| Angular Misalignment |
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| Angular misalignment is where the driver and driven shafts are not parallel. Resulting in uneven loading of the tensile cords. The tensile cords on the high tension side are often overloaded which may cause edge cord failure which would be transmitted across the width of the belt. This misalignment also results in high belt tracking forces which causes excessive belt edge wear. |
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Aramid – Tensile Cord
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Aramid fibers (most commonly known as 'Kevlar' or 'Twaron'), which have been commercially available since the 1960s, have found a wide field of applications.
Aramid fibers offer the following properties:
- Low density
- High tensile modulus
- High tensile strength
- Good vibration damping
- High energy absorption
- High impact resistance
- Low material fatigue
- Good temperature resistance
- Good chemical resistance
- Low thermal conductivity
- Low compression strength
- Moderate adhesive properties
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| Axial Run Out |
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| The total deviation of the axial reference surface noted during one revolution of the work piece. It is expressed as TIR (Total Indicator Reading). |
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| BackLash |
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| Backlash can be defined as free play between two mating parts. A prime example of backlash is sloppy steering in a car, where the steering wheel can be turned left or right a small amount with no change in the direction of the car. Backlash in a synchronous belt results from clearance between belt teeth and pulley grooves. This clearance is needed to allow the belt teeth to enter and exit the grooves smoothly with a minimum of interference; the amount of clearance necessary depends upon the belt tooth profile. Too much clearance creates positional inaccuracy, while too little can generate excessive noise, vibration and wear. |
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| Belt Pitch Length |
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| Belt pitch length is the total length (circumference) in millimeters as measured along the pitch line |
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| Belt Pitch Line |
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| Belt pitch line in theory lies within the tensile member of the belt |
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| Belt Pitch |
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Belt pitch is the distance in millimeters between two adjacent tooth centers as measured on the pitch line of the belt
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| Clearance Values |
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| The clearance between the synchronous belt teeth and the matching synchronous pulley teeth is the principal indication of backlash in a drive. Proper clearance between a belt tooth and a pulley groove lets the tooth enter and exit smoothly. |
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| CTD Profile |
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| (CTD: Conti Torque Drive) is the symbiosis of the HTD and the STD profile and combines both profile advantages in a single profile. The arch-shaped pulley-entry geometry, on the one hand, and the higher tooth, on the other, makes for ideal conditions for use on dynamic drives with simultaneously high tension load. |
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| Elongation |
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| Elongation of a belt or stretch occurs naturally when a belt is placed under tension. The total tension exerted within a belt results from installation, as well as working loads. The amount of belt elongation is a function of the belt tensile modulus, which is influenced by the type of tensile cord and the belt construction.
A belt with a high tensile modulus stretches less and improves positional accuracy, however in order to maintain the belts life the tensile cord must also exhibit flexibility. |
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| Flanges |
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Timing belts typically track to one side of a pulley during operation, and would slide off the pulley if it was not flanged.
FLANGING GUIDELINES:
Two Pulley Drives - On simple two pulley drives, either one pulley should be flanged on both sides, or each pulley should be flanged on opposite sides
Multi Pulley Drives - On multiple pulley drives (i.e. more than two pulleys, serpentine), either every other pulley should be flanged on both sides, or every pulley should be flanged on alternating sides around the system
Vertical Shaft Drives - On vertical shaft drives, at least one pulley should be flanged on both sides, and the remaining pulleys should be flanged on at least the bottom side
Long Span Drives - Belts on drives with long spans, typically (8x) times the diameter of the smaller pulley require both pulleys to be flanged |
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| Flank (Pulley Tooth Flank) |
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The surface area of a pulley tooth between the pitch circle and the bottom land along the length of the tooth, including the fillet. |
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| Fiberglass - Tensile Cord |
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Fiberglass tensile cords are the most common reinforcement in synchronous belts.
Fiberglass fibers offer the following properties:
- Low tensile modulus
- High bending flexibility
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| HTD Profile |
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(HTD: High Torque Drive) offers especially good protection from belt ratcheting. This is thanks to the height of its teeth and their semi-rounded geometry. However, because of the larger belt teeth which require substantial clearance (generating backlash) to enter and exit the pulley groove cleanly. The HTD profile is typically used on applications that require minimal positional accuracy. The HTD profile or curvilinear profile is admirably suited to transmitting high torque. |
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| Pulley Pitch |
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| Parallel Misalignment |
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| Pulley Pitch Circle |
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| Pulley pitch circle coincides with the belt pitch line mating with it |
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| Pulley Pitch Diameter |
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| PolyChain GT Profile |
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The PolyChain GT profile is a modified curvilinear profile that improves upon the HTD profile. Unlike the HTD profile the PolyChain GT profile features reduced tooth depth, increased flank angle, and minimized clearance. Translating into excellent ratcheting resistance and increased load carrying capacity. |
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| Ratcheting (Tooth Jumping) |
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Ratcheting in a synchronous belt drive occurs when the drive is under-tensioned. The belt pitch begins to mismatch the driven sprocket pitch. Modified curvilinear tooth profiles (deeper and steeper teeth) have better anti-ratcheting characteristics compared to trapezoidal tooth profiles. Combining the modified curvilinear profile with a polyurethane construction increase the resistance of tooth deflection, thus providing greater anti-ratcheting characteristics than rubber belts. |
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| Radial Run Out |
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| Registration (Positioning) |
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Registration is the difference in angular position between two sprockets and can be classified as STATIC or DYNAMIC. The three factors contributing to registration/positioning errors are:
- Belt Elongation
- Backlash
- Tooth Deflection
STATIC registration is defined as the movement from an initial static position to a secondary static position. In designing a system it is only critical for how accurately and consistently the movement stops at its secondary position. Potential registration errors occurring during movement is of no concern. The only concern is backlash, and the effects of belt elongation and tooth deflection will not influence the final outcome.
DYNAMIC registration is defined as the movement required to perform a registering function while in motion with torque loads varying as the system operates. In designing a system it is critical that the rotational position of the drive pulleys in relation to each other is known at every point in time. In a system experiencing dynamic registration, all three factors (belt elongation, backlash and tooth deflection) will contribute to registration errors. |
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| STD Profile |
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| (STD: Super Torque Drive) provides optimum engagement performance thanks to its arched geometry. Even at high belt speeds, drives with the STD profile exhibit very good running precision and are extremely quiet in operation |
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| Synchronous Belts (Timing Belts, Cog Belts) |
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| Synchronous belt drives operate by positively engaging the teeth molded to the belt with the corresponding teeth of a pulley. Synchronous belts do not rely on friction to transmit power (i.e. V-belts), and should not be confused with mold-notch V-belts which transmit power by the wedging action of the V-shape. The positive engagement of the two sets of meshing teeth allows synchronous belts to transmit large torques and withstand large accelerations. Because to the positive engagement there is little relative motion and most importantly NO SLIP between the meshing teeth. Synchronous belts are extremely useful in applications where indexing, positioning or a constant speed ratio is required. |
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| Taper Bushing |
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Tapered bushings work on a wedging action that when the tapered bushing is tightened by screws the I.D. taper of the pulley and the matching O.D. taper of the bushing are drawn together which at the same time contracts the inside diameter securing it to the shaft. The I.D. contraction is the equivalent to a press fit (interference fit) that offers a higher strength alternative for transmitting torque and resisting slippage than single or even multiple setscrew style connections. Another advantage is that it eases maintenance for installation and removal and will not mar shafts as set screws would.
The three main types of tapered bushings are:
- Flanged Bushing
- Flangeless Bushing
- Keyless Bushings (Shaft Locking Devices)
The two most popular types of flanged bushings are SPLIT TAPER and QD.
SPLIT TAPER
The tapered portion of the split tapered bushing is split in two places, with the split ending before it enters the flange. The bushing is keyed to the shaft, and the outside diameter of the barrel is keyed to the pulley component as well
Advantages:
- The double keyed connection allows the drive to continue to operate (transmit torque) even if the fasteners connection the mating tapers comes loose.
Disadvantages:
- The “flange” on the pulley increases weight and requires more space for mounting
- The split taper bushing fasteners can only be inserted from the flange side only
QD
The tapered portion of a QD bushing is split in one place only, with the split extending through the flange. There is also only one keyed connection between the bushing and shaft.
Advantages:
- The QD bushing can be inserted from either direction for standard or reverse mounting
Disadvantages:
- The “flange” on the pulley increases weight and requires more space for mounting
- Since the only keyed connection is between the bushing and the shaft. If the fasteners securing the bushing to the pulley become loose the bushing could slip within the pulley.
Are typically called TAPER LOCK bushings and are differentiated by not having a flange.
Advantages:
- Larger taper angle permits tightening the pulley with less displacement along the shaft. This makes it easier to accurately locate the pulley on the shaft where precise positioning is required.
- Full length of the bushing supports the pulley
- The flangeless design allows for use of less shaft space.
Disadvantages:
- Difficult installation, requiring the installation instructions be fully read and followed.
Keyless bushings convert clamping action between inner and outer tapered rings into circumferential uniform radial pressure that locks the device to the shaft and the pulley. As the name implies keyless bushings do not have a keyway.
Advantages:
- Use of smaller diameter shaft sizes or hollow shaft sizes for weight reduction
- Allows for infinite adjustment for timing purposes
- Can be used over damaged shaft keyways
- Ability to transmit high torques
- bility to take high axial forces and shocks
Disadvantages:
- Cost is substantially more than other taper bushings
- Limited number of bore sizes
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| Tensile Member |
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| The tensile cord provides the backbone of the belt. These cords give the synchronous belt high strength, excellent flex life and high resistance to elongation. Commonly used materials include fiberglass, aramid (kevlar), steel, polyester and carbon fiber. Tensile cord members are constructed of several filaments twisted around each other. The twisting of the filaments into a cord is commonly referred to as “S” twist or a “Z” twist. The two different twists will cause the belt to track to one side of the pulley depending on which twist is employed. To counteract this force a belt is typically constructed with the use of both twists wound in opposite directions around the belt. However, with that said the belt still has the tendency to favor the tracking of the “S” twist. |
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| Tooth Deflection (Tooth Deformation) |
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No matter which tooth profile is chosen if it deforms or deflects under a torque load, it causes lost motion which increases positional inaccuracy. The main factors contributing to synchronous belt tooth deflection include torque loading, pulley size, and installation tension and belt material type.
As can be imagined the harder the belt material type the less the deflection. However, using to hard of a material negatively impacts the belt’s flex fatigue characteristics and increases drive noise. |
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| Trapezoidal Profile |
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| Trapezoidal profiles provide a high degree of precision indexing or registration. Unlike curvilinear profile belts which have full flank contact, trapezoidal belts contact the pulley in the root radius (radius of the root circle which contains the bottom of the tooth spaces) and in the upper flank area only. The load carrying capacities of the trapezoidal tooth profile is severely limited and makes the belt highly susceptible to ratcheting (tooth jumping). |
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