Timing (synchronous) belts failures and breaking analysis

 

 

 

 

 

If you change your belts at least more than one time per year, it is time to investigate your drive.
From belt crimping damage to important belt installation stress to sprocket misalignment and adverse environmental conditions, this guide will help you to identify reasons behind untimely failure and makes recommendations on corrective and protective measures.

Timing belt failure are consequences of ever-decreasing performances and often expensive belt replacement. A careful analysis of your drive’s concealed issues will finally reduce you maintenance time and money.

Part 1: Common Causes of Belt Failure

Recognizing the cause of belt deterioration could be a challenge. In this section,we’ll determine, demonstrate and make a diagnosis of some of the most popular common culprits, so you’ll be ready to fix the problem and take preventive protocol for the future.

Normal Belt Wear and Failure

A failure that happens when a belt attains its last tensile cord fatigue life, after moving for a period of 2 or 3 years could be considered as standard. Belt tensile failure caused by cord fatigue after a long using time is considered as exemplar.

Timing belt fatigue breakFigure 1 shows a jagged 45-degree belt failure, that is characteristic of traction cord at the end of its lifetime.
Timing belt teeth could also break, but it is considered to be an imperfect type of broken belt. After a certain run time, belt teeth could be used, although they should retain their original size and form.

Timing belt fuzzy tooth appearance

Protruding fibers from the jacket could give belt teeth a fuzzy appearance, as show in Figure 2.
No remedial action is needed for belts using for a long period. Belt life may vary expressively, depending of the application, cause by multiple factors including the environment, sprocket condition, the transmitted power level, belt installation tension, shaft alignment, and even how the belt was handled prior to and during installation.

Belt Crimp Failures

A “crimp” type belt failure mostly looks like to a straight tensile break as showed in Figure 3. This type of break could happen when belt traction cords are installed around a very small diameter. A short rotation angle bring much compressive forces on the traction cords causing individual fibers to reducing the ultimate tensile strength of the belt.
Belt crimping damage is often the result of belt mishandling, inadequate belt installation tension, sub-minimal sprocket diameters, and/or entry of foreign objects within the belt drive. Belt crimping due to mishandling may appear after improper storage practices, bad packaging, and belt handling prior to and during installation.

Belts using in an under tensioned can permit belt teeth to ride out of the sprockets till a nominal belt tension is achieved. This is called “self-tensioning.”

Self-tensioning can happen at the point of lowest dynamic span tension, or where the teeth are entering the driven sprocket grooves. When a belt is self-tensioning, the teeth goes out the sprocket grooves. Then span tension forces the belt teeth back down into the sprocket grooves. The moment at which the belt teeth are getting back down into the sprocket grooves frequently results in a sharp, momentary point of bending that can result in belt traction cord failures. This point of traction cord default looks like a crimp. If the tight side tension does not force the teeth back down into the sprocket, the belt will ratchet. Belt ratcheting can also comes from traction cord crimp and belt tooth failure.

Subjecting belts to sub-minimal bend diameters can also result in belt traction cord failure, or crimping (sprockets or flat backside idlers in sub-minimal sizes), hand bending a belt too sharply.
Foreign items detected between the belt and sprocket can lift the belt away from the sprocket , creating a sharp angle, making a point of traction cord crimp. The same may arrive with tools used to force belts onto sprockets (screwdrivers or bars). Belts subjected to foreign items or inappropriate use of tools during installation could not fail immediately after being damaged;however, the overall belt lifetime will be decreasing.

Shock Load

These shock loads are the reaction in high belt tensions and may act as a catalyst for belt damages. While conventional V-belt drives may show irregular slip under peak torque load conditions, synchronous belt drives should transmit the global magnitude of the peak loads.

Timing belt shock load breakImportant shock loads may result in belt tensile breaks with a ragged and odd look as illustrated in Figure 4. The belt teeth engaged in the sprocket at the moment of the shock load could also develop root cracks and/or exhibit tooth shear. If the shock load happen only once, or was periodical and redundant at a specific area around the belt, the remaining belt teeth could appear normal.

Timing belt root crackFigure 5 shows how root cracks caused by shock loading may proliferate through the teeth.

Cracks forming at the tooth roots can move towards the tooth tips. Teeth having multiple cracks could then shear, leaving only a portion of the tooth behind.
The shock loads could be created by the driven equipment operation or could result from  occasional condition such as jamming. If the drive shock loads cannot be deleted, the belt stress strength may need to be increased or the synchronous belt drive changed with a more forgiving V-belt drive system capable of intermittent slip.

Part 2: Improper Belt Installation Tension

In this part, we’ll focus on the effects of inappropriate belt tensioning – from applying excessive installation tension to insufficient tension – to help prevent premature belt failure.

High Belt Installation Tension

Applying excessive installation tension to a synchronous belt may result in belt tooth shear or sometimes a stress break. Many belts that have been exaggerated tensioned show evident signs that sprockets have worn the belt land areas.
Timing belt crushed land areasFigure 6 shows crushed land areas and a crack that formed at the root of the tooth. A root crack propagate along  the tensile member until the next root crack. Individual belt teeth will then separate from the body of the belt and frequently fall off.

Timing belt land area wearFigure 7 shows a belt that had been much tensioned on large sprockets. Excessive belt land area wear  may be caused by high belt land pressures, ultimately revealing individual tensile cords. So as to anticipate belt wear problems like these, a proper belt installation tension levels must be found and set accurately.

Low Belt Installation Tension

Assign a low installation tension to belts operating on moderately to heavily loaded drive systems may also result in untimely damages. Insufficient belt installation tension could create “tooth rotation”. When belt teeth climb out of their respective sprocket grooves (self tensioning), drive loads are not  applied at their roots. Drive loads pushing further down the belt tooth sides cause the belt teeth to curve (like a diving board) and “rotate.” Belt tooth rotation can result in rubber laceration at the base of the belt teeth down the tensile member.
Tooth separation failureAs rubber lacerations propagates, belt teeth currently begin to split from the belt body in strips, as shows in Figure 8. Damages due to important tooth rotation may resemble failures caused by a low rubber adhesion to the traction cords. If the cables are installed properly, there is no items of rubber that may peel off.

As belt teeth get out of their sprocket grooves to self tension, belt ratcheting or tooth jumping may appear, then rubber tearing and belt tooth separation may occurs. Belt traction cord failure from ratcheting can cause untimely belt tensile damages. These tensile failures may similar to crimp breaks (clean and straight) as well as shock load breaks (angled and jagged). If belt ratcheting does not happen and belts continue to turn while self tensioning, important belt tooth wear often happen.
Timing belt tooth wearThis tooth wear is referred to as “hook wear” and results from bad belt tooth meshing with the sprockets, as illustrated in Figure 9. Hook wear type belt damages result from low belt installation tension or from fragile drive structures. If center distance flexing while the drive system is under load, the tension is decreasing. Increasing belt installation tension levels often anticipates untimely belt damages due to tooth rotation and hook wear. If ameliorate the belt installation tension level does not avoid this type of damage, the drive structure could not be rigid enough to anticipate deflection. Added structural support could be necessary to upgrade belt performance. If it is not practical to increase belt installation tension levels, improve the sprocket diameters will allow higher drive loads to be transmitted with a lower belt tension.

Part 3: Belt Drive Hardware Problems

In this part, we’ll analyze the negative effects that problems with belt drive hardware have on the operation and lifetime of your belts.

Sprocket Misalignment

Belts running on drives with angular shaft misalignment or tapered sprockets generally exhibit an irregular wear pattern across the belt tooth sides and irregular compaction in the land areas (in between belt teeth) cause by the irregular application of load to the belt. Belt damages often occur from tooth root cracks or lacerations initiating on the side of the belt that is carrying the most of tension and proliferating to the belt width, finally resulting in tooth split. One edge of the belt could also show significant wear cause by important tracking force and could even roll up or attempt to climb the sprocket flange.
Timing belt extreme belt wear
Figure 10 illustrates important belt edge wear from a grave tracking force.

Belts running on flanged sprockets with parallel misalignment (offset sprockets) could show exaggerated belt edge wear on both edges if the belt is pinched between facing flanges. Belt failures could then happen by tooth root cracks or tears initiating from both edges of the belt. These laceration could probably extend across the whole width of the belt, resulting in tooth shear.

Belts operating on an association of flanged and non-flanged sprockets with parallel misalignment may run or track partly off of the non-flanged sprocket(s). The part of the belt remaining engaged with the non-flanged sprocket(s) will support the entire operating load and could develop a concentrated area of wear after running this way for a certain period.

Timing belt uneven belt wearFigure 11 illustrates concentrated wear across the major part of the belt tooth face with a portion relatively unworn.
A root crack has also developed below the worn area. This may ultimately result in untimely belt damage cause by either stress or tooth fatigue.

Sprocket(s) Out of Specification

Untimely belt failures resulting from sprockets either built or worn outside of design conditions are difficult to be identifying. This is partially cause by the fact that sprockets are almost never checked closely when a belt fails. Untimely belt damages are almost identifying to be only the belt’s fault.

Timing belt tooth wear wom sprocketsBelts running on sprockets that are out of dimensional requirements frequently show a high level of tooth side wear with the jacket flank exposing a fuzzy or flaking appearance, as illustrated in Figure 12.

Timing belt land desintegrationCurvilinear belts (HTD and GT) running on sub-minimal sprocket diameters basically fail by land disintegration, as shown in Figure 13, and tensile breaks. V-belts (XL, L, H) will mainly fail by tooth root cracks and tooth shear; however, tensile breaks are not rare.
A higher rate of sprocket wear could happen from belts that have been mounted with high installation tension. After a long time, running belts sometimes tooth jacket or facing worn away. Belts in this condition show that significant sprocket wear could have also happened. Belts worn to this point also occasionally allow belt traction members to contact the sprockets can generate a grooved wear around the outside sprocket circumference.

Excessive sprocket wearA ridge along the tip of sprocket could appear, as showed in Figure 14. Use caution: important worn surfaces on sprocket faces could become very sharp. It is better to use a screwdriver or another tool to feel for the ridge in order to anticipate finger cuts. When a ridge on the sprocket face is found, the sprockets should be changed.

The most quickly and gravely worn sprockets are most usually found in abrasive atmospheres. Important worn sprockets generally show a reduction in the outside finish diameter as well as groove wear. An usual belt failure on worn sprockets exhibits polished land wear and could have teeth worn to the point of severe dimensional distortion (hook wear).

To extend the sprocket life in abrasive atmospheres, it can be plated with a hard chrome finish.

Another sign of important sprocket wear is when new belt lifetime is evidently reduced compared to previous belts. When this happens, sprockets must be check thoroughly for severe wear.

Excessive Sprocket Run-Out

Belts running on sprockets with radial run-out are exposed to a cyclic rise and fall in belt tension as the sprockets turn. Cyclic peak tensions create land areas with a crushed appearance. Tooth shear and crushed land areas are both visible in Figure 15. A crushed land area condition could appear similar to belts running on moderate size sprockets under severe high tensions.

Belts under  important cyclic belt tension variations generally fail from either tooth shear or tensile break.

Severe sprocket run-out is generally happening when sprockets are installed improperly on bushings or when minimum plain bore sprockets are improperly re-bored and installed.

Part 4: Negative Effects of Environmental Conditions

In this part, we’ll concentrate to the environmental conditions – foreign items , heat degradation, abrasive atmosphere and chemical degradation– that can negatively impact your belts.

Abrasive Atmosphere

Belts running in abrasive atmospheres on applications like foundry shakers, taconite processing equipment, and phosphate mining machines generally show a high level of belt land and tooth side wear. Worn areas commonly have a polished appearance.
Timing belt wear fromp abrasionFigure 16 shows an important worn Gates Poly Chain® GT®2 belt that function in a severe abrasive environment. Sprocket wear is basically quick in abrasive environments; therefore, sprockets must be changed along with belts. To increase the lifetime of belts and sprockets, a sealed guard that is pressurized with clean air may be mounted to help dissipation of abrasive particles and contaminates.

Heat Degradation

When rubber belts are running in high temperatures (greater than 185°F) for improve periods of time, the rubber hardens resulting in back cracking due to bending. These cracks normally remain parallel to the belt teeth and basically happens over land areas (in between belt teeth), as showed in Figure 17. Timing belt cracking from high temperatureBelts basically fail because of tooth shear, which often leads to traction cord fracture.

High-temperature rubber belt constructions are available. These special belt conception help to ameliorate belt work. To define if a special high-temperature belt conception will improve the belt performance, contact a Gates Representative.

The urethane material used in belts such as Poly Chain® GT® Carbon® belts is thermoplastic and has a melting point. When submitted to environmental temperatures in more than 185°F, the teeth could begin to deform and melt. Furthermore, the traction cord to urethane adhesion loses its integrity.
Timing belt melting from high temperature

Figure 18 shows a Poly Chain® GT®2 belt that was submitted to a severe environmental temperature.

 

Chemical Degradation

Rubber belts submitted to either organic solvent vapors or ozone will be similar as belts that have been submitted to severe environmental temperatures. The rubber will harden and belts will show back cracking. The cracking pattern will differ, though, in that the compound hardening happens usually at a surface level allowing cracks to form in both lateral and longitudinal directions. A “checkered” appearance could result.

Foreign Objects

Timing belt cord failure from debrisThe introduction of foreign items between a belt and sprocket often damages both belt teeth and traction cords. Traction cords often fracture internally (see Figure 19) or fail Timing belt crimp failure due to debrislater cause to crimping, as illustrated in Figure 20.
Once a portion of the traction cords have fractured, the remaining tensile strength of the belt has been decrease seriously. This generally results in a hard reduction in belt lifetime. If belt damage from debris is noticeable, the belt must be changed and the sprockets inspected for damage. Damaged sprockets must also be changed.