Power Transmission by Belt: Part II

Editor's note: This is the second of a two-part series on belt power transmission.


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Continuing our discussion of belt power transmission, we now apply the 18 considerations for a successful flat belt drive that were developed in “Power Transmission by Belt: Part I,” Gas Engine Magazine, April 2005.

Lets assume we want to run a 10 kilowatt, 1,800 RPM DC generator with our antique 30 HP tractor. We are told we'll need about 17 HP to power the generator at the full 10-kilowatt output. The tractor's belt pulley capability will undoubtedly be sufficient since this rating is always less than the engine's rating by just a few horsepower. This is because of the slight amount of loss attributable to gears and bearings. The rated speed of the belt pulley is 1,000 RPM. The pulley is 10 inches in diameter and 7 inches wide.

Some time ago, we mounted a generator in a shed with a hinged door on the side for a belt to enter. We placed the generator in such a position that the following criteria were met: 1) the pulley-end of the generator will be in a people-safe area; 2) the tension side of the belt will be on the bottom when the generator is being turned in the proper direction; 3) the tractor can easily be maneuvered and placed so that the belt length will fall into an acceptable range. Ten feet is about all the pulley-to-pulley separation we can get in this case. Certainly not very far, but it will be okay.

First we'll check belt speed. The tractor pulley may have to be changed if the belt speed is not in our desired range at the rated speed of 1,000 RPM. Factory installed pulleys are normally sized to give belt speeds in the correct range. Belt speed is calculated by multiplying pi (3.14) by the diameter of the pulley by the RPM. In our case the belt speed is 31,400 IPM or about 2,600 FPM. This is good. Since the RPM of the generator shaft must be 1.8 times the tractor shaft, the load pulley must be smaller than the PTO pulley by a factor of 1/1.8.

Cross-multiplying and dividing both sides by 1,800 gives approximately 5-1/2 inches for the generator pulley diameter.

A 5-1/2-inch pulley would be nice, but most likely a 5- or 6-inch pulley will be located. Either one could be used, but the following concerns must be evaluated: The larger pulley is better as far as power transmission is concerned, but the tractor pulley will now need to run at 1,080 RPM. If 5 inches is used the tractor pulley can run at 900 RPM. The question in this latter case is – can the tractor supply 17 HP at this reduced speed?

Two minor correction factors must be figured in at this point. Recall that we will have about 1 percent creep. And the overall efficiency of the belt drive will be around 95 percent. Taking these into account, we must increase tractor speed by about 10 RPM to account for creep and we must add about 5 percent to the power that the tractor must supply to account for inefficiency, thus about 17-3/4 HP total.

So, if we use a 5-inch pulley the tractor must run at 909 RPM and with a 6-inch pulley tractor RPM must be about 1,090. It must provide 17-3/4 HP in either case.

Now, what belt width do we need? If we use the 3 HP per-inch figure, we need a 6-inch belt. The tractor's 7-inch wide pulley is fine and we need to be sure whichever generator pulley we decide to use is about 7 inches wide.

If we don't find an pre-made belt, we cut a piece and properly square the ends and join them either by gluing or by using the wire-type lacing hooks and rawhide pin, as described in Part 1.  

Take time to properly align the pulleys. A 3- or 4-foot straight stick placed against the side of one pulley will be a big help in aiming the side of one pulley to the corresponding side of the opposing pulley. If this is done on both pulleys, you can be sure the pulleys are not only lined up side-to-side but the shafts are parallel. Put the belt on, smooth-side down, and with the feather edge, if glued, facing away from the direction of travel. Initial tension, per side, should be around 40 pounds per inch of width. The tractor is aligned and chocked so that it exerts about 480 pounds pull on the generator

After the belt is properly adjusted and checked, pull the belt by hand to be sure that it is tracking properly. The tractor is started and the generator is slowly starting to rotate. The tractor's throttle is adjusted so that the generator runs at 1,800 RPM, with the expected electrical load applied. Once this is set, the tractor's governor should hold the generator speed fairly constant even with somewhat varying load. Electrical/electronic regulators are available to maintain very close control of output voltage. Hand-held tachometers are widely available and should be used as often as required to check speeds.

If an AC generator is being considered, a frequency meter connected to the electrical output will serve as a tachometer. You will find that frequency control is difficult and the frequency can be expected to wander by a few hertz from time-to-time. If you plan to use an AC generator be sure to check on the RPM requirement! Here again, regulators are available to control the voltage to a very close tolerance.


Unlike flat belts, these belts make use of their angled sides to wedge into the groove in the pulley or sheave (pronounced shiv). Rumor has it the word “sheave” does not have an equivalent, in other major languages, and translation difficulties may force the word “pulley” into favor.

The wedging action can be visualized best by flexing an actual belt and observing that as the belt bends, as if going into a sheave, the angle changes as the top is stretched and the bottom compressed. This makes the belt sides steeper, i.e., less angle. As the belt enters the sheave groove the belt is immediately restrained and the angle cannot change, but the effect is to grip the sides of the belt very tightly. V-belt drives can operate with tight-side to slack-side ratios around 5-to-1, i.e., T2 = 1/5(T1). They can handle high horsepower for their size.

Main Types of V-belts:

1. Light Duty (sometimes called fractional horsepower): The nominal angle of the groove in the sheave is the same for the whole series (38 degrees), but must be decreased as the diameter decreases because of the significant change in angle as the belt is bent around the small diameter sheaves. The smallest sheaves allowable require angles of about 30 degrees. These sizes and angles are published in the literature, e.g., Machinery's Handbook. The horsepower ratings of the 4L and 5L series is about 2 HP with sheaves not smaller than about 4 inches and belt speeds around 4,000 FPM.

2. Multiple or Industrial V-belts: The name is misleading. They can be used singly but they are made to closer tolerances so that they can be used in sets if desired. For critical applications they should be ordered in matched sets. This type is measured in pitch length, not outside length like the light-duty type.

The nominal sheave angle is 38 degrees, decreasing to about 34 degrees for the minimum pitch diameters shown. Horsepower ratings of this type reach into the hundreds for the larger sizes. To avoid confusion, I like to refer to these as the “letter type.”

3. SAE Standard V-belts: These are used in automotive applications and are covered in the literature. Widths are 3/8-inch, 1/2-inch, 11/16-inch, 3/4-inch, 7/8-inch and 1-inch. The nominal sheave angle is also 38 degrees, decreasing with small sheaves.

4. High Profile Wedge V-belts: These belts have a taller cross-section than all other belt types and have somewhat higher horsepower ratings because of the greater side contact area. They go by the designations 3V, 5V and 8V, and are 3/8-inch, 5/8-inch and 1-inch wide respectively. They also have a nominal groove angle of 38 degrees for sheaves in the range of about 6-12 inches. This is reduced for smaller diameters as with the light duty and “letter type” belts, but must be increased to as much as 42 degrees for the larger diameters. In addition, the groove depth may need to be increased in a given sheave to use these belts. All these dimensions should be studied in the manufacturer's literature before any usage.

It is my recommendation that unless a need exists for the highest possible horsepower capacity or for replacement, that light duty or “letter type” (multiple) belts be used, depending on expected load.

5. Double-sided V-belts: These belts are for applications where the belt must drive on the top as well as the bottom. It contacts some pulleys with one side and others with the other side. The net effect is reversal of direction of some pulleys. The belt resembles two regular v-belts glued back-to-back. These are often used on farm equipment and serve well, but they can't be used on small pulleys or where reverse rotation is required on shafts that are close together. The height of these belts makes them susceptible to damage from excess flexing. One popular type is designated AA, BB, CC, etc.

6. Detachable Link V-belts: This is another type of belt that is kind of a do-it-yourself belt, where you assemble individual links to form a belt of any length. These belts perform about as well as the regular v-belts and have the major advantage of being able to be installed in cases where endless belts can't be used. They are available in 3/8- to 7/8-inch widths.

7. Modern Thin Flat Belts: In the 1950s, a new variety of very thin, toothed flat belt commonly known as Gilmer belts appeared. They could handle high tensile loads at high speeds and around small pulleys. Over the years these have been improved and 100 HP per inch is common. Speeds range up to 20,000 FPS. These belts are used as final drive belts on high-powered motorcycles and other heavy-use applications. These belts have teeth that engage grooves in the pulley, which means there is zero creep and the relative position of the pulleys does not change. These belts typically have fiberglass, Kevlar or steel strength members and stretch is virtually eliminated. This makes alignment somewhat critical. They are about 99 percent efficient because of the small amount of bending and are available as thin as 1/32-inch. They are extensively used in office equipment and for automobile timing belts.

Ribbed belts are another variety of thin flat belts and are used commonly for vehicle fan belts. They are often used in a serpentine configuration. They have a series of small v-belt-like projections that run in corresponding grooves in the pulleys. In automotive applications these belts handle up to about 10 HP at speeds well above 10,000 FPM.  

General V-belt Considerations

All v-belt series are available in the notched design. These belts have the same horsepower ratings as the standard belts and are particularly useful with small sheaves.

The force required to deflect the belt 1/64-inch per inch of span usually judges v-belt tension. This is measured at the center of the span. These forces are in the neighborhood of 4 pounds for size A, 5 for B, 12 for C and 25 for D. For more precise tension data consult the manufacturer's specifications. New belts are set up a bit tighter than the desirable final tension because they quickly wear-in. Tension is not too critical, as long as there is enough so that no slip occurs at the full load expected.

If the load is intermittent and/or highly inertial, chirping may occur. This is okay, but squealing is not and must be corrected immediately.

Oil and grease contamination is very detrimental to a v-belt drive and must be prevented. Belt dressing should be avoided unless extremely dusty conditions exist. Creep occurs to about the same extent as the flat belt drives, maybe 1 percent. Steel reinforced belts exhibit less creep. Idlers should be used only if absolutely necessary. Back-side idlers are particularly bad. The problem arises from the fact that the belts are thick and the extra flexing causes premature cracking. If an idler must be used it should be as large a diameter as possible.

Special thanks to George B. Loughery and the Hay Creek Valley Historical Assn. for permission to publish this article, which originally appeared in the Association's Late Summer 1994 issue of The Journal. 

For a complete list of references used for this article, please visit the Gas Engine Magazine website at: www.gasenginemagazine.com  

Contact George Loughery at: g19605@hotmail.com 

Contact the Hay Creek Valley Historical Assn. at: www.haycreek.org