V-Belts and Flats Belts: Learn Their Types, Advantages & Differences

Types of Flat Belt Drives

Now, let us learn how the Flat belt drives are arranged for different conditions. These drives can be arranged in two ways:

(i) Open Belt Drives: When the direction of rotation of both the pulleys is required in the same direction, then we can use an Open Belt Drive. Here, the tension in the lower side belt will be more than that in the upper side belt. 

The lower side belt (because of more tension) is known as the tight side whereas the upper side belt (because of less tension) is known as the slack side. Consider the image shown below, fig 3A and fig 3B.

Fig 3A: Diagrammatic representation of Open Belt Drive

Fig 3B: Real-life example of Open Belt Drive

(ii) Crossed Belt Drives: When the direction of rotation of pulleys is required in opposite direction, then we can use a Cross Belt Drive. 

In this case, the driver pulls the belt from the side RQ and delivers it to the other side LM. Thus, the tension in the belt RQ will be more than that in the belt LM. Refer to the diagrammatic representation below.

Fig 4: Diagrammatic Representation of Cross Belt Drive

(iii) Quarter turn belt drive: The quarter turn belt drive (also known as right angle belt drive) as shown in Fig 5, is used with shafts arranged at right angles and rotating in one definite direction. 

To prevent the belt from leaving the pulley, the width of the face of the pulley should be greater or equal to 1.4b, where b is the width of the belt.

Fig 5: Quarter-turn belt drive

In case the reversible motion is desired then a quarter-turn belt drive with a guide pulley, as shown in Fig 5 (b) can be employed.

(iv) Belt drive with idler pulleys: A belt drive with an idler pulley (also known as jockey pulley

drive) as shown in Fig 6A, is used when an open belt drive can not be used due to a small angle of contact on the smaller pulley. 

Fig 6A: Belt drive with single idler pulley

To transmit motion from one shaft to several shafts arranged parallelly, a belt drive with many idler pulleys, as shown in Fig 6B, may be employed. 

Fig 6B: Belt drive with multiple idler pulley

(v) Compound belt drive: A compound belt drive as shown in Fig 7, is used when power is

transmitted from one shaft to another through multiple numbers of pulleys.

Fig 7: Compound belt drive

We now have an idea of Flat Belts and Flat belt drives. We just discussed how Flat belt drives can be arranged. What may be the types of pulleys used for Flat belts if there are specific pulleys for V-Belts?

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Types of Pulleys for Flat Belts

A commonly known fact is that pulleys are used to transmit power from one shaft to another employing flat belt, V-belts or ropes. The pulleys must be in perfect alignment to allow the belt to travel in a line normal to the pulley faces. 

We shall now discuss the most commonly used pulleys.

1. Cast Iron Pulleys: The cast materials should have good friction and wear characteristics. The cast iron pulleys are generally made with rounded rims. This slight convexity is known as crowning. (Refer to Fig 8A)

The crowning tends to keep the belt in the centre on a pulley rim while in motion.

2. Steel Pulleys: The pulleys made of pressed steel are lighter than cast pulleys and have great strength and durability. They are made to run at high speeds. But in many cases, they have lower friction and may produce excessive wear. 

Consider this example of an Aircraft Engine pulley in Fig 8B.

Advantages and Disadvantages of Flat Belt Drive

Some main advantages and disadvantages of flat belt drive are discussed below. 

Advantages:

• Simplicity, low cost, smoothness of operation, ability to absorb shocks, flexibility and efficiency at high speeds.
• Protects the driven mechanism against breakage in case of sudden overloads owing to belt slipping.
• Low maintenance and easy service.
• Possibility to transmit power over a moderately long distance.

Disadvantages

• Flat belt drive is not positive as slip and creep occur frequently. 
• Comparatively large size.
• Stretching of belt calling for resewing when the centre distance is constant.
• Belt joints reduce the life of the belt. 
• High bearing loads, high belt stresses, and less efficiency.

Introduction to V-Belts

V-belts or V-belt drives is mostly used in factories and workshops where a great amount of power is to be transmitted from one pulley to another with a V-groove. 

Whenever the centre distance between the pulleys is small, Flat Belt Drives fail to be the optimal solution. In such areas, V-Belt Drives have taken complete control. 

As the V-Belts have a trapezoidal cross-section that comes in contact with the sides of the pulley grooves and not at the base, they transmit greater power in the same directions. Refer to Fig. 1 to have a look at how V-Belts look in real life.

The V-belts are made of fabric and chords moulded in rubber and covered with fabric in the form of a trapezium as shown in Fig 9. 

The chords are located near the centre of gravity of the belt and transmit the force from the driving to the driven pulley. Since the chords are nearer to the neutral axis of the belt, the bending stress of the belt is almost negligible. 

The V-belts are particularly suitable for short drives. The included angle for the V-belt is usually from 30° to 40°. The exact value of this angle depends on the belt section, the sheave diameter, and the angle of contact. 

Types of V-Belts

According to the Indian Standards (IS: 2494 – 1974), the V-belts are made in five types i.e., A, B, C, D and E. The dimensions for standard V-belts are shown in the table below:

Table 1: Dimensions of standard V-Belts according to IS: 2494-1974

Let us have a sneak-peak at the V-Grooved Pulleys primarily used with V-belts to make them V-belt drives.

V-Grooved Pulley

V-belt pulleys are devices which transmit power between two parallel axles by the use of a V-belt. The most notable difference between a V-belt pulley and other types of pulleys (round, flat, etc.) would be the geometry of the groove.

Consider the images below. Fig 10A represents the sectional diagram of the V-grooved pulley and Fig 10B is a simple part diagram to help you better understand the pulley structure.

These grooves guide and gain traction on a V-belt. A clearance must be provided at the bottom of the groove as shown in Fig 10A, to prevent touching the bottom as it becomes narrower from wear.

Advantages and Disadvantages of V-Belt Drives

Following are some advantages and disadvantages of V-belt drives. 

Advantages

• The V-belt drive gives compactness due to the small distance between the centres of pulleys.
• The drive is positive because the slip between the belt and the pulley groove is negligible.
• Since the V-belts are made endlessly and there is no joint trouble, therefore the drive is smooth.
• It provides a longer life of up to 3-5 years.
• It can be easily installed and removed.
• The operation of the belt and pulley is quiet.
• The belts can cushion the shock when machines are started.
• A higher velocity ratio (maximum 10) can be obtained.
• The V-belt may be operated in either direction, with the tight side of the belt at the top or bottom. 

Disadvantages

• The V-belt drive cannot be used with large centre distances, because of the larger weight per unit length. 
• The V-belts are not as durable as flat belts. 
• The construction of pulleys for V-belts is more complicated than pulleys for flat belts.
• Since the V-belts are subjected to a certain amount of creep, these are not suitable for constant-speed applications such as synchronous machines and timing devices. 
• The belt life is greatly influenced by temperature changes, improper belt tension and mismatching of belt lengths. 
• The centrifugal tension prevents the use of V-belts at speeds below 5 m/s and above 50 m/s.

Now, we have a clear understanding of the V-belt drives. Let us understand the difference between V-belts and Flat belts.

Difference between V-Belt and Flat Belt Drives

Let us compare between V-Belt and Flat Belt drives and understand the differences between them. The following table gives the basic differences between V-belt and Flat belt drives