When designing a belt drive system, the first step is to choose the most suitable belt for the application. But pulleys also play an important role in belt performance, especially in synchronous belt drive systems, where the proper fit of the belt teeth to the pulley grooves can affect everything from the amount of torque that can be transmitted to the speed of belt wear patterns. and possible failures.
Synchronous pulleys are typically designated by the number of grooves (analogous to the number of belt teeth), groove pitch (analogous to the belt tooth pitch), and pulley width.
The required tooth pitch (and therefore groove pitch) is determined during belt selection, based on design torque and speed. Torque and speed are also the main factors in determining belt width and therefore pulley width. (The recommended pulley width is usually slightly larger than the belt width and takes into account the clearance required for the pulley flanges.) The number of pulley grooves is determined by the required speed ratio.
For pulleys used with synchronous (toothed) belts, it is important that the pulley grooves match the belt tooth profile: trapezoidal, curved or modified curve.
Dimensional information also includes the type and size of hub for attaching the pulley to the crankshaft. Common pulley mounting options include locking taper bushings, split taper bushings, QD (quick release) bushings, or plain holes with or without grooves.
Although the pulley diameter is not explicitly specified during selection, it can be determined by the number of pulley grooves and their pitch, which are used to calculate the pitch diameter of the pulley. The pitch diameter is slightly larger than the outer diameter of the pulley and corresponds to the pitch line of the belt, which is the line formed by the belt traction cable.
pd = pulley pitch diameter (mm, in.)
P = pulley groove pitch (also belt tooth pitch) (mm, in.)
N = number of grooves on the pulley
Once the pitch diameter has been calculated, the outside diameter of the pulley can be determined by finding the distance from the belt start line to the bottom of the tooth profile (a value specified by the belt manufacturer) and subtracting twice that distance from the pitch diameter of the pulley. belt. pulley.
DE = external diameter of the pulley (mm, in.)
U = distance from the belt slope line to the bottom of the tooth profile (mm, in.)
Some manufacturers offer pulleys in the form of rods that can be cut and machined by the user. While this can be a cost-effective solution for small quantity prototyping, pulley accuracy is critical to ensure proper belt tracking, speed ratio, and efficiency.
Acceptable pulley tolerances are specified by trade associations (such as the Mechanical Power Transmission Association, MPTA) and the International Standards Organization (ISO). In some cases, major belt manufacturers also specify tolerances for specific tooth profiles.
Key manufacturing tolerances for synchronous pulleys include:
- pulley outer diameter
- eccentricity between the pulley hole and the outer diameter of the pulley
- parallelism between the pulley hole and the vertical faces of the pulley
- groove pitch accuracy
- parallelism between grooves and hole.
It is important to note that pulleys may also need to undergo static or dynamic balancing after manufacturing.
Synchronous pulleys can be made from a variety of materials, including aluminum, steel, cast iron, and various plastics. The pulley material determines its weight and inertia and therefore affects the dynamic performance of the belt drive system. Material selection also affects the amount of noise generated by the system, with polycarbonate (thermoplastic polymer) pulleys generating more noise during operation than metal pulleys.
To prevent the belt from slipping off the pulley and to resist lateral forces caused by sideways movement of the belt, synchronous drives typically require flanged pulleys.
