Comparison of MXL and S2M Timing Belt Transmissions (Trapezoidal vs. Curvilinear Teeth)

1. Introduction

Trapezoidal-tooth timing belts were first developed by the American company Uniroyal Rubber in 1964 and were gradually adopted for mechanical transmission applications.

Figure 1: Trapezoidal-tooth synchronous belt tooth profile

In this type of timing belt—characterized by a trapezoidal tooth profile—the flanks of the teeth are straight lines. This specific profile geometry leads to severe stress concentration at the tooth roots, which, at high speeds, reduces both the belt's service life and its load-carrying capacity. Furthermore, during transmission, these belts generate high levels of noise and vibration, thereby limiting the maximum operating speed. Subsequent photoelastic stress analysis revealed that the stress distribution within the trapezoidal tooth profile is highly uneven, exhibiting significant stress concentration at the root; this makes the teeth prone to fracture and failure. Moreover, the actual load-bearing contact area of the trapezoidal tooth occupies only about one-third of the total tooth flank surface, indicating that this design fails to fully utilize the entire load-carrying potential of the belt teeth.

In 1973, Uniroyal developed a single-curvilinear-tooth timing belt (known as HTD; corresponding to the domestic standard JB/T 7512.1). This design effectively reduced the "polygon effect" inherent in belt transmissions and achieved a more rational stress distribution, leading to its increasingly widespread adoption. Subsequently, however, it was observed that as transmission speeds increased beyond a certain threshold, these curvilinear-tooth belts would once again generate noise and exhibit a noticeable decline in transmission efficiency. Consequently, in 1977, the American company Goodyear modified this tooth profile to create the flat-top curvilinear-tooth (STPD) timing belt. This design significantly improved airflow dynamics at the precise moment of engagement between the belt teeth and the pulley teeth, thereby substantially mitigating the air-damping resistance—a phenomenon typically associated with high-speed meshing.

Figure 2: STPD curved-tooth synchronous belt tooth profile

In recent years, China has witnessed rapid growth in the domestic sectors for security surveillance equipment, video conferencing systems, printers, and photocopiers, leading to an increasingly widespread application of timing belts within these devices. Since these compact devices typically involve relatively low power transmission requirements and moderate operating speeds, they predominantly utilize small-sized belts. Beyond the requirement for low noise levels, the most critical performance metrics for these applications are service life and angular transmission precision. The MXL, S1.5M, and S2M series of timing belts are specifically designed to meet these exact requirements. Currently, domestic manufacturers—including renowned ones—still widely utilize MXL timing belt drives. In contrast, manufacturers abroad—particularly in Japan—have already widely adopted STPD-profile timing belt drives for small-scale electronic devices, such as the internal mechanisms of financial equipment, printers, and photocopiers.

Although the tooth profiles of MXL and S2M timing belts differ, their pitches are very similar: the MXL pitch is 2.032 mm, while the S2M pitch is 2.000 mm. Consequently, they can be utilized in the same drive systems. For this reason, I have selected these two belt types for a comparative analysis, allowing for a clear assessment of the respective advantages and disadvantages of these two distinct timing belt categories. Based on over a decade of research and comparative testing within the drive systems of financial and security surveillance equipment, the S2M belt consistently outperforms the MXL belt in terms of both service life and transmission precision.

2. Comparative Analysis

2.1 Tooth Profile Comparison

First, let us compare the tooth profiles of the two belts. In the figure below, the dark line represents the tooth profile of the S2M timing belt, while the light line represents that of the MXL timing belt. The base thickness of both belts is identical at 0.6 mm; however, the teeth of the S2M belt are noticeably wider than those of the MXL belt—particularly at the root. Furthermore, the fillet radius of the S2M tooth (R0.2) is larger than that of the MXL tooth (R0.13). Assuming the belt materials are identical, the S2M belt possesses superior structural strength compared to the MXL belt, enabling it to transmit greater power. Conversely, if the power transmission requirements are identical, the S2M belt will offer a longer service life than the MXL belt.

Relative to the pulleys, the structural strength and wear resistance of the timing belt itself tend to be slightly lower; consequently, the belt often represents the limiting factor regarding the overall service life and load-bearing capacity of the drive system. This is because the belt's body typically incorporates reinforcing elements—such as steel wires or fiberglass cords—leaving the teeth as the relatively weaker structural components.

Figure 3: Comparison of two synchronous belt tooth profiles (MXL trapezoidal teeth and S2M curved teeth)

The arc-tooth timing belt drive system addresses this issue by increasing the thickness of the belt teeth to enhance the belt's structural integrity, while simultaneously slightly reducing the thickness of the pulley teeth (thereby slightly reducing the pulley's structural strength). This design approach aligns more effectively with practical application requirements, resulting in an overall improvement in both service life and load-bearing capacity. We can also obtain specific data regarding this point from the technical manuals provided by Mitsuboshi Belting (Japan); please refer to the tables below (a partial selection is shown):

• Figure 4: Basic Power Transmission Capacity of MXL Timing Belts (Partial)

• Figure 5: Basic Power Transmission Capacity of S2M Timing Belts (Partial)

• Figure 6: Allowable Transmission Torque for MXL Timing Belts (Partial)

• Figure 7: Allowable Transmission Torque for S2M Timing Belts (Partial)

Note that the standard width for the S2M timing belt shown above is 4 mm, while the standard width for the MXL timing belt is 6.4 mm. Whether considering allowable transmission power or allowable transmission torque, the S2M belt outperforms the MXL belt by at least 50%. If these figures were normalized to a common belt width, the performance advantage of the S2M belt over the MXL belt would be even more pronounced.

2.2 Meshing Characteristics Comparison

Next, let us compare the meshing characteristics of the timing belts and their corresponding pulleys.

• Figure 8: Comparison of Meshing Conditions for Two Timing Belt Types (MXL vs. S2M)

• Figure 9: Basic Dimensions of Two Timing Pulley Types (MXL vs. S2M)

Comparison of Actual Meshing Photographs:

• Figure 10: Actual Meshing of MXL Timing Belt and Pulley

• Figure 11: Actual Meshing of S2M Timing Belt and Pulley

As can be observed—and consistent with the theoretical tooth profiles—there is a noticeable clearance between the MXL timing belt and its pulley; when the belt is manually nudged, a distinct, slight slippage is perceptible. In contrast, no visible clearance exists between the S2M timing belt and its pulley; when the belt is nudged, no slippage is discernible to the naked eye. The MXL timing belt exhibits distinct clearances at both the tooth flanks and the tooth tips, resulting in a more pronounced "polygon effect" compared to timing belts featuring a curvilinear tooth profile.

As illustrated in the figures above, the tooth profile of the MXL timing belt is smaller than the profile of the pulley groove. The theoretical sum of the clearances on both sides of the belt tooth is approximately 0.2 mm, meaning the clearance accounts for roughly 18% of the tooth width. Furthermore, the tooth height of 0.51 mm is less than the corresponding groove depth of 0.64 mm. When the S2M timing belt engages with its pulley, the dimensions of the belt teeth and the pulley grooves are nearly identical, resulting in minimal clearance. Theoretically, the combined clearance on both sides of a single belt tooth amounts to approximately 0.08 mm—a gap equivalent to about 6% of the tooth width. Furthermore, the tooth height and groove depth match perfectly at 0.76 mm, ensuring zero clearance in the vertical (tooth-height) direction.

Based on this analysis of engagement clearance, it is evident that if the tension applied to the timing belt is insufficient—leading to slippage between the belt and the pulley—transmission precision will inevitably suffer. Theoretically, the transmission precision of an MXL timing belt system is somewhat lower than that of an S2M system. Strictly from a geometric perspective, the angular error for an MXL belt is approximately (where Z2 is the number of teeth on the driven pulley) 18% × 360°/Z2; conversely, the angular error for an S2M belt is approximately 6% × 360°/Z2. However, in practical applications, belt tension is often maintained at a relatively high level—minimizing slippage—meaning the actual precision achieved typically exceeds these theoretical values.

With MXL timing belts, contact occurs solely between the tooth roots and the pulley crests; this leads to a relatively severe concentration of stress, thereby limiting the maximum tension the belt can withstand. In contrast, S2M timing belts make contact with the pulley at both the tooth tips and the tooth roots, resulting in a more uniform distribution of force. This significantly mitigates the "polygon effect." Furthermore, the engagement process—facilitated by the belt's curvilinear tooth profile—is notably smoother; consequently, S2M belts featuring these curvilinear teeth offer superior load-bearing capacity and extended service life.

3. Empirical Testing

For this evaluation, a PTZ (Pan-Tilt-Zoom) surveillance camera was selected to assess the precision of its preset positioning capabilities. Two distinct transmission systems were tested: one utilizing MXL-profile teeth and the other utilizing S2M-profile teeth. Both systems employed identical motor models; aside from the specific drive pulleys and belts, all other components within the assembly remained exactly the same. The camera's transmission system operates via two rotational axes, as illustrated in Figure 12. Specifically, the camera lens is capable of continuous 360° rotation around the Z-axis, as well as reciprocating rotation within a 110° range around the Y-axis; both movements are driven by timing belts. For the purpose of this test, we focused on the vertical rotation mechanism, which features a transmission ratio of 1:4 (comprising a 20-tooth drive pulley and an 80-tooth driven pulley). The MXL belt used was the "180MXL" model (with a pitch length of 365.76 mm), while the S2M belt used was the "S2M364" model (with a pitch length of 364 mm). The camera lens was positioned at a distance of 15.6 meters from the reference scale used for measurement.

A preset position is a method of linking a key area under surveillance to the operational status of a PTZ camera. Under manual or programmed control, the PTZ camera can rotate to any angular position, which can then be configured and stored as a preset position. Recalling a preset position is a standard function of surveillance PTZ cameras. Regardless of the direction in which the camera lens is currently facing, once a pre-configured preset position is recalled, the camera will rapidly rotate to that specific location.

Figure 12: Test Scheme for Comparing the Precision of Timing Belt Transmissions

When a preset position is recalled via software, the PTZ camera rotates to the corresponding location; however, a deviation from the originally configured position often occurs. Aside from the influences of the software and the motor, the primary factor contributing to this deviation is the belt transmission error within the PTZ camera mechanism. To eliminate the influences of the software and the motor, the tests were conducted using a single PTZ camera unit equipped with the same motor model throughout the process. Only the timing pulleys and timing belts were interchanged; thus, the pulleys and belts were the sole variables in the experiment. The actual deviation measured during these tests serves as a direct indicator of the real-world transmission precision achieved by the MXL timing belt system versus the S2M timing belt system.

As indicated by the test data in Table 1, the S2M timing belt demonstrates significantly higher precision than the MXL timing belt under identical operating conditions. When the belt tension exceeds 25 N, increasing the tension further yields only marginal improvements in precision; beyond 30 N, the impact of increased tension on precision becomes even less significant. Notably, the S2M timing belt is capable of achieving superior transmission precision even under relatively low tension conditions. Furthermore, the damping characteristics of the motion system were observed to have a substantial impact on transmission precision.

Table 1: Test Data on Deviations When Recalling Preset Positions (refer to original data in the article)

4. Tensile Strength Test Data

As illustrated by the data provided by Bando Chemical Industries in the figure below: Both the MXL and S2M timing belts are composed of rubber materials and share an identical base thickness of 0.6 mm. Given their identical material composition, their tensile strengths (referred to as "breaking strength" in the technical drawings) are essentially equivalent. (Note: The belt thickness values shown in the diagrams include the height of the teeth; specifically, while both the MXL and S2M belts have a base thickness of 0.6 mm, the total thickness—including teeth—is 1.1 mm for the MXL belt and 1.31 mm for the S2M belt.)

• Figure 13: MXL Timing Belt Materials and Structure

• Figure 14: S2M Timing Belt Materials and Structure

5. Summary and Discussion

In summary, the S2M curvilinear-tooth timing belt outperforms the MXL trapezoidal-tooth timing belt in terms of transmission precision; this constitutes a major advantage of curvilinear-tooth belts. The installation requirements for curvilinear-tooth belts are similar to those for trapezoidal-tooth belts; however, they offer a longer service life and accommodate a wider range of tension settings. From a cost perspective—although the weight per unit width of the S2M timing belt is slightly higher, and the cost for an equivalent width may be marginally greater (approximately 1.18:1)—it can be manufactured with a narrower profile. Consequently, the actual cost is not necessarily higher than that of the MXL timing belt; in fact, it can often be lower, offering distinct advantages in terms of reducing the overall volume of the transmission system. Based on transmission capacity calculations, a 4 mm wide S2M timing belt can effectively replace the vast majority of 6.4 mm wide MXL timing belt drives, potentially reducing actual costs by over 25%.

Although China has already established national standards for HTD-series curvilinear-tooth belts, their adoption remains relatively uncommon; furthermore, there is a notable lack of established standards for micro and small-scale curvilinear-tooth timing belts (specifically those smaller than the 3M series).

The curvilinear-tooth design has further evolved into the RPP series, featuring a concave-top parabolic tooth profile that generates even lower noise levels, as illustrated in Figure 15.

Figure 15: RPP Concave-Top Parabolic Tooth Profile

6. Conclusion

Trapezoidal-tooth belts have a longer history of application in China and remain more widely utilized. However, the inherent superiority of curvilinear-tooth belts is undeniable. Therefore, policies should be vigorously implemented to promote and support the adoption of curvilinear-tooth belts as replacements for trapezoidal-tooth belts—particularly in the small-scale sector—in order to accelerate the formulation of relevant standards and facilitate the industrial upgrading and modernization of the domestic manufacturing sector.