Modern Machine-Shop Practice Part 180

[Ill.u.s.tration: Fig. 2626.]

In Fig. 2625 is shown a patent self-adjusting compression clamp, which is peculiarly adapted to connect shafting that is of proper gauge diameter. It consists of a sleeve A made in two halves, each embracing nearly one-half of the shaft circ.u.mference and being bored parallel and slightly smaller than the diameter of the shaft ends. Over this sleeve pa.s.ses at each end a ring D E, bored conical and fitting a similar cone on the external diameter of the sleeve. On each end of the sleeve is the nut F G, which by forcing the cone ring up the taper of the sleeve causes the two halves of the latter to close upon and grip the shaft.

For shafts less than two inches in diameter there are provided in the sleeve two pins to enter holes in the shaft ends in place of keys, but for sizes above that keys are employed. All parts of this coupling being cylindrical it is balanced. The separate parts of this coupling are shown in Fig. 2626.

[Ill.u.s.tration: Fig. 2627.]

[Ill.u.s.tration: Fig. 2628.]

[Ill.u.s.tration: Fig. 2629.]

[Ill.u.s.tration: Fig. 2630.]

In Figs. 2627 to 2630 are shown a side elevation and sectional view of another form of shaft coupling. A is the sleeve, B B nuts on the ends of the sleeve, and C C cones fitting taper holes in the sleeve. These cones are split, as shown in Fig. 2629, to permit them to close upon the shaft ends. The shaft ends themselves are matched with a half dovetail, as in Fig. 2630, which dispenses with the employment of a key.

In coupling shafts of different diameters it is usual to reduce the diameter of the end of the larger to that of the smaller shaft, and to employ a size of coupling suitable for the smaller shaft; but in this case it is necessary that the coupling be placed on the same side of the hanger or bearing as the smaller shaft, otherwise it is obvious that the strength of the larger would, between its bearings, be reduced to that of the smaller shaft.

The couplings for line shafting are usually placed as near to the bearings or hangers as will leave room for the removal of the couplings by sliding them along the shaft.

The couplings on the length of shaft receiving power from the motor are placed outside the bearings, hence on the succeeding lengths there will be one coupling between each pair of bearings, the couplings being in each case as close to each bearing as will allow the coupling to be moved towards the bearing sufficiently to permit the length to be removed without disconnecting the adjacent length from its bearings.

[Ill.u.s.tration: Fig. 2631.]

Fig. 2631 represents a very superior form of coupling for line shafts.

The ends of the line shaft are reduced to half diameters as shown, and lapped with a horizontal joint at an angle to the axis of the shaft as denoted by the dotted line, which prevents end motion; the ends of the shaft and their ab.u.t.ting surfaces are dovetailed, as shown A and B, and, therefore, perform driving duty. A sleeve envelops the whole joint and is secured by a key. This coupling accomplishes all that can be desired, but requires very accurate workmanship, and on this account is expensive to make.

[Ill.u.s.tration: Fig. 2632.]

Fig. 2632 represents a form of coupling suitable for light shafting. It consists of two halves A A, of cast iron, which are drawn together by the bolt C; the centre of the coupling is recessed to enable the coupling to take a better hold on the shaft, which is prevented turning by the pins D D. This coupling has no projections to catch clothes or belts, and is quickly applied or removed.

[Ill.u.s.tration: Fig. 2633.]

Fig. 2633[38] represents a form of coupling for heavy duty, the transmitting capacity only being limited by the strength of the projections A. If the shafts are not axially in line, this form of coupling accommodates the error, since the projections A may slide in their recesses, while if the axial lines of the shafts should vary from flexure of the bearings or foundations, as in steamships, clearance between the ends of A and the bottom of the recesses may be allowed, as shown at B.

[38] From Rankine's "Machinery and Millwork."

[Ill.u.s.tration: Fig. 2634.]

In Fig. 2634 is shown a coupling (commonly known as the universal joint coupling) which will transmit motion either in a straight line, or at any angle up to 45.

It is formed of two double eyes, such as A, connected to a yoke or crosspiece B as shown at C. It is mainly used for connecting shafts or arms carrying tools of some kind, such as rubbers for polishing stone, tools for boring, or other similar purposes in which the tool requires to be rotated at varying angles with the driving shaft.

CHAPTER x.x.xI.--PULLEYS.

Pulleys for the transmission of power by belt may be divided into two princ.i.p.al cla.s.ses, the solid and the split pulley. The former is either cast in one entire piece, or the hub and arms are in one casting, and the rim a wrought-iron band riveted on. The latter is cast in two halves so that they may be the more readily placed upon or removed from the shaft.

On account of the shrinkage strains in large pulley castings rendering them liable to break, it is usual to cast pulleys of more than about 6 feet in halves or parts which are bolted together to form the full pulley. On account of these same shrinkage strains it was formerly considered necessary to cast even small pulleys with curved arms, so that the strains might be accommodated or expended in bending or straightening the curves of the respective arms. It is found, however, that by properly proportioning the amount of metal in the hub, arms, and rim of the pulley, straight arm pulleys may be cast to be as strong as those with curved arms, and being lighter they are preferable, as causing less friction on the shafting journals, and, therefore, being easier to drive.

It is obvious that a pulley for a double belt requires to be stronger than is necessary for a single one, but the difference is not sufficiently great to give any practical advantage by making separate pulleys for single and double belts; hence all pulleys are made strong enough for double belts.

Pulleys are weaker in proportion to their duty as the speed at which they rotate is increased, because the centrifugal force generated by the rotation acts in a direction to burst the pulley asunder, so that if the speed of rotation be continuously increased a point will ultimately be reached at which the centrifugal force generated will be sufficient to cause the wheel to burst asunder. But the speed at which pulleys are usually run is so far within the limits of the pulley's strength, that the element of centrifugal force is of no practical importance except in the case of very large pulleys, and even then may be disregarded provided that the pulleys be made in a sufficient number of pieces to avoid undue shrinkage strains in the castings, but if solid pulleys are rotated at high velocities the internal strains due to unequal cooling in the mould has been known to cause the wheels to fly asunder when under high speeds.

Fig. 2635 represents a solid pulley, the tapered arms meeting the rim in a slightly rounded corner or fillet, and the rim being thickened at and towards its centre. When the width of rim is excessive in proportion to one set of arms a double set is employed as in Fig. 2636.

In some forms of pulley the arms and hub are cast in one piece and the rim is formed of a band of wrought iron riveted to the arms. By this means shrinkage strains are eliminated and a strong and light pulley is obtained.

Fig. 2637 represents a split pulley in which the two halves are bolted together after being placed on the shaft.

Variable motion may be transmitted by means of an oval driving pulley, as in Fig. 2638, it being obvious that the belt velocity will vary according to the position of the major axis of the oval. Arrangements of this kind, however, are rarely met with in practice.

In Fig. 2639 is shown an expanding pulley largely employed on the drying cylinders of paper machinery, and in other similar situations where frequent small changes of revolution speed is required. Each arm of the wheel carries a segment of the rim, and is moved radially to increase or diminish the rim diameter by sliding in slots provided in the hub of the wheel, a radial screw operated by bevel gears receiving motion from the hand wheel and gear-wheels shown in the engraving. It is obvious that in this case the driving belt must be made long enough to embrace the pulley when expanded to its maximum diameter, the slack of the belt due to reduction of diameter being taken up by a belt tightener.

[Ill.u.s.tration: Fig. 2640.]

[Ill.u.s.tration: Fig. 2641.]

[Ill.u.s.tration: Fig. 2642.]

In Fig. 2640 is shown a wooden pulley having a continuous web or disk instead of arms. It is built up of segments, the web being secured to the shaft as follows. In Figs. 2641 and 2642 A, B are clamping plates, and C a split sleeve fitting easily to the shaft and pa.s.sing through A, B, while receiving the nut E on the other side. The web of the pulley fits on the shoulder J, and the f.l.a.n.g.e B fits on the shoulder K, so as to keep these parts true or concentric to A. The bore of A is taper to fit the taper of C; hence the nut E in drawing C through A, causes C to close upon and grip the shaft, while the f.l.a.n.g.es A, B grip the pulley and hold it to C.

In Figs. 2643 and 2644 are represented the Otis self-oiling loose pulley, designed to automatically oil itself upon its starting or stopping.

[Ill.u.s.tration: _VOL. II._ =EXAMPLES OF PULLEYS.= _PLATE XII._

Fig. 2635.

Fig. 2636.

Fig. 2637.

Fig. 2638.

Fig. 2639.]

[Ill.u.s.tration: Fig. 2643.]

[Ill.u.s.tration: Fig. 2644.]

The hub D is cored out in such manner as to form within it an annular chamber or cavity B B, entirely surrounding the bore, and serving as a reservoir to contain oil or other lubricating liquid.

This chamber or reservoir has no direct communication with the bore of the hub, but a communication is formed between it and the bore through one or more chambers C C, which are termed supply chambers, and which are part.i.tioned off within the bore from the reservoir B, by coring the hub in a suitable manner.

These supply chambers have openings N N in their sides or ends communicating with the reservoir B, and also openings C C communicating with the bore of the pulley. These supply chambers are filled with wick or other fibrous or capillary material, which is also inserted into the openings N N, to draw the oil from the reservoir by capillary attraction and supply it in moderate quant.i.ties between the bore of the pulley and the shaft on which it runs. Three or more openings are provided in the outer sh.e.l.l of the hub for the introduction of oil into the reservoir B, which openings are closed by thumb-screws, plugs, or other stoppers E E.

There being three of these openings, one will always be at or near the top when the pulley is at rest, and through this the oil may be introduced without difficulty. It is not intended that the reservoir should at any time contain more than one-third its capacity of oil, so that whenever the pulley is at rest the surface of the oil will be below the lowest point of the bore, thus preventing any waste of oil at such times.