V-belts standards

In addition to the standard types of V belts (see Figure 3-19), many manufacturers make V belts that arc specifically designed to have a higher horsepower capacity. 4 hese V belts are shown and compared to standard V-belt cross-sections. These superpower V belts allow shorter center distances and narrow er sheaves without imposing any extra total bearing stresses. Such belts are able to reduce the drive by 30-. )0% and less for horsepow er capacity. In addition, speeds can be increased up to 6000 ft/ min without dynamic balancing of the sheaves because the sheaves arc smaller. 

Standard V-belt pulleys are specified with exact dimensions as opposed to nominal. The industry adopted the datum system in 1988 as the standard for specifying classic V-belt pulleys. Therefore, all pulley specifications should be in datum dimensions rather than pitch. Individual pulley dimensions are shown in Figure 58.2. 

Extreme heat In the manufacturing process, belts are cured with scientifically controlled heat for given periods of time. If standard belts are operated below 140°F, their materials of constmction are not affected. However, at temperatures above 140°F, over-curing will occur and belt life will be shortened. Therefore, the use of standard V-belts above this temperature should be avoided. Often, adequate shielding between the heat source and belts can be provided. 

Joined or power band belts provide a good alternative in pulsating drives where standard V-belts have a tendency to turn over. They function like a... 

Positive-drive belts are sometimes called timing belts because they are often used in operations when timing a piece of equipment is critical. However, they are also used in applications where heavy loads cause standard V-belts to slip. They are flexible and provide the same benefit as standard V-belts, but their alignment is more critical. For an example, see Figure 23.6. 

Since it is good practice to maintain a selected inventory of spare parts for drives, economy can be achieved by standardizing conveyor drives throughout the plant. For example, intermediate speed reduction by means of V belts, sheaves or chains, and sprockets can frequently permit using the same speed-reducer size for several drives. Thus, it may be necessary to keep only one repair-stock speed reducer for a number of conveyors. 

The load-transmitting capacities of a single V-belt, at 180° arc of contact, are provided by the belt manufacturer as standard selection data for the user for different areas of belt cross-sections and speed of the faster shaft. We have provided this data for a leading manufacturer, for belts of section D, in Table 8.4, to illustrate the selection of V-belts for the drive of Example 8.2. 

Transmission of load and suitability of bearings 8/213 Table 8.9 Nominal inside lengths, pitch lengths L for all standard sizes of multiple V-belts... 

Normal electric motor speeds run from the standard induction speeds for direct connection of 3600, 1800 and 1200 rpm to the lower speed standards of the synchronous motors, and then to the somewhat arbitrary speeds established by V-belt or gear drives. For some cases, the pump speed is set by the type of drivers available, such as a gasoline engine. 

Modern V belts are nearly all of the closed-loop continuous type which come in standard lengths. 

Table 58.1 Classical V-belt standard datum length (ANSI/RMA PI-20, 1988)...

V belts operate over a wide range of speed and provide a broad choice of center distance. While V belts can operate at very high speeds, standard sheaves are normally limited to a 6500 ft/min (33 m/s) rim speed maximum for safety purposes. Beyond this speed, special materials and balancing may be required. 

In addition to exhibiting excellent oil and heat resistance, most classical and narrow industrial V belts meet industry standards for static conductivity. This allows for safe operation in potentially hazardous environments. 

V-belt drive, flat-belt, chain, and gear drives, in accordance with NEMA standards... 

Most of the chemical fume hoods considered here consist of a cabinet or enclosure set at waist level (above a table or storage cabinet) that is connected to a blower located above the hood or external to the hood through a duct system. The cabinet has an open side (or sides) to 2illow a user to perform work within. A movable transparent sash separates the user from the work. Most chemical fume hoods have a sill that functions as an airfoil at the work surface below the sash. The connection to the blower might be by use of a v-belt, or it may be direct drive. This allows provision of a smooth flow of air with minimal turbulence. In some installations, axially mounted blowers are used, especially if multiple hoods are ducted into a common blower. Baffles located in the rear of the cabinet provide control of the air flow patterns, and can usually be adjusted to provide the best air flow around the experiment or procedure being performed. Many chemical fume hoods are equipped with air flow indicators, low flow monitors and alarms, and differenti2d pressure sensors to allow the user to operate safely. The major types of chemical fume hoods include the standard/conventional, W2dk-in, bypass, variable air volume, auxiliary air, or ductless types. Additional types include snorkels and canopies that are portable. Each type must be understood to be operated most efficiently within specifications (see the section below on safe operation). 

Buckets for continuous elevators are V-shaped in cross-section and carry their maximum load when the inclination of the elevator is such that the front and rear of the buckets are inclined equally to the vertical. Single-chain and belt elevators usually are operated at about double the speed of continuous bucket elevators so their inclination does not have to be so pronounced. Double-chain elevators when run in a vertical plane have their buckets suspended between the chains, to permit the required snubbing, but inchned elevators of the standard type, when inclined, have their buckets attached to the chains on their backs. Chains should be of the short-pitch variety to avoid pulsations as they pass about the end sprockets and when handhng abrasive material likely to raise dust they should be bushed. Supporting sprockets and rollers are used on inclined elevators to prevent the loaded side sinking and interfering with the return run of the elevator. 

Some belt manufacturers, and some specialty belts, may still use the older, couventioual matching system. Each belt is measured under industry standard tension in V sheaves and marked with a punch or match number designating the small increment of length within the... 

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