An Introduction to Machine Drawing and Design Part 2

[Ill.u.s.tration: FIG. 7.]

[Ill.u.s.tration: FIG. 8.]

III. SCREWS, BOLTS, AND NUTS.

_Screw Threads._--The various forms of screw threads used in machine construction are shown in fig. 9. The _Whitworth_ =V= thread is shown at (_a_). This is the standard form of triangular thread used in this country. The angle between the sides of the =V= is 55, and one-sixth of the total depth is rounded off both at the top and bottom. At (_b_) is shown the _Sellers_ =V= thread, which is the standard triangular thread used by engineers in America. In this form of thread the angle between the sides of the =V= is 60, and one-eighth of the total depth is cut square off at the top and bottom. The _Square_ thread is shown at (_c_).

This form is princ.i.p.ally used for transmitting motion.

[Ill.u.s.tration: FIG. 9.]

Comparing the triangular and square threads, the former is the stronger of the two; but owing to the normal pressure on the =V= thread being inclined to the axis of the screw, that pressure must be greater than the pressure which is being transmitted by the screw; and therefore, seeing that the normal pressure on the square thread is parallel, and therefore equal to the pressure transmitted in the direction of the axis of the screw, the friction of the =V= thread must be greater than the friction of the square thread. In the case of the triangular thread there is also a tendency of the pressure to burst the nut. The _b.u.t.tress_ thread shown at (_e_) is designed to combine the advantages of the =V= and square threads, but it only has these advantages when the pressure is transmitted in one direction; if the direction of the pressure be reversed, the friction and bursting action on the nut are even greater than with the =V= thread, because of the greater inclination of the slant side of the b.u.t.tress thread. The angles of the square thread are frequently rounded to a greater or less extent to render them less easily damaged. If this rounding is carried to excess we get the _Knuckle_ thread shown at (_d_). The rounding of the angles increases both the strength and the friction.

EXERCISE 11: _Forms of Screw Threads._--Draw to a scale of three times full size the sections of screw threads as shown in fig. 9.

The pitch for the Whitworth, Sellers, and b.u.t.tress threads to be 3/8 inch, and the pitch of the square and knuckle threads to be 1/2 inch.

_Dimensions of Whitworth Screws._

+-----------------------------------+ Diameter Number Diameter of screw of threads at bottom per inch of thread +----------+------------+-----------+ 1/8 40 .093 3/16 24 .134 1/4 20 .186 5/16 18 .241 3/8 16 .295 7/16 14 .346 1/2 12 .393 5/8 11 .508 3/4 10 .622 7/8 9 .733 1 8 .840 1-1/8 7 .942 1-1/4 7 1.067 1-3/8 6 1.162 1-1/2 6 1.286 1-5/8 5 1.369 1-3/4 5 1.494 1-7/8 4-1/2 1.590 2 4-1/2 1.715 2-1/4 4 1.930 2-1/2 4 2.180 2-3/4 3-1/2 2.384 3 3-1/2 2.634 3-1/4 3-1/4 2.856 3-1/2 3-1/4 3.106 3-3/4 3 3.323 4 3 3.573 4-1/4 2-7/8 3.805 4-1/2 2-7/8 4.055 4-3/4 2-3/4 4.284 5 2-3/4 4.534 5-1/4 2-5/8 4.762 5-1/2 2-5/8 5.012 5-3/4 2-1/2 5.238 6 2-1/2 5.488 +-----------------------------------+

_Gas Threads_[1] (_Whitworth Standard_).

[1] Used for wrought-iron and bra.s.s tubes.

+-------------------------------------------------------------+ Diameter of Screw 1/8 1/4 3/8 1/2 5/8 3/4 1 +-------------------+-----+-----+-----+-----+-----+-----+------ Number of threads per inch 28 19 19 14 14 14 11 +-------------------------------------------------------------+

+-------------------------------------------------+ Diameter of Screw 1-1/4 1-1/2 1-3/4 2 +-------------------+-------+-------+-------+-----+ Number of threads per inch 11 11 11 11 +-------------------------------------------------+

_Representation of Screws._--The correct method of representing screw threads involves considerable trouble, and is seldom adopted by engineers for working drawings. For an explanation of the method see the author's Text-book on Practical Solid Geometry, Part II., problem 134. A method very often adopted on working drawings is shown in fig. 15; here the thin lines represent the points, and the thick lines the roots of the threads. At fig. 16 is shown a more complete method. The simplest method is ill.u.s.trated by figs. 10, 11, 13, and 14.

Here dotted lines are drawn parallel to the axis of the screw as far as it extends, and at a distance from one another equal to the diameter of the screw at the bottom of the thread.

[Ill.u.s.tration: FIG. 10.]

[Ill.u.s.tration: FIG. 11.]

_Forms of Nuts._--The most common form of nut is the hexagonal shown in figs. 10, 13, 14, 15, and 16; next to this comes the square nut shown in fig. 11. The method of drawing these nuts will be understood by reference to the figures; the small circles indicate the centres, and the inclined lines pa.s.sing through them the radii of the curves which represent the chamfered or bevelled edge of the nut. In all the figures but the first the chamfer is just sufficient to touch the middle points of the sides, and in these cases the drawing of the nut is simpler.

[Ill.u.s.tration: FIG. 12.]

[Ill.u.s.tration: FIG. 13.]

[Ill.u.s.tration: FIG. 14.]

_Forms of Bolts._--At (_a_), fig. 12, is shown a bolt with a square head and a square neck. If this form of bolt is pa.s.sed through a square hole the square neck prevents the bolt from turning when the nut is being screwed up. Instead of a square neck a snug may be used for the same purpose, as shown on the cup-headed bolt at (_b_). The snug fits into a short groove cut in the side of the hole through which the bolt pa.s.ses.

At (_a_) the diagonal lines are used to distinguish the flat side of the neck from the round part of the bolt above it. At (_c_) is shown a tee-headed bolt, and at (_d_) an eye-bolt. Fig. 13 represents a hook bolt. A bolt with a countersunk head is shown in fig. 11. If the countersunk head be lengthened so as to take up the whole of the unscrewed part of the bolt, we get the taper bolt shown in fig. 14, which is often used in the couplings of the screw shafts of steams.h.i.+ps.

The taper bolt has the advantage of having no projecting head, and it may also be made a tight fit in the hole with less trouble than a parallel bolt. Bolts may also have hexagonal heads.

[Ill.u.s.tration: FIG. 15]

[Ill.u.s.tration: FIG. 16]

_Studs_, or _stud bolts_, are shown in figs. 15 and 16; that in fig. 15 is a _plain stud_, while that in fig. 16 has an intermediate collar forged upon it, and is therefore called a _collared stud_.

_Proportions of Nuts and Bolt-heads._--In the hexagonal nut the diameter D across the flats is 1-1/2_d_ + 1/8, where _d_ is the diameter of the bolt. The same rule gives the width of a square nut across the flats. A rule very commonly used in making drawings of hexagonal nuts is to make the diameter D, across the angles equal to 2_d_. H, the height of the nut, is equal to the diameter of the bolt. In square and hexagonal headed bolts the height of the head varies from _d_ to 2/3_d_; the other dimensions are the same as for the corresponding nuts.

_Washers_ are flat, circular, wrought-iron plates, having holes in their centres of the same diameter as the bolts on which they are used. The object of the washer is to give a smooth bearing surface for the nut to turn upon, and it is used when the surfaces of the pieces to be connected are rough, or when the bolt pa.s.ses through a hole larger than itself, as shown in fig. 10. The diameter of the washer is a little more than the diameter of the nut across the angles, and its thickness about 1/8 of the diameter of the bolt.

EXERCISE 12.--Draw, full size, the views shown in fig. 10 of an hexagonal nut and washer for a bolt 1-1/4 inches in diameter. The bolt pa.s.ses through a hole 1-3/4 1-1/4. All the dimensions are to be calculated from the rules which have just been given.

EXERCISE 13.--Draw, full size, the plan and elevation of the square nut and bolt with countersunk head shown in fig. 11, to the dimensions given.

EXERCISE 14.--Draw, full size, the elevation of the hook bolt with hexagonal nut shown in fig. 13 to the dimensions given, and show also a plan.

EXERCISE 15.--Draw, to a scale of 4 inches to a foot, the conical bolt for a marine shaft coupling shown in fig. 14. All the parts are of wrought iron.

EXERCISE 16.--Fig. 15 is a section of the mouth of a small steam-engine cylinder, showing how the cover is attached; draw this full size.

EXERCISE 17.--Fig. 16 shows the central portion of the india-rubber disc valve which is described on page 68. A is the central boss of the grating, into which is screwed the stud B, upon which is forged the collar C. The upper part of the stud is screwed, and carries the guard D and an hexagonal nut E. F is the india-rubber. The grating and guard are of bra.s.s. The stud and nut are of wrought iron. Draw full size the view shown.

_Lock Nuts._--In order that a nut may turn freely upon a bolt, there is always a very small clearance s.p.a.ce between the threads of the nut and those of the bolt. This clearance is shown exaggerated at (_a_), fig.

17, where A is a portion of a bolt within a nut B. Suppose that the bolt is stretched by a force W. When the nut B is screwed up, the upper surfaces of the projecting threads of the nut will press on the under surfaces of the threads of the bolt with a force P equal and opposite to W, as shown at (_b_), fig. 17. When in this condition the nut has no tendency to slacken back, because of the friction due to the pressure on the nut. Now suppose that the tension W on the bolt is momentarily diminished, then the friction which opposes the turning of the nut may be so much diminished that a vibration may cause it to slacken back through a small angle. If this is repeated a great many times the nut may slacken back so far as to become useless.

[Ill.u.s.tration: FIG. 17.]

[Ill.u.s.tration: FIG. 18.]

A very common arrangement for locking a nut is shown at (_a_), fig. 18.

C is an ordinary nut, and B one having half the thickness of C. B is first screwed up tight so as to act on the bolt, as shown at (_b_), fig.

17. C is then screwed on top of B. When C is almost as tight as it can be made, it is held by one spanner, while B is turned back through a small angle with another. The action of the nuts upon the bolt and upon one another is now as shown at (_b_), fig. 18. It will be seen that the nuts are wedged tight on to the bolt, and that this action is independent of the tension W in the bolt. The nuts will, therefore, remain tight after the tension in the bolt is removed.

It is evident that if the nuts are screwed up in the manner explained, the outer nut C will carry the whole load on the bolt; hence C should be the thicker of the two nuts. In practice, the thin nut, called the lock nut, is often placed on the outside, for the reason that ordinary spanners are too thick to act on the thin nut when placed under the other.

Another very common arrangement for locking a nut is shown in fig. 19. A is the bolt and B the nut, the lower part of which is turned circular. A groove C is also turned on the nut at this part. The circular part of the nut fits into a circular recess in one of the parts connected by the bolt. Through this part pa.s.ses a set screw D, the point of which can be made to press on the nut at the bottom of the groove C. D is turned back when the nut B is being moved, and when B is tightened up, the set screw is screwed up so as to press hard on the bottom of the groove C. The nut B is thus prevented from slackening back. The screw thread is turned off the set screw at the point where it enters the groove on the nut.

[Ill.u.s.tration: FIG. 19]

The use of the groove for receiving the point of the set screw is this: The point of the set screw indents the nut and raises a bur which would interfere with the free turning of the nut in the recess if the bur was not at the bottom of a groove. Additional security is obtained by drilling a hole through the point of the bolt, and fitting it with a split pin E.

Locking arrangements for nuts are exceedingly numerous, and many of them are very ingenious, but want of s.p.a.ce prevents us describing them. We may point out, however, that many very good locking arrangements have the defect of only locking the nut at certain points of a revolution, say at every 30. It will be noticed that the two arrangements which we have described are not open to this objection.

EXERCISE 18.--Draw, full size, a plan, front elevation, and side elevation of the arrangement of nuts shown in fig. 18, for a bolt 7/8 inch diameter.

EXERCISE 19.--Draw the plan and elevation of the nut and locking arrangement shown in fig. 19. Make also an elevation looking in the direction of the arrow. Scale 6 inches to a foot.