When you need to put light loads in the right place, check out miniature synchronous belts
Belt drives for office machines, household appliances, and small machine tools face some common challenges: they have to transfer motion and position light loads in small spaces. In many cases, running smoothly at high speeds is another criteria.
Pint-sized synchronous (timing) belts usually satisfy the need for precision in such equipment. Where smooth operation and speed are important, small V-belts or V-ribbed belts may be better suited. Whatever the type, special sizes and materials help to meet specific design requirements.
When size matters
Mini extra-light-pitch (MXL) synchronous belts transfer motion and synchronize small components in business machines such as check sorters, printers, copiers, plotters, and computer tape feeds. Their trapezoidal tooth shape minimizes lost motion from backlash, which is the clearance between belt teeth and sprocket grooves.
The tooth profile is smaller than the 40-deg-angle trapezoidal tooth of standard size synchronous belts. And it has a standard 0.080-in. pitch, which is recognized by both the Rubber Manufacturers Association (RMA) and the International Organization for Standardization (ISO).
These miniature belts come in three standard widths -- 1/8, 3/16, and 1/4 in. -- with peak torque ratings from 0.29 to 2.01 lb-in. They operate on sprockets as small as 0.255-in. pitch diameter. Speed reduction ratios range from 1:1 to 8:1 depending on the minimum distance between sprocket centers.
Similar to MXL belts, though not interchangeable, are 40DP (diametral pitch) belts with 0.0816-in. pitch. This design is not recognized by RMA.
Newer belt designs have either curvilinear (HTD) or modified curvilinear shaped teeth. In general, drives with trapezoidal teeth (MXL) have little backlash, whereas curvilinear tooth belts (HTD) provide improved torque capacity and resist ratcheting (jumping teeth). Curvilinear tooth belts are not acceptable for most precision indexing or registration applications because they have higher backlash.
Modified curvilinear tooth belts offer both improved torque capacity and low backlash -- comparable to that of MXL belts. They are available in 2, 3, and 5-mm pitches.
Synchronous belts produce low levels of vibration that may be objectionable in sensitive equipment. This vibration stems from the teeth as they mesh with sprockets and from the belt's high tensile modulus (stiffness), a characteristic that holds consistent belt pitch under load. To reduce such vibration, choose sprockets with diameters larger than the minimum size available because the belt teeth mesh more smoothly.
V-belts have no teeth, and they aren't as stiff as synchronous belts. As a result, they run smooth. However, small discontinuities in rubber and overlaps in fabric covers may cause rough running as speeds increase and pulley diameters decrease. V-ribbed belts generally provide smooth operation due to their precision formed longitudinal ribs.
All three miniature belt types (synchronous, V, and V-ribbed) can operate at high speeds, over 1,200 ft/min. However, V-belts nearly always run quieter than synchronous belts. Joined V-belts are especially stable, turning shafts at speeds over 30,000 rpm on equipment such as computer peripherals, dental grinders, drilling machines, and lathe drives. V-ribbed belts drive small machine tools, precision grinders, food slicers, juice mixers, and pumps.
Flexing fatigue can shorten belt life at high speeds. Therefore, use small pitch synchronous designs that flex easily without generating high fatigue stress. Large sprocket diameters also reduce flexing. Belts with 2 and 3-mm pitch and modified curvilinear tooth profiles suit high speed applications because their curved teeth enter and leave the sprocket grooves smoothly.
Tension is also critical with high speed synchronous drives. Low tension lets a belt ride out of the driven sprocket, causing faster tooth and sprocket groove wear. High tension, on the other hand, pulls a belt further into the sprocket, causing premature wear of the tooth lands and possible tensile failure.
Equipment such as office copiers require serpentine drives with belts that drive from both sides. Similar in construction to regular synchronous belts, a double-sided belt has nyloncovered teeth on both sides to synchronize motion from both driving surfaces.
A double-sided belt can transmit 100% of its rated load from either side of the belt, or in a combination where the sum of the loads exerted on both sides doesn't exceed the maximum rating.
These belts are available in trapezoidal and curvilinear tooth versions. Trapezoidal types come in 1/5, 3/8, and 1/2-in. pitches and pitch lengths up to 270 in. Their capacity ranges from 0.003 to 67 hp. They operate at speeds up to 10,000 rpm with speed ratios to 8.57:1.
Curvilinear tooth versions are available in 3, 5, 8, and 14-mm pitches and pitch lengths up to 270 in. Their capacity ranges from less than 0.01 to 190 hp. These belts offer speeds to 14,000 rpm and speed ratios to 8.73:1.
Common sense dictates using standard miniature belts whenever possible. However, some operations need made-to-order (MTO) belts and hardware. One example is a 1/12-in. or 0.0833-in. pitch synchronous belt used on printer head drives. The non-standard pitch matches a step motor driving the head to ensure the proper number and spacing of characters on a page.
Other examples include special materials that minimize belt wear particles for "clean" applications. A 2-mm "bare-tooth" synchronous belt keeps computer printers running cleaner. The bare-tooth belt has a nylon tooth facing, but without the rubber coating usually found on synchronous belt teeth. This nylon facing reduces noise and cuts down on visible wear particles generated by the belt.
In another application, urethane belts are used in clean rooms, where microprocessor chips and drugs are manufactured. Urethane belts generate less wear debris than rubber belts.
Material handling processes (paper and plastic film), and sensitive electronic equipment cannot tolerate electrical discharges. These applications require conductive belts that let static charges generated by the belts dissipate through the sprockets to ground. Special rubber belts are made with conductive carbon for this purpose.
Tips on tensioning
Synchronous belts usually require less tension than V-belts for the same load. Belt manufacturers can recommend specific values for various operating conditions.
In general, a synchronous belt that transfers motion at low torque usually requires just enough tension to make the belt teeth conform to and mesh with the sprocket grooves. If accurate registration is required, the belt needs more tension to minimize registration errors. Higher tension increases the belt tensile modulus as well as the degree of tooth mesh, both reducing backlash. The best way to ensure that a belt drive achieves the desired performance characteristics for such an application is to determine the required tension value experimentally.
Synchronous belts that operate with uniform loads most of the time may only need enough tension for the uniform loads, even though intermittent peak loads occur. When such a belt encounters peak loads, it self-generates tension as needed to carry the load. This higher tension may cause the belt teeth to ride out of the sprocket grooves as they enter the driven sprocket on the slack side, causing increased belt tooth and sprocket wear. As long as the belt teeth don't ratchet, however, the lower tension value will usually give satisfactory results.
Sprockets and pulleys for synchronous belts and V-belts respectively are made of cast aluminum, sintered steel, and plastics. Aluminum offers light weight (2/3 the weight of steel), oxidation resistance, excellent heat dissipation, and high electrical conductivity. Some grades can be heat treated for higher strength and hardness.
Sintered steel offers excellent strength and wear resistance. Machined sprockets are generally quieter than molded ones because of material density, resonance characteristics, and dimensional accuracy.
Injection-molded plastic sprockets and pulleys are economical because they're ready to use (without machining) after molding. However, they usually have less strength, accuracy, and heat dissipation than metallic parts. Common materials include nylon, Nylatron, and polycarbonate, with optional fiberglass reinforcement for all three types. These materials are rust-proof and can be molded around a metal insert.
Belt drives for light applications commonly have fixed centers between sprockets or pulleys, with no means of adjusting belt tension. Precise registration applications, on the other hand, usually require belt adjustment or takeup.
For a fixed center design (no takeup), hold the center distance as accurately as possible, typically within 0.002-0.003 in. This may require a stamped steel frame to get consistent accuracy throughout a production run. To verify that such drives will perform as expected, test belts produced over their tolerance range for the maximum center-distance variation.
Idlers are commonly used to adjust belt tension for precise registration applications. This increases belt bending and fatigue, but proper design procedures will minimize this effect. For example, it's usually better to place an idler inside a belt loop rather than outside, and on the slack side rather than the tight side. Grooved idlers generally work better than flat ones. Other important design factors include the idler location in the belt span, and its diameter.
Brent Oman is an application engineer, Power Transmission Product Application Dept., The Gates Rubber Co., Denver, Colo.