Why choose a timing belt over a flat belt or V-belt?

This is a question commonly asked by engineers looking to design or specify power transmission systems. And the answer to this question is straightforward: timing belts offer precision registration and timing with no loss of high torque carrying capacity. They also usually have higher mechanical efficiency and power density than flat and V-belts, which means smaller belt and pulley drives can be employed for the transmission of equivalent loads

However, applications will only benefit if timing belt drives are correctly sized and specified for the particular application.

[Learn more about imperial-sized timing belts on GlobalSpec.com.]

Application requirements

As with most design processes, when considering the design of a timing belt, engineers must first address the application requirements. For example, designers must know how much torque (and system transmission power) is needed to do the intended work or accomplish a particular motion.

After determining the peak torque and system transmission power, designers must account for unusual momentary or shock loads that may occur during operation. These unusual loads can be accounted for by using a specified overload factor, Ks. The overload factor values can be found in the timing pulleys and belt catalogs provided by manufacturers.

Once designers know the overload factor for their application requirements, the total system design power can be calculated as follows.

Equation 1: Timing belt overload factor.Equation 1: Timing belt overload factor.

Timing belt tooth profile

Although belt manufacturers typically offer exclusive tooth designs, most of these designs are based on three basic profiles:

  • Trapezoidal belt tooth profile
  • Curvilinear belt tooth profile
  • Modified curvilinear tooth profile

Belts with trapezoidal tooth profiles offer good force transmitting capabilities and are ideal for linear positioning and conveying applications. They also produce very little backlash, mitigating the risk of slipping under low to moderate torques. However, keep in mind that this belt profile causes high-stress concentration at the belt-pulley interface, resulting in high wear rates under high torque and speed applications.

Curvilinear tooth profiles alleviate the stress concentrations in trapezoidal tooth profiles. They have a smoother transition at their edges compared to trapezoidal tooth profiles, making them ideal for applications requiring high torque transfer capabilities such as mechanical hoists or elevators.

Modified curvilinear tooth profiles are quite similar to the curvilinear tooth profile; they only differ in their tooth depth and flank angle. Belts with modified curvilinear tooth profiles have a smaller tooth depth and greater flank angle compared to belts with curvilinear tooth profiles, making them ideal for applications requiring extremely high torque transmitting capabilities at high speeds.

Belt length, width and center-to-center distance

The required belt length can be calculated using.

Equation 2: Timing belt length equation.Equation 2: Timing belt length equation.

Where:

C = Approximate center distance between the two shafts

N1 = Number of teeth in the larger pulley

N2 = Number of teeth in the smaller pulley

T = Tooth pitch

Table 1 presents the tooth pitches of different timing belts

Table 1: Tooth pitches of different timing belts.Table 1: Tooth pitches of different timing belts.

The actual distance between shafts can then be estimated using:

Equation 3: Timing belt distance between shafts.Equation 3: Timing belt distance between shafts.

For specifying belt width, designers are advised to check belt manufacturers' catalogs. They usually provide rating tables and guidelines to help designers choose the belt width for their desired applications.

[Learn more about metric timing belts on GlobalSpec.com.]

Conclusion

Although there are many other factors to consider when sizing and specifying timing belt drives, these three factors are great starting points for building a timing belt drive system.

The design process for timing belt drive demands several iterative steps and modifications to arrive at a system that meets the application requirements. Therefore, engineers are advised to reach out to manufacturers to discuss their application requirements.

To contact the author of this article, email engineering360editors@globalspec.com