Round Belt Pulleys Information

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Selecting round belt transmissionRound belt pulleys are mechanical devices which transmit rotational motion around an axle. They are used to lift loads or to apply/transmit power. The round belt pulley accommodates a belt, cable, or rope in a groove around the circumference of the device to guide the drive element. Round belt pulleys are very efficient (approx. 95%), they can produce a mechanical advantage, and they are recognized as one of the six simple machines.


Round Belt Pulley Operation




In the accompanying video, the demonstrator accurately explains the mechanical advantage achieved by the use of a pulley system. 


Video credit: Charles Marzzacco via YouTube


Power Transmission


In many incarnations, a pulley transmission greatly resembles a roller chain drive. Most transmission applications utilize a belt to transmit energy between pulleys. A pulley transmission offers several advantages over roller chains drives. Not only can pulleys account for axial misalignment, but axles can be perpendicular or reside at any other angle. It's also common to employ a pulley transmission in a crossover drive to reverse the direction of the receiving pulley.


Selecting pulleys  Selecting crossover pulley

Images Credits: Orbit Bearings | Scholar Express 


Other advantages:


  • Less expensive than roller chain drives.
  • Selecting pulleys round beltEasy maintenance, with no lubrication required.
  • Protects from overload.
  • Quiet operation with vibration dampening.
  • Invulnerable to oils/petroleum.
  • Can be easily disengaged; can serve as a clutch.
  • Very little wear on pulleys.

Disadvantages do exist. Drive belts easily stretch and slip, so idler pulleys or a new belt must be implemented. Also, pulley drives are the weakest form of power transmission; in any configuration where the belt does not wrap 180° around the pulley significant power loss occurs (remedied by idler pulleys, toothed belts, or an increase in drive power). Belt drives are limited to moderate speeds (35 m/s) and by power transmission (500 hp maximum). Finally, operating temperatures are often restricted to -31°—185°F.


Round Belt Pulley Components




Grooves on a round belt pulley are intended to provide guidance for the line as it travels the perimeter of the pulley, and in transmission applications they are also responsible for maintaining friction on the line to transmit motion. The grooves of a pulley can often serve a range of line diameters. Two different types of grooves exist: the round groove, which offers minimal friction and torque transmission, but is adequate for guidance; and the trapezoidal groove, which wedges a belt between opposing surfaces to optimize friction and is preferred for transmission applications. Rounded-groove pulleys are sufficient for tackle configurations, or as idler pulleys to reroute or stabilize transmission belts. Pulleys may utilize adjacent circumferential grooves to accommodate multiple drives on one axle.


Round groove         Trapezoidal groove           Multiple groove pulley

Rounded groove; trapezoidal groove; dual-groove pulley

Images credits: Durabelt; Harsiddhi; Durabelt




The line is the physical linkage which exerts force around a pulley's axle. Round belts and o-ring belts are explicitly meant to cooperate with round belt pulleys and are the preferred choice for transmitting motion between axles. Nitrile rubber, neoprene, polyurethanes, and natural rubbers are the most common materials of construction. These elastomeric belts are typically between 1/16" to 3/4" of an inch thick, and must be taught upon installation to maintain friction against the pulleys. The segment of the round belt in contact with the pulley experiences some deformation from its typical round cross section. Belts are also abrasion resistant, easily replaces, non-marking, and can feature ESD or UV resistance or ridges to better engage the pulley.


Images credit: Durabelt 


In block and tackle configurations, the line has two distinct ends, one of which is attached to a stationary point above the load. The line is threaded through at least one pulley (although at least two pulleys are required to create an advantage) and force is exerted on the loose end of the line to create lift. In this instance, ropes and cables are exceptionally more useful and can withstand much heavier burdens. An unconstrained but manageable length of line is helpful due to the additional travel needed to raise a weight to the required height.


Selecting pulleys round

Image credit: Wise Geek




Primarily, the bore diameter of the pulley must be a geometric match with the mating shaft. The shaft about which the pulley rotates is an axle, and the most common type of accomodating this axle is by the use of a simple bore, whereas the pulley is allowed to freely rotate about the axis with no torque transmitted. This bore is used in idler pulleys and in tackle applications, but is otherwise useless for power transmission. Other mounting styles include:


  • Set screw: A through-hole allows a screw to be perpendicularly tightened against the axle.
  • Keyway: An offset slot or shaft ensures an intimate fit along the axle to transfer torque between components.
  • Press-on: An offset slot or shaft ensures an intimate fit along the axle to transfer torque between components
  • Weld-on: The pulley's hub is directly attached to the axle via welding.
  • Taper bushing: A bolt-on tapered hub locks around the shaft.
  • Clamped hub: A split hub is tightened around the axle by the use of a clamp.


Round Belt Pulley Production


The accompanying video depicts the production of pulleys, from raw material to finished product.


Video credit: How It's Made via YouTube


For large, metallic designs the center rim of the pulley may be partially removed or cut away to reduce the pulley's overall weight. This reduces the inertia and starting torque of the pulley, but correspondingly limits the load capability. Aluminum, cast iron, steel, stainless steel, plastic, brass, and zinc compositions account for the construction materials of most pulleys.


Round Belt Pulley Specifications


Pulley geometry is represented in metric and avoirdupois units, but there is no other structural or design difference between the types. Other considerations for integrating pulleys include:


  • Selecting pulleyBelt dimensions: The size range of belt/line diameters that the pulley can contain between its grooves, as well as the appropriate length of the belt when accounting for the center distance of the pulleys, the pulley diameters, and the stretch of the belt. Manufacturer Durabelt has developed a belt calculator to assist in these considerations. 
  • Pulley diameter: Based on the application, axle distances, HP transmitted, and mechanical advantage, procuring round belt pulleys is an arduous decision. A pulley-sizing calculator has been established to determine the optimal diameters for pulleys based on the aforementioned factors.

  • Pitch diameter: Pitch or datum diameter for a pulley is the diameter of the line of action. Typically it is at or near the center of the circular pulley groove.

Round Belt Pulley Standards


These accompanying standards may prove useful when considering round belt pulleys.


- ISO 254 Quality of belt-drive pulleys

- BS 4018 Specifications for pulley blocks with 25 ton cable capacity

- BS 3790 Specifications for belt drives and their corresponding pulleys

BS 13157 Safety for hand-powered cranes




Durabelt - Round Belts


Wikipedia - Pulley; Block and tackle;


Gizmology - Belts and pulleys


PIC Design - Belts and Pulleys (.pdf) 


Image Credits:


Peter Valdivia | Direct Industry | Motionco