You probably know what the source of power is in, say, a lawn mower. It is the engine. Without that, you may as well be pushing one of those hand-powered lawn mowers.

The engine is what creates the power that spins the blades fast enough to cut your grass. But do you know what transmits that power from the engine to the blades?

A v-belt is what transmits energy from a source (the engine) to an application (lawn mower blades). V-belts are so-called because they are shaped like a trapezoid. They most commonly come in three different categories:

• Fractional horsepower v-belts
• Classic v-belts
• Narrow v-belts

There are many other styles, but these are the three main ones that you will commonly find in America. In essence, v-belts are responsible for putting the power of any given engine or motor to good use. Without the v-belt, a motor will just run and run and will not accomplish anything or get anywhere.

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Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing, or operating.Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Different Kinds of V-Belts

All v-belts are made with a combination of synthetic tensile chords that are twisted together from strong fibrous materials and encased in rubber. This structure is enclosed, meaning the ends are brought together to form a circle.

If you are going to understand v-belts, you first need to know the different kinds of belts and what they are typically used for. All v-belts are so-called because they are shaped like a “v” or, more accurately, a trapezoid (it is easier to say v-belt as opposed to trapezoid-belt).

V-belts can have different dimensions within that shape, and those dimensions allow the different belts to be used in different kinds of engines. What determines which kind of belt gets used in which kind of engine is the amount of horsepower of the engine. It breaks down like this:

• Fractional horsepower v-belts are used for motors of less than a horsepower
• Classic v-belts are used for engines that range from a fraction of a horsepower to 500 horsepower
• Narrow v-belts are used for engines of 1 horsepower to 1,000 horsepower

While all v-belts perform the same duty, different types of v-belts work for different kinds of engines. 

Fractional Horsepower V-Belts

Motors that generate less than a horsepower of energy are small motors and often electric motors, the kind that power:

• Household fans
• Washing machines
• Refrigerators

The most important thing to know about fractional horsepower v-belts is that you should never use one to replace a classic or narrow v-belt, even if you think you can get away with it. Fractional v-belts can only handle a fraction of one horsepower and will break when forced to bear heavier loads.

The reason is that the dimensions of the cross-section of these belts are quite small. Below you will find an example of fractional v-belts from as small as they get to as large as they get:

• .25 inches top width by .16 inches high
• 3.75 inches top width by .22 inches high
• .5 inches top width by .31 inches high
• .66 inches top width by .375 inches high

The four sizes of fractional horsepower v-belts are designated by a number that indicates the eighth inch to which the top width corresponds, followed by the letter L. Such as 2L, 3L, 4L, and 5L.

Classic V-Belts

Since classic v-belts range from a fraction of one horsepower all the way up to 500 horsepower, they have a variety of applications, from lawn mowers to classic cars. They are the oldest style of v-belts, dating back to when they were made of leather, and are also used in industrial and agricultural machines.

They cannot handle the heavy loads that narrow v-belts can handle, so, even in the variety listed above, they are limited in their application.

Nevertheless, they are sturdy and can take a lot of abuse, being able to operate in dirty and less-than-ideal environments, hence the preference for their use in industrial and agricultural equipment.

Classic v-belts are designated by the letters A through E together with the inside length of the belt. The standard dimensions of the classic v-belt cross-sections are:

• A: .5 inches top width by .31 inches high
• B: .66 inches top width by .41 inches high
• C: .88 inches top width by .53 inches high
• D: 1.25 inches top width by .75 inches high
• E: 1.5 inches top width by .91 inches high

This style of belt also carries designations of W, Y, and Z, but those sizes are not commonly found in America.

Narrow V-Belts

Unlike classic v-belts, which have a fairly wide top and bottom width, narrow v-belts have—you guessed it—a comparatively narrow top and bottom width.

This gives them the strength to handle the higher horsepower applications for which they can be used. 1,000 horsepower, for example, is the top limit of high-end sports cars.

Narrow v-belts are designated with a number that corresponds to the belt’s top width, the letter “v,” and a number that corresponds to the belt’s outside length, such as 5V500.

Three designations are typically found in America, and their cross-sectional dimensions are:

• 3V: .38 inches top width by .32 inches high
• 5V: .62 inches top width by .54 inches high
• 8V: 1 inch top width by .88 inches high

Narrow v-belts have a range from 1 horsepower to 1,000 horsepower, which means they can handle three times as much power as the classic v-belts. 

What is the Difference Between a Mower Deck Belt and a Drive Belt?

With all this talk of belts and lawn mowers, you may be wondering, what is the difference between the deck and drive belts? Some explanation is required for this, and it involves understanding a little about your lawn mower.

Lawn mowers that are riding mowers will typically have two belts that transmit the power of the engine to their necessary functions. 

• The drive belt
• The deck belt

Because they do two different functions, they are best thought of (and bought) as two different belts, even though they both might be classic v-belts or narrow v-belts. 

The Lawn Mower Drive Belt

Just like it sounds, the drive belt is responsible for transmitting the power of the motor to the wheels of the tractor, thus making it drive forward or backward. If you were to separate the motor from your tractor, you will find the drive belt still attached to the tractor.

Drive belts are typically constructed using polyester material to construct the tensile chords that are wound together and lie within the rubber construction of the belt.

The Lawn Mower Deck Belt

In one of those mysteries of terminology, the deck belt would still be attached to the motor if you separated the two, but its function would be on the “deck” of the mower. The deck belt is responsible for turning the mower blades at speeds high enough to cut the grass.

As odd as it may sound, deck belts should be of stouter construction than drive belts. The reason is that mower blades often come into contact with thick or high grass. When the blades hit that grass, they slow down, but the belt will not want to slow down.

As a result, the belt receives a high degree of friction because of the tension between the two forces. This is why it is a good idea to have a deck belt constructed with chords made from kevlar rather than polyester. They can take more of a beating.

If you’re having problems with your lawn mower belt staying on, read this article about looking for problems on your lawn mower deck.

Conclusion

Powerful engines are great and fun to listen to, but if you want them to do anything, you need a v-belt. V-belts transmit the power from the engine to the thing that the engine is running. In the case of riding lawn mowers, the engine is running the wheels and the mower blades. 

Knowing how v-belts work helps you appreciate everything from a household fan to a roaring classic car.