Synchronous belt drives dominate motion control designs for positioning and other precision functions. Here we detail V belts and other belt types used in industrial, robotic, and consumer designs for both linear and rotary axes.
Modern flat belts are either endless (welded or otherwise closed into a hoop by the manufacturer) or open. Common on grinders, fans, grocery conveyors, and other power-transmission applications, drives based on flat belts rely on precisely set tension for maintaining the proper the friction coefficient between belt and drive pulley.
Even today, many flat belts are made of natural materials as well as synthetic yarns featuring various filament structures. Flat belts made of polyurethane are common on the ends of conveyors consisting of roller arrays — to gang powered rollers (integrating a motor in its cylindrical body) to passive nonpowered rollers. Flat belts with polyester tension members excel where high tension (but little stretch) is required; coatings of PVC, polyurethane, and rubber enable high friction for use on high-speed axes running to 22,000 feet per minute.
One specialty type of flat belt indispensable in settings subject to high temperatures and corrosive washdown (or other chemicals) is that made of thin stainless steel. These flat belts are precision welded closed to traverse just a few centimeters to dozens of meters — and often perforated to accept the positive engagement of studded pulleys. Flat metal belts also exhibit no stretch or creep, so allow precision positioning of workpieces … and can protect workpieces sensitive to electrostatic charges with grounding.
Round belts (sometimes called O-ring belts or O belts) have a circular cross section; they’re common on axes of consumer-grade electronics with moving elements, office-grade printers and scanners, and light industrial equipment such as tabletop robotics with modest to moderate power-transmission requirements. Most round belts are extruded from neoprene, propylene, or cross-linked urethane (either reground or virgin) and then butt welded together into endless loops. Their elasticity makes them more forgiving of suboptimal installations, but at a sacrifice of power capability. Mating pulleys have semicircular grooves and diameters no less than sixfold the belt’s cross section. Texturized O-belts have lower coefficients of friction but are better able to resist abrasion and overheating.
Industrial power-transmission belt creep, slip, and tension
The caveat with these belt types (and really any belts not employing positive engagement via teeth) is that they can exhibit slip and creep.
Creep is a cyclical elongation of belt (with some measure of elasticity) as it travels around the loaded side to the slack side of its circuit … and is considered normal. Proper tension holds the dimensional changes of creep to within 0.5% of the belt’s normal length and cross section. There is a cyclical stressing associated with creep as well as the flexing of belt around its pulleys — which does ultimately limit belt life but doesn’t induce dramatic temperature increases.
In contrast, belt slip due to improper tension or (worse yet) improper design can quickly generate heat buildup. Simple measurements taken of belt temperature along with geometry, vibrations, and sound generated (including squealing upon startup) can accurately indicate the amount of tensioning or retensioning required. Belts transmitting high power require greater tensioning or risk slip and other modes of improper operation.
Design tensions are often defined by the ratio of belt drive-side tension divided by that of the slack side — along with a constant wedging design factor (for V belts) and the belt-to-pulley dynamic friction coefficient. Design tension ratios for V belts tend to be higher than those for flat belts.
A V belt’s primary subsection is its tension-bearing top. This top includes fiber cords for strength to pull actual traction load. Modern tension-member cords are often aramid, polyester, fiberglass, or steel … and pre-stretched variations help minimize stretch. The cords embed into the main belt material that serves to hold the belt body together and shed heat. The working side of V belts (which engages the pulley) is a compression section designed to wedge into pulley grooves for reliable shock-damping engagement. In many instances, a rubberized fabric cover ruggedizes the belt surface and prevents slipping (which in turn prevents overheating tension cords).
One additional note here: Though V-belt slip is usually detrimental, it can be a helpful behavior on axes that are truly jammed — serving to protect more expensive components in the drivetrain.
Though versatile and forgiving, friction-based belt drives that are improperly sized can slip tangentially on the pulley — a form of lost motion — and axially creep. Both of these issues can in turn cause unreliable speed output. Just remember if a V-belt drive makes the most sense for a motion axis: Output torque depends on belt resistance to tension and belt-pulley adherence. The latter is why oils and greases must be kept away from belt drives … or else failure due to slipping may arise.
The special case of cogged V belts
Not to be confused with toothed (synchronous) belts for motion-control applications are cogged V belts. These have notches on the working a.k.a. pulley-contacting belt surface to:
• Allow airflow for cooler operation
• Boost flexibility — to travel around pulleys having diameters smaller than otherwise allowable
These notched V belts are available in a wide array of classic and narrow configurations. In addition, they often have a raw-edge design (sans cover) for more space in the belt cross section for load-carrying cord. Any standard V belt that is cogged will have a name with an X suffix — such as BX or 3VX, for example.
Though most associated with heavy power-transmission applications, V belts do in fact find use in precision motion designs as well. Embrace the oxymoron: Static motion designs — those that only depend on consistent end-of-move positioning — can tolerate the errors of friction-belt drives. In contrast, dynamic motion designs require axes that move predictably over their complete strokes — even if load varies during operation. Here, engineers typically specify low-backlash toothed belts needing shallow clearance for pulley engagement. Single V belts for these motion applications often take the form of light-duty or fractional horsepower V belts denoted by 2L, 3L, 4L, or 5L codes with the latter dimensionally resembling so-called classical A and B-coded V belts.
More on common V belt standards
V-belt specification generally references cross-section geometries including the belt’s width at its widest, V angle, depth of engagement, height, and overall pitch length. The latter geometry is the circumferential length along a belt’s pitch line. Final V-belt specification is then determined by which correctly dimensioned belts have sufficient power ratings (defined by rpm and sheave speed) to satisfy the application’s nominal horsepower (at the driving motor’s output shaft) and the applicable service factor.
To simplify V-belt specification, suppliers have come to publish (and include in design software) information about various V-belt service factors that accommodate various applications’ typical demands … as well as losses from variable speeds and loads as well as detrimental environmental conditions such as shock, vibration, and heat.
V belts with so-called classical geometries just mentioned are a rugged if moderate-efficiency option. In the U.S., standardized geometries are coded A and B (most common) as well as lesser-used C, D, and E having progressively larger cross sections. Narrow V belts are named with progressively higher numbers for progressively larger belts — and have V suffixes. Double-sided V belts are those with a double-angle or so-called hexagonal geometry for winding through and driving serpentine drive arrangements; these are coded AA, BB, CC, and so on. These codes are listed in specification software and manufacturer catalogs as well as printed on the belts themselves … usually followed by a dash and then a number denoting the total working length of the belt in inches. Even International Organization for Standardization standards such as ISO 8419 (dictating the standards for narrow V belts) lists values in millimeters that reflect these standards first established in Imperial units.
Of course, the latter (length) code does not appear on adjustable V belts — those made of a series of interlocking sections joined by tabs or other fasteners like industrial chain. These linked belts (suitable for even high-power and high-speed axes to many thousands of rpm) are sold in open sections that are cut to length and then closed by the installer in the field.
Another V-belt design is that of joined V belts. Unlike ribbed V belts (covered in other articles) that have a common foundation of thickness sufficient to feature reinforcement throughout, joined V belts have discrete trapezoidal sections. These sections are themselves reinforced but joined by just a thin layer of tension material. Such joined V-belts are easier to specify than matched sets and far less problematic than separate arrays of V belts running in parallel — especially on axes subject to intermittent forces and speeds. That’s especially true on axes that might otherwise require a dozen or more V belts in parallel for sufficient power transmission.
Axes run off 2-hp or smaller motors under the control of variable speed drives (never prompting more than fivefold speed increases) accept V belts having 4L, 5L, A, or B notations. Axes with more dramatic speed variations necessitate other V belt that’s also standardized and coded — in this case, with four-digit values and a V (for narrow) suffix.
Specifying V belts by horsepower and geometry
Standards established by the Rubber Manufacturers Association (RMA) and the Mechanical Power Transmission Association (MPTA) inform specification approaches that first satisfy design horsepower requirements. Using this approach, a service factor applied to nominal (rated) motor or axis horsepower (to accommodate friction, vibration, heat, and other losses) ensures reliable and efficient belt-drive operation. These service factors are published for machine and drive types typical to various industries.
Power ratings are well documented for all standard V belt and pulley sizes and speeds. But arc and length correction factors (along with the belt-installation center distance and speed ratio covered in a moment) affect this basic power rating. Center distance is often presumed for set pulley combinations. That said, long pulley-to-pulley center distances yield high power ratings and shorter yield lower ratings.
Next, the driven-to-drive pulley speed ratio (based on any difference in their diameters) is calculated to yield output belt speed (sometimes called rim speed). High speed ratios magnify the effect of center distance changes on drive power ratings. Then the maximum output rpm or fpm (based on the axis geometry, pulley construction, and level of balancing) is calculated — with multiplane dynamic balancing increasing this value. Finally, work output for a given time (based on horsepower x 1.341) yields a value expressed in kilowatts.
One last design consideration for machines employing V belts is the level of balancing required. Refer to Balancing the pulleys of belt drives on linearmotiontips.com for more on this topic.