Note: Descriptions are shown in the official language in which they were submitted.
~o~6Q3
SPEC IFICATION
This invention relates to the driven-type fastener art,
and is more particularly concerned with new and irnproved tapered
6crews or bolts.
Lag screws are a type of screw or bolt commonly used
5 because of their convenience, in particular where it would be
difficult to fasten a bolt by means of a nut or where a nut on the
surface would be objectionable. Commonly available lag screws
range from about 0. 2 to 1-1/4 inches in diameter and from 1 to
16 inches in length. The length of the threaded portion of the
10 screw varies in the conventional lag screws from about 3/4 inch
in ~e shortest screws to about half the length for all lengths
greater than 10 inches.
For best holding results lag screws should be started in
prèbored holes of the proper size wherein the lead-in portion
15 of the hole should be of the same diameter as the unthreaded
portion of the shank adjacent to its head end and the diameter
of the hole to receive the threaded part should be of a smaller
diameter depending upon the density of the wood into which the
screw is to be driven. Then the lag screw should be inserted
20 by turning with a wrench and not by driving with a hammer.
However, in actual practice, lag screws are commonly driven
into the wood with a hammer to the full length of the thread
on the lag screws and then turned with a wrench.
Inasmuch as conventional lag screws have straight shanks,
25 that is on a common diarneter from adjacent the head or un-
threaded portion to the tip of the screw, the hammering in of
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the screw substantially shatters the wood fibers along the bore
formed by the screw, with a generally shearing, tearing action
to the extent that the screw is driven. Generally the driving
is effected to the full length of the threaded portion, then the
screw is threaded in the remainder of the way, but this results
in only the last three or four turns of the threads, depending on
pitch and the length of the tip of the screw, will turn into
unshattered fibers of the wood to provide substantially all of
the holding power. Except for those three or four turns adjacent
the tip, the remaining extent of the threads of the screws are
virtually ineffective in retaining the screws because of the
cxtremely damaged condition of the wood fibers. Another disad-
vantage of the conventional lag screws is that because of the
straight shanks mere is a tendency, especially with the smaller
siæs of lag screws, to buckle while being harnmered in. They
also tend to fail during wrench torquing.
It is therefore the object of the present invention to
overcome the disadvantages, deficiencies, inefficiencies, short-
comings, and problems encountered with prior screws or bolts
and to provide new-and improved lag screws which:
a) will employ less material than standard screws or
bolts of the same type,
b) will require substantially less impact energy to install,
c) will have substantially greater withdrawal holding
strength than the conventional screws or bolts,
d) will drive more efficiently into the wood under head
imposed torquing, and
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105'~ ;3
e) will have substantially greater lateral resistance to
loads in thc smallcr sizes of the screws wllile
maintaining at least as great efficiency in lateral
loading as larger sizes of conv~ntional screws
or bolts.
Another object of the invention is to provide new and
S irnproved screws or bolts equipped to substantially resist unin-
tentional turnout.
According to features of the invention, a screw or bolt
adapted to be driven into a wooden member by combination of
long~tudinal impact and torque driving forces comprises an
elongated shank having a head at one end and a penetrating tip
at the opposite end. A limited length unthreaded neck section
of the shank is contiguous to the head and a major length threaded
section extends from the neck section to the tip. A spiral thread
extends along the threaded section from the neck section to the
tip and the thread has a first thread surface facing generally
toward the head and a second thread surface facing generally
toward the tip. The surfaces meet at a root diarneter and
converge to a pitch diameter at the crest of the thread. The
conve~gent angle of the first su~ace is substantially less relative
to a planè normal to the shank axis than the angle of the second
sur~ace relative to said plane, and the shank is tapered toward
~e tip throughout at least the major extent of the threaded
sect~. The surface of the thread of the screw or bolt which
iaces toward the head end is disposed at a convergent angle
toward the opposite surface at from 20 to 25 relative to a
plane normal to the shank axis and the opposite surface is angled
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at 65 to 70 to said plane. The depth of the thread is substan-
tially uniform throughout the length of the tapered shank of the
screw or bolt, and the taper of the shank is in a range from
1-1/2 to 2-1/2 from a cylindrical projection from the neck
section of the screw or bolt.
Still another feature of the invention resides in a minor
portion of the threaded section of the screw or bolt adjacent to the
neck section of the shank being of substantially the same cylindri-
cal diameter as the neck section.
Yet another feature of the invention comprises a turnout
lock in the threaded section of the shank.
It is also a feature of the invention to provide a new and
improved method of driving into a wooden member a screw or bolt
e~nbodying the foregoing features, by applying longitudinal impact
driving force through the head toward the tip of the screw and
1:hereby effecting penetration of the threaded section of the screw
into the wooden member up to substantially the neck section and
forming a tapered bore in the wooden member substantially
conforming to the tapered shank of the screw, and thereafter
applying torque to the screw through said heàd and driving the
screw substantially the remainder of its length into the wooden
member by spiral cutting of said thread into the wall of the bore
along substantially the entire length of the threaded section.
The dlreaded section may include minor cylindrical area con-
tîguous to the neck, and in wrench-torquing the screw the
cylindrical threaded area is driven into the adjacent end portion
of the bore formed by the tapered extent of the shank. The
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shank may be provided with a turnout lock which automatically
effects a locked relationship with the fibers of the wooden member
into which the screw or bolt is driven.
Other objects, features and advantages of the invention
S will be readily apparent from the following description of repre-
sentative embodiments thereof taken in conjunction with the
accompanying drawing although variations and modifications may
be effected without departing from the spirit and scope of the
novel concepts embodied in the disclosure and in which:
Fig. 1 is a sectional elevational view, showing a screw
or bolt embodying features of the invention driven into a wooden
member.
Fig. 2 is an enlarged sectional view similar to Fig. 1
but showing the screw or bolt in vertical diametral section.
Fig. 3 is a top plan view taken substantially along the
....
lines III-III of Fig. 1.
Fig. 4 is a transverse sec~ional detail view through the
shank of the screw or bolt of Fig. 1, and
Fig. 5 is a sectional view similar to Fig. 4 but showing
a turnout lock feature for the screw.
A screw or bolt 10 (Figs. 1-3), exemplified by a lag
~; screw, and embodying features of the invention is depicted as
driven into a wooden member 11 such as a utility pole for
securing a metal brace 12. It will be understood that the
wooden mernber 11 and the attached member 12 are merely
representative of members that may be secured together by means
o~ ~he lag screw 10.
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As is customary with lag screws, at one end the screw
10 h~s a fairly massive driving head 13 provided with suitable
wrench faces for applying driving torque to the screw by means
of a suitable wrench. Centrally from the head 13 extends a
S sha~; 14 of suitable length and diameter or gauge provided at its
oppo6ite end with a pointed penetrating tip which may be of the
generally girnlet type. Adjacent to the head 13, the shank 14
may, as is customary, have a straight cylindrical section com-
prising about 1/6 of the total length of the shank and the re-
10 main~ng section of the shank from the cylindrical section 17 tothe tip 15 being threaded as by means of a single thread 18 of
suitable pitch for the size and intended use of the screw.
A substantially new and improved structure of the
threaded portion of the shank 14 and of the thread 18 not only
15 facilitates substantially improved driving of the screw 10 into the
wood~n member 11, and improves the torque force resistance and
pullffut resistance of the screw, but also affords desirable
econ~nies in material from which the screw- is made. To this
end~ the threaded length of the shank 14 is uniforrnly tapered
20 throughout its major extent from adjacent the head end to the tip
end o~ the screw. A substantially uniform depth of the thread 18
has ~een found desirably with the pitch and root diarneters
parallel to each other and to the shank taper.
Further structural and functional improvement in the
25 screu 10 is attained by having the opposite surfaces of the
thread 18 joined at the root diameter and converging at the
lOS'~ 3
crest or pitch diameter of the thread. Especially desirable
results are obtained where the surface of the thread 18 which
faces generally toward the head 13 in on a 20 to 25 angle to
a plane normal to the longitudinal axis of the screw and the
angle of the surface of the thread which faces generally toward
the tip 15 is about 65 to 70 relative to such plane, as
represented in Fig. 2.
Additional improvement in the screw 10 resides in having
the cylindrical neck section 17 as short as practicable such as
on the order of 1/6 the length of the shank 14, represented by
the double headed arrow 19 in Fig. 1. About an equal extent
- sectîon of the threaded portion of the shank 14 contiguous to the
neck 17 is also desirably of substantially cylindrical form as
represented by the double headed arrow 20 in Fig. 1. The
remainder of the threaded section of the shank 14 from the
cylindrical section 20 to the tip 15 is tapered at 1-1/2 to 2
.
relative to the cylindrical projection of the neck 17.
Although excellent results are obtained where the screw
thread 18 is continuous and unbroken from one end to the other,
~reatly improved resistance to turn out of the screw 10 after it
has been fully driven into the wooden member 11 is attained
where the screw is provided with a turnout lock 21 as depicted
in Fig. S and in dash-line in Fig. 1. Such lock is formed in
the thread 18 as by partially trimming off a narrow longitudinal
strip area 22 along the length of at least the tapered portion of
the threaded section of the shank 14 and generally tangent to the
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root diameter of the thread 18. The strip area 22 joins a
coe~tensive diametral shoulder 23 facing in the turnout direction
on de screw, that is away from the pitch of the screw.
Thereby, the shoulder 23 presents no interference to torque
driv~ng of the screw, nor any interference to longitudinal hammer
dri~ring of the screw, and the generally tangent area 22 provides
a relief cam surface facilitating torque driving without any
catching on the wood fibers of the member 11 into which the
screw is driven. After the screw has been driven into the wood,
the shoulder 23 engages with the wood fibers and strongly resists
turr~ut.
~or driving the screw into the wooden member 11, it may
be simply spotted at the point into which it is to be driven and
then harnmered in until only the neck portion 19 remains exposed
substantially as indicated in dash outline in Fig. 1. Then by
applying wrench generated torque to the head 13, the screw may
be driven fully home into the wood member 11. If preferred,
a starting hole of about the length of the neck portion 19 may be
~ drilled into the wood member 11 before driving in of the screw
; 20 sha~ A starting hole is especially desirable for hard wood
and where it is particularly desirable to avoid splitting of the
wood during driving in of the lag screw.
Tests have demonstrated ~ha~ because of the tapered shank
14, and the convergently angular form of the screw thread 18,
as described, greatly improved driving efficiency and retaining
efficiency are attained with the lag screw 10. The tapered form
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03
of the shank 14, and the angular shape of the tip-facing surface
of the thread 18 substantially facilitate hammering in of the
screw as compared to conventional straight lag screws.
After the screw 10 has been fully set, tests have shown
about 13% greater resistance to pullout in comparison to an
equivalent straight lag scre~. This important result is
apparently attained because after the tapered lag screw 1~ has
been partially driven into the wood to the usual extent that
conventional lag screws are driven, such as to the point where
only the neck portion 17 is exposed, there is minimum tearing
damage to the wood into which the screw is driven., There has
been observed at least some voluntary turning or screwing in
of the screw during axial impact driving resuiting apparently
because of the substantial exposure of the high angle thread
lS surface. The wood fibers are to a substantial extent turned
toward the tip end of the screw by the high angle surface of the
thread which faces toward the tip 15. This action takes place
along the entire tapered bore formed in the wood by and
generally conforming to the tapered portion of the shank 14.
Then, as the screw is driven to its final depth by application of
torque force tO the head 13, the thread 18 cuts and retainingly
, engages in a complementary spiral groove 24 in fresh otherwise
unsevered wood along the self-formed tapered bore provided by
the axial impact initial driving of the screw. Such cutting of the
spiral groove 24 into fresh unsevered wood occurs along the
entire len~th of the thread 18 including that, part of the thread
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which is on the tapered portion of the shank as well as that
part of the thread which is on the generally cylindrical portion
20. The wood fibers compressibly displaced in the driving-in
of the screw expand into the groove of the thread and thrust
generally toward the low angle thread surface which faces toward
the head end of the screw, substantially as shown in Figs. 1
and 2. As a result, the wood fibers into which the thread 18
is turned afford maximum resistance to pulling out of the screw.
By virtue of the tapered form of the shank 14 and the
sub~tantially uniform depth of the thread 18, it will be noted that
the shar~c 14 diminishes quite gradually in cross-sectional mass
from the neck toward the tip end, thus providing maximum
resi~tance not only to axial driving in of the screw but also to
torque stresses imposed on the screw during final wrenching in
of the screw. Such greater mass in the head end portion of the
threaded portion of the shank 14 is also beneficial in lateral load
resistance on the especially smaller gauges of lag screws down
to a diameter of about 1/4 inch at the head end portion of the
sharik. As much as 13% improvement in lateral load resistance
has been experienced with lag screws embodying the present
invention.
An additional important advantage of lag screws according
to the present invention resides in the substantially less material
required because of the advantageous tapered construction, it
having been found ~hat as much as 14% less steel per screw in
comparison to standard lag screws of the same general size will
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suffice and p~rmit attainment of the new and improved results
already described.
Atthough the embodiment of the invention selected for
~llustration comprises a lag screw or bolt, other screw
fasteners in which the invention may be advantageously utilized
comprise wire holders, drive hooks, pole steps, washer head
lag studs, cross arm pins, clevis wire holders or eye bolts,
and the like.
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