As belts operate, belt tension applies a wedging force perpendicular to their tops, pushing their sidewalls against the sides of the sheave grooves, which multiplies frictional forces that allow the drive to transmit higher loads.
Figure 2 How a V-belt fits into the groove of the sheave while operating under tension impacts its performance. Figure 2 — Vertical force Fv applied perpendicular to belt top creates high sidewall forces Fn to transmit higher loads.
V-belts are made from rubber or synthetic rubber stocks, so they have the flexibility to bend around the sheaves in drive systems. Fabric materials of various kinds may cover the stock material to provide a layer of protection and reinforcement. V-Belt Profiles Cross Sections V-belts are manufactured in various industry standard cross-sections, or profiles, including the following: Classical Narrow Metric Fractional Horsepower The classical V-belt profile dates back to industry standards developed in the s.
Belts manufactured with this profile come in several sizes A, B, C, D, E and lengths Figure 3 , and are widely used to replace V-belts in older, existing applications. Figure 3 — Classical V-Belt Profiles. V-belts with a narrow profile 3V, 5V, 8V have more steeply angled sidewalls than classical V-belts Figure 4 , providing increased wedging action and higher load capacity up to 3x that of comparable classical V-belts..
Figure 4 — Narrow V-belt Profiles. They are used to replace belts on industrial machinery manufactured in other parts of the world. Figure 5 — Metric V-belt Profiles. The fractional horsepower V-belt profile is designed for light-duty applications such as lawnmowers, snow blowers, attic or furnace fans, etc. These belts have a thinner cross-section and lighter gauge tensile cord Figure 6 , making them more flexible and able to bend around small sheaves. Figure 6 — Fractional Horsepower V-belt Profiles.
Notches reduce bending stress, allowing the belt to wrap more easily around small diameter pulleys and allowing better heat dissipation. Excessive heat is a major contributor to premature belt failure.
Engineering a notched belt is a balancing act between flexibility, tensile cord support, and stress distribution. Precisely shaped and spaced notches help to evenly distribute stress forces as the belt bends, thereby helping to prevent undercord cracking and extending belt life Figure 7. Figure 7 — Bending stress red area is evenly distributed in a well-engineered notched V-belt, while the tensile cord between red and yellow bands remains well supported, all without sacrificing flexibility.
For applications with vibrating or pulsating loads, especially with long center distances, joined V-belts may be the answer.
A joined V-belt is, in essence, a number of single V-belts joined together with a continuous tie-band across the back see Figure 8. Figure 8 — Joined V-belt. A joined V-belt increases lateral rigidity to reduce belt whip and maintain stability under shock loads. It also simplifies installation and tensioning compared with multiple single belts.
V-Belt Construction and Material Figure 9 describes the constructional components of standard and notched V-belts. Each component has a vital role to play in how well V-belts perform and how long they last.
Different materials and configurations can influence belt performance characteristics in specific applications. Figure 9 — Anatomy of a V-belt. The tensile cord is the load-carrying component of a V-belt.
Most V-belts are made with polyester cords, although some belts are constructed with aramid or Kevlar? In a well-engineered V-belt, the tensile cords and rubber body of the belt are chemically bonded to form one unit, allowing for equal load distribution and longer belt life. Tensile cords are supported by rubber stocks, both above over cord and beneath under cord. Various synthetic rubber stocks are used by different manufacturers to provide heat resistance and reduce wear.
Some high-performance synthetic rubber compounds, such as ethylene, significantly extend a belt's operating temperature range and resist hardening, cracking, and premature failure.
A well-engineered V-belt will have transverse rigidity, which means a high level of rigidity across its width so that the tensile cords will transfer the load equally. At the same time, the belt must be highly flexible along its length to reduce heat and bending stresses, which in a superior belt is accomplished by parallel alignment of fibers in the rubber compound.
Adhesion gum is the element that forms a strong chemical bond between the tensile cord and the rubber stock. It bonds the belt together so that it acts as a single unit. The gum also absorbs cord stresses and avoids cord pullout.
To protect the core of the belt from destructive environmental forces such as oil, grime and heat, as well as from general wear and tear, some V-belts have a fabric cover, or band ply. In a well-engineered belt, this flexible fabric is treated to form a chemical bond with the belt core materials, allowing it to withstand the stress of constant bending over time and prolonging cover life.
This nominal horsepower refers to the motor horsepower or required horsepower for the application. A Service Factor takes into account such situations as power losses due to vibration, shocks, heat and other related factors caused by the motor and the application. The Design Power is calculated based on these considerations thus producing more accurate results to ensure that the drive will function more efficiently. In a belt drive system, a ratio is used to determine the speed relation between two v-belt pulleys.
The speed ratio would be stable if slippage did not occur; however as belt slip is inevitable, the ratio varies and is therefore only theoretical.
In both cases, the ratio is obtained using the dimensions of the input drive driver pulley and the output driven pulley. The most work accomplished in the least amount of time, equals greater power. This formula also shows the relation between torque and HP. To determine whether dynamic balancing is advisable, perform the following calculation. Note: If the sheave or pulley is to be operated at a higher speed, a two plane balance is recommended.
Service Factors for different kinds of driven machines combined with different types of driving units are shown in Table 1 below. The driven machines are representative examples only. Select a driven machine whose load characteristics most closely ressemble those of the machine being considered. Line Shafts. The use of a service factor of 2.
Occasional inspection however, will ensure optimal drive efficiency, and one sign of wear to look for, that is often overlooked, is the rounding or deepening of V-belt pulley hereafter referred to as sheaves grooves.
Are one or more of the belts slack, while others are tight in a proper tensioned drive? This is a sign some may be riding higher than others due to rounded grooves. Or, is the bottom of the groove shiny? Belts, depending on the type, can ride at or above the highest point of the sheave, but they should never be allowed to touch the bottom of the groove. If they do, the sheave should be replaced immediately. How to be sure? A handy, inexpensive tool to have on hand is a Sheave Gage Part If the wearing is premature, it could be caused by misalignment between the sheaves, thus forcing the belts in and out at an angle to the groove.
Here are some considerations when consulting the tables. Adjusted power rating The basic power rating is evaluated according to the size of the sheaves and the speed of the drive.
In the following tables, the adjusted power rating is presented, including both correction factors, compared to other existing tables that only show basic horsepower values. For other center distances than those suggested in the tables, here are certain factors that should be taken into consideration: Longer center distances mean slightly higher power ratings and inversely, shorter center distances mean lower power ratings. For high ratio drives, small center distance changes may affect the power rating considerably.
Minimum recommended diameter According to the RMA, there are minimum recommended sheave diameters depending on the belt used. Drives not adhering to this standard have been calculated but are shown with a shaded area for practical purposes. Maximum sheave rim speed Standard Maska sheaves are designed to operate up to 6, FPM as per industry standards. In such cases, please contact our Technical Support Department for assistance at For more detailed drive calculations and possibilities, please run our Drive Selection Program software available online at www.
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