Belt failure

Several causes of V-belt failure and the action required to correct the problem are described in this section. Table 58.6 provides a troubleshooting overview. 

Keep pulleys aligned or excessive belt and pulley wear will occur. Shafts that are not parallel are very common and are the biggest contributor to early belt failure. In such cases, belts work harder on one side than on the other and wear faster. As a result, the entire belt set will need to be replaced. Misalignment in the pulley only is indicated by belt cover and pulley wear. 

Most electronic equipment shares the television set s need for a number of differing voltages for the operation of individual components. This alone may be sufficient justification for the inclusion of a direct current to alternating current converter in fuel cell power systems. In addition, alternating current electric motors are more suitable in most applications. They tend to operate at a rotational speed controlled by the frequency of the current. If completely unloaded they speed up to this fixed velocity and accelerate no further. Many types of direct current motors, if operated unloaded, will continue to accelerate until they fail. A belt driven fan operated by an alternating current motor is undamaged by the failure of the belt. A direct current motor will require a special safety circuit to shut it down in case of belt failure. If the belt and the safety circuit both fail, the motor will speed up until it destroys itself. 

Four different types of belt are considered synchronous belts, V-belts, V-ribbed belts and conveyor belts. Each type of belt has a distinct set of failure modes and so each is considered in turn below. Only failures which can be considered to be belt failures have been considered, rather than belt/pulley system failures, so that failures as a result of pulley misalignment, for instance, have not been included. For adhesion related failure modes, methods of predicting belt failure where they have been developed, are outlined. 

Test results from experimental studies on belt life suggest the following as the major belt failure modes tooth root cracking, wear, cord failure and fabric separation [14,15,16, 17, 18], and this classification has support from field data [19, 20]. Figure 12.2 shows examples of tooth root cracking, cord delamination and fabric separation failures. 

Figure 12.2 Synchronous belt failure modes, a) tooth root cracking, b) cord delamination, c) fabric separation...

Fabric separation failure occurs when the belt teeth and fabric land become detached from the belt cords [17] and is essentially seen as purely an adhesion failure, although there may be links between this failure mode and the tooth root cracking failures observed by lizuka [18], originating from cracks developed in the cord itself through internal delamination. Wear causes belt failure through changing the tooth profile to such an extent that the belt teeth can no longer support the required load [25]. 

In attempting to identify parameters which allow the belt life to be predicted within the adhesion related failure modes identified above, the most common approach has been to use measures of belt distortion. Dalgarno [17] examined belt life data from belt failures within the tooth root cracking, fabric delamination and cord separation failure modes. 

The most common symptoms, causes, and corrections of V-belt and synchronous-belt failures... 

V-Belts Measure the tension of 1 Identify the proper Premature belt failures through 100% ) 20 vs. 1... 

Work carried out to identify how the performance of power transmission belts may be improved is described. An examination is made of which mechanical properties of the elastomer compound are of significance in determining belt performance, and to identify how the significant elastomer compound properties may be changed to improve belt performance. This work encompasses the examination of belt failure and modelling of belt operation to ensure that the current limits of belt performance are fully understood. It is concluded... 

Insoluble Sulfur. In natural mbber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of mbber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

Backstops. A backstop is a device that permits rotation of the pulley in the forward direction but automatically prevents rotation in the opposite direction. A backstop should be installed at the headshaft of an inclined belt to prevent the belt from moving in reverse if the power to the motor is intermpted or if there is a failure in the mechanical drive system. 

The weight of material in the buckets on the loaded side of an elevator chain causes the elevator to momentarily mn backwards if, during operation, the power is intermpted or there is a failure in the driving system. Because this could be a ha2ard to operating personnel, as well as damage to the elevator, a backstop, similar to that described for a belt conveyor, should be used. 

Electric motors with V-belt intermediate drive display the same failure modes as those described previously. However, the unique V-belt frequencies should be monitored to determine if improper belt tension or misalignment is evident. 

Table 44.1 Belt Drive Failure Symptoms, Causes, and Corrective Actions...

Most of the forcing functions generated by V-belt drives can be attributed to the elastic or mbber band effect of the belt material. This elasticity is needed to provide the traction required transmitting power from the drive sheave (i.e., pulley) to the driven sheave. Elasticity causes belts to act like springs, increasing vibration in the direction of belt wrap, but damping it in the opposite direction. As a result, belt elasticity tends to accelerate wear and the failure rate of both the driver and driven unit. 

Belt-drive fault frequencies are the frequencies of the driver, the driven unit, and the belt. In particular, frequencies at 1X the respective shaft speeds indicate faults with the balance, concentricity, and alignment of the sheaves. The belt frequency and its harmonics indicate problems with the belt. Table 44.1 summarizes the symptoms and causes of belt-drive failures, as well as corrective actions. 

Cracking Cracking can be caused by exposure to high temperature and/or dust. Cracking on the bottom sections of belts does not cause a loss of strength or operating efficiency, but can lead to eventual failure. Nevertheless, there is no need to replace belts simply because bottom cracking has been detected. 

Cause of failure Belt has evenly spaced deep bottom cracks from use of substandard backside idler (Figure 58.13). 

Cause of failure Back of the belt rubbing on a belt guard or other component (Figure 58.14). 

Cause of failure Tensile breaks caused by high shock loads, foreign objects lodged under the belt, or damage sustained during installation (Figure 58.15). 

Cause of failure Excessive exposure to oil or grease causing the belt to become soft and swell, resulting in the bottom envelope seam splitting open (Figure 58.16). 

Cause of failure Cut bottom and sidewall indicates that the belt was pried over the pulley and damaged during installation (Figure 58.18). 

Cause of failure Worn pulley grooves allow the joined belt to ride too low thus cutting through to the top band (Figure 58.20). 

Cause of failure Split on side at the belt pitch-line indicates use of a pulley with a substandard diameter (Figure 58.21). 

Cause of failure Web fabrics wear caused by improper belt and pulley fit (Figure 58.23). 

Cause of failure Flange wear on belt (Figure 58.24). 

Cause of failure Tooth shear caused by belt overload condition from improper application or shock loads (Figure 58.25). 

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