Modern Machine-Shop Practice Part 183

When the amount to which the leather has been stretched is an unknown quant.i.ty (as is commonly the case), the workman cuts the belt too short, to an amount dictated solely by judgment, following no fixed rule. If, as in the case of narrow belts, the stretching be done by hand, the belt is placed around the pulleys, stretched by hand, and cut too short to an amount dictated by judgment, but which may be stated as about 2-1/2 per cent. of its length.

But the stretch of a belt after it is put to work proceeds very much more rapidly if it has been stretched in the piece and not in the strip, hence it gets slack in the course of a few hours, or of a day or more, according to how much it has been stretched; whereas one properly stretched in the strip will last for weeks, and sometimes for months, without getting too slack.

[Ill.u.s.tration: Fig. 2660.

+----------------+----------------+-----------------+----------------+ |2,000 1/4 3. |2,050 3/16 3.1|2,150 3/16 3.2 |2,175 1/4 3.3 | +----------------+----------------+-----------------+----------------+ |1,400 9/32 2.12|2,000 1/8 3. |2,625 3/16 3.4 |2,325 7/32 3.4 | +----------------+----------------+-----------------+----------------+ |2,000 1/4 2.11|2,075 3/16 3.1|2,375 7/32 3.4 |2,175 7/32 3.5 | +----------------+----------------+-----------------+----------------+ |2,075 1/4 2.12|2,700 7/32 3.3|2,600 7/32 3.4 |2,275 5/32 3.7 | +----------------+----------------+-----------------+----------------+ |2,450 1/4 2.13|3,025 9/32 3.7|2,575 11/32 3.8 |2,225 7/32 3.10| +----------------+----------------+-----------------+----------------+ |2,475 1/4 3. |2,975 5/16 3.6|3,200 9/32 3.10|2,175 3/8 3.10| +----------------+----------------+-----------------+----------------+ |2,575 11/32 3.2 |2,875 9/32 3.7|3,475 11/32 3.13|1,850 11/32 3.11| +----------------+----------------+-----------------+----------------+ |2,675 11/32 3.2 |3,075 11/32 3.8|3,450 9/32 4. |1,950 1/4 3.11| +----------------+----------------+-----------------+----------------+ |2,650 3/8 3.2 |2,900 9/32 3.6|3,150 3/16 3.15|2,225 1/4 3.10| +----------------+----------------+-----------------+----------------+ |2,800 1/4 3.1 |3,050 5/16 3.6|2,850 1/4 3.13|2,275 3/16 3.7 | +----------------+----------------+-----------------+----------------+ |2,700 1/4 3. |3,150 7/32 3.5|3,000 3/16 3.10|2,600 1/4 3.5 | +----------------+----------------+-----------------+----------------+ |2,650 1/4 2.13|3,000 7/32 3.4|3,400 1/8 3.6 |2,550 1/4 3.4 | +----------------+----------------+-----------------+----------------+]

The results of some experiments made by Messrs. J. B. Hoyt & Co. on the strength of the various parts of a hide are given in Fig. 2660. One side of the part of the hide used for leather belting was divided off into 48 equal divisions, each piece being 11-3/4 inches long, and two inches wide, the results of each test being marked on the respective pieces.

The first column is the strain under which the piece broke; the second column is the amount in parts of an inch that the piece stretched previous to breaking; and the third column is the weight of the piece in ounces and drachms.

From the table it appears that the centre of the hide which has the most equal stretch has the least textile strength, while in general that which has the most stretch has the greatest textile strength, but at the same time the variations are in many cases abrupt.

A single belt is one composed of a single thickness of leather put together, to form the necessary length, in pieces, riveted and cemented together at the joint, or sewed or pegged as hereafter described.

A double belt is similarly constructed, but is composed of two thicknesses of leather cemented and riveted, pegged, or sewed together throughout its whole length, as hereafter described. The object of a double belt is to increase the strength without increasing the width of the belt. Belts are usually made in long lengths coiled up for ease of transportation, the length of belt required being cut from the coil.

To find the length in a given coil that is closely rolled--Rule: the sum of the diameter of the roll and the eye in inches, multiplied by the number of turns made by the belt, and this product multiplied by the decimal .1309, will equal length of the belt in feet.

[Ill.u.s.tration: Fig. 2661.]

The grain or smooth side of the leather is the weakest, as may be readily found by chamfering it to a thin edge, when it will tear like paper, and a great deal more easily than will the flesh side under similar treatment. Again, it will crack much more readily: thus, take a piece of leather and double it close with the grain side outward, and it will crack, as shown in Fig. 2661 at C, whereas if doubled, however closely, on the flesh side no cracks will appear. If the edge of a clean-cut piece of leather be examined, there will be found extending from the grain side inward a layer of lighter color than the remainder of the belt; and this whole layer is less fibrous and much weaker than the body of the belt, the strongest part of which is on the flesh side.

If the grain side is shaved off thin and stretched slightly with the fingers it will exhibit a perfect network of small holes showing where the hair had root. Here, then, we have weakness and excessive liability to crack on the grain side of the leather, and it is obvious that if this side is the outside of the belt, as in Fig. 2662, at A, the tendency is to stretch and crack it, especially in the case of small pulleys, whereas if the grain side were next to the pulley the tendency would be to compress it, and therefore, rather to prevent either cracking or tearing. Furthermore, very little of the belt's strength is lost by wearing away its weakest side.

[Ill.u.s.tration: Fig. 2662.]

Another and important consideration is, that the grain side will lie closest and have most contact over a given area with the pulley surface.

In making double belts of extra good quality, it is not uncommon to cut away or shave off the grain side of both belts, and place those surfaces together in making up the belts.

If the grain side of a belt is the outside when on the pulleys, and a crack should consequently start, the destruction of the belt proceeds rapidly, because the line of crack is the weakest part of the belt, and the belt has less elasticity as a continuous body, and more at the line of crack. Cracking may, to some extent, be provided against by oiling the belt, and for this purpose nothing is better than castor oil. In the manufacture of belts, extra pliability is induced by an application of fish oil and tallow, applied when the belt (after having been wetted), is in a certain stage of progress toward drying. The oil and tallow are supposed to enter the pores of the leather and supply the place of the evaporated water.

LENGTH OF BELTS.--Since the stretch of a belt is variable in different belts of the same length, no rule can be given for the amount to which a belt should be cut shorter than the measured length around the pulleys, and it follows, therefore, that the length of a belt cannot be obtained precisely by calculation. In practice the necessary length for a belt to pa.s.s around pulleys already in their places upon the shaft is usually obtained by pa.s.sing a tape line or cord around the pulleys, the stretch of the tape line being allowed as that necessary for the belt. Then when the belt is placed around the pulleys it is shortened if it should appear to require more tension. If, however, the belt length for pulleys not in position is required, it may be obtained as follows, the error being so slight as to be within the margin of difference of stretch in different belts, and therefore of no practical moment:--

[Ill.u.s.tration: Fig. 2663.]

For open belts let the distance between the shaft centres, as _a_ _b_ in Fig. 2663, be the base of a right angle triangle, and the difference between the semi-diameters, as _b_ _c_, the perpendicular. Square the base and the perpendicular, and the square root of the sum of the two will give the hypothenuse, and this multiplied by 2 and added to one-half the circ.u.mference of each pulley is the required length for the belt. This will give a belt too long to the amount to be cut out of the belt to give it the necessary tension when on the pulleys.

_Example._--Let the distance between centres in Fig. 2663 be 48 inches; diameter of large pulley 24 inches; diameter of small pulley 4 inches--

Here distance between centres 48 " " " 48 --- 384 192 ---- 2304 Square of perpendicular 100 ---- 2404 Square root of 2404 = 49.03 Multiply by 2 2 ----- 98.06 Half circ.u.mference of large pulley 37.699 ------- 135.759 Half circ.u.mference of small pulley 6.283 ------- Length of belt 142.042

A simpler rule which gives results sufficiently accurate for practical purposes is as follows:--

_Rule._--Add the diameter of the two pulleys together, divide the result by 2, and multiply the quotient by 3-1/4, then add this product to twice the distance between the centres of the shafts, and you have the length required.

When the length of a crossed belt is required, and the pulleys are not erected upon the shafts, it is, on account of the abstruseness of a calculation for the purpose, preferred in workshop practice to mark off by lines the pulleys set at their proper distance apart (either full size or to scale), and measure the length of the side of the belt, supposing the belt to envelop one-half the circ.u.mference only of each pulley, and to add to this one-half the circ.u.mference of each pulley; or if there is a great difference between the relative diameters of the pulleys and the distance apart of the shafts is unusually small, the lengths of the straight sides of the belt are measured and the arcs of contact around the pulleys are stepped around by compa.s.ses, the set of the compa.s.ses being not more than about one-tenth the circ.u.mference of the pulleys. This gives a more near result than that obtained by calculation, because although it will give a belt shorter than by calculation, yet the belt will be too long on account of the stretch necessary to the tension required for ordinary conditions.

[Ill.u.s.tration: Fig. 2664.]

In narrow belts, as, say, three inches and less in width, the belt may be cut to the length of a tape line pa.s.sed over the pulleys, and when placed over the pulleys it may be strained under a hand pull and cut as much shorter as the tension under hand pressure indicates as being necessary.

But if the belt is a wide one a stretching clamp, such as shown in Fig.

2664, is employed, the screws being right hand at one end and left hand at the other, so that operating them draws the clamps, and therefore the ends of the belt, together.

The stretch of a belt not stretched in the piece proceeds slowly when the belt is at work, hence if laced at first to a proper degree of tension it will get slacker in a few hours or in a day or so, and must be tightened, or taken up as it is termed, by cutting a piece out. For this purpose a b.u.t.t joint possesses the advantage that the piece to be taken out may be less, and still leave the end clear for new holes to be punched, than is the case with a lap joint, which occurs because the b.u.t.t joint occupies a shorter length of the belt than is the case with a lap joint.

[Ill.u.s.tration: Fig. 2665.]

[Ill.u.s.tration: Fig. 2666.]

When a belt is under tension upon two pulleys and at rest, the friction or grip of the belt upon the respective pulleys (supposing them to be of the same diameter and therefore to have the same arc and area of contact) will depend upon the relative positions of the pulleys; thus suppose one pulley to be above the other as in Fig. 2665, the upper pulley P will have the grip due to the tension of the belt added to that due to the weight of the belt, whereas if placed horizontally, as in Fig. 2666, the weight of the belt will fall equally on the two pulleys, and for this reason vertical belts of a given width require to have a greater tension to transmit the same amount of power as the same belt would if placed horizontally. But as soon as motion was transmitted, by the belt, from one pulley to the other, the belt on one side of the pulley would be under greater tension then that on the other.

[Ill.u.s.tration: Fig. 2667.]

Suppose, for example, a belt to transmit motion and power from pulley A in Fig. 2667, to pulley B, then the side C of the belt is that which drives or pulls B, and it is therefore called the driving side of the belt, the resistance to rotation offered by B causing the driving side of the belt to be the most strained; and hence the straightest, whereas the side D will be free of the tension due to the resistance of B.

[Ill.u.s.tration: Fig. 2668.]

But if the direction of motion be reversed as in Fig. 2668, A still being the driving pulley, the side D will be the one most tightly strained, and therefore, the driving side of the belt; or, in other words, the driving side of a belt is always that side which approaches the driving pulley, and the slack side is always that which recedes from the driving pulley. In horizontal belts, however, the driving side of the belt is not a straight line, because of the belt sagging from its own weight no matter how tightly it may be strained, but the shorter the belt the less the sag.

[Ill.u.s.tration: Fig. 2669.]

It is always, therefore, desirable, so far as the driving power of the belt is concerned, to have the lower half (of belts running horizontally) the driving side, because in that case the sag of the belt causes it to envelop a greater arc of the pulley, which increases its driving power. If the circ.u.mstances will not permit this and the sag of the belt operates to practically incapacitate the belt for its duty, what is termed an idle wheel or idler may be employed as shown in Fig.

2669 at E, serving to prevent the sag and to cause the belt on the driving side to envelop a greater portion of the pulley's circ.u.mference, and hence increase its friction on the pulley and therefore its driving power. In the example the two pulleys A and B are of equal diameters; hence the idle wheel is placed midway between them, but when such is not the case the idle wheel should be located according to the circ.u.mstances and the following considerations. The idle wheel requires a certain amount of power to drive it, and this amount will be greater as the idle wheel is nearer to the smallest wheel of the pair connected; but on the other hand, the closer the idle wheel to the small pulley (all other factors being equal) the greater the arc of small pulley surface enveloped by the belt, and hence the greater the belt's driving power.

When therefore a maximum increase of driving power is required, the idler must be placed near to the smallest pulley, the desired effect being paid for in the increased amount of motive power required to rotate the driving pulley.

But under equal conditions the larger the diameter of the idle wheel the less the power required to drive it, because the less its friction on its journal bearing. A belt tightener should whenever practicable be placed on the slack side of the belt.

Belt tighteners are sometimes used to give intermittent motion, as in the case of trip hammers; the belt being vertical is made long enough to run loose, until the tightening pulley closes the belt upon the pulley, taking up its slack and increasing the arc of contact.

[Ill.u.s.tration: Fig. 2670.]

When the direction of rotation of the driven pulley requires to be reversed from that of the driving pulley, the belt is crossed as in Fig.

2670. A crossed belt has a greater transmitting power than one uncrossed (or, as it is termed, than an "open belt") because it envelops a greater arc of both pulleys' circ.u.mference. This is often of great advantage where the two pulleys are of widely varying diameter, especially if the small pulley requires to transmit much power, and be of very small diameter.

But a crossed belt is open to the objection that the surfaces of the belt rub against each other at the point of crossing, which tends to rapidly wear out the laced joint of the belt. By crossing a vertical belt the lower pulley receives part of the weight of the belt.

When a belt connects two pulleys whose respective planes of revolution are at an angle one to the other, it is necessary that the centre line of the length of the belt shall approach the pulley in the plane of the pulley's revolution, which is sufficient irrespective of the line of motion of the belt when receding from the pulley. This is shown in Fig.

2671, which represents what is known as a quarter twist; A, B are two pulleys having their planes of revolution at a right angle, the belt travelling as denoted by the arrows, then the centre line C of the belt being in the plane of rotation of A on the side on which it advances to A, the belt will continue to run upon the same section of A. If the pulley positions be reversed, as in Fig. 2672, the same rule applies, and the side D in the figure being that which advances upon B must travel to B in the plane of B's rotation, otherwise the belt would run off the pulley; hence it is obvious that the belt motion must occur in the one direction only.

[Ill.u.s.tration: Fig. 2671.]

[Ill.u.s.tration: Fig. 2672.]

[Ill.u.s.tration: Fig. 2673.]

Shafts at any angle one to another may have motion communicated from one to the other by a similar belt connection, providing that a line at a right angle to the axis of one shaft forms also a right angle with the axis of the other. Thus in Fig. 2673 the axis of shaft A may be set at any required angle to the plane of rotation of pulley B, provided that the axial line of A be made to lie at a right angle to the imaginary line _l_, which is at a right angle to the axis of the shaft of B, and that the side of the driving pulley which delivers the belt (as C, Fig.

2671) is in line with the centre line of the driven pulley, as denoted by the dotted line C.