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Keeping timing belts in synch
By: Andrew Dittmer
As published 04/12/2006 in Machine Design Magazine

How tooth profiles, belt materials, and sprocket tolerances influence positioning accuracy? Let's explore.
Keeping timing belts in synch

Power transmission drives on equipment such as printers, robots, and x-y coordinate machines require accurate registration.

Around and Around

A belt connects two equal-sized sprockets. Exerting a force F on sprocket A causes it to rotate?°, which, through a reaction force in the belt, causes sprocket B to rotate ?°a°.

Trapezoidal Profile

A trapezoidal belt tooth makes line contact with the sprocket groove in the root radius and upper flank under load. Concentrated stress along these points can increase belt wear and decrease positioning accuracy.

High Tourque Drive Profile

HTD profiles provide excellent torque capacity, but require substantial clearance.

Modified Curvilinear Profile

A modified curvilinear drive system combines the registration accuracy of a trapezoidal drive with the increased load-carrying capacity of an HTD drive.

Candle Profile

Positioning belt drives are found in motion transfer applications, such as conveyors and packaging equipment. Here, they use a unique foam backing to grasp and transport candles for cooling.

Synchronous belts originally known as timing belts precisely match, or synchronize, the motion of two or more shafts. They keep shafts in synch because they have a toothed surface that meshes with grooves in the associated sprockets. This makes them ideal for indexing, positioning, and constant-speed applications, eliminates backlash, and lets relatively small drive forces operate over great distances.

Like any multiaxis timing function, synchronous belt drives require highly accurate positioning, or registration. Registration is the difference in angular position between two sprockets and can be classified as static or dynamic. Static registration concerns how accurately a drive moves from its initial to secondary position, and is determined primarily by backlash. Dynamic registration, on the other hand, is a measure of accuracy over an entire cycle and is subject to belt elongation, backlash, and tooth deflection. Both types of positioning accuracy must be considered when selecting belt drives.

Profile types
In a synchronous belt drive, the shape and fit of the belt tooth in the sprocket groove, or profile, are the starting point when determining positioning accuracy. Common profiles include trapezoidal, high torque drive (HTD), and modified curvilinear. Proper clearance between a belt tooth and sprocket groove lets the tooth enter and exit smoothly. Clearance values set by belt manufacturers vary depending on pitch size, but generally range from 0.002 to 0.007 in. Too much clearance creates inaccuracy, while too little can generate excessive noise, vibration, and wear. An ideal tooth profile minimizes clearance without harming belt operations.

Trapezoidal profiles originated in the 1940s and are so named because of the shape they assume inside sprocket grooves under load. While this fit provides accuracy initially, concentrated stress along the points of contact accelerates belt wear, decreasing longevity and accuracy. In addition, the tooth profile s typically shallow depth and low flank angle limit load-carrying capabilities and make the belt vulnerable to ratcheting (tooth jumping). Nonetheless, trapezoidal profile systems are still viable today, particularly for positioning applications on lightly loaded drives.

The HTD profile, developed in the 1970s, addresses load-carrying limitations of trapezoidal belt drives. Its capacity advantage stems from deep, rounded tooth shapes having a higher (steeper) flank angle and greater contact area than trapezoidal profiles. These larger belt teeth, however, require substantial clearance to enter and exit the sprocket groove cleanly. The high backlash inherent in this design generally relegates HTD profiles to applications requiring little positioning accuracy.

In the early 1980s, another profile emerged, the modified curvilinear. A refinement of the HTD design, it features reduced tooth depth, increased flank angle, and minimized clearance. What s more, the belt land area (in between teeth) shares tensile load support with the belt tooth compression in the sprocket grooves, resulting in excellent ratcheting resistance.

Deflection and elongation

Two other factors affecting positional accuracy are tooth deflection and elongation. Regardless of the tooth profile, if it distorts or deflects under load, it causes lost motion, which increases positional inaccuracy.

Although difficult to quantify, the amount of belt tooth deflection depends largely on loading, sprocket size, and belt installation tension. In general, the harder the tooth material, the less the deflection. Two common materials used in power transmission belts are chloroprene (Neoprene) and polyurethane, both of which can be hardened by varying their chemical formulas. A word of caution: Material that s too hard may negatively impact the belt s flex fatigue characteristics and increase drive noise.

Elongation, an even bigger concern, is caused by placing a belt under a large tensile load. Installation and working-tension loads contribute to a belt s total tension. The amount a belt stretches depends on the load applied, and more so, on the modulus of the belt/tensile cord reinforcement. A higher modulus belt stretches less and improves positional accuracy. However, an increased tensile-cord modulus must balance with sufficient belt flex-fatigue characteristics to maintain longevity. Modulus varies depending on the belt pitch size and material construction.

Fiberglass is the most common reinforcement in synchronous belts. It provides a high tensile modulus for many applications, as well as excellent flexibility. Aramid and steel tensile cords offer higher modulus, but sacrifice bending flexibility. Carbon tensile cords are fairly new and only available for special applications. Carbon cords provide an excellent balance between high tensile modulus and bending flexibility.

Sprocket tolerances
All sprockets need tight tolerances to fit and mesh with a synchronous belt, and the material they re made of directly affects this. Standard metallic sprockets with machined grooves, for instance, often suffice for registration drives, but plastic molded sprockets may not, due to their reduced wear resistance. Sometimes, designers increase sprocket accuracy by reducing clearance grooves as a way to increase belt drive positional accuracy. While somewhat beneficial, this can cause accelerated belt wear and noise in high-speed applications.

Another tactic to improve positional accuracy is slightly oversizing the sprocket outside diameter (O.D.) so its pitch is just larger than the belt s. (On a sprocket, pitch is the distance between groove centers and is measured on the pitch circle of the sprocket.) When using this tactic, designers typically oversize sprocket O.D. by the standard tolerance and set the new O.D. tolerance at one half the original. This improved fit between the belt teeth and sprocket grooves yields greater interference, improving positional accuracy. For example, if the standard sprocket O.D. is 3.000 +0.003/-0.000 in., then the new sprocket O.D. would be 3.003 +0.0015/-0.000 in.

Sprocket plating is a third technique for improving belt drive positional accuracy. A 0.0005 to 0.002-in. layer of nickel-plating increases sprocket O.D. slightly while reducing groove clearance.

Selecting a drive design
The right pitch size and service factor also help improve positioning accuracy. When determining pitch size as well as sprocket size and belt width designers should consider the application s load and speed requirements. To further assist with selection, belt manufacturers also provide design manuals and software programs.

Depending on the severity of a load s characteristics, a service factor may be applied. Generally, 2.0 or higher is recommended for drives requiring precise positioning. In drives that have not been properly sized, the belt can lift in the sprocket grooves, increasing positioning errors.

Based on the drive selected, belt designs may be available in more than one pitch. For example, one drive option may use an 8-mm pitch, 62-mm belt, while another uses a 14-mm pitch, 37-mm belt. Smaller pitches are usually preferred because they provide better positional accuracy, due to reduced clearance.


Tensile modulus
Bending flexibility
Aramid (Kevlar)
Various materials can comprise a synchronous belt's reinforcement, each delivering specific moduli and flexibility.

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