V-Belts: Types, Terminology and Overview

Chapter 1: What is a V-Belt?

A V-belt is a versatile and effective component used for transmitting power between shafts. It features a distinctive trapezoidal design that locks securely into the sheaves of a shaft. This specific shape allows V-belts to fit snugly within the grooves of the sheave, enhancing surface contact and providing better stability.

When tension is applied, vertical forces acting perpendicular to the top of a V-belt press its sides against the sheave's grooves. As these forces intensify, the belt becomes more firmly wedged into the sheave, increasing friction between the belt and sheave surfaces. This enhanced grip facilitates higher torque transmission and reduces power loss due to slippage.

The ability to transmit greater loads is attributed to the multiple frictional forces generated. The effectiveness of a V-belt is largely influenced by how securely it engages with the sheave groove under increased tension.

V-belts are crafted from both synthetic and natural rubber, providing the necessary flexibility and resilience to conform to sheaves. They are constructed by compressing different fibrous tensile cords into the distinctive V-belt shape, which imparts remarkable strength and durability. Some V-belt designs feature added cogs to enhance bending resistance and reduce operating temperatures.

Chapter 2: What is an overview of belt drives?

Belt drives facilitate the transfer of power between two or more rotating shafts, typically with parallel axes. Belts are looped around pulleys mounted on both the driving and driven shafts. These pulleys are positioned at specific distances to maintain the necessary belt tension. The friction between the belt and the pulley ensures a secure grip during operation.

As the driver pulley rotates, it increases tension on one side of the belt, creating a taut side. This tension generates a tangential force on the follower pulley, which then applies torque to the driven shaft. Conversely, the other side of the belt, known as the slack side, experiences reduced tension.

Many types of belt drives are used today. The earliest types were flat belts made from leather or fabric. Flat belts operate satisfactorily in low-power applications such as farm equipment, mining, and logging. However, at higher loads and speeds, they tend to slip on the surface of the pulleys and climb out of the pulley.

Advancements in technology have addressed and corrected the limitations of traditional flat belts. Modern innovations have enabled these belts to operate at higher speeds while reducing shaft loads. Contemporary flat belts are designed to be thin, efficient, and effective in minimizing energy loss. They are constructed from advanced materials like extruded polyamide, polyester, or aramid fabric, which significantly boost their durability and performance.

Historically, rope drives made from cotton or hemp were used on pulleys with a V-shaped groove. This design prevented the rope from slipping off the pulley, allowing belt drives to be employed over longer distances. This innovation eventually led to the development of round belts, made from elastomeric materials such as rubber, nylon, or urethane, further enhancing belt drive technology.

The most important improvement in belt materials was the development of long-lasting elastomeric materials, such as natural rubber, synthetic rubber, and various polymers, that gave belts the strength and endurance to withstand the constant stress and torque of the forces produced by a belt drive. V-belts, ribbed belts, multi-groove belts, and timing belts were produced to solve the problems of the previous belt drives.

Belt drives are often preferred over other power transmission systems like gears and chain drives because they offer:

• Ability to absorb power fluctuations, shocks, and overloads: Since belt drives rely on friction to maintain coupling with the driver and follower pulleys, shocks and overloading can be dissipated by allowing the belt and pulley surfaces to slip from one another. This prevents excessively high torques from being transmitted to driven parts, preventing damage to the machine.
• Ability to change speed and torque: Speed and torque can be varied by changing the diameters of the pulleys. Similar to gears and chain drives, belt drives produce mechanical advantage expressed by:
  $$\begin{equation} \ MA = \frac{τ_b}{τ_a} = \frac{r_b}{r_a} = \frac{ω_b}{ω_a} \end{equation}$$

  Where MA is the mechanical advantage, τb and τa are the torques, rb and ra are the radii of the pulleys, and ωa and ωb are the angular speeds. These equations are true in an ideal scenario where there is no power loss between pulleys.

  

  
  
• Low noise and vibration: Aside from timing belts, belt drives have no backlash. The surfaces of the belt and pulleys contact efficiently during operation. Moreover, belts typically have rubber surfaces with high impact resistance. This makes them quieter than gears and chains that operate with metal-to-metal contact.
• Economic Value: Belt drives are the most convenient option for transmitting power over relatively long distances because they are cheaper than gears and chains. When coupling two pulleys separated by long distances, the incremental cost only depends on the cost of the additional length of the belt.
• Compatibility with non-parallel shafts: Belts are flexible, unlike gears and chains; thus, they can be twisted to conform with non-parallel shafts. This eliminates the need for intermediate components, adding cost and complexity to the drive system.
• Can serve non-parallel shafts. Belts are flexible, unlike gears and chains. Thus, they can be twisted to conform with non-parallel shafts. This eliminates the need for intermediate components which adds cost and complexity to the drive system.

  

  
  
• Ability produce an opposite rotation: Belts can be crossed so that rotating the driver causes a reverse rotation to the follower. This simplifies the construction since there is no need for an additional gearing system.
• Unaligned and offset pulleys: Again, owing to the flexibility of belts, pulleys can have a slight axial offset. This is particularly useful for having multiple pulleys fitted side-by-side with different diameters for varying the follower speed.
• Lack of lubrication: Belt drives do not need lubrication to operate, unlike gears and chain drives. This means simpler maintenance and improved cleanliness.

  

  
  

  However, belt drives also come with disadvantages:

• Relatively high power loss: Because of the tendency of the belt to slip, belt drives have lower power transmission efficiency than other mechanical drives. The power lost is turned into heat and noise generated by the friction between the surfaces of the belt and pulley. V-belts solve this problem since they have a higher grip on the pulley.
• Cannot be used for synchronized applications: Because of slippage, they cannot be used in applications where the follower must rotate at a specific angle relative to the rotation of the driver. This problem is solved by toothed or timing belts, which function similarly to chain and sprocket drives.

  

  
  
• Specific operating speed range: The power transmission efficency of belt drives fallas signifiantly at high speeds. This is due to belt whipping, stretching, and increased vibration as the speed increases. Stretching also makes the speed of the belt erratic. At low speeds, slipping can easily occur due to the relatively low tensile force.
• Shorter life span: Belts are constantly being stretched and abraded during operation. Wear and tear is inevitable for belt drives, which comes sooner than that of metal gears and chains. Moreover, since they are made from elastomeric materials, they are easily affected by high temperatures. They are usually the weakest components in a drive system.

  

  
  
• High radial loads on shafts and bearings: Belts need sufficient tension to minimize slippage. This increased tension is transferred to the bearings and shaft, which induce additional loads. Too much tension can shorten the life of bearings. Shafts can also be bent, which can produce high vibrations.

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Chapter 3: How is a V-belt constructed?

A V-belt is constructed from a blend of natural and synthetic rubbers, along with various polymers, all reinforced for added strength. In operation, a V-belt experiences both tensile and compressive stresses. The upper side of the belt endures longitudinal tensile forces, while the lower side is compressed due to its interaction with the pulley grooves and the bending of the belt as it wraps around the pulleys. The belt's surface is made from materials that offer a high friction coefficient and enhanced wear resistance.

V-Belt Fabric Cover

The outer fabric layer of V-belts interacts directly with the sheave surface, designed to endure significant abrasion and resist contaminants. This fabric layer shields the inner elastomer and tension cords from exposure to chemicals, corrosion, and elevated temperatures.

Known as wrapped V-belts, these coverings provide a consistent appearance, texture, and smooth operation. A well-designed cover also reduces operational noise. The abrasion-resistant properties of the V-belt enhance its durability, especially since it typically contacts the sheave at high speeds.

Aside from the obvious benefits of texture and appearance, wrappings or coverings increase friction with the surface of the sheave to prevent slippage. When torque spikes happen, V-belts are forced to make an immediate response. Raw V-belts buckle and break under such conditions, while wrapped or covered V-belts will slip before sending power back to the gearbox or drive as a safety precaution.

V-Belt Tension Cord or Member

Tension cords, embedded within the rubber matrix of a V-belt, serve as the power-transmitting elements. Positioned at the pitch line of the belt's cross-section, they enhance tensile strength. These cords are typically made from materials like polyester, steel, or aramid fibers. In certain V-belt designs, the tension cord is securely bonded to the core using an adhesive rubber compound.

For added strength, tension cords are manufactured from continuous, joint-free materials such as wire, providing critical reinforcement and the tensile strength necessary to endure torque transmission.

V-belts are engineered to maintain high rigidity across their width, necessitating the use of tensile cords that distribute the load evenly. Flexibility in the tension cord is vital for reducing heat and stress during bending. This flexibility is achieved through the parallel alignment of the tension cords.

An adhesion gum holds the tension cord in place, creating a bond with the elastomer core, ensuring that all components work together as a cohesive unit.

V-Belt Elastomer Core

The elastomer core is integral to maintaining the structure of a V-belt, providing its characteristic trapezoidal cross-section. It is composed of various materials known for their shock absorption, high flexural strength, and thermal stability. Commonly used elastomers include neoprene, EPDM, and polyurethane.

In certain V-belt configurations, the elastomer core is split into two distinct sections, with the tension cord situated between a top rubber cushion and a bottom compression rubber layer. These sections are crafted from different rubber compounds to withstand the specific stresses they encounter.

Wrapped and Raw Edge V-Belts

V-belts are structurally divided into two main categories: wrapped belts and raw edge belts. Wrapped V-belts are traditional V-belts, fully encased in a fabric cover. These belts offer enhanced protection against external factors and operate more quietly. However, they have a lower coefficient of friction, which can result in some power loss. Wrapped V-belts are suitable for applications that may involve slight slippage without causing belt damage.

On the other hand, raw edge V-belts feature exposed flanks without fabric covers, allowing the elastomer core to directly contact the pulley surface. This exposure increases the belt's coefficient of friction, providing a better grip. The elastomer core in raw edge V-belts is more wear-resistant than in their wrapped counterparts. Raw edge V-belts can be further classified into three distinct types:

• Raw Edge Plain (REP): With raw edge plain, the top surface is covered with one or more layers of a fabric cover with covering at the bottom side present or not, depending on the design.

  

  
  
• Raw Edge Laminated (REL): Raw edge laminated types of v-belts are similar to REP but have additional layers of laminate fabric at the elastomer core. The addition of the laminated fabrics helps reduce noise.

  

  
  
• Raw Edge Cogged (REC): Raw edge cogged, also known as raw edge notched V-belts, have cogs or notches at the bottom side of the belt. Cogs improve the flexibility of the belt, allowing use for pulleys with small diameters. The increased surface area at the bottom creates better heat dissipation, making them suitable for high-temperature applications.

  

  
  

Chapter 4: What is the terminology used for V-belt geometry?

The typical cross-section of a V-belt is trapezoidal, featuring parallel top and bottom edges. The specific dimensions of this trapezoid help determine the type of V-belt and are crucial for ensuring compatibility with the correct pulley.

In addition to the trapezoidal shape, V-belts are characterized by other geometric factors such as the pitch line location and the internal and external lengths. Knowing these dimensions is essential for choosing the right V-belt to match the application requirements accurately.

• Top Width: This is the larger side of the trapezium, parallel with the shorter side.
• Pitch Line or Pitch Zone: When bent, an unloaded v-belt experiences both tensile and compressive stresses. The outer side is subjected to tension, while the inner side is under compression. The line where the stress is zero is known as the pitch line or pitch zone.
• Top to Pitch: This is the length between the top side and the pitch zone.
• Pitch Width: This is the width of the trapezium measured at the pitch.
• Height: This is the distance between the top and bottom sides of the trapezium.
• Relative Height: Relative height is a non-dimensional characteristic that is defined as the ratio of the height to pitch width.
• Outside Length: This is the circumference of the belt measured along the top side.
• Inside Length: This is the length of the belt measured along the bottom side.
• Pitch Length: This is the length of the belt along the pitch line.
• Included Angle: This is the angle made by the flanks when extended. The included angle of most v-belt sections is 40°.

Chapter 5: What are the types of V-belts?

V-belts come in various types, and this section will categorize them based on the dimensions of their cross-sectional shape. The most prevalent cross-sections include standard, wedge, narrow, fractional horsepower, banded, cogged, and double. These dimensions are standardized by organizations such as ISO, BS, and DIN.

• Standard V-Belt: The standard v-belt, also known as classical or conventional v-belt, is the earliest forms of V-belt and is widely used in power transmission. Standard v-belts have various dimensions designated as Y, Z, A, B, C, D, and E. When using DIN standards, their designation is denoted by numbers equal to the belt‘s top width in millimeters. All sizes have an included angle of 40° and a top width to height ratio of 1.6:1. The table below summarizes these designations.

  

  
  

  Designation BS, ISO, IS, JIS	Designation DIN	Top Width in mm	Height in mm	Recommended Minimum Pulley Pitch Diameter in mm
  	5	5	3	20
  Y	6	6	4	28
  	8	8	5	40
  Z	10	10	6	50
  A	13	13	8	13
  B	17	17	11	125
  	20	20	12.5	160
  C	22	22	14	200
  	25	25	16	250
  D	32	32	19	355
  E		38	23	500
  	40	40	24	500

• Wedge V-Belt: Wedge belts are a primarily used for high power transmission with reduced space requirements. They can operate at 1.5 to 2 times the load of classical v-belts with the same top width. Because of the higher power rating, fewer wedge belts are needed to transmit the load. Like classical v-belts, the included angle of wedge belts is also 40°, but they have a different top width to height ratio of 1.2:1. They have better cord construction and placement, providing the highest strength while in motion. Wedge belts are designated as SPZ, SPA, SPB, and SPC.

  Designation	Top Width in mm	Height in mm	Recommended Minimum Pulley Pitch Diameter in mm
  SPZ	10	8	63
  SPA	13	10	90
  SPB	17	14	140
  SPC	22	18	224

• Narrow V-Belt: Narrow belts are similar to wedge belts. They are also used for transmitting larger loads in a smaller form. The designations used for narrow belts are 3V, 5V, and 8V. The numbers denote the top width of the belt multiplied in terms of 1/8 of an inch. Like other belt sections, its included angle is also 40°. Narrow belt sections are standardized and mostly used in the North American region. They partially conform to the profile of a wedge belt. Section 3V corresponds to SPZ and 5V to SPB. 3V and 5V belts can be used for SPZ and SPB pulleys, respectively. However, using SPZ and SPB pulleys on American standard pulleys is not recommended.

  

  
  

  Designation	Top Width in mm	Height in mm	Recommended Minimum Pulley Pitch Diameter in mm
  3V	9.7 (3/8")	8	63
  5V	15.8 (5/8")	14	140
  8V	25.4 (1")	23	335

• Double or Hexagonal V-Belt: These are similar to two mirrored v-belts with their top sides as the adjoining side. The tension cord is placed between the two V-shaped sections. Double v-belts are used for drives with one or more reverse bends since the two compression cores allow the belt to be bent from either side. This property makes double v-belts suitable for drives with multiple pulleys that must be driven either clockwise or anti-clockwise. Double v-belt sections are designated as AA, BB, and CC.

  

  
  

  Designation	Top Width in mm	Height in mm	Recommended Minimum Pulley Pitch Diameter in mm
  AA	13	10	80
  BB	17	14	125
  CC	22	17	224

• Banded V-Belt: A banded belt is several v-belts joined together in parallel by a fabric cover or band at the top side. Each V-section can have the dimensions of classical, wedge, or narrow belts. Banded belts are mostly used in high-power applications. They are designated by an H followed by the v-belt section number. Examples of banded v-belts are HA, HB, HSPA, HSPB, H3V, and H5V.
• Fractional Horsepower V-Belt: These types of v-belts are used for light-duty applications. Examples of such applications are household appliances and machine shop equipment where the power requirement is about 1 horsepower or less. Common fractional horsepower belt sections are 2L, 3L, 4L, and 5L. The number before the L denotes the top width of the belt multiplied in terms of 1/8 of an inch.

  Designation	Top Width in mm	Height in mm
  2L	1/4	1/8
  3L	3/8	7/32
  4L	1/2	5/16
  5L	21/32	3/8

• Cogged V-Belt: As discussed earlier, these belts have cogs or notches at the bottom side, which allows them to be bent at a smaller radius. They are not fully wrapped with fiber cover, unlike the previous types. Cogged belts can take the cross-section dimension of classical, wedge, narrow, banded, and fractional horsepower v-belts. Cogged belts are designated with an X after the v-belt section number, except for wedge belts. Example designations are ZX, AX, 3VX, 5VX, HAX, H3VX, etc. Cogged wedge belts are designated as XPA, XPB, and so on.

  

  
  
• Double Cogged V-Belt: This design has the combinations of principles behind a double v-belt and a cogged v-belt. They are used in applications that require high belt flexibility for a small pulley radius. The cogged construction at the top side of the belt allows it to be bent in a serpentine-like path. This is used for driving multiple pulleys. Double cogged v-belts dimensions depend on manufacturer standards.

  

  
  
• Agricultural V-Belt: These are wrapped belts designed for more extreme abrasion from dust, sand, grains, and others. Also, they are exposed to rain and sunlight, which can easily degrade ordinary rubber compounds. Because of these, agricultural v-belts are made of more durable polyurethane blends for the elastomer core and Kevlar fibers for the tensile cords. Some manufacturers mix their specifications with classical, narrow, double, and banded section v-belts. When referring to ISO standards, agricultural v-belts are designated as HI, HJ, HK, HL, and HM.
• Poly-V Belt: Poly V is the common market term for V-ribbed, multi-groove, or poly-groove belts. Unlike banded v-belts, they do not have the standard section dimensions of classical, wedge, and narrow v-belts. They have a more compact construction than banded v-belts. They have improved flexibility because of their reduced thickness, making them suitable for driving multiple pulleys. Poly V-belts can take a serpentine path with the help of idlers. Poly V-belts are designated as PH, PJ, PK, PL, and PM.

  

  
  
• Variable Speed V-Belt: This is a raw edge cogged v-belt with a wider cross-section than classical belts. They are designed to be used with variable speed pulleys. Their section can be made into standard or non-standard sizes. Designations for variable speed belts vary from each manufacturer. They are usually made from chloroprene rubber (Neoprene) or EPDM.

Conclusion

• A v-belt is a flexible machine element used that transmits power between a set of grooved pulleys or sheaves. They are characterized by their trapezium cross-section.
• V-belts are used because of their ability to wedge tightly into the grooves of the pulley. This breaks higher surface friction, reducing slip and power loss.
• V-belts can be classified as wrapped or raw edge belts. Wrapped v-belts are fully covered with a fiber cover, while raw edge belts have bare flanks.
• V-belts can also be categorized according to the cross-section. The most common cross-sections are standard, wedge, narrow, fractional horsepower, banded, cogged, and double.

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