GatesFacts Technical Information Library Gates Compass Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA

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1 Specialized Synchronous Belts David E. Roos Machine Design June 8, 1989 Uncommon belts perform operation that standard belts cannot. And if an unusual design is not readily available, one can be made to order. Synchronous or toothed belts fill a performance gap between V-belts and chains. However, on each side of this technological divide, there is gray area where chains or V-belts might work as well as synchronous belts. With no drive having a clear, significant advantage, selecting the best type is difficult. One alternative to a compromise drive is a specialized synchronous belt. Over the years, users requested enough unusual belts so that what was once an oddball drive is now relatively commonplace and readily available. Double-sided drives Formerly, only chains were used in serpentine timing drives, those that traveled S-shaped paths to turn sprockets in opposite directions. While chains are capable of high torque, they are limited to low shaft speeds, not much over 4,000 rpm. Also, they require lubrication. While these operating conditions are acceptable for heavy-duty equipment like industrial conveyors, they are intolerable in business machines. To accommodate small machines without resorting to chains, belt manufacturers mold teeth to both sides of a synchronous belt. This eliminates lubricating serpentine drives and pushes shaft speeds to 14,000 rpm. Some automakers are using these double-sided belts on motors to turn balance shafts in opposite directions. Also, the belts can transmit 100% of maximum rated load from either side or, for example, 80% of full load from one side and 20% from the other. Page 1

Double-sided belts are available with standard trapezoidal teeth in XL, L, and H (0.2, 0.375, and 0.5-in.) pitches. They come in lengths up to 146 in. and are rated to 67 hp.

2 Double-sided belts are available with standard trapezoidal teeth in XL, L, and H (0.2, 0.375, and 0.5-in.) pitches. They come in lengths up to 146 in. and are rated to 67 hp. Newer curvilinear teeth, commonly called HTD belts, are also available in double-sided designs. Long Lengths Not all power transmission requirements fall neatly within published sizing tables. For a recent long-stroke requirement, designers considered a pneumatic apparatus and a rack and pinion arrangement. But compressed air was not available, and a steel rack was too heavy and expensive. The final design used a long-length belt in place of the rack and a sprocket instead of a pinion. This alternative is now preferred because it installs quickly and is less expensive than the compromise drives. Long-length or an ended-belt construction is the same as that for standard endless synchronous belts. They consist of a neoprene body, fiberglass tensile cords, and a nylon fabric facing on the teeth. Standard lengths, however, are up to 100 ft; 500 ft is available on special order. Long-length belting can also be used as a conveyer belt, particularly where synchronization of moving parts in a production environment is necessary. Page 2

Splicing together the ends of one of these long belts gives designers an endless belt of nonstandard length. The only drawback here is that splices reduce power ratings.

[Since this article field splicing is no longer available.

3 Splicing together the ends of one of these long belts gives designers an endless belt of nonstandard length. The only drawback here is that splices reduce power ratings. For example, a factory splice carries only 75% of the industry rated power for a similar design, but shorter, endless belt. Field splices are further derated to 25% of the endless belt rating. [Since this article field splicing is no longer available.] Made to order Despite the specialization mentioned so far, only a custom design or made-to-order belt (MTO) can fill other operating requirements. Most often, the specialization deals with a second belt function while the teeth are usually a standard pitch trapezoidal or HTD design. However, unusual pitches also require a custom belt. Other features that MTO belts provide are: Page 3

Special Backings: Extra thickness is usually used on conveyors and to grip objects. One example of using a belt as a gripper is in a machine that tightens jar lids on a filling line.

4 Special Backings: Extra thickness is usually used on conveyors and to grip objects. One example of using a belt as a gripper is in a machine that tightens jar lids on a filling line. Two belt drives, separated by about the lid width, spin the lids on as the jar passes between the belts. Uniform or close-tolerance thickness produces minimum vibration in applications with backside idlers. A special grinding process gives a uniform thickness. Nonmarking backings may be required when the belt transports paper, cloth, or an easily marred material. Abrasion resistance is required when objects with a rough surface are transported. Special Formulations: Belt conductivity is improved by adding carbon black with other compounds to a belt-rubber formulation. In grain elevators and computer equipment, this feature allows potentially hazardous static charges to drain safely to ground. Nonconductivity or belt-insulation properties can be improved for home appliances or power tools. Resistance to special lubricants provides longer operating life on machine tools. Special compounds allow belts to maintain flexibility in cold applications down to -65 F. Similarly, in hostile environments to 250 F, a belt molded from a modified compound can out perform an off-the-shelf formulation. Nonstandard pitches: Page 4

Occasionally, standard pitches cannot fill a strict requirement, usually related to spacing or timing. For example, spacing the characters on a print-head drive properly is aided with a 1/12-in. or 0.

5 Occasionally, standard pitches cannot fill a strict requirement, usually related to spacing or timing. For example, spacing the characters on a print-head drive properly is aided with a 1/12-in. or in. pitch belt. In this case, the designer must also match the belt pitch to the stepper motor. In another example, mm pitch belt body with an 8-mm pitched tooth satisfies special geometry and load requirements on some automotive overheadcamshaft drives. Small Drives As products are downsized to shed weight and improve efficiency, so too are motors and their belt drives. Small or minipitch urethane belts were developed for the business machine market, but are used wherever small motors require speed reduction. Mini-extra-light or MXL belts are available in a variety of polyurethane compounds with excellent abrasion resistance. Despite their small size, the belts provide good strength characteristics through the use of polyester or Kevlar tension members. Tension member selection is based on loading, drive geometry, and flexibility. Typically, minipitch belts are used in office and computer-related machines such as typewriters, check sorters, printers, and optical equipment. The belts are also ideal for nonmarking applications. Additionally, they exhibit excellent abrasion resistance, making them suitable for precision laboratory equipment and clean rooms. Page 5

6 The tooth profile on the minipitch belts is a miniature version of the standard timing belt s 40 tooth angle. These designs use a.080-in. pitch, recognized by the Rubber Manufacturers Association and the International Standards Organization (ISO). Minipitch belts can have a speed ratio of 8:1 with a center distance of as little as 1.5 in.; or a 1:1 ratio with a minimum center distance of 0.84 in. on sprockets as small as in. Specialized Sprockets and Hardware Most synchronous drive sprockets are made of ductile or cast iron. While these are suitable for most industrial equipment, different materials work better in nontypical applications. Aluminum is a good choice for drives that require low weight. These applications can include high speed or low inertia, or both. Steel is a better choice than iron when the drive will exceed the safe operating limits for cast iron (6,500 fpm) or ductile iron (8,000 to 10,000 fpm). Nonmetallics offer low-cost alternatives where power requirements are low as in office machines or home appliances such as vacuum cleaners. Nonmetalic sprockets may also be acceptable if power requirements are moderate but belt service life is short, as in some power tools, or lawn and garden equipment. Page 6

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GatesFacts Technical Information Library Gates Compass Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA

GatesFacts Technical Information Library Gates Compass Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA Gary L. Miller Plant Engineering November 7, 1991 DIFFERENCES IN SYNCHRONOUS BELTS All toothed belts are not the same; subtle differences in tooth profiles and construction affect performance. Synchronous