Modern Machine-Shop Practice Part 179

COUPLINGS FOR LINE OR DRIVING SHAFTS.--The couplings for connecting the ends of line shafts should accomplish the following objects:--

1. To hold the two shaft ends axially true one with the other.

2. To have an equal grip along the entire length of shaft enveloped by the coupling.

3. To have a fastening or locking device of such a nature that it will not be liable to work loose from the torsional strains due to the flexure of the shaft, which is caused by the belts springing or straining the axial line of shafting out of the straight line.

[Ill.u.s.tration: Fig. 2610.]

4. To be capable of easy application and removal, so as to permit the erection or disconnection of the lengths of shafting with as little disarrangement of the hangers and bearings as possible, and to be light, run true, and be balanced.

To these requirements, however, may be added that, since it is well-nigh impracticable to obtain lengths of lathe-turned shafting of exactly equal diameter, couplings for such shafting require to fill the following further requirements:

5. The piece or pieces gripping the shaft ends must be capable of concentric and parallel closure along the entire area, enveloping the end of each shaft, and must do this at each end independently of the other, and the piece or pieces exerting the closing or compressing pressure must grip the closing piece or pieces, enveloping the shafting over the entire area.

[Ill.u.s.tration: Fig. 2611.]

[Ill.u.s.tration: Fig. 2612.]

[Ill.u.s.tration: Fig. 2613.]

If, for example, a sleeve be split at four equidistant parts of its circ.u.mference, and from each end nearly to the middle of its length, as in Fig. 2611, any pressure that may be applied to its circ.u.mference to cause it to grip the shaft it envelops will cause it to grip the shaft with greater force at one part than at another, according to the diameter of the shaft and the location of the external pressure. Thus, if the pressure be applied equally along the length A B, the weaker end B will close most readily, while at A the support afforded by the unsplit section offers a resistance to closure at the ends A of the split, hence the shaft, even though a working fit to the sleeve bore, will be gripped with least force at the end A. If the shaft were simply a close fit, as, say, just movable by hand on the sleeve bore, the form of the coupling bore would, when compressed upon the shaft, be as shown in Fig. 2612, the bend on the necks _a_, _b_, _c_, _d_, being magnified for clearness of ill.u.s.tration. If the compressing piece covered the compressed sleeve for a lesser distance, the grip would be more uniform, because there would be a greater length of the sleeve to afford the curves _a_, _b_, _c_, _d_, as shown in Fig. 2613. The grip may be more equalised by boring the sleeve of slightly smaller diameter than the shaft.

[Ill.u.s.tration: Fig. 2614.]

Fig. 2614 represents a sleeve carrying out this principle. It is composed of two halves, as shown, bored slightly smaller than the shaft diameter, and is to be compressed on the shaft, which, acting as a wedge, would spring open the sides of the bore until the crown of the bore bedded against the shaft. This, in the case of parallel shaft ends of equal diameter, would hold them with great force axially true, and with equal force and bearing, thus meeting all the requirements. If, however, the end of one shaft were of larger diameter than the end of the other (as has. .h.i.therto been supposed to be the case), the end accomplished by boring the sleeve of smaller diameter than the shaft is, that the end of the sleeve is afforded the extra elasticity due to the transverse spring of the sleeve, which permits the edges of each half of the sleeve to bear along a greater length of the shaft end than would otherwise be the case; but the bearing is in this case mainly at and near the edges of the split.

It will be perceived, then, that under this principle of construction, when applied to shaft ends of varying diameters, the metal is left to spring and conform itself to the shape of the parts to be connected, and that there is nothing outside of the condition of relative diameter of shaft to sleeve bore to determine what the direction of the spring or closure of sleeve shall be; but, on the other hand, the principle possesses excellence in that the sleeve being cylindrical and its closure taking place equally at similar points of contact the shafts will be held axially true, one with the other; or in other words, the movements of the metal while sleeve closure is progressing are equally radial to the axis of the sleeve, and there is no element tending to throw the shaft axis out of line one with the other.

If a sleeve have a single split, the manner in which it will grip a shaft smaller than the sleeve's bore depends upon the manner in which the compression is effected.

In Fig. 2615, for example, is a ring supposed to be compressed by a pressure applied at A and at B, causing the ring to a.s.sume the form shown by the dotted lines. The centre of the ring bore would therefore be moved from C to D. Now, suppose that the end of one section of shafting were to fit the sleeve bore, then compressing the sleeve upon it would not practically move the centre of the bore; but if the shaft at the other end of the sleeve were smaller than the sleeve bore, the compression of the sleeve to grip the shaft would move the centre of the bore, and, therefore, of the shaft towards D, hence the axial lines of the shafts would not be held true one with the other. To accomplish this latter object, the compression must be equal all round the sleeve, or it may be applied at the points E and F, Fig. 2616, although it is better to have the compression area embrace all the circ.u.mferential area possible of the sleeve, and to have the movement that effects the compression simultaneous and equal at all points on the circ.u.mference of the ring or sleeve, because if these movements are independent, more movement or compression may be given at one point than at another, and this alters the centre of the bore; thus, if more pressure were exerted at E than at F, in figure, the centre of the bore would be thrown towards F, or _vice versa_. If the pressure be concentric, the single split ring or sleeve grips the shaft all round its circ.u.mference; hence it is only necessary in this case to maintain the circ.u.mference of the sleeve in line to insure that the shaft ends be held axially true one with the other; and if the pressure on the ring be applied equally from end to end its closure will also be parallel and equal, and the shaft will be held with equal force along that part of its length enveloped by the coupling. It is obvious, however, that the piece or sleeve gripping the end of one shaft must be independent of that gripping the other, so as to avoid the evils shown in Fig. 2612, while at the same time the casing or guide enveloping the two independent rings or sleeves must guide and hold them axially true, one with the other.

[Ill.u.s.tration: Fig. 2615.]

[Ill.u.s.tration: Fig. 2616.]

[Ill.u.s.tration: Fig. 2617.]

In Fig. 2617 is shown an excellent form of _plate coupling_, in which most of the requirements are obtained. A and B are the ends of the two lengths of shafting to be connected, C and D are the two halves of the coupling driven or forced on the ends of the shafting, and further secured by keys. The end of one half fits into a recess provided on the other half, so as to act as a guide to keep the shafts axially true one with the other, and also to keep the two halves true one with the other, while drilling the holes to receive the bolts E which bolt the coupling together. The objections to this form are, that it is costly to make, inasmuch as truth cannot be a.s.sured unless each half coupling is fitted and keyed to the shaft, and turned on the radial or joint faces afterwards. Furthermore, if the coupling were taken off in order to get a solid pulley on the shaft, the coupling is apt to be out of true when put together again, and, therefore, to spring the shaft out of true.

Also, that the bearing, support, or hanger must be open-sided to admit the shaft, and that each coupling, being fitted and turned to its place, would be apt to run out of true if removed and applied to another shaft, whether the same be of equal diameter or not; but if each half coupling be provided with a feather instead of the usual key, the coupling may be readily removed and will remain true when put on again.

[Ill.u.s.tration: Fig. 2618.]

Fig. 2618 represents a plate coupling, in which one end of the shaft pa.s.ses into the bore of the half coupling on the other length of shaft, which serves to keep the shafts in line one with the other.

[Ill.u.s.tration: Fig. 2619.]

Fig. 2619 represents a single cone coupling composed of an external sleeve having a conical bore and a split internal sleeve bored to receive the shaft, and turned on its outer diameter to the same cone as the bore of the outer or encasing sleeve. The bolts pa.s.s through the inner sleeve, the bolt head meeting the radial face of the inner sleeve while the nut meets the radial face of the outer sleeve, so that s.c.r.e.w.i.n.g up the nut forces the inner sleeve into the outer and closes the bore of the former upon the shaft. This coupling is open to the objection that it cannot grip the ends of the shafts equally unless both shafts be of exactly equal diameter, and the bearing on the smaller shaft will be mainly at the outer end only, as explained in Fig. 2611.

As a result, the transverse strains on the shaft will cause the couplings to come loose in time.

[Ill.u.s.tration: Fig. 2620.]

Fig. 2620 represents a coupling composed of a cylindrical sleeve split longitudinally on one side, as at _d_; the bolts _c_ pa.s.s through the split. Diametrally opposite is another split pa.s.sing partly through as at _b_. A key is employed at right angles to the two splits as shown.

Here, again, the pressure on a shaft that is smaller than the other, of the two shafts coupled, will be mainly at one end, but separation of the shaft ends is provided against by means of two cylindrical pins on the ends of the key fitting into corresponding holes drilled in the shaft, as shown in the side elevation in the figure.

[Ill.u.s.tration: Fig. 2621.]

[Ill.u.s.tration: Fig. 2622.]

In Fig. 2621 is shown a coupling whose parts are shown in Fig. 2622. It consists of a cylindrical ring turned true on the outside and bored conical from each end to the middle of its length, as shown. The split cones are bored to receive the shaft and contain a keyway to receive a spline provided in the shaft ends, and are turned on the external diameter to fit the conical borings in the sleeve. Three square bolts pa.s.s through the split cones, which, being square, are prevented from rotating while their nuts are being screwed up.

To put the coupling together one split cone is pa.s.sed over the end of one shaft and the other over that of the other. The sleeve is then put between the ends of the shaft, the position of the shaft adjusted for length and the split cones pushed up into the sleeve; the bolts are then pa.s.sed through and screwed up. The forcing of the split cones into the conical borings of the sleeve causes the former (from being split) to close upon the shaft ends and grip them equally tight, notwithstanding any slight difference in the diameters of the shaft, there being left between the ends of the split cones sufficient s.p.a.ce to allow them to pa.s.s through the conical borings sufficiently to close upon the respective ends of the shafts; the pressure being parallel and equal on each shaft end, because when the cone has gripped the largest shaft the whole movement due to s.c.r.e.w.i.n.g up the nuts is transferred to the cone enveloping the smaller shaft, and by reason of the cones fitting, the closure of the holes in the cones is parallel, giving an even grip along the shaft end and an equal amount of grip to each shaft end.

To remove the coupling the bolts are removed, and the sleeve being moved endways the cones open from their spring and relieve the grip upon the shaft.

It is evident that in their pa.s.sage through the sleeve casing the cones will move with their axial lines true with the axial line of the casing; and it is equally evident that the taper on the cone accurately fitting the taper in the sleeve bore, the closure of the cone bores must be equal; while at the same time the pressure on the two cones upon the respective shaft ends must be equal, because it is the friction of the cone bores upon the shaft ends which equally resists the motion of both, while the pressure applied to the respective cones is derived from the same bolts, and hence is equal and simultaneous in its action.

To loosen this coupling for removal the bolts must be stacked back and a few blows on the exterior of the outer sh.e.l.l with a billet of wood may loosen the coupling; but if not, a wedge or a cold chisel may be driven in the splits of the cones to loosen them, but such wedge or chisel should not have contact with the sides of the split, either near the bore or near the perimeter, for fear of raising a burr.

[Ill.u.s.tration: Fig. 2623.]

[Ill.u.s.tration: Fig. 2624.]

In Fig. 2623 is shown a patent internal clamp coupling. It is formed of a cylindrical piece containing a pair of separate clamps, and between these clamps and the outer casing are four screws, two to each clamp; these screws are tapered so as to close the clamp when screwed up and release it when screwed outwards. The holes to receive the shaft ends are bored somewhat smaller than the shafts they are to fit, and the clamps opened to permit the easy insertion of the shaft ends by means of wedges A driven in the split B of each clamp, as shown in Fig. 2624.

The lower edge of the wedges should be slightly above the bore of the clamp to prevent the formation of a burr or projection of metal when the wedge is driven in. When placed upon the shaft ends and in proper position the wedges are removed and the clamp bore will have contact at and near the edges of the longitudinal split and on the opposite sides of the bore where the keyway is shown, but the pressure of the tape screws will spring the clamps on the side of the longitudinal splits, and increase the bearing area at those points.

The main features of this device are that though the bore be made a driving fit to the shaft, it can, by the employment of the wedges, be put on the shaft with the same ease as if it were an easy fit, while the clamps being separated by a transverse groove may open and close upon the shaft independently of each other, so as to conform separately to any variation in the diameters of the two shaft ends it couples. But it may be noted that since the circ.u.mference of each shaft end has a bearing along the line of the coupling bore diametrally opposite to the longitudinal splits, the shafts will not be held quite axially true one with the other unless there be as much difference in the diameters of the separate clamp bores as there is in the diameters of the shaft ends; because to hold two shafts of different diameters axially true one with the other the longitudinal planes of the two circ.u.mferences must not at any part of the circ.u.mferences form a straight line, as would be the case at that part of the coupling bore at and near the keyway.

It is to be noted, however, that this coupling is formed of one solid piece, and that the strain on the tightening bolts or screws is one of compression only, which tends to hold them firmly and prevent their coming loose.

If the workmanship of a plate coupling, such as in Fig. 2617, be accurately and well done, and the proportions of the same are of correct design, so that the strain placed on the same in keying and coupling it up does not distort it, the coupling and the shaft will run true, because the strain due to the key pressure will not be (if properly driven) sufficient to throw the coupling out of true. But the degree of accuracy in workmanship necessary to attain this end is greater than can be given to the work and compete in the market with work less accurately made, because the difference in the quality of the workmanship will not be discernible save to the most expert and experienced mechanic, and not to him even unless the pieces be taken apart for examination. If the bore of the coupling be true and smooth and of proper fit to the shaft the key pressure, if the key fits on its top and bottom, will not, as stated, be sufficient to throw the coupling out of true. It is true, however, that such pressure is exerted on one half the bore _of the coupling_ only, being the half bore opposite to the key. On the other diametral side of the coupling the strain due to the key is exerted on the top face of the key.

If, therefore, the key seats in the shaft and in the couplings are in line or parallel, and both therefore in the same plane, the strain due to the key may throw the coupling out of true to the amount that the key pressure may relieve the bore of the coupling (on the half circ.u.mference of the shaft of which the key is the centre) from contact or pressure with the shaft. As a result, the coupling may run to that extent out of true, but the shaft would run true nevertheless so long as the nature of the surfaces on the shaft and on the coupling bore was such that the key pressure caused no more compression or closer contact in the case of one half coupling than in the case of the other.

It is obvious that a plate coupling will require at least as much force to remove it from the shaft as it took to put it on, and sometimes, from rusting of the keys, &c., it requires more. If it be removed by blows it becomes damaged, and damage is likely to be also caused to the shaft, while the surfaces having to slide in contact under the pressure of the fit the surfaces abrade and compress, and the fit becomes impaired. But in couplings such as shown in Fig. 2621, the gripping pieces are relieved of pressure on the shaft by the removal of the bolts, and the removal of the coupling becomes comparatively easy.

The interchangeability of plate couplings is further destroyed by the fact already stated, that turned shafting is not, as a rule, of accurate gauge diameter, and the least variation in the pressure or fit of the coupling to its shaft is apt to cause a want of truth when the key bears on its top and bottom. The fit of the coupling to its shaft may be, it is true, relied on to do the main part of the driving duty, and the key fitting on the sides only may be a secondary consideration, but in proportion as the fit is relied on to drive, that fit must be tighter, and the difficulty of application and removal is increased.

Another and important disadvantage of the plate coupling in any form is that it necessitates the use of hangers open on one side to admit the shaft, because the couplings must be fitted upon the shaft before the same is erected and should not be removed after being fitted, as would be necessary to slide the end of the shaft through the bearing.

When plate couplings are constructed as in Fig. 2617, the removal of a section involves either the driving back of one-half of the coupling so that the other half will clear it, or else the moving endwise of the whole line to effect the same object.

With a plate coupling the half coupling on one end of the shaft must be removed when it is required to put an additional pulley on the shaft, unless, indeed, a split pulley be used, whereas with a clamp coupling, such as shown in Fig. 2621, the half coupling at each end may be slacked and moved back, one end of the shaft released, a solid pulley placed on the shaft and the coupling replaced, when it will run as true as before, and the pulley may be adjusted to its required position on the length of shafting.

It is to be remarked, however, that a well-made plate coupling, such as in Fig. 2618, makes a good and reliable permanent job that will not come loose under any ordinary or proper conditions.

[Ill.u.s.tration: Fig. 2625.]