106. Horse-power of Belting.—A belt is an apparatus for the transmission of power from one shaft to another. The driving pulley exerts a certain pull in the belt and this pull is transmitted by the belt and exerted on the rim of the driven pulley.

The power transmitted by any belt depends on two thingsthe effective pull of the belt tending to turn the wheel, and the speed with which the belt travels. From the preceding pages, it is easily seen that these include the three items necessary to measure power. The pull of the belt is the force. The speed, given in feet per minute, includes both distance and time. Force, distance and time are the three items necessary for the measurement of power.

The total pull that a belt will stand depends on its width and thickness. It should be wide enough and heavy enough to stand for a reasonable time the greatest tension put upon it. This is, of course, the tension on the driving side. This tension, however, does not represent the force tending to turn the pulley. The force tending to turn the pulley (or the Effective Pull, as it is called) is the difference in tension between the tight and the slack sides of the belt.

The effective pull that can be allowed in a belt depends primarily on the width, thickness, and strength of the leather, or whatever material the belt is made of. Besides, we must consider that every time a belt causes trouble from breaking or becoming loose, it means a considerable loss in time of the machine, of the men who are using it, and of the men required to make the repairs and, therefore, it should not be loaded as heavily as might otherwise be allowed. Leather belts are called "single," "double," "triple," or "quadruple," according to whether they are made of one, two, three, or four thicknesses of leather. Good practice allows an effective pull of 35 lb. in a single leather belt per inch of width. In a double belt a pull of 70 lb. per inch of width may be allowed. The pull times the width gives the total effective pull or the force transmitted by the belt.

The force times the velocity, or speed, of the belt in feet per minute will give the foot-pounds transmitted by it in 1 minute. One horse-power is a rate of 33,000 foot-pounds per minute; hence, the horse-power of a belt is obtained by dividing the foot-pounds transmitted by it per minute by 33,000. The velocity of the belt is calculated from the diameter and revolutions per minute of either one of the pulleys over which the belt

travels, as explained in Chapter VII. From these considerations, the formula for the horse-power that a belt will transmit may be written

where H=horse-power

P-effective pull allowed per inch of width

W width in inches

=

V = velocity in feet per minute

Stated in words, this formula would read as follows: "The horsepower that may be transmitted by a belt is found by multiplying together the allowable pull per inch of width of the belt, the width of the belt in inches, and the velocity of the belt in feet per minute and then dividing this product by 33,000.

Example:

Find the horse-power that should be carried by a 12-in. double leather belt, if one of the pulleys is 14 in. in diameter and runs 1100 R. P. M.

Explanation: To get the horsepower, we must first find the values of P, W, and V. We will take P as 70 lb. since this is a double belt. W is given, 12 in. V, the velocity, is obtained by multiplying the circumference of the pulley by the R. P. M., which gives us 4032. Multiplying these three together gives 3,386,880 foot-pounds per minute, and dividing by 33,000 we have 102+ as the horse-power that this belt might be required to carry.

107. Widths of Belts. It is possible, also, to develop a formula with which to calculate the width of belt required to transmit a certain horse-power at a given velocity.

One horse-power is 33,000 foot-pounds per minute. Then the given number of horse-power multiplied by 33,000 gives the number of foot-pounds to be transmitted per minute.

Foot-pounds per minute=33000 XH

If we know the velocity in feet per minute, we can divide the foot-pounds per minute by the velocity; the quotient will be the force or the effective pull in the belt.

Now the force can be divided by the allowable pull per inch of width of belt. The result will be the necessary width.

Stated in words, this formula would read: "To obtain the width of belt necessary for a certain horse-power; multiply the horsepower by 33,000 and divide by the product of the allowable pull per inch of width of belt times the velocity of the belt in feet per minute."

Example:

Find the width of a single belt to transmit 10 horse-power at a speed of 2000 ft. per minute.

108. Rules for Belting. 1. Belt Thickness.-It is generally advisable to use single belting in all cases where one or both pulleys are under 12 in. in diameter, and double belting on pulleys 12 in. or larger. Triple and quadruple belts are used only for main drives where considerable power is to be transmitted and where a single or double belt would have an excessive width. A triple belt should not be run on a pulley less than 20 in. in diameter, nor a quadruple belt on a pulley less than 30 in. in diameter.

2. Tension per Inch of Width.—An effective pull of 35 lb. per inch of width of belt is allowable for single belts. For double belts an effective pull of 70 lb. per inch is allowable unless the belt is used over a pulley less than 12 in. in diameter, in which case only 50 lb. per inch should be allowed. A prominent manufacturer of rubber belting recommends 33 lb. per inch of width of belt for 4-ply belts and 43 lb. for 6-ply rubber belts.

3. Belt Speeds.-The most efficient speed for belts to run is from 4000 to 4500 ft. per minute. Belts will not hug the pulley and therefore will slip badly if run at a speed of over one mile per minute. These figures are seldom reached in machine shops.

Belts for machine tool drives run from 1000 to 2000 ft. per minute, while main driving belts for line shafts are more often run about 3000 ft. per minute. On wood-working tools we find higher speeds, usually 4000 ft. per minute or over.

4. Distance between Centers.-The best distance to have between the centers of shafts to be connected by belting is 20 to 25 ft. For narrow belts and small pulleys this distance should be reduced.

5. Arrangement of Pulleys.-It is desirable that the angle of the belt with the floor should not exceed 45 degrees; that is, the belt should be nearer horizontal than vertical. Fig. 66 shows

Driver

FIG. 66.

FIG. 67.

Driver

the effect of having a belt nearly vertical. Any sag in the belt causes it to drop away from the lower pulley and lose its grip on it. Fig. 67 shows the best arrangement. Have the belt somewhere near horizontal and have the tight side of the belt underneath, if possible. This will increase the wrap of the belt around the pulleys. If the lower side is the loose side, the wrap will be decreased by the sag.

It is also desirable, whenever possible, to arrange the shafting and machinery so that the belts will run in opposite directions from the shaft, as shown in Fig. 68. This arrangement balances somewhat the belt pulls, and reduces the friction and wear in the bearings.

For belts which are to be shifted, the pulley faces should be flat; all other pulleys should have the faces crowned (high in the center) about in. per foot of width.

FIG. 68.

6. Grain and Flesh Sides.-The grain side of the leather is the side from which the hair is removed. It is the smoothest but weakest side of the leather, and should run next to the pulley surface. It will wrap closer to the pulley surface and thus get a better grip on the pulley. Furthermore, the flesh side, being stronger, is better able to stand the stretching which must occur in the outside of the belt in bending around a pulley.

7. Belt Joints.—Whenever possible, the ends of belts should be fastened together by splicing and cementing. Never run a wide cemented belt onto the pulleys as one side is liable to be stretched out of true. Rather lift one shaft out of the bearings,