Timing Belts and Pulleys – Operations

9.1 LOW-SPEED OPERATION
Synchronous drives are especially well-suited for low-speed, high torque applications. Their positive traveling nature helps prevent potential slippage associated with V-belt drives, and even allows significantly better torque carrying capacity. Small pitch synchronous drives working at speeds of 50 ft/min (0.25 m/s) or much less are considered to be low-speed. Care ought to be taken in the get selection process as stall and peak torques can sometimes be very high. While intermittent peak torques can often be carried by synchronous drives without special considerations, high cyclic peak torque loading should be carefully reviewed.

Proper belt installation tension and rigid get bracketry and framework is essential in stopping belt tooth jumping in peak torque loads. Additionally it is beneficial to design with more than the normal the least 6 belt tooth in mesh to make sure adequate belt tooth shear strength.

Newer era curvilinear systems like PowerGrip GT2 and PowerGrip HTD ought to be found in low-acceleration, high torque applications, as trapezoidal timing belts are even more prone to tooth jumping, and also have significantly much less load carrying capacity.

9.2 HIGH-SPEED OPERATION
Synchronous belt drives are often used in high-speed applications despite the fact that V-belt drives are typically better suited. They are often used due to their positive traveling characteristic (no creep or slip), and because they require minimal maintenance (don’t stretch significantly). A substantial drawback of high-rate synchronous drives is certainly travel noise. High-quickness synchronous drives will almost always produce even more noise than V-belt drives. Small pitch synchronous drives working at speeds more than 1300 ft/min (6.6 m/s) are considered to end up being high-speed.

Special consideration ought to be directed at high-speed drive designs, as several factors can significantly influence belt performance. Cord fatigue and belt tooth wear will be the two most crucial factors that must be controlled to ensure success. Moderate pulley diameters should be used to reduce the rate of cord flex exhaustion. Developing with a smaller pitch belt will often offer better cord flex exhaustion characteristics than a bigger pitch belt. PowerGrip GT2 is especially well suited for high-speed drives because of its excellent belt tooth entry/exit characteristics. Even interaction between the belt tooth and pulley groove minimizes wear and noise. Belt installation pressure is especially crucial with high-quickness drives. Low belt tension allows the belt to trip from the driven pulley, resulting in rapid belt tooth and pulley groove wear.

9.3 SMOOTH RUNNING
Some ultrasensitive applications require the belt drive to operate with only a small amount vibration aspossible, as vibration sometimes impacts the system procedure or finished produced product. In such cases, the features and properties of most appropriate belt drive products should be reviewed. The ultimate drive system selection should be based upon the most significant style requirements, and could need some compromise.

Vibration is not generally considered to be a problem with synchronous belt drives. Low degrees of vibration typically derive from the procedure of tooth meshing and/or as a result of their high tensile modulus properties. Vibration caused by tooth meshing is certainly a standard characteristic of synchronous belt drives, and cannot be totally eliminated. It could be minimized by staying away from small pulley diameters, and rather choosing moderate sizes. The dimensional accuracy of the pulleys also influences tooth meshing quality. Additionally, the installation stress has an impact on meshing quality. PowerGrip GT2 drives mesh extremely cleanly, leading to the smoothest feasible operation. Vibration resulting from high tensile modulus could be a function of pulley quality. Radial go out causes belt stress variation with each pulley revolution. V-belt pulleys are also manufactured with some radial go out, but V-belts possess a lesser tensile modulus resulting in less belt tension variation. The high tensile modulus found in synchronous belts is necessary to maintain appropriate pitch under load.

9.4 DRIVE NOISE
Drive noise evaluation in virtually any belt drive system should be approached with care. There are numerous potential resources of sound in a system, including vibration from related components, bearings, and resonance and amplification through framework and panels.

Synchronous belt drives typically produce even more noise than V-belt drives. Noise results from the process of belt tooth meshing and physical connection with the pulleys. The sound pressure level generally increases as operating quickness and belt width increase, and as pulley diameter decreases. Drives designed on moderate pulley sizes without excessive capability (overdesigned) are usually the quietest. PowerGrip GT2 drives have already been found to be considerably quieter than various other systems due to their improved meshing characteristic, see Figure 9. Polyurethane belts generally produce more noise than neoprene belts. Proper belt installation tension can be very Dry Screw Vacuum Pump important in minimizing get noise. The belt ought to be tensioned at a level which allows it to run with as little meshing interference as feasible.

Drive alignment also has a significant effect on drive sound. Special attention should be given to reducing angular misalignment (shaft parallelism). This assures that belt tooth are loaded uniformly and minimizes side tracking forces against the flanges. Parallel misalignment (pulley offset) isn’t as critical of a problem as long as the belt isn’t trapped or pinched between contrary flanges (see the particular section dealing with drive alignment). Pulley materials and dimensional accuracy also influence travel noise. Some users have found that steel pulleys are the quietest, accompanied by lightweight aluminum. Polycarbonates have been found to become noisier than metallic components. Machined pulleys are generally quieter than molded pulleys. The reason why because of this revolve around materials density and resonance characteristics and also dimensional accuracy.

9.5 STATIC CONDUCTIVITY
Little synchronous rubber or urethane belts can generate a power charge while operating about a drive. Factors such as humidity and operating speed influence the potential of the charge. If decided to become a problem, rubber belts can be stated in a conductive construction to dissipate the charge into the pulleys, and to ground. This prevents the accumulation of electrical charges that might be harmful to material handling procedures or sensitive consumer electronics. It also greatly reduces the potential for arcing or sparking in flammable environments. Urethane belts cannot be produced in a conductive building.

RMA has outlined standards for conductive belts in their bulletin IP-3-3. Unless usually specified, a static conductive structure for rubber belts is definitely on a made-to-purchase basis. Unless normally specified, conductive belts will be created to yield a resistance of 300,000 ohms or much less, when new.

Nonconductive belt constructions are also available for rubber belts. These belts are generally built specifically to the customers conductivity requirements. They are generally used in applications where one shaft should be electrically isolated from the additional. It is necessary to note a static conductive belt cannot dissipate a power charge through plastic material pulleys. At least one metallic pulley in a drive is required for the charge to be dissipated to surface. A grounding brush or equivalent device could also be used to dissipate electrical charges.

Urethane timing belts are not static conductive and can’t be built in a particular conductive construction. Unique conductive rubber belts ought to be utilized when the presence of a power charge can be a concern.

9.6 OPERATING ENVIRONMENTS
Synchronous drives are suitable for use in a wide variety of environments. Unique considerations could be necessary, however, depending on the application.

Dust: Dusty environments usually do not generally present serious problems to synchronous drives as long as the particles are good and dry out. Particulate matter will, however, act as an abrasive producing a higher rate of belt and pulley use. Damp or sticky particulate matter deposited and loaded into pulley grooves could cause belt tension to increase considerably. This increased pressure can effect shafting, bearings, and framework. Electrical fees within a get system will often catch the attention of particulate matter.

Debris: Debris should be prevented from falling into any synchronous belt drive. Debris captured in the get is generally either forced through the belt or results in stalling of the system. In either case, serious damage occurs to the belt and related get hardware.

Drinking water: Light and occasional contact with water (occasional clean downs) shouldn’t seriously affect synchronous belts. Prolonged get in touch with (constant spray or submersion) results in significantly reduced tensile strength in fiberglass belts, and potential length variation in aramid belts. Prolonged contact with drinking water also causes rubber compounds to swell, although less than with oil contact. Internal belt adhesion systems are also gradually broken down with the existence of water. Additives to drinking water, such as for example lubricants, chlorine, anticorrosives, etc. can have a more detrimental influence on the belts than clear water. Urethane timing belts also have problems with drinking water contamination. Polyester tensile cord shrinks considerably and experiences lack of tensile power in the presence of drinking water. Aramid tensile cord maintains its strength fairly well, but encounters duration variation. Urethane swells a lot more than neoprene in the existence of water. This swelling can increase belt tension significantly, causing belt and related equipment problems.

Oil: Light connection with natural oils on an occasional basis won’t generally harm synchronous belts. Prolonged connection with oil or lubricants, either straight or airborne, outcomes in considerably reduced belt service existence. Lubricants trigger the rubber compound to swell, breakdown internal adhesion systems, and decrease belt tensile strength. While alternate rubber substances might provide some marginal improvement in durability, it is advisable to prevent oil from contacting synchronous belts.

Ozone: The existence of ozone could be detrimental to the substances used in rubber synchronous belts. Ozone degrades belt materials in quite similar way as excessive environmental temperature ranges. Although the rubber materials used in synchronous belts are compounded to resist the effects of ozone, ultimately chemical substance breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation is dependent upon the ozone focus and duration of exposure. For good performance of rubber belts, the next concentration levels should not be exceeded: (parts per hundred million)
Standard Construction: 100 pphm
Nonmarking Construction: 20 pphm
Conductive Construction: 75 pphm
Low Temperatures Building: 20 pphm

Radiation: Contact with gamma radiation can be detrimental to the substances used in rubber and urethane synchronous belts. Radiation degrades belt materials much the same way excessive environmental temperatures do. The quantity of degradation is dependent upon the strength of radiation and the exposure time. Once and for all belt performance, the next exposure levels should not be exceeded:
Standard Construction: 108 rads
Nonm arking Construction: 104 rads
Conductive Construction: 106 rads
Low Temperatures Construction: 104 rads

Dust Era: Rubber synchronous belts are known to generate little quantities of good dust, as a natural consequence of their operation. The amount of dust is normally higher for new belts, as they run in. The time period for run in to occur is dependent upon the belt and pulley size, loading and acceleration. Factors such as pulley surface end, operating speeds, installation pressure, and alignment influence the amount of dust generated.

Clean Area: Rubber synchronous belts may not be ideal for use in clean room environments, where all potential contamination must be minimized or eliminated. Urethane timing belts typically generate significantly less debris than rubber timing belts. However, they are recommended only for light operating loads. Also, they cannot be stated in a static conductive structure to allow electrical costs to dissipate.

Static Sensitive: Applications are sometimes delicate to the accumulation of static electrical charges. Electrical fees can affect materials handling functions (like paper and plastic film transport), and sensitive digital apparatus. Applications like these need a static conductive belt, so that the static charges generated by the belt can be dissipated into the pulleys, and also to ground. Standard rubber synchronous belts do not fulfill this requirement, but can be manufactured in a static conductive construction on a made-to-order basis. Normal belt wear resulting from long term procedure or environmental contamination can influence belt conductivity properties.

In delicate applications, rubber synchronous belts are favored over urethane belts since urethane belting cannot be produced in a conductive construction.

9.7 BELT TRACKING
Lateral tracking characteristics of synchronous belts is usually a common area of inquiry. While it is normal for a belt to favor one part of the pulleys while running, it is abnormal for a belt to exert significant drive against a flange leading to belt edge put on and potential flange failing. Belt tracking is normally influenced by several factors. To be able of significance, conversation about these elements is as follows:

Tensile Cord Twist: Tensile cords are formed into a single twist configuration during their manufacture. Synchronous belts made with only solitary twist tensile cords monitor laterally with a significant power. To neutralize this monitoring power, tensile cords are stated in right- and left-hand twist (or “S” and “Z” twist) configurations. Belts made out of “S” twist tensile cords monitor in the contrary direction to those built with “Z” twist cord. Belts made out of alternating “S” and “Z” twist tensile cords monitor with reduced lateral force since the tracking features of the two cords offset one another. This content of “S” and “Z” twist tensile cords varies slightly with every belt that is produced. Consequently, every belt comes with an unprecedented tendency to track in either one path or the additional. When an application requires a belt to track in a single specific direction only, an individual twist construction is used. See Figures 16 & Figure 17.

Angular Misalignment: Angular misalignment, or shaft nonparallelism, cause synchronous belts to track laterally. The position of misalignment influences the magnitude and path of the monitoring pressure. Synchronous belts have a tendency to monitor “downhill” to a state of lower stress or shorter middle distance.

Belt Width: The potential magnitude of belt monitoring force is directly linked to belt width. Wide belts tend to track with more push than narrow belts.

Pulley Size: Belts operating on small pulley diameters can tend to generate higher tracking forces than on large diameters. This is particularly true as the belt width approaches the pulley size. Drives with pulley diameters significantly less than the belt width are not generally recommended because belt tracking forces may become excessive.

Belt Length: Due to the way tensile cords are applied to the belt molds, short belts can tend to exhibit higher tracking forces than long belts. The helix angle of the tensile cord decreases with increasing belt length.

Gravity: In travel applications with vertical shafts, gravity pulls the belt downward. The magnitude of this force is minimal with small pitch synchronous belts. Sag in long belt spans should be prevented by applying adequate belt installation tension.

Torque Loads: Sometimes, while functioning, a synchronous belt can move laterally from side to side on the pulleys rather than operating in a constant position. Without generally regarded as a substantial concern, one description for this is normally varying torque loads within the travel. Synchronous belts sometimes track in a different way with changing loads. There are many potential reasons for this; the primary cause relates to tensile cord distortion while under great pressure against the pulleys. Variation in belt tensile loads may also cause adjustments in framework deflection, and angular shaft alignment, resulting in belt movement.

Belt Installation Pressure: Belt tracking is sometimes influenced by the amount of belt installation pressure. The reason why for this are similar to the result that varying torque loads have got on belt tracking. When issues with belt tracking are experienced, each one of these potential contributing elements ought to be investigated in the order they are outlined. In most cases, the principal problem will probably be discovered before moving totally through the list.

9.8 PULLEY FLANGES
Pulley guidebook flanges are necessary to preserve synchronous belts operating on the pulleys. As talked about previously in Section 9.7 on belt tracking, it is regular for synchronous belts to favor one part of the pulleys when working. Proper flange design is important in preventing belt edge wear, minimizing sound and stopping the belt from climbing out from the pulley. Dimensional recommendations for custom-made or molded flanges are contained in tables coping with these issues. Proper flange placement is important to ensure that the belt is usually adequately restrained within its operating system. Because style and layout of little synchronous drives is so diverse, the wide variety of flanging situations potentially encountered cannot quickly be protected in a straightforward set of guidelines without acquiring exceptions. Despite this, the following broad flanging guidelines should help the developer in most cases:

Two Pulley Drives: On basic two pulley drives, each one pulley should be flanged in both sides, or each pulley should be flanged on reverse sides.

Multiple Pulley Drives: On multiple pulley (or serpentine) drives, either almost every other pulley should be flanged on both sides, or every pulley ought to be flanged on alternating sides around the machine. Vertical Shaft Drives: On vertical shaft drives, at least one pulley ought to be flanged on both sides, and the remaining pulleys should be flanged on at least underneath side.

Long Span Lengths: Flanging suggestions for small synchronous drives with lengthy belt span lengths cannot conveniently be defined due to the many factors that may affect belt tracking characteristics. Belts on drives with long spans (generally 12 times the diameter of the smaller pulley or more) often require more lateral restraint than with short spans. Due to this, it really is generally smart to flange the pulleys on both sides.

Large Pulleys: Flanging large pulleys can be costly. Designers frequently wish to leave huge pulleys unflanged to lessen price and space. Belts generally tend to require much less lateral restraint on huge pulleys than little and can often perform reliably without flanges. When deciding whether or not to flange, the previous guidelines should be considered. The groove encounter width of unflanged pulleys should also be greater than with flanged pulleys. See Table 27 for recommendations.

Idlers: Flanging of idlers is normally not necessary. Idlers designed to carry lateral side loads from belt tracking forces can be flanged if needed to offer lateral belt restraint. Idlers used for this function can be utilized on the inside or backside of the belts. The previous guidelines should also be considered.

9.9 REGISTRATION
The three primary factors contributing to belt drive registration (or positioning) errors are belt elongation, backlash, and tooth deflection. When evaluating the potential registration features of a synchronous belt drive, the system must first be motivated to end up being either static or dynamic when it comes to its sign up function and requirements.

Static Registration: A static registration system moves from its preliminary static position to a secondary static position. Through the process, the designer is concerned only with how accurately and consistently the drive arrives at its secondary position. He/she is not worried about any potential registration errors that take place during transportation. Therefore, the principal factor adding to registration error in a static registration system is certainly backlash. The consequences of belt elongation and tooth deflection don’t have any influence on the registration precision of this kind of system.

Dynamic Sign up: A powerful registration system must perform a registering function while in motion with torque loads varying as the machine operates. In this instance, the designer is concerned with the rotational position of the travel pulleys regarding each other at every point in time. Therefore, belt elongation, backlash and tooth deflection will all contribute to registrational inaccuracies.

Further discussion on the subject of each of the factors contributing to registration error is as follows:

Belt Elongation: Belt elongation, or stretch, occurs naturally whenever a belt is placed under stress. The total pressure exerted within a belt results from set up, along with operating loads. The amount of belt elongation is certainly a function of the belt tensile modulus, which can be influenced by the type of tensile cord and the belt construction. The typical tensile cord found in rubber synchronous belts can be fiberglass. Fiberglass includes a high tensile modulus, is dimensionally stable, and has excellent flex-fatigue features. If an increased tensile modulus is needed, aramid tensile cords can be considered, although they are generally used to provide resistance to harsh shock and impulse loads. Aramid tensile cords used in little synchronous belts generally possess only a marginally higher tensile modulus in comparison to fiberglass. When required, belt tensile modulus data is certainly available from our Software Engineering Department.

Backlash: Backlash in a synchronous belt drive results from clearance between the belt teeth and the pulley grooves. This clearance is needed to allow the belt tooth to enter and exit the grooves smoothly with a minimum of interference. The amount of clearance necessary depends upon the belt tooth account. Trapezoidal Timing Belt Drives are known for having fairly small backlash. PowerGrip HTD Drives have improved torque carrying capability and withstand ratcheting, but possess a significant quantity of backlash. PowerGrip GT2 Drives possess even more improved torque having capability, and also have as little or much less backlash than trapezoidal timing belt drives. In special cases, alterations can be made to travel systems to help expand lower backlash. These alterations typically lead to increased belt wear, increased travel noise and shorter travel life. Contact our Program Engineering Division for more information.

Tooth Deflection: Tooth deformation in a synchronous belt drive occurs as a torque load is put on the machine, and individual belt teeth are loaded. The quantity of belt tooth deformation depends upon the quantity of torque loading, pulley size, installation stress and belt type. Of the three major contributors to registration mistake, tooth deflection is the most difficult to quantify. Experimentation with a prototype get system is the best method of obtaining practical estimations of belt tooth deflection.

Additional guidelines which may be useful in designing registration essential drive systems are the following:
Select PowerGrip GT2 or trapezoidal timing belts.
Style with large pulleys with more tooth in mesh.
Keep belts limited, and control stress closely.
Design frame/shafting to be rigid under load.
Use high quality machined pulleys to minimize radial runout and lateral wobble.

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