriver to prevent the hand from being scratched. In this case, the pulley should be replaced.
In corrosive air, the pulley is most easily worn quickly. Severely worn pulleys usually exhibit wheel groove wear and reduction in outer diameter of the pulley. Wear belt pulleys cause belt failures that are typical of worn-to-bottom damage of the belt and regional dimensional deformation of the teeth. Hard-chrome plated pulleys can extend their life in corrosive air. In the other case, if the replacement new belt has a lower life than the previous belt, it is also necessary to carefully check whether the lower pulley is excessively worn.
When the belt is running in a radial run-out pulley system, the tension of the belt is periodically raised and lowered as the pulley rotates. The greater the beating, the higher the maximum tension. Affected by this, the bottom of the tooth appears to be crushed, as shown in Figure 15. The crushed tape body may be similar to the phenomenon caused by too high belt tension running in a moderately sized pulley. Extremely periodic tension changes often result in tearing of the belt teeth or tensile fracture of the belt.
Figure 15 Pulley
When the pulley is installed on the taper sleeve, or is installed after re-expansion on the basis of the smallest pulley hole, it is often prone to beating. How to install according to the standard and how to ensure the fit of the shaft hole are detailed in the related design manual.
When belts are used in corrosive air applications such as tipping sanders, iron ore processing equipment, and phosphate mining conveyors, the toothed sides and the belt bottoms will wear at high heights, and wear areas are often bright. Figure 16 shows a worn belt in a highly corrosive environment. Pulleys usually wear faster under corrosive air, so the pulleys and belts should be replaced together. In order to prolong the life of the belt and the pulley, it is better to install a seal cover with clean air pressure to prevent corrosive dust objects from entering and contaminating.
Figure 16 Corrosion air
High temperature degradation
When the rubber belt is operated at a high temperature (above 85°C) for a long period of time, the rubber component becomes hard, and the back cracks due to bending during operation. These cracks are parallel to the direction of the teeth and often occur in the middle of the teeth. As shown in Figure 17. Belts can also cause tensile fractures due to shearing of the teeth.
The high-temperature rubber belt structure can be used in such applications requiring a high temperature environment. These special belt structures increase the service life of the belt.
Figure 17 Rubber cracking due to excessive temperature
Domestic PU polyurethane belts, body materials are thermoplastic polyurethane materials (with a melting point). When the temperature exceeds 85°C, the belt teeth may become soft and deformed. In addition, the core will completely lose its ability to support the belt. Figure 18 shows a polyurethane timing belt exposed to high temperatures.
Figure 18 High temperature degradation
The failure of the rubber belt in the volatile organic solvent and ozone environment is similar to that in the high temperature environment. The rubber component will harden and the belt will crack back. However, the form of cracking is not very consistent because the hardening of the rubber structure occurs almost at the belt surface, and this may cause the belt to crack longitudinally and longitudinally, and various shapes may occur.
Foreign matter entry
When foreign matter enters between the pulley and the belt, it will cause damage to the belt teeth and the core wire. The core is usually broken internally (shown in Figure 19) or soon afterwards due to excessive bending (Figure 20). After some of the belt cores break, the tensile strength of the entire belt will be greatly reduced. This also leads to a significant drop in the life of the belt. If the belt is found to be damaged, the belt should be replaced and the pulley checked. If a damaged pulley is found, it should be replaced.
Figure 19 Foreign matter entry Figure 20 Flex failure