Modern Machine-Shop Practice Part 185

There are many different methods of lacing a belt, but those here described are generally preferred. Thus referring to Fig. 2687 the lace is first pa.s.sed through holes G and D, the ends being of equal length from the belt and emerging on the side that is to be the outside of the belt, thence each end of the lace is laced towards the edge of the belt, the dotted lines in the cut showing the path of the lace. It is then laced back to the middle of the belt, the second inside lacing exactly overlaying the first, the laces never crossing; the outside appearing as in Fig. 2688. The ends are in some cases tied in a knot on the outside, and in others fastened as shown in Fig. 2689, in which case the ends are merely held by friction, which will serve very well unless for a belt that is tightly strained.

By this method of lacing all the crossing of the lace is on the outside of the belt, which is an advantage, because from the creep of the belt the lace undergoes considerable friction, which is apt to rapidly wear out the lace, especially if it be crossed on the side of the bed that meets the pulley surface.

[Ill.u.s.tration: Fig. 2690.]

Fig. 2690 shows a method of lacing in which the crossing of the lace is entirely avoided, the knot being on the outside at _a_ _a_. The path of the lace on one side of the belt is shown in full lines, and on the other side in dotted lines.

The objections to lacing are that the lace lifts the belt from the pulley surface, which throws all the wear on the lace, causing it eventually to break, and which also reduces the area of belt (at the joint) in contact with the pulley surface and reduces the driving power of the belt at the time the joint is pa.s.sing over the pulley. In fact, in running belts this reduction of transmitting capacity is not great, because of the rapidity with which the joint pa.s.ses over the pulley, but in slow moving belts slip is very apt to occur when the lace meets the pulley, especially if the power transmitted is great in proportion to the width of the belt.

[Ill.u.s.tration: Fig. 2691.]

[Ill.u.s.tration: Fig. 2692.]

There are considerable movement and friction between the lace and the belt, more especially when the latter pa.s.ses over a pulley of small diameter, and this with the friction due to whatever amount of slip the belt may experience, wears away the lace so that in time it breaks.

Sometimes a cover is employed as shown in Fig. 2691 at A, to protect the lace, the cover being riveted or cemented to the belt on the side that is to meet the pulley surface. A similar means is also sometimes employed to make a b.u.t.t joint. Thus in Fig. 2692 A is the cover riveted or cemented to the two ends B C, of the belt so as to dispense with lacing.

[Ill.u.s.tration: Fig. 2693.]

Fig. 2693 represents an excellent method of joining very thin belts, the operation being as follows:--

Place the two ends of the belt together with the edges fair one with the other, and with an awl make a row of holes at _a_, through both ends; then take about half a yard of strong twine (in some cases a lace or gut is better) and draw half the length through the first hole, then pa.s.s each end of the twine through the second hole, one end to the right and the other to the left, and draw both tight at the same time, and so on until the last hole is reached, when one end only of the twine is pa.s.sed through; the two ends of the twine are then knotted tight together and the excess cut off.

The middle sketch shows the joint when the belt is stretched. The lower sketch shows it pa.s.sing over a small pulley, where it will be seen that in the act of bending over the curve there is no friction between the lace and the belt, and this is the reason of its superiority over other methods, where there is always more or less friction between the lace and the belt when bending over a curve. Another advantage is, that in this system the lace does not come into contact with the pulley, so that whatever friction or slipping may take place between the belt and the pulley, the lacing is perfectly unaffected by it.

[Ill.u.s.tration: Fig. 2694.]

A lap joint is one in which the two ends of the belt overlap, as in Fig.

2694. The overlap is cut down to a plain bevel so as to reduce the joint to nearly or quite the same thickness as the main body of the belt. The lap joint is employed to join together the strips of leather forming the belt, and to fasten the ends of the finished belt together. In making the belt the overlap is cemented and riveted, while in joining the ends it may be cemented, or riveted, or laced.

The advantage of rivets lies simply in that they are easily applied.

Their disadvantages are that they grip but a small area of the belt, namely, that portion beneath the rivet head and washer surface; hence, when rivets are used the joint should always be cemented also. A more important defect is, however, that the heat generated by the compression of the rivet while riveting it is sufficiently great to _burn the leather_ beneath the rivet-head. The reason that the leather under the head and not under the washer or burr at the riveted end of the rivet burns is, that although the heat due to riveting is most at the burr end of the rivet, its pa.s.sage from the rivet to the washer is less rapid than it is through the body of the rivet, because in the one case it has to be transferred from one body to another (from the rivet to the burr), while in the other its pa.s.sage is uninterrupted and continuous.

[Ill.u.s.tration: Fig. 2695.]

Rivets for lap joints are usually placed about, as in Fig. 2695, the rows A and C being about 1/2 inch from the edges B and D respectively, and the row F about 3/8 inch from the edge F of the lap, while the rivets are about 5/8 inch apart in the rows.

For comparatively narrow belts as, say, four inches wide, a single row G would be placed in the middle, additional middle rows should for wider belts be about 1-1/4 inches apart.

The rivet holes should be a close fit to the rivets, the latter being left just long enough to hold the washer or burr and sink with it, in the riveting, to the level surface of the belt.

The heads of the rivets should be on the side of the belt that is to run next to the pulley.

The strongest method of forming a belt is by means of small taper wooden pegs, such as are used in boot and shoe manufacture, the joint being cemented, and the pegs inserted. In this case the belt is merely pierced with an awl, hence none of the leather is removed.

[Ill.u.s.tration: Fig. 2696.]

The arrangement of wooden pegs should be as in Fig. 2696, the rows A and B being respectively about 5/8 inch from the edges C D, the row E being about 1/4 inch from the edge of the joint, and H about 3/4 inch from that edge. The pegs are placed about 1/2 inch apart in the rows.

A cemented and pegged joint is the strongest made, and it preserves a more equal tension throughout the belt than any other, while the belt is strong, since the hole for the pegs may be pierced with an awl, which does not remove any leather from the belt, as is the case with punched holes.

The length of the lap in some of the best practice is as follows:

When the strips of leather are cut from the hide in such lengths that the part termed the shoulder of the hide is utilised, a uniform lap of 8 inches is employed for all widths of belt. When the strips do not contain the shoulder of the hide, the following are the respective lengths of lap:--

Width of single belt. Length of lap.

1 to 4-1/2 inches 4-1/4 inches.

5 inches 5 "

6 to 8 inches 6 "

9 inches 6-1/2 "

10 to 14 inches 7 "

15 to 24 " 8 "

All double belts are given a 6 inch lap.

[Ill.u.s.tration: Fig. 2697.]

Another and excellent method of joining a belt, or of fastening two thicknesses together to form a double belt, is to sew it together with lace leather, as shown in Fig. 2697. The lace is in this case about 1/4 inch wide, the holes being pierced so as to have the lace diagonal, as shown in the cut. Sometimes four rivets are added at the joint as shown in the cut.

[Ill.u.s.tration: Fig. 2698.]

Other methods of fastening the ends of leather belts are by means of metal hooks of various forms. Fig. 2698 represents a fastening of this kind, the appearance of both sides of the joint being shown in the figure. In this case considerable leather is removed from the belt, but this is to some extent compensated for, because the hook holds each end of the belt in two places; that is to say, in the crook of the hook as well as at the end. This, however, while it has the effect of increasing the grip of the hook on the belt, still leaves the belt as a whole weaker, by reason of the removal of leather to form the holes.

[Ill.u.s.tration: Fig. 2699.]

[Ill.u.s.tration: Fig. 2700.]

In Figs. 2699 and 2700 is shown a belt screw, intended to take the place of rivets, and thus avoid the burning of the leather which accompanies the use of rivets. It consists of two screws, one having a right and the other a left-hand thread. The former is of bronze, and has a coa.r.s.e exterior thread cut conically, while it is hollow with a fine thread tapped inside. The latter is of steel, and has a conical shoulder underneath. The heads of both screws are slightly rounded and formed with circular grooves on the under side, to give them a firm grip on the leather. The conical screw is first run into the leather, and the steel screw is then introduced. The belt is run with the head of the latter on the inner side.

If the body of a narrow belt is riveted it contains two rows only of rivets; but as the width of the belt increases, other rows are introduced, all the rows running the entire length of the belt. In some cases two separate single belts running one over or outside the other are employed in place of an ordinary double belt, and the arrangement works well.

Two single belts applied in this manner are especially preferable to a double belt when used upon a small pulley, because they will bend to the curvature of the pulley more readily, being more pliable; whereas a double belt will from its resistance to bending not envelop as much of the circ.u.mference of the belt as is due to the relative sizes of the pulleys, and the distance apart of their axes.

Round leather belts are made in two forms, the solid and the twisted.

The first consists of a simple leather cord, hence its diameter cannot exceed the thickness of the leather. The second consists of a strip of leather twisted into cylindrical form, the grain side of the leather being outside.

The ends of round belts are usually joined by means of cylindrical hooks and eyes, which are threaded so as to screw on to the end of the belt, but for twisted round belts it is better to place in the centre of the belt a small core of soft wood. The ends of the belt should be slightly tapered, and the hook and eye screwed firmly home. Sometimes from the smallness of the pulleys the inflexibility of the hook and eye becomes objectionable, and a simple hook is employed on solid round belting.

The length of twisted round belting may be altered by twisting or untwisting it, which renders it unnecessary to cut the belt for a small amount of shortening.

Round belts should bear upon the sides, and not on the bottom of the pulley-groove, which increases their transmitting power. Thus, if the groove is a semicircle of the same radius as is the belt when new, the stretch of the belt as it wears decreasing its diameter, it will then touch only on the bottom of the groove. Furthermore, when the belt bears on the sides only of the groove it becomes wedged to a certain extent in the sides of the pulley groove.

[Ill.u.s.tration: Fig. 2701.]

[Ill.u.s.tration: Fig. 2702.]

[V]-belting is formed of strips of leather welted together, as shown in Figs. 2701 and 2702, the latter showing the joint or splice of the belt.

The pulleys are [V]-grooved as shown. The tension of the belt causes it to grip the sides of the groove on the wedge principle, and the belt is flat at the apex of the [V] so that it shall not bottom in the groove, which would impair its wedging action. This cla.s.s of belt is largely employed for connecting shafts at an angle, especially in cases where the distance between the shafts is small, in which case it will last much longer than a flat belt.