CA1052603A - Tapered lag screw or bolt - Google Patents

Abstract

A B S T R A C T
The tapered screw or bolt of this invention differs from conventional substantially straight screws by having at least the major threaded portion of the screw shank uniformly tapered to the tip with the thread of substantially uniform depth throughout its length and the opposite surfaces of the thread both sloping from the crest and meeting at a root juncture. A minor length area of the threaded portion of the shank adjacent the screw head may be substantially straight to the upper end of the tapered portion. A turnout lock may be provided on the threads.

Description

~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 ~Q5;~03 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 . ;
"

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 - ~ _3 i(~5`~t;03 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 ~5'~03 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.
.

1~15'~03 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 `\

~OS;Z~iO3 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 :` :

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 ~05'~03 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 , -10-105;~03 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.

.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A tapered type screw or bolt adapted to be driven into a wooden member by combination of longitudinal impact and torque driving forces, and comprising; an elongated shank having a head at one end and a penetrating conical tip at the opposite end; a limited length unthreaded neck section of the shank contiguous to the head and a major length threaded frusto-conical section extending from the neck section to the conical tip; a spiral thread extending along said threaded frusto-conical section from said neck to said tip and said thread having a first thread surface facing generally toward said head and a second thread surface facing generally toward said conical tip, said surfaces meeting at a root diameter and converging to a pitch diameter at the crest of the thread; the convergent angle of said first surface facing toward the head being about 20° to 25°
relative to a plane normal to the shank axis, said second surface facing toward the tip being angled about 65° to 70°
relative to said plane; and the shank being tapered toward said conical tip throughout at least the major extent of said threaded section at about 1-1/2° to 2-1/2° from a cylindrical projection from said neck section.--

2. A screw according to claim 1, wherein said neck section is about 1/6 of the total length of said shank.

3. A lag screw according to claim 1, wherein said neck section is substantially cylindrical, and a portion of said unthreaded section contiguous to said neck section being of substantially the same cylindrical diameter as said neck section, and the taper of said shank extending from said cylindrical portion of the threaded section to said tip.

4. A screw according to claim 1, wherein said thread extends to a substantially uniform depth throughout the shank taper.

5. A screw according to claim 1, including a turnout lock formed in said thread.

6. A screw according to claim 5, wherein said turnout lock comprises a longitudinal strip cut through said thread along the tapered extent of the shank and providing turnout shoulder structure facing in the turnout direction and cam surface structure leading from said shoulder structure in the same direction to facilitate threading in action of the screw into a wooden member, but said shoulder structure engaging with wood fibers of the wooden member and resisting turnout of the screw relative to the wooden member.

7. A screw according to claim 1 and a wooden member into which the screw has been driven partway by axial impact on said head whereby the tapered shank has formed a tapered bore, and into which bore the screw has been driven the remainder of the way by torque applied through said head, said thread substantially throughout its length engaging in a groove cut in the wall of the bore by the thread incident to the torque driving.

8. A method of driving into a wooden member a screw or bolt having an elongated shank with a head at one end and a penetrating tip at the opposite end, a limited length unthreaded neck section of the shank contiguous to the head and a major length threaded section extending from the neck section to the tip, a spiral thread extending along said threaded section from said neck section to said tip and said thread having a first thread surface facing generally toward said head and a second thread surface facing generally toward said tip, said surfaces meeting at a root diameter and converging to a pitch diameter at the crest of the thread, the convergent angle of said first surface being about 20° to 25° relative to a plane normal to the shank axis, and the angle of said second surface relative to said plane, being about 65° to 70° and the shank being tapered toward said tip throughout at least the major extent of said threaded section at about 1-1/2° to 2-1/2° from a cylindrical projection from said neck section, and comprising: inserting said shank into the wooden member up to substantially said neck section at least in part by applying longitudinal impact driving force through said head toward said tip and forming a tapered bore substantially conforming to said tapered shank into said wooden member; and thereafter applying torque to the screw through said head and driving the screw substantially the remainder of its length into the wooden member by spiral cutting of said thread into the wall of said bore along substantially the entire length of said threaded section.

9. A method of driving into a wooden member a screw or bolt having an elongated shank with a head at one end and a penetrating tip at the opposite end, a limited length unthreaded neck section of the shank contiguous to the head and a major length threaded section extending from the neck section to the tip, a spiral thread extending along said threaded section from said neck section to said tip and said thread having a first thread surface facing generally toward said head and a second thread surface facing generally toward said tip, said surfaces meeting at a root diameter and converging to a pitch diameter at the crest of the thread, the convergent angle of said first surface being about 20° to 25° relative to a plane normal to the shank axis, and the angle of said second surface relative to said plane, being about 65° to 70° and the shank being tapered toward said tip throughout at least the major extent of said threaded section at about 1-1/2° to 2-1/2° from a cylindrical projection from said neck section, and comprising: applying longitudinal impact driving force through said head toward said tip and thereby effecting penetration of the threaded section of the screw into the wooden member up to substantially said neck section and forming a tapered bore in said wooden member substantially conforming to said tapered shank; and thereafter applying torque to the screw through said head and driving the screw substan-tially the remainder of its length into the wooden member by spiral cutting of said thread into the wall of said bore along substantially the entire length of said threaded section.

10. A method according to claim 9, wherein said threaded section has a limited length portion adjacent to said neck section which is of substantially a common cylindrical diameter with said neck section, and in the torque driving of the screw spirally cutting the thread along said cylindrical portion into the adjacent end portion of the wall of said tapered bore.

11. A method according to Claim 9, wherein said threaded section has a turnout lock, and effecting locking of said turnout lock with fibers of the wooden member to resist turnout of the screw relative to the wooden member.