Choosing special-purpose synchronous belts

Sept. 1, 2000
Though many standard synchronous belts are available, none of them may be exactly right for your application. Check out specialized belts because they could be the answer.

Standard synchronous belt drives, which transmit power through trapezoidal or curvilinear-shaped teeth, satisfy the need for precision in many timing and power transmission applications. But a substantial number of such applications require non-standard belts and sprockets with special features. Knowing the types of specialized synchronous belt components can help you solve troublesome drive design problems. These components include:
• Urethane mini-pitch belts.
• Double-sided belts.
• Long-length belting.
• Made-to-order (MTO) belts.
• Sprockets with non-standard materials and special design features.

Here’s a rundown on these special components with practical applications for each.

Urethane mini-pitch belts

Available in a variety of urethane compounds, mini extra-light-pitch (MXL) belts offer excellent motion transfer properties. These belts, Figure 1, generally have either polyester or aramid tensile members, depending on the required load, flexibility, and drive geometry.

Their tooth profile is a miniature version of the 40-deg-angle tooth in standard timing belts. These mini-pitch belts are made with a 0.080-in. pitch that is recognized by both the Rubber Manufacturers’ Association (RMA) and the International Organization for Standardization (ISO).

Mini-pitch belts offer speed ratios up to 8:1 with a center distance as little as 1.5 in., or speed ratios as low as 1:1 with a center distance of as little as 0.84 in. They operate on sprockets as small as 0.255-in. pitch diameter. There are three standard belt widths — 1/8, 3/16, and 1/4 in. Peak torque ratings range from 0.29 to 2.01 lb-in.

These belts are found in motion control or positioning systems, which are often computer numerically controlled (CNC). They accurately transfer motion with minimal torque and minimum lost motion from backlash.

Mini-pitch belts are used even more frequently on office equipment, such as check sorters, printers, copiers, plotters, and computer tape feeds, plus coin changers and photographic equipment.

They are well-suited for applications requiring clean operation, such as where a drive belt must move paper without marking it. Their abrasion resistance eliminates the tiny black wear particles found inside copy machines that use rubber belts.

Also available, though not recognized by the RMA, are 40DP (diametral pitch) belts with an 0.0816-in. pitch. Though 40DP belts are not interchangeable with MXL belts, they are used in similar applications. Double-sided belts These belts typically have nylonjacket- covered teeth molded on both sides of the belt, Figure 2. This feature lets the belt transmit up to 100% of the maximum rated load from either side of the belt, or from both sides in combination when the sum of the loads does not exceed the belt rating. If both sides of a belt are not fabric covered, it will achieve less than the published load ratings. Belt construction is similar to that of comparable standard single-sided belts, with a neoprene body and fiberglass tensile member. Double-sided belts are available in XL, L, or H trapezoidal timing belt configurations and standard HTD curvilinear tooth profile types.

Timing belt versions come in 1/5, 3/8, and ½-in. pitches, and either standard pitch lengths up to 140 in. or special lengths to 270 in. Power transmission ratings range from 0.003 to 67 hp. These belts offer speed ratings up to 10,000 rpm with speed ratios to 8.57:1.

HTD versions are available in 3, 5, 8, and 14-mm pitches and pitch lengths up to 270 in. Power transmission ratings range from less than 0.01 hp up to 190 hp. These belts offer speeds up to 14,000 rpm and speed ratios to 8.73:1.

Common uses of double-sided belts include serpentine drives in roller mills, printing press chill rolls, feed mills, and textile machinery. Such applications almost always require reversed direction of rotation. These belts also operate in riding- mower deck drives using counter-rotating cutting blades, and in many applications where they replace gears.

Long-length belting

Available in lengths to 400-ft, longlength belting comes with L and H trapezoidal tooth profiles and the 8-mm HTD profile. Its basic construction is the same as standard endless synchronous belts, which have a neoprene body, fiberglass tensile cords, and a nylon fabric facing on the teeth.

Capabilities of long-length belting include power transmission, synchronization, and positioning. This belting is commonly applied where the drive center distance exceeds the length of standard endless belts, and where non-standard lengths are needed.

Long-length belting serves as a conveyor belt in certain applications, particularly where the movement of parts must be synchronized. In another example, it replaces rack-and-pinion gear systems in moving tables for medical X-ray equipment, machine tools, and X-Y coordinate plotters.

Be careful in sizing these belts. Factory spliced belts carry only 75% of the industry rated horsepower for a similar endless belt.

Made-to-order belts

Because of their special capabilities, made-to-order (MTO) belts play a particularly important role in machinery design. Generally available in most trapezoidal and HTD configurations, these belts offer a variety of special features as follows.

Special material or construction.
• Conductive belt material prevents static discharge, which could disrupt operation of computer peripherals and equipment in potentially hazardous environments, such as grain silos, coal mines, and munitions plants. Both the belt body and tooth facing fabric contain conductive carbon, which provides a ground path for any static charge.
• Non-conductive material acts as an electrical insulator for home appliance and power tool users.
• Oil-resistant compounds minimize belt softening and degradation caused by oil and chemicals in machine tool applications.
• Temperature-resistant compounds permit efficient operation in high (250 F) or low (265 F) ambient temperatures.
• Tensile members incorporate aramid for high shock loads or steel for more precise registry (positioning). Though most belts use alternate righthand/ left-hand (s/z) twist cord construction, belts can be produced with all-right or all-left twist cord to accommodate special tracking requirements.

Special backing.

• Belts with extra thickness or special backing material are used to convey parts or grip objects. In the first case, extra material, Figure 3, lets the belt wear more before it must be replaced. In an example of the second case, two belts run back-toback in opposite directions and a special backing material applies force to a jar lid, literally screwing the lid on the jar.
• Consistent backing thickness avoids vibration on backside idler applications. Normally a thickness tolerance of + 0.025 in. is suitable. However, for very smooth power transmission, as in machine tool spindle drives, belts are ground to a tolerance as close as + 0.005 in.
• Non-marking belts touch finished products, such as paper, cloth, or pharmaceutical containers, without leaving black marks. This is accomplished by using a non-carbon compound, or by gluing a non-marking plastic on a rubber backing.
• Abrasion resistance avoids belt wear, especially where abrasive objects contact the back of a conveyor belt. This feature is usually achieved by using a wear-resistant compound in conjunction with a thick backing.

Nonstandard sizes. • Though standard pitch belts are usually preferred, there are exceptions. For example, a 1/ 12-in. or 0.0833-in. pitch belt used with a step motor on a printer head drive ensures the proper number and spacing of characters per width of page. In another example, a 9.525-mm pitch belt with an 8-mm pitch tooth profile satisfies the geometry and load requirements of automotive overhead camshaft drives. With this configuration, the teeth are spaced further apart than normal so users could switch to a curvilinear type belt while retaining the original 3/ 8-in. (9.525 mm) tooth spacing. • Nonstandard widths or lengths meet specific drive design conditions or geometry requirements.

Modified curvilinear tooth profile.
• Besides the HTD (curvilinear) profile, special rubber and high-capacity urethane belts are available with a modified curvilinear tooth shape in 2, 3, 5, 8, and 14-mm pitches. Such belts, Figure 4, often serve as quiet, lubrication-free replacements for gears or steel roller chain drives.

Special sprockets

For industrial applications, most synchronous belt drive sprockets are made of either cast or ductile iron. However, aluminum, steel, and non-metallic materials, Figure 5, are better suited for certain applications:

Aluminum is used for drives with high speed and low inertia, or where weight must be minimized. But excessive tooth wear may occur with sprockets.

Steel is best for drive applications that exceed the safe operating limits for cast iron (6,500 fpm) or ductile iron (10,000 fpm). Steel sprockets can operate at speeds up to 20,000 fpm, depending on the type of steel.

Non-metallic materials are used in office machines, power tools, and lawn and garden equipment where little torque is transmitted or where service life requirements are short. Tooth wear and structural strength must be considered with these sprockets.

Other specialized features for synchronous drive sprockets include special hub mounts and flanges, or tooth profile modifications to meet positioning tolerances such as in the robotics industry.

David E. Roos is the manager of application engineering at The Gates Rubber Co., Denver, Colo.

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