_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 deg., 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 deg., 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 principally used for transmitting motion.
[Illustration: 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
_Buttress_ 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 buttress 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 buttress 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 brass 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 illustrated 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.
[Illustration: FIG. 10.]
[Illustration: 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 passing 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.
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
[Illustration: 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 passed 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 passes.
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 steamships.
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.
[Illustration: FIG. 15]
[Illustration: 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 passes 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 passes through a hole 1-3/4 x 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 brass. 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 space 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.
[Illustration: FIG. 17.]
[Illustration: 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 passes 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.
[Illustration: 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 space 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 deg.. 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.