Timing Belts and Pulleys - Operations

9.1 LOW-SPEED OPERATION
Synchronous drives are specially well-suitable for low-speed, high torque applications. Their positive driving nature prevents potential slippage associated with V-belt drives, and even allows significantly greater torque carrying capacity. Little pitch synchronous drives operating at speeds of 50 ft/min (0.25 m/s) or much less are believed to be low-speed. Care ought to be taken in the drive selection process as stall and peak torques can sometimes be high. While intermittent peak torques can frequently be carried by synchronous drives without particular considerations, high cyclic peak torque loading should be carefully reviewed.

Proper belt installation tension and rigid drive bracketry and framework is vital in preventing belt tooth jumping under peak torque loads. It is also helpful to design with more than the normal the least 6 belt tooth in mesh to ensure adequate belt tooth shear power.

Newer generation curvilinear systems like PowerGrip GT2 and PowerGrip HTD ought to be used in low-swiftness, high torque applications, as trapezoidal timing belts are more susceptible to tooth jumping, and have significantly 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 suitable. They are generally used due to their positive generating characteristic (no creep or slip), and because they require minimal maintenance (don't stretch considerably). A substantial drawback of high-swiftness synchronous drives is usually drive noise. High-acceleration synchronous drives will almost always produce more noise than V-belt drives. Small pitch synchronous drives working at speeds more than 1300 ft/min (6.6 m/s) are believed to be high-speed.

Special consideration should be given to high-speed drive designs, as a number of factors can significantly influence belt performance. Cord fatigue and belt tooth wear are the two most significant elements that must be controlled to ensure success. Moderate pulley diameters should be used to reduce the rate of cord flex fatigue. Developing with a smaller pitch belt will most likely offer better cord flex fatigue characteristics than a bigger pitch belt. PowerGrip GT2 is especially well suited for high-rate drives due to its excellent belt tooth entry/exit characteristics. Clean interaction between the belt tooth and pulley groove minimizes wear and sound. Belt installation stress is especially critical with high-acceleration drives. Low belt stress allows the belt to trip out of the driven pulley, resulting in rapid belt tooth and pulley groove wear.

9.3 SMOOTH RUNNING
Some ultrasensitive applications require the belt drive to use with as little vibration aspossible, as vibration sometimes has an effect on the system operation or finished produced product. In these cases, the features and properties of most appropriate belt drive products ought to be reviewed. The ultimate drive program selection should be based upon the most critical style requirements, and could require some compromise.

Vibration isn't generally considered to be a issue with synchronous belt drives. Low levels of vibration typically derive from the procedure of tooth meshing and/or as a result of their high tensile modulus properties. Vibration resulting from tooth meshing is a standard characteristic of synchronous belt drives, and can't be totally eliminated. It could be minimized by staying away from small pulley diameters, and instead choosing moderate sizes. The dimensional precision of the pulleys also influences tooth meshing quality. Additionally, the installation pressure 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 tension variation with each pulley revolution. V-belt pulleys are also produced with some radial go out, but V-belts possess a lesser tensile modulus leading to less belt pressure variation. The high tensile modulus within synchronous belts is essential 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 various potential sources of noise in something, 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 boosts as operating acceleration and belt width boost, and as pulley size reduces. Drives designed on moderate pulley sizes without excessive capability (overdesigned) are generally the quietest. PowerGrip GT2 drives have been found to be significantly quieter than additional systems because of their improved meshing characteristic, see Figure 9. Polyurethane belts generally produce more noise than neoprene belts. Proper belt installation tension is also very essential in minimizing travel noise. The belt should be tensioned at a level which allows it to run with as little meshing interference as possible.

Drive alignment also offers a significant influence on drive sound. Special attention ought to be given to Dry Screw Vacuum Pumps reducing angular misalignment (shaft parallelism). This assures that belt tooth are loaded uniformly and minimizes part monitoring forces against the flanges. Parallel misalignment (pulley offset) is not as essential of a problem provided that the belt isn't trapped or pinched between contrary flanges (start to see the special section coping with travel alignment). Pulley materials and dimensional precision also influence get noise. Some users possess discovered that steel pulleys are the quietest, followed closely by light weight aluminum. Polycarbonates have already been discovered to become noisier than metallic components. Machined pulleys are generally quieter than molded pulleys. The reasons because of this revolve around materials density and resonance features along with dimensional accuracy.

9.5 STATIC CONDUCTIVITY
Little synchronous rubber or urethane belts can generate an electrical charge while operating in a drive. Elements such as for example humidity and operating speed influence the potential of the charge. If identified to become a problem, rubber belts can be produced in a conductive building to dissipate the charge in to the pulleys, and to floor. This prevents the accumulation of electrical charges that may be harmful to materials handling processes or sensitive electronics. In addition, it greatly reduces the potential for arcing or sparking in flammable conditions. Urethane belts cannot be produced in a conductive construction.

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

Nonconductive belt constructions are also available for rubber belts. These belts are usually built particularly to the clients conductivity requirements. They are generally used in applications where one shaft must be electrically isolated from the additional. It is important to note that a static conductive belt cannot dissipate an electrical charge through plastic 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 may also be used to dissipate electrical charges.

Urethane timing belts aren't static conductive and cannot be built in a particular conductive construction. Special conductive rubber belts ought to be used when the existence of an electrical charge is usually a concern.

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

Dust: Dusty conditions usually do not generally present serious complications to synchronous drives as long as the particles are good and dry out. Particulate matter will, however, become an abrasive resulting in a higher rate of belt and pulley put on. Damp or sticky particulate matter deposited and loaded into pulley grooves can cause belt tension to increase considerably. This increased stress can influence shafting, bearings, and framework. Electrical costs within a drive system can sometimes appeal to particulate matter.

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

Drinking water: Light and occasional contact with drinking water (occasional clean downs) should not seriously affect synchronous belts. Prolonged get in touch with (constant spray or submersion) results in significantly reduced tensile strength in fiberglass belts, and potential duration variation in aramid belts. Prolonged contact with water also causes rubber compounds to swell, although less than with oil get in touch with. Internal belt adhesion systems are also steadily broken down with the existence of drinking water. Additives to drinking water, such as lubricants, chlorine, anticorrosives, etc. can have a far more detrimental effect on the belts than pure water. Urethane timing belts also suffer from drinking water contamination. Polyester tensile cord shrinks significantly and experiences lack of tensile power in the existence of drinking water. Aramid tensile cord keeps its strength fairly well, but experiences duration variation. Urethane swells a lot more than neoprene in the presence of drinking water. This swelling can increase belt tension significantly, leading to belt and related hardware problems.

Oil: Light connection with natural oils on an occasional basis will not generally damage synchronous belts. Prolonged connection with essential oil or lubricants, either straight or airborne, outcomes in considerably reduced belt service existence. Lubricants cause the rubber compound to swell, breakdown inner adhesion systems, and reduce 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 compounds found in rubber synchronous belts. Ozone degrades belt materials in quite similar way as excessive environmental temperature ranges. Although the rubber components found in synchronous belts are compounded to withstand the consequences of ozone, ultimately chemical substance breakdown occurs and they become hard and brittle and begin cracking. The quantity of degradation depends upon the ozone concentration and duration of publicity. For good overall 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 could be detrimental to the substances found in rubber and urethane synchronous belts. Radiation degrades belt materials quite similar way extreme environmental temperatures do. The quantity of degradation depends upon the intensity of radiation and the exposure time. For good belt performance, the following exposure levels shouldn't be exceeded:
Standard Construction: 108 rads
Nonm arking Construction: 104 rads
Conductive Construction: 106 rads
Low Temperatures Construction: 104 rads

Dust Generation: Rubber synchronous belts are recognized to generate small quantities of fine dust, as a natural consequence of their operation. The amount of dust is typically higher for new belts, because they operate in. The time period for run directly into occur is dependent upon the belt and pulley size, loading and swiftness. Elements such as for example pulley surface finish, operating speeds, set up pressure, and alignment impact the number of dust generated.

Clean Area: Rubber synchronous belts might not be suitable for use in clean space environments, where all potential contamination must be minimized or eliminated. Urethane timing belts typically generate significantly less particles than rubber timing belts. Nevertheless, they are recommended limited to light working loads. Also, they cannot be produced in a static conductive structure to permit electrical fees to dissipate.

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

In sensitive applications, rubber synchronous belts are preferred over urethane belts since urethane belting can't be produced in a conductive construction.

9.7 BELT TRACKING
Lateral tracking characteristics of synchronous belts is certainly a common area of inquiry. While it is normal for a belt to favor one side of the pulleys while running, it is abnormal for a belt to exert significant pressure against a flange leading to belt edge use and potential flange failure. Belt tracking can be influenced by many factors. To be able of significance, discussion about these factors is as follows:

Tensile Cord Twist: Tensile cords are formed into a solitary twist configuration throughout their produce. Synchronous belts made with only single twist tensile cords track laterally with a substantial force. To neutralize this tracking push, tensile cords are produced in right- and left-hand twist (or "S" and "Z" twist) configurations. Belts made out of "S" twist tensile cords monitor in the opposite direction to those constructed with "Z" twist cord. Belts made with alternating "S" and "Z" twist tensile cords monitor with minimal lateral force since the tracking characteristics of both cords offset each other. The content of "S" and "Z" twist tensile cords varies slightly with every belt that is produced. As a result, every belt comes with an unprecedented tendency to track in each one path or the various other. When an application requires a belt to monitor in a single specific direction just, a single 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 tracking push. Synchronous belts have a tendency to monitor "downhill" to circumstances of lower pressure or shorter center distance.

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

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

Belt Length: Because of the way tensile cords are applied to the belt molds, short belts can tend to exhibit higher tracking forces than longer 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 little pitch synchronous belts. Sag in lengthy belt spans should be prevented by applying sufficient belt installation tension.

Torque Loads: Sometimes, while functioning, a synchronous belt will move laterally from side to side on the pulleys instead of operating in a consistent position. While not generally considered to be a substantial concern, one explanation for this is normally varying torque loads within the travel. Synchronous belts occasionally track in different ways with changing loads. There are plenty of potential reasons for this; the primary cause is related to tensile cord distortion while under great pressure against the pulleys. Variation in belt tensile loads may also cause changes in framework deflection, and angular shaft alignment, leading to belt movement.

Belt Installation Tension: Belt tracking may also be influenced by the level of belt installation pressure. The reason why for this are similar to the effect that varying torque loads have on belt tracking. When issues with belt monitoring are experienced, each one of these potential contributing elements ought to be investigated in the order that they are outlined. Generally, the primary problem will probably be determined before moving totally through the list.

9.8 PULLEY FLANGES
Pulley information flanges are necessary to hold synchronous belts operating on the pulleys. As discussed previously in Section 9.7 on belt tracking, it really is regular for synchronous belts to favor one part of the pulleys when working. Proper flange style is essential in stopping belt edge use, minimizing sound and avoiding the belt from climbing out from the pulley. Dimensional recommendations for custom-produced or molded flanges are included in tables dealing with these problems. Proper flange placement is important to ensure that the belt is usually adequately restrained within its operating system. Because style and design of little synchronous drives is indeed different, the wide selection of flanging situations potentially encountered cannot quickly be covered in a simple set of rules without getting exceptions. Not surprisingly, the following broad flanging guidelines should help the designer generally:

Two Pulley Drives: On simple two pulley drives, either 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 every other pulley should be flanged in both sides, or every single pulley ought to be flanged in 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 rest of the pulleys ought to be flanged on at least the bottom side.

Long Period Lengths: Flanging suggestions for little synchronous drives with lengthy belt span lengths cannot conveniently be defined because of the many factors that can affect belt tracking qualities. Belts on drives with long spans (generally 12 times the diameter of small pulley or even more) often require even more lateral restraint than with brief spans. Because of this, it is generally smart to flange the pulleys on both sides.

Huge Pulleys: Flanging huge pulleys can be costly. Designers frequently wish to leave large pulleys unflanged to lessen price and space. Belts tend to need much less lateral restraint on large pulleys than little and can frequently perform reliably without flanges. When choosing whether or not to flange, the previous guidelines is highly recommended. 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 generally not necessary. Idlers made to carry lateral part loads from belt tracking forces can be flanged if needed to offer lateral belt restraint. Idlers utilized for this purpose can be used inside or backside of the belts. The previous guidelines should also be considered.

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

Static Sign up: A static registration system moves from its preliminary static position to a secondary static position. During the process, the designer is concerned just with how accurately and regularly the drive finds its secondary position. He/she is not concerned with any potential sign up errors that occur during transport. Therefore, the primary factor contributing to registration error in a static registration system is certainly backlash. The consequences of belt elongation and tooth deflection do not have any influence on the sign up accuracy of this kind of system.

Dynamic Registration: A powerful registration system is required to perform a registering function while in motion with torque loads different as the system operates. In this case, the designer is concerned with the rotational position of the drive pulleys regarding each other at every point in time. Therefore, belt elongation, backlash and tooth deflection will all donate to registrational inaccuracies.

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

Belt Elongation: Belt elongation, or stretch out, occurs naturally whenever a belt is positioned under tension. The total tension exerted within a belt results from set up, along with working loads. The amount of belt elongation is certainly a function of the belt tensile modulus, which is normally influenced by the kind of tensile cord and the belt construction. The standard tensile cord used in rubber synchronous belts is usually fiberglass. Fiberglass has a high tensile modulus, is dimensionally stable, and has excellent flex-fatigue characteristics. If a higher tensile modulus is necessary, 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 small synchronous belts generally possess only a marginally higher tensile modulus compared to fiberglass. When required, belt tensile modulus data is definitely 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 required to allow the belt teeth to enter and exit the grooves smoothly with a minimum of interference. The quantity of clearance required depends upon the belt tooth account. Trapezoidal Timing Belt Drives are recognized for having fairly little backlash. PowerGrip HTD Drives have improved torque holding capability and resist ratcheting, but have a significant amount of backlash. PowerGrip GT2 Drives possess even further improved torque having capability, and also have only a small amount or much less backlash than trapezoidal timing belt drives. In particular cases, alterations can be made to travel systems to further lower backlash. These alterations typically lead to increased belt wear, increased drive sound and shorter drive life. Get in touch with our Application Engineering Division for more information.

Tooth Deflection: Tooth deformation in a synchronous belt drive occurs as a torque load is applied to the system, and individual belt teeth are loaded. The quantity of belt tooth deformation is dependent upon the quantity of torque loading, pulley size, installation pressure and belt type. Of the three major contributors to registration error, tooth deflection is the most challenging to quantify. Experimentation with a prototype travel system is the best method of obtaining practical estimations of belt tooth deflection.

Additional guidelines which may be useful in designing registration vital drive systems are the following:
Select PowerGrip GT2 or trapezoidal timing belts.
Design with large pulleys with an increase of tooth in mesh.
Keep belts limited, and control stress closely.
Design body/shafting to end up being rigid under load.
Use top quality machined pulleys to reduce radial runout and lateral wobble.

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