Machinery's Handbook, 31st Edition
Flat Belts and Pulleys 2567 small machines from a large common power source such as a steam engine. As electric motors became smaller, more efficient, and more powerful, and new types of belts and chains were made possible by modern materials and manufacturing processes, flat belts fell out of favor. Flat belts are still used for some drive purposes, but leather has been re- placed by other natural and synthetic materials such as urethanes, which can be reinforced by high-strength polyamide or steel fabrics to provide properties such as resistance to stretching. The high modulus of elasticity in these flat belts eliminates the need for peri odic retensioning that is usually necessary with V-belts. Driving belts can be given a coating of an elastomer with a high coefficient of friction, to enable belts to grip pulleys without the degree of tension common with earlier materi- als. Urethanes are commonly used for driving belts where high resistance to abrasion is required, and also resist attack by chemical solvents of most kinds. Flat belts having good resistance to high temperatures are also available. Typical properties of polyurethane belts include tensile strength up to 40,000 psi (276 MPa), depending on reinforcement type and Shore hardness of 85 to 95. Most polyurethane belts are installed under ten- sion. The amount of tension varies with the belt cross section, being greater for belts of small section. Belt tension can be measured by marking lines 10 inches, or 100 mm, apart on an installed belt, then applying tension until the separation increases by the desired percentage. For 2 percent tension, lines on the tensioned belt would be 10.2 inches, or 102 mm, apart. Mechanical failure may result when belt tensioning is excessive, and 2 to 2.5 percent elongation should be regarded as the limit. Flat belts offer high load capacities and are capable of transmitting power over long dis tances, maintaining relative rotational direction, can operate without lubricants, and are generally inexpensive to maintain or replace when worn. Flat belt systems will operate with little maintenance and only periodic adjustment. Because they transmit motion by friction, flat belts have the ability to slip under excessive loads, providing a fail-safe action to guard against malfunctions. This advantage is offset by the problem that friction drives can both slip and creep so that they do not offer exact, consistent velocity ratios nor preci sion timing between input and output shafts. Flat belts can be made to any desired length, being joined by reliable chemical bonding processes. Increasing centrifugal force has less effect on the load-carrying capacity of flat belts at high speeds than it has on V-belts, for instance. The low thickness of a flat belt, com - pared with a V-belt, places its center of gravity near the pulley surface. Flat belts therefore may be run at surface speeds of up to 16,000 or even 20,000 ft/min (81.28 to 101.6 m/s), although ideal speeds are in the range of 3,000 to 10,000 ft/min (15.25 to 50.8 m/s). Elasto meric drive surfaces on flat belts have eliminated the need for belt dressings that were often needed to keep leather belts in place. These surface coatings can also contain an- tistatic materials. Belt pulley wear and noise are low with flat belts, shock and vibration are damped, and efficiency is generally greater than 98 percent, compared to 96 percent for V-belts. Driving belt load capacities can be calculated from torque T = F ( d /2) and horsepower HP = T × rpm /396,000, where T is the torque in in-lb, F is the force transmitted in lb, and d is the pulley diameter in inches. Pulley width is usually about 10 percent larger than the belt, and for good tracking, pulleys are often crowned by 0.012–0.10 inch (0.305– 2.54 mm) for diameters in the range of 1.5–80 inches (3.8–203 cm). Before a belt specification is written, the system should be checked for excessive startup and shut-down loads, which sometimes are more than 10 percent above operating condi tions. In overcoming such loads, the belt will transmit considerably more force than dur- ing normal operation. Large starting and stopping forces will also shorten belt life unless they are taken into account during the design stage. Flat Belt Pulleys.— Flat belt pulleys are usually made of cast iron, fabricated steel, paper, fiber, or various kinds of wood. They may be solid or split and in either case the hub may be split for clamping to the shaft.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online