Introduction

This article contains all the information you need to know about V-Belts.

Read further to learn more about topics such as:

• What is a V-Belt?
• Overview of Belt Drives
• V-Belt Construction
• V-belt Geometry Terminologies
• Types of V-Belts
• And much more…

Chapter 1: What is a V-Belt?

A v-belt is a flexible and efficient power transmission device capable of transferring power from one shaft to another. It is known for its trapezoidal shape that wedges securely into the sheaves of a shaft. The unique shape of V-belts helps them fit tightly and snugly into the grooves of a sheave, giving them additional surface contact and increased stability.

When there is belt tension, vertical forces perpendicular to the top of a V-belt push its walls against the grooves of the sheave. As the forces increase, the belt wedges tightly into the sheave grooves, which increases the friction between the surfaces of the belt and the sheave walls. The ever-growing connection allows for a higher torque to be transmitted, while the increased friction minimizes the loss of power through slippage.

The multiple frictional forces allow a drive to transmit higher loads. The performance of a V-belt is determined by how tightly it fits into the groove of the sheave when placed under higher tension.

V-belts are made from synthetic and natural rubber, which gives them the flexibility and elasticity to bend into the sheaves. The various fibrous tensile chords are compressed to the shape and form of a V-belt, a process that gives V-belts their exceptional strength and durability. Certain designs of V-belts increase bending resistance and lower their operating temperature by adding cogs.

Chapter 2: Overview of Belt Drives

Belt drives transmit power between two or more rotating shafts, usually with parallel axes of rotation. The belts are looped over pulleys attached to the driver and follower shafts. The pulleys are placed at a measured distance to create tension on the belt. The friction causes the belt to grip the pulley when in operation.

The rotation of the driver pulley increases the tension on one side of the belt, creating a tight side. This tight side applies a tangential force to the follower pulley. Torque is then applied to the driven shaft. Opposite the tight side is the slack side, where the belt experiences less 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.

The flaws and drawbacks of the original flat belts have been mitigated and resolved by modern technology. The many developments and improvements to their design have enabled them to perform at higher speeds, generating fewer shaft loads. Modern flat belts are thin, efficient, and able to prevent energy loss. They are made of extruded polyamide, polyester, or aramid fabric, materials that significantly enhance their longevity and performance.

Another early type of belt drive was a rope drive made from cotton or hemp rope, which was used on two pulleys with a V-shaped groove. The use of ropes solved the problem of climbing out of the pulley, enabling belt drives to be used over large distances and leading to the development of round belts made from elastomeric materials such as rubber, nylon, or urethane.

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.

Generally, belt drives are desired over other power transmission mechanisms such as gears and chain drives due to their:

• 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:
  
  $$MA=\frac{{\tau }_b}{{\tau }_a}=\frac{r_b}{r_a}=\frac{{\omega }_b}{{\omega }_a}$$

  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.

Chapter 3: V-Belt Construction

A V-belt is a composite of different types of rubber, synthetic rubber, and polymers that are combined with reinforcements. In its usual application, a V-belt is subjected to combined tensile and compressive stresses. The top side of a V-belt is subjected to a tensile force directed longitudinally, while the bottom side is compressed due to the compression against the grooves and bending as a belt segment passes the pulley. The surface of the belt needs different types of materials with a high coefficient of friction and increased wear resistance.

V-Belt Fabric Cover

The fabric cover of V-belts makes contact with the surface of the sheave. It is made of a material capable of withstanding high abrasion and is resistant to contaminants. It protects the elastomer and tension cord from the harmful effects of chemicals, corrosion, and high temperatures.

Often referred to as wrapped V-belts, coverings give V-belts a uniform look, feel, and smoothness. The proper covering suppresses noise from the belt when it is in operation. The abrasion resistance of V-belts increases their durability since contact with the sheave normally occurs at exceptionally 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 are embedded into the rubber compound of a V-belt, creating a composite structure, and are power-transmitting components. They are positioned at the pitch diameter of the belt cross-section to increase tensile strength. Tension cords are made of polyester, steel, or aramid fibers. In some V-belt constructions, the tension cord is bonded into the core by an adhesion rubber.

To increase the strength of tension cords, they are made of continuous material, like wire, without joints. The design provides essential reinforcement, tensile strength, and durability to withstand the transmission of torque.

V-belts are designed to transfer rigidity, which is at high levels across the width of a V-belt and requires tensile cords to transfer the load equally. The flexibility of the tension cord is necessary to reduce heat and stress from bending. All of these factors are accomplished by the parallel arrangement of the tension cord.

The tensile cord is held in place by adhesion gum that forms a bond between it and the rubber stock elastomer core. The bonding of the two elements forces the different components to perform as a single unit.

V-Belt Elastomer Core

The elastomer core holds the components together and gives a V-belt its trapezium cross-section. It is made from a variety of materials with shock resistance, high flexural strength, and temperature stability. Common elastomers used are neoprene, EPDM, and polyurethane.

In some designs, the elastomer core is divided into two sections separated by the tension cord placed between a top cushion of rubber above and compression rubber below. The two sections are made from different types of rubber because of the stresses they experience.

Wrapped and Raw Edge V-Belts

In terms of structure, v-belts can be categorized as wrapped belts and raw edge belts. Wrapped v-belts are considered standard v-belts with all sides wrapped in a fabric cover. Wrapped v-belts have a higher resistance against external elements and quieter operation. However, the downside is a lower coefficient of friction resulting in power loss. Wrapped v-belts are used on applications that require some amount of slippage without damaging the belt.

In contrast with wrapped v-belts, raw edge v-belts do not have covers at their flanks. This means the elastomer core is exposed and in contact with the surface of the pulley. The elastomer core has a higher coefficient of friction than the fabric-covered ones, allowing for better grip. The elastomer core of raw edge v-belts has higher wear resistance than the core found on wrapped v-belts. Raw edge v-belts are further divided into three 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: V-Belt Geometry Terminology

The cross-section of a V-belt is generally defined as a trapezium with parallel top and bottom sides. The dimensions of this trapezium can define the type of V-belt. These dimensions are necessary for matching the belt with the appropriate pulley.

V-belts are also defined by other geometries, such as the location of the pitch line and the inside and outside lengths. Understanding these dimensions is necessary when selecting a V-belt to ensure the right size and dimensions are used to fit an application.

• 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: Types of V-Belts

V-belts are available in different types. This chapter focuses on the classification of v-belts according to the dimensions of their cross-section. The most common cross-sections are standard, wedge, narrow, fractional horsepower, banded, cogged, and double. These cross-sections are standardized by different 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.