From Team 449 Wiki
Belt is one of the main methods of motion transfer in FRC. Belting is primarily used to transfer rotational motion from one shaft to another. Somewhat less-commonly, belting can be used as a method to move game pieces around a robot. Belts consist of strips of material running between pulleys.
The most common use of belt is to transfer rotational power from one shaft to another. The belting used to do this is called "timing belt." Timing belt has teeth that mesh with a pulley, providing torque transfer much in the same way as chain.
Almost all timing belts used in FRC are made by Gates, and have their size and form specified by three values: "profile," "pitch," and "width."
"Profile" refers to the shape of the belt's teeth. In FRC, the two most-commonly used profiles are "HTD" and "GT2." Team 449 almost exclusively uses HTD, as that is what is most-supported by suppliers AndyMark and VexPro.
"Pitch" refers to the size of the belt's teeth. In FRC, this is almost always 5mm.
"Width" refers to the width of the belt. In FRC, this is almost always either 9mm or 15mm.
A typical belt specification will read like "HTD-5m-15," where "HTD" is the profile, "5mm" is the pitch, and "15mm" is the width.
Selecting timing belts and pulleys
The following section is written under the assumption that all belts are HTD, 5mm pitch.
The main concern for selecting a timing belt for a given mechanism is whether it can effectively transfer the torque loading required without "ratcheting," or skipping teeth. The torque rating for a given belt/pulley combination depends primarily on three things: belt pitch, belt width, and diameter (or tooth count) of the pulley. Torque rating for a given belt pitch and width scales roughly linearly with diameter - that is, a 40-tooth pulley can transfer roughly twice as much torque as a 20-tooth pulley. Torque rating also scales linearly with belt width, so, all else being equal, a 15mm belt transfers approximately 1.9 times as much torque as a 9mm belt.
Additionally, belts require some minimum number of teeth in mesh to function properly - in general, with any fewer than 6 teeth in mesh with the pulley, the load rating of the belt begins to drop (details can be found in the document linked below).
Official torque ratings for Gates belt/pulley combinations can be found here. Bear in mind, however, that the official specs from Gates are somewhat conservative, since in actual industrial uses belts are expected to last for orders of magnitude longer than in FRC. Thus, it is acceptable to load belts somewhat more than the official specs indicate (a good rule of thumb is that the "actual" load limit is around 1.5 times what is seen in the official specs).
Notes on drive belt selection
As with other contexts, belts used in the robot's drive must be selected so that they will not "ratchet." A few rules of thumb are useful for selecting drive belts, in particular:
For a traction-limited drive (i.e., one in which the motors can deliver enough torque to break static wheel friction) determining factors for torque on the belt are the weight of the robot, diameter of the wheel and the coefficient of friction (CoF) of the wheel. Given that the CoF of most FRC wheels is about 1, and robot weight is almost always around 120lb, the critical factors for determining drive belt setup are the belt width and the ratio of the pulley size to the wheel size. To see this, imagine we hold this ratio constant. If we double the size of both the wheel and the pulley, the torque on the shaft goes up by a factor of two, but so does the rated torque of the belt.
As it works out, while 15mm belts are relatively safe unless you are doing something really weird (exceptionally big wheels and small pulleys), 9mm belts can ratchet when used in drives if one is not careful. As a rule of thumb, 9mm-wide belts should only be used in a drive to connect outer wheels to a direct-driven center wheel (such as in a West Coast drive). In this use, they are likely safe from ratcheting so long as the pulley size/wheel diameter ratio remains larger than 8 teeth/inch (i.e., a 24-tooth pulley for a 3" wheel).
In order to use timing belts effectively, it is essential that they be properly-tensioned. A properly-tensioned belt has no visible slack, but also yields slightly to the touch - it should be "snug." Overly-tensioned belts introduce additional friction and are more likely to fail, while under-tensioned belts are likely to ratchet. For specific technical advice on belt tensioning, see here.
To ensure proper belt tensioning, a belt length calculator should be used whenever possible while designing mechanisms to determine correct center-to-center pulley distances.
Belt versus chain
Belt and chain serve very similar mechanical purposes, and thus one is often presented with the problem of choosing between the two for a given mechanism. Some relevant concerns in making this decision are listed below:
- Chain can generally transfer much higher loads than belt, so mechanisms involving large forces likely should use chain.
- Belt is generally quieter and slightly more efficient than chain.
- Chain tends to stretch when used due to wearing in the mechanical joints between the links. Belt does not stretch appreciably in FRC applications. Thus, mechanisms which use chain must be designed to allow tensioning (either by providing for movement of one sprocket or including a tensioner), while with belt one can "set and forget."
- Chain is (arguably) somewhat more sensitive to alignment issues than belt - i.e., chain is more likely to jump off a misaligned sprocket than belt from a misaligned pulley.
- The length of a run of chain can be modified through breaking and the use of master links. The length of a belt is fixed. This makes chain somewhat more flexible in the case of later design changes than belt.
- Belt is generally very reliable, but it can fail if crimped/creased. Thus, do not play with spare belting.
Belt for moving gamepieces
Belting is often used to move game pieces within robots. The belts used for this purpose are fundamentally different than the timing belts discussed above, and are generally quite large and not toothed.
One popular solution in FRC for moving gamepieces with belt is called "Polycord," and involves running polyurethane (or similar) tubing as a belt between a pair of pulleys.