Modern Machine-Shop Practice Part 31

[Ill.u.s.tration: Fig. 404.]

A plain tap bolt should be turned up along its body, because if out of true the hole it pa.s.ses through must be made large enough to suit the eccentricity of the bolt, or else a portion of the wrench pressure will be expended in rotating the bolt in the hole instead of being expended solely in s.c.r.e.w.i.n.g the bolt farther into the work.

It is obvious therefore, that if a tap bolt be left black the hole it pa.s.ses through must be sufficiently large to make full allowance for the want of truth in the bolt. For the same reasons the holes for tapped bolts require to be tapped very true.

Black studs possess an advantage (over tap bolts) in this respect, inasmuch as that if the holes are not tapped quite straight the error may be to some extent remedied by s.c.r.e.w.i.n.g them fully home and then bending them by hammer blows.

Nuts are varied in form to suit the nature of the work. For ordinary work, as upon bolts, their shape is usually made to conform to the shape of the bolt head, but when the nut is exposed to view and the bolt head hidden, the bolt end and the nut are (for finished work) finished while the bolt heads are left black.

[Ill.u.s.tration: Fig. 405.]

[Ill.u.s.tration: Fig. 406.]

The most common form of hexagon nut is shown in Fig. 405, the upper edge being chamfered off at an angle of about 40. In some cases the lower edge is cut away at the corners, as in Fig. 405 at A, the object being to prevent the corners of the nut from leaving a circle of bearing marks upon the work, but this gives an appearance at the corners that the nut does not bed fair. Another shape used by some for the end faces of deep nuts, that is to say, those whose depth exceeds the diameter of the bolt, is shown in Fig. 406. Nuts of extra depth are used when, from the nut being often tightened and released, the thread wear is increased, and the extra thread length is to diminish the wear.

To avoid the difficulty of having some of the bolt ends project farther through some nuts than others on a given piece of work, as is liable to occur where the f.l.a.n.g.es to be bolted together are not turned on all four radial faces, the form of nut shown in Fig. 407 is sometimes employed, the thread in the nut extending beyond the bolt end.

[Ill.u.s.tration: Fig. 407.]

[Ill.u.s.tration: Fig. 408.]

As an example of the application of this nut, suppose a cylinder cover to be held by bolts, then the cylinder f.l.a.n.g.e not being turned on its back face is usually of unequal thickness; hence to have the bolt ends project equally through the nuts, each bolt would require to be made of a length to suit a particular hole, and this would demand that each hole and bolt be marked so that they may be replaced when taken out, without trying them in their places. Another application of this nut is to make a joint where the threads may be apt to leak. In this case the mouth of the hole is recessed and coned at the edge; the nut is chamfered off with a similar cone, and a washer W, Fig. 408, is placed beneath the nut to compress and conform to the coned recess; thus with the aid of a cement of some kind, as red or white lead (usually red lead), a tight joint may be made independent of the fit of the threads.

When the hole through which the bolt pa.s.ses is considerably larger in diameter than the bolt, the f.l.a.n.g.e nut shown in Fig. 409 is employed, the f.l.a.n.g.e covering the hole. A detached washer may be used for the same purpose, providing that its hole fit the bolt and it be of a sufficient thickness to withstand the pressure and not bend or sink into the hole.

[Ill.u.s.tration: Fig. 409.]

[Ill.u.s.tration: Fig. 410.]

Circular nuts are employed where, on account of their rotating at high speed, it is necessary that they be balanced as nearly as possible so as not to generate unbalanced centrifugal force. Fig. 410 represents a nut of this kind: two diametrically opposite flat sides, as A, affording a hold for the wrench. Other forms of circular nuts are shown in Figs. 411 and 412. These are employed where the nuts are not subject to great strain, and where lightness is an object.

[Ill.u.s.tration: Fig. 411.]

[Ill.u.s.tration: Fig. 412.]

That in Fig. 411 is pierced around its circ.u.mference with cylindrical holes, as A, B, C, to receive a round lever or rod or a wrench, such as shown in Fig. 459.

That shown in Fig. 412 has slots instead of holes in its circ.u.mference, and the form of its wrench is shown in Fig. 461.

When nuts are employed upon bolts in which the strain of the duty is longitudinal to the bolt, and especially if the direction of motion is periodically reversed, and also when a bolt is subject to shocks or vibrations, a single nut is liable to become loose upon the bolt, and a second nut, termed a check nut, jamb nut, or safety nut, becomes necessary, because it is found that if two nuts be employed, as in Fig.

413, and the second nut be screwed firmly home against the first, they are much less liable to come loose on the bolt.

Considerable difference of practice exists in relation to the thickness of the two nuts when a check nut is employed. The first or ordinary nut is screwed home, and the second or check nut is then screwed home. If the second nut is screwed home as firmly as the first, it is obvious that the strain will fall mainly on the second. If it be screwed home more firmly than the first, the latter may be theoretically considered to be relieved entirely of the strain, while if it be screwed less firmly home, the first will be relieved to a proportionate degree of the strain. It is usual to screw the second home with the same force as applied to the first, and it would, therefore, appear that the first nut, being relieved of strain, need not be so thick as the first, but it is to be considered that, practically, the first nut will always have some contact with the bolt threads, because from the imperfections in the threads of ordinary bolts the area and the force of contact is not usually the same nor in the same direction in both nuts, unless both nuts were tapped with the same tap and at about the same time.

When, for example, a tap is put into the tapping machine, it is at its normal temperature, and of a diameter due to that temperature, but as its work proceeds its temperature increases, notwithstanding that it may be freely supplied with oil, because the oil cannot, over the limited area of the tap, carry off all the heat generated by the cutting of a tap rotated at the speeds usually employed in practice. As a result of this increase of temperature, we have a corresponding increase in the diameter of the tap, and a variation in the diameter of the threads in the nuts. The variation in the nuts, however, is less than that in the tap diameter, because as the heated tap pa.s.ses through the nut it imparts some of its heat to the nut, causing it also to expand, and hence to contract in cooling after it has been tapped, and, therefore, when cold, to be of a diameter nearer to that of the tap.

Furthermore, as the tap becomes heated it expands in length, and its pitch increases, hence here is another influence tending to cause the pitches of the nut threads to vary, because although the temperature of the tap when in constant use reaches a limit beyond which, so long as its speed of rotation is constant, it never proceeds; yet, when the tap is taken from the machine to remove the tapped nuts which have collected on its shank, and it is cooled in the oil to prevent it from becoming heated any more than necessary, the pitch as well as the diameter of the tap is reduced nearer to its normal standard.

[Ill.u.s.tration: Fig. 413.]

[Ill.u.s.tration: Fig. 414.]

So far, then, as theoretical correctness, either of pitch or diameter in nut threads, is concerned, it could only be attained (supposing that the errors induced by hardening the tap could be eliminated) by employing the taps at a speed of rotation sufficiently slow to give the oil time to carry off all the heat generated by the cutting process. But this would require a speed so comparatively slow as not to be commercially practicable, unless followed by all manufacturers. Practically, however, it may be considered that if two nuts be tapped by a tap that has become warmed by use, they will be of the same diameter and pitch, and should, therefore, have an equal area and nature of contact with the bolt thread, supposing that the bolt thread itself is of equal and uniform pitch. But the dies which cut the thread upon the bolt also become heated and expanded in pitch. But if the temperature of the dies be the same as that of the tap, the pitches on both the bolt and in the nut will correspond, though neither may be theoretically true to the designated standard.

In some machines for nut tapping the tap is submerged in oil, and thus the error due to variations of temperature is practically eliminated, though even in this case the temperature of the oil will gradually increase, but not sufficiently to be of practical moment.

Let it now be noted that from the hardening process the taps shrink in length and become of finer pitch, while the dies expand and become of coa.r.s.er pitch, and that this alone precludes the possibility of having the nut threads fit perfectly to those on the bolt. It becomes apparent, then, that only by cutting the threads in the lathe, and with a single-toothed lathe tool that can be ground to correct angle after hardening, can a bolt and nut be theoretically or accurately threaded.

Under skilful operation, however, both in the manufacture of the screw-cutting tools and in their operation, a degree of accuracy can be obtained in tapped nuts and die-threaded bolts that is sufficient with a single nut for ordinary uses, but in situations in which the direction of pressure on the nut is periodically reversed, or in which it is subject to shocks or vibrations, the check nut becomes necessary, as before stated.

An excellent method of preventing a nut from slackening back of itself is shown in the safety nut in Fig. 414; it consists of a second nut having a finer thread than the first one, so that the motion of the first would in uns.c.r.e.w.i.n.g exceed that of the second, hence the locking is effectually secured.

[Ill.u.s.tration: Fig. 415.]

Work may be very securely fastened together by the employment of what are called differential screws, the principle of whose action may be explained with reference to Fig. 415, which is extracted from "Mechanics." It represents a piston head and piston rod secured together by means of a differential screw nut. The nut contains an internal thread to screw on the rod, and an external one to screw into the piston head, but the internal thread and that on the rod differ from the external one, and that in the head by a certain amount, as say one tenth of the pitch. The nut itself is furnished with a hexagonal head, and when screwed into place draws the two parts together with the same power as a screw having a pitch equal to the difference between the two pitches.

[Ill.u.s.tration: Fig. 416.]

When putting the parts together the nut is first screwed upon the rod B.

The outside threads are then entered into the thread in the piston C, and by means of a suitable wrench the nut is screwed into the proper depth. As shown in the engraving, the nut goes on to the rod a couple of threads before it is entered in the piston. The tightening then takes place precisely as though the nut had a solid bearing on the piston and a fine thread on the rod, the pitch of which is equal to the difference between the pitches of the two threads. Fig. 416 shows its application to the securing of a pump plunger upon the end of a piston-rod. In this case, as the rod does not pa.s.s through the nut, the latter is provided with a cap, which covers the end of the rod entirely.

[Ill.u.s.tration: Fig. 417.]

The principle of the differential screw may be employed to effect very fine adjustments in place of using a very fine thread, which would soon wear out or wear loose. Thus in Fig. 417 is shown the differential foot screws employed to level astronomical instruments. C D is a foot of the instrument to be levelled. It is threaded to receive screw A, which is in turn threaded to receive the screw B, whose foot rests in the recess or cup in E F. Suppose the pitch of screw A is 30 per inch, and that of B is 40, and we have as follows. If A and B are turned together the foot C D is moved the amount due to the pitch of A. If B is turned within a the foot is moved the amount due to the pitch of B. If A is turned the friction of the foot of B will hold B stationary, and the motion of C D will equal the difference between the pitches of the threads of A and B.

Thus one revolution of A forward causes it to descend through C D 1/30 inch (its pitch), tending to raise C D 1/30 inch. But while doing this it has screwed down upon the thread of B 1/40 inch (the pitch of B) and this tends to lower C D, hence C D is moved 1/120 inch, because 1/30-1/40 = 1/120.

To cause a single nut to lock itself and dispense with the second or jamb nut, various expedients have been employed. Thus in Fig. 418 is shown a nut split on one side; after being threaded the split is closed by hammer blows, appearing as shown in the detached nut. Upon s.c.r.e.w.i.n.g the nut upon the bolt the latter forces the split nut open again by thread pressure, and this pressure locks the nut. Now there will be considerable elasticity in the nut, so that if the thread compresses on its bearing area, this elasticity will take up the wear or compression and still cause the threads to bind. Sometimes a set screw is added to the split, as in Fig. 419, in which case the split need not be closed with the hammer.

Another method is to split the nut across the end as shown in Fig. 420, tapping the nut with the split open, then closing the split by hammer blows. Here as before the nut would pa.s.s easily upon the bolt until the bolt reached the split, when the subsequent threads would bind. In yet another design, shown in Fig. 421, four splits are made across the end, while the face of the nut is hollowed, so that a flat place near each corner meets the work surface. The pressure induced on these corners by s.c.r.e.w.i.n.g the nut home is relied on in this case to spring the nut, causing the thread at the split end to close upon and grip the bolt thread.

Check nuts are sometimes employed to lock in position a screw that is screwed into the work, thus screws that require to be operated to effect an adjustment of length (as in the case of eccentric rods and eccentric straps) are supplied with a check nut, the object being to firmly lock the screw in its adjusted position.

The following are forms of nuts employed to effect end adjustments of length, or to prevent end motion in spindles or shafts that rotate in bearings.

Fig. 422 shows two cylindrical check nuts, the inner one forming a f.l.a.n.g.e for the bearing. The objection to this is that in s.c.r.e.w.i.n.g up the check nut the adjustment of the first nut is liable to become altered in s.c.r.e.w.i.n.g up the second one, notwithstanding that the first be held by a lever or wrench while the second is screwed home.

Another method is to insert a threaded feather in the adjustment nut and having at its back a set screw to hold the nut in its adjusted position, as in Fig. 423. In this case the protruding head of the set screw is objectionable. In place of the feather the thread of the spindle may be turned off and a simple set screw employed, as in Fig. 424; here again, however, the projecting set screw head is objectionable. The grip of an adjustment nut may be increased by splitting it and using a pinching or binding screw, as in Fig. 425, in which case the bore of the thread is closed by the screw, and the nut may be countersunk to obviate the objection of a projecting head. For adjusting the length of rods or spindles a split nut with binding screws, such as shown in Fig. 426, is an excellent and substantial device. The bore is threaded with a right-hand thread at one end and a left-hand one at the other, so that by rotating the nut the rod is lengthened or shortened according to the direction of rod rotation. Obviously a clamp nut of this cla.s.s, but intended to take up lost motion or effect end adjustment, may be formed as in Fig. 427, but the projecting ears or screw are objectionable.

[Ill.u.s.tration: Fig. 429.]

Where there is sufficient length to admit it an adjustment nut, such as in Fig. 428, is a substantial arrangement. The nut A is threaded on the spindle and has a taper threaded split nut to receive the nut B. Nut A effects the end adjustment by s.c.r.e.w.i.n.g upon the spindle, and is additionally locked thereon by s.c.r.e.w.i.n.g B up the taper split nut, causing it to close upon and grip the spindle.

[Ill.u.s.tration: Fig. 430.]

Lost motion in square threads and nuts may be taken up by forming the nut in two halves, A and B, in Fig. 429 (A being shown in section) and securing them together by the screws C C. The lost motion is taken up by letting the two halves together by filing away the joint face D of either half, causing the thread in the nut to bear against one side only of the thread of the screw. The same end may be accomplished in nuts for [V]-shaped threads by forming the nut either in two halves, as shown in Fig. 430, in which A is a cap secured by screws B, the joint face C being filed away to take up the lost motion. Or the nut may be in one piece with the joint C left open, the screws B crossing the nut upon the screw by pressure. In this case the nut closes upon the circ.u.mference of the thread, taking up the wear by closing upon both sides of the thread instead of on one side only as in the case of the square thread.

[Ill.u.s.tration: Fig. 431.]

[Ill.u.s.tration: Fig. 432.]