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Two-step process eliminates V-belt service
Rotary vulcanization and polyester tension cord stabilization eradicate wrapped-wedge belt maintenance
By John Lewis, Northeast Technical Editor -- Design News, June 17, 2002
Addison, IL — Engineers typically use belts to transmit rotary motion between shafts. Tension on the belts creates friction between belt and pulley. This allows the belt to grip the pulley, and rotate without slipping. Belts are either flat or V-shaped. Flat belts run on pulleys and are more suitable for transmitting power between shafts that are far apart. V-belts run in a grooved pulley, or sheave, and are good for connecting shafts that are closer together.
Because a V-belt interfaces with a sheave on its flanks, it has more surface area contacting the pulley than flat belts do, and subsequently achieves greater friction with less tension, and in a more compact space. The wedging action of a V-belt increases the force of the belt against the sheave groove and helps prevent slip. Consequently, tension is key for transmitting power and torque, and ultimately dictates the efficiency and life of the drive.
Slippage decreases tension, drive efficiency, and life, and is the biggest problem with V-belts due to normal stretching during operation. As a result, tension must be frequently checked and reset throughout a belt's operating life. That is, unless you're using the new Red Power II Service Free V-belts from Optibelt Corp., which never need retensioning. "Stretch is so low that once set correctly, it maintains optimum tension throughout its service life," explains Optibelt President Philip Carlson.
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For engineers designing belt drives, high power ratings generally translate into fewer belts per drive, narrower pulleys and lower overhung loads. |
Thanks to new materials, improved tolerance levels, and better temperature control in the manufacturing process, the service-free belt needs to be correctly tensioned only once. Other benefits include: up to 42% greater power transmitting capacity, improved operating efficiency, and longer life. Though it costs more than standard V-belts, the belt generates lower operating costs over its lifetime, thanks to less maintenance and greater efficiency, the company says. While this may benefit retrofit applications, engineers designing new belt drives with the service-free belt instead of a standard grade belt may realize the greatest payback.
"Because Red Power II transmits more power than standard belts of the same size, there are many ways an engineer can improve a drive design from a value engineering standpoint," explains Optibelt Technical Manager Eric Steele. Using such belts in a multi-belt drive system, for example, lets the engineer decide between extending belt life, decreasing pulley cost, or reducing overhung loads on the pulley bearings.
For longer belt life, an engineer may use the same number of belts to increase the drive system service factor. Theoretically, a greater service factor means the belts will last longer. To decrease the cost of the pulley, the engineer could use fewer belts, decreasing the number of sheaves required. Because power transmission capability increases with pulley diameter, another way to solve the same problem is to use an equal number of belts with a smaller diameter pulley to reduce the overhung load. "It's up to the individual engineer to determine which parameter saves him the most amount of money or achieves the highest payback," Steele adds.
The first step to fully understanding this product line and its inherent advantages, notes Carlson, is understanding how it's made. The ORV (Optibelt Rotary Vulcanization) process differs considerably from traditional vulcanization presses in that it vulcanizes belts in the same mode of motion and shape as they are during service—in other words, while rotating in v-grooves on a heated, vulcanized drum. "This process gives the smoothest running V-belt, with a true trapezoidal cross-section," Carlson explains. "Near-zero tolerance in the flanks of the belt compared to the cross-section of the pulley provides 100% contact."
After the vulcanization process, the belts are subsequently cooled during an electronically controlled cord stabilization process that sets the length of each belt. This process gives Optibelt standard as well as Red Power II Service Free V-belts, their unique SetConstant feature that allows V-belts of similar nominal lengths to be assembled into matched sets at random, without further measurement or code matching, Carlson explains.
While existing belts continue to stretch over their lifetime, Red Power II belts are designed not to stretch using a transversely oriented fiver mixture of polychloroprene positioned above and below the new polyester service-free tension cord. The key to a service-free belt is taking up all of the stretch during installation. Belt tension is typically set twice: once after initial installation on the pulley, then a second time after a brief run-in period. "Initial tension may be up to 30% greater than run-in tension after the belt sinks down into the sheave," Steele says. "But once you retension the Red Power II, you're done. For all intents and purposes, you never have to retension it over its lifetime."
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