Using synchronous belt drives for positioning

Neoprene or polyester urethane or synchronous belts (also called timing belts) with reinforcing tensile cords are widely used on rotary motion axes and other power transmission applications. Recall that timing belts areconstructed much like flat and V belts plus have arrays ofevenly spaced teeth on their inner surface to work via positiveengagement. This kind of operation (core to chain and gear drives as well) is defined by almost zero relative differential of motion between the pulley or sprocket driving the belt and the belt itself.

But besides their use for simple PT and rotary motion, the belts are also indispensable in two precision-motion application types:

• Linear motion axes inside machinery necessitating servo-level motion control

• Conveyors that need to position workpieces and other discrete items.

The reinforcing tensile cords embedded in these belts are usually made of steel or Kevlar to prevent stretch, even when transmitting large torques and propelling loads with high acceleration; that complements the way in which the belt teeth prevent creeping because of their positive-drive nature. That’s especially true for urethane belts, which exhibit minimal tooth deflection.

The synchronous belt here is a Poly Chain GT Carbon belt from Gates Corp. These excel on axes on low-speed high-torque machine in industrial applications.

Such operation makes for a constant speed ratio and in some cases the ability to execute relative axis positioning with the belt — both useful in automated machinery that must execute predefined strokes.

The distance between belt teeth (from centerline to centerline) is the belt pitch; this in turn is measured at the belt pitch line defined by the belt’s tensile cords and neutral bending axis. The neutral axis is that plane of the synchronous belt that neither compresses or stretches so remains free of stress during operation. Pitch circle intersects with this pitch line.

Such definitions are key to properly specifying belts and pulleys that are compatible — and quantifying the dynamics of a synchronous belt employed in moving workpieces. For more on this topic and the specific case of driving a load-bearing carriage along a linear axis, visit linearmotiontips.com.

APPLYING SYNCHRONOUS BELTS

Some general guidelines are applicable to all timing belts, including miniature and double-sided belts. First, engineers should always design these belt drives with a sufficient safety factor—in other words, with ample reserve horsepower capacity. Tip: Take note of overload service factors. Belt ratings are generally only 1/15 of the belt’s ultimate strength. These ratings are set so the belt will deliver at least 3,000 hours of useful life if the end user properly installs and maintains it. The pulley diameter should never be smaller than the width of the belt.

As mentioned, belts are quieter than other power-transmission drive options … but they’re not silent. Noise frequency increases proportionally with belt speed, and noise amplitude increases with belt tension. Most belt noise arises from the way in which belt teeth entering the pulleys at high speed repeatedly compresses the trapped pockets of air. Other noise arises from belt rubbing against the flange; in some cases, this happens when the shafts aren’t parallel.

Pulleys are metal or plastic, and the most suitable depends on required precision, price, inertia, color, magnetic properties and the engineer’s preference based on experience. Plastic pulleys with metal inserts or metal hubs are a good compromise.

Another tip: Make at least one pulley in the belt drive adjustable to allow for belt installation and tensioning. Also note that in a properly designed belt drive, there should be a minimum of six teeth in mesh and at least 60° of belt wrap around the drive pulley. Other tips:

Pretension belts with the proper recommended tension. This extends life and prevents belt ratcheting or tooth jumping.

Align shafts and pulleys to prevent belttracking forces and belt edge wear. Don’t crimp belts beyond the smallest recommended pulley radius for that belt section. Select the appropriate belt for the design torque.

Tensioner idler images courtesy BRECOFLEX

SYNCHRONOUS BELT CONVEYOR BENEFITS AND USES

When designing a conveyor, one of the first considerations is the conveying media to be used, which depends on the size, shape, and weight of the product being handled and on the process requirements, such conveying distance and route, transport speed, and positioning accuracy. For assembly and automation applications, the most popular conveying media are belts, chains, flat-top chains, and powered rollers.

Of these media choices, belts are arguably the most versatile. They can be manufactured in virtually any width and can span long conveying distances, and they operate with lower noise and less required maintenance than chains or rollers. Belts and their mating rollers (or pulleys) are also available in many different materials and finishes to meet specific application requirements and environmental conditions.

Traditional belt-driven conveyors use flat belts that rely on friction between the belt and rollers to transmit power. They also rely on friction between the belt surface and the product to hold the product at a specific location on the conveyor. This friction-based design is flexible and economical, but it leads to a potentially variable product positioning and orientation. Case in point: Tthe belt can slip on its drive rollers if working conditions exceed rated load, speed, or acceleration, and the product can move, or slip, on the belt during starts and stops or any time there’s a change in acceleration.

When accurate product location and orientation is required, synchronous timing belt conveyors are typically the best choice.

Timing belt conveyors typically use belts that are reinforced with steel or Kevlar tensile cords to provide increased load carrying capability and enable high acceleration rates (which result in high forces on the belt) without causing the belt to stretch.

To improve grip, the top (carrier) side of the belt can be coated to increase friction between the product and the belt surface and reduce the possibility of the product slipping on the belt. Belt and pulley materials and finishes can also be selected to meet specific application requirements and environmental conditions, such as ESD-compatible, food-grade, or clean environments.

When application requirements call for a very accurate product location with no loss of position, cleats or fixtures can be attached to the belt carrier surface to hold the product in a specific location and with the correct orientation.

Another option to ensure precise location and orientation is to transport the product on a pallet. Locating the product on a fixed pallet, together with the accurate, no-slip conveying of the timing belt, provides the highest level of position accuracy and certainty.

A further benefit of timing belt conveyors is that the movements — and therefore the products being carried — of multiple conveyors can be synchronized. This makes timing belt designs ideal for dual- or multipleline configurations and allows large, heavy loads to be transported, even when accurate positioning is required.

RATCHETING IN SYNCHRONOUS BELT DRIVES

Synchronous belts (also referred to as toothed, cogged, timing, or high torque belts) use profiled teeth that mesh with a pulley or sprocket to deliver power transmission – most notably for applications that require high torque. Where V-belts rely on friction between the sidewalls of the belt and the sides of the pulley to transmit power, synchronous belts rely on the engagement between pulley teeth and belt teeth to transmit power.

While synchronous belts can transmit high torque without slippage when properly tensioned, using a belt with insufficient tension for the required operating parameters can cause the belt to jump teeth – a condition known as belt ratcheting.

When belt tension is too low, the belt may begin to self-tension with the belt teeth riding out of the pulley and causing increased tension on the belt. When this tension becomes too high, it will force the belt back down into the pulley grooves, which results in a brief but pronounced period of bending that can damage the belt tensile cords in a manner referred to as crimping. However, if the force of the “self-tension” does not cause the belt to slip back into the pulley grooves, the belt will ratchet, which can also cause crimping to the belt tensile cords and result in premature failure.

Proper tension of synchronous belts is the tension at which the belt will transmit the required power without ratcheting when the drive system experiences full load.

Synchronous belts have three general tooth profiles: trapezoidal, curvilinear, and modified curvilinear. Trapezoidal profiles are arguably the most common and provide good force capabilities with low backlash. Curvilinear (also called high torque drive) profiles have a geometry that is rounder and deeper than trapezoidal profiles, with a higher flank angle and greater contact area. This allows for better stress distribution and higher overall loading on the belt, but at the expense of higher backlash.

Modified curvilinear tooth profiles have a smaller tooth depth and an even greater flank angle, which provides the highest load carrying capacities of the three tooth profiles. But one of the primary advantages of this design is that the areas of the belt between teeth share loadcarrying duties with the teeth that are engaged in the pulley. This gives modified curvilinear belts the best anti-ratcheting properties, even under extremely high loads.