US3793786A

US3793786A - Screw anchor

Info

Publication number: US3793786A
Application number: US00178712A
Authority: US
Inventors: E Jahnke
Original assignee: Joslyn Manufacturing and Supply Co
Current assignee: Joslyn Corp
Priority date: 1971-08-31
Filing date: 1971-09-08
Publication date: 1974-02-26
Anticipated expiration: 1991-02-26
Also published as: CA967717A

Abstract

A screw anchor for penetration and holding in soft soil comprising an elongated shank rotatable about its longitudinal axis and having a lower end adapted to penetrate into the earth upon rotation. A helical flight is secured to the shank for holding engagement in the earth and includes a crowned upper surface extending outwardly of the shank and sloping downwardly adjacent the outer edge thereof, thereby increasing the holding strength of the flight by providing a larger, frustoconical, shear area in the soil rather than a smaller cylindrical shear area, as with conventional anchors.

Description

United States Patent [1 1 Jahnke SCREW ANCHOR [75] Inventor: Earl M. Jahnke, Elmhurst, 111.

[73] Assignee: Joslyn Mfg.-and Supply Co.,

Chicago, 111.

[22] Filed: Sept. 8, 1 971 [21] Appl. No.: 178,712

[52] US. Cl. 52/157 51 Int. Cl E02d 5/74 [58] Field of Search 52/155-166; 61/5368 [56] References Cited UNITED STATES PATENTS 1,336,773 4/1920 Caldwell 85/46 816,631 /1906 Widmer.... 52/157 3,318,058 5/1967 Sullivan 52/157 3,295,274 l/1967 Fulton 52/157 1,109,020 9/1914 Skiff et a1. 52/157 FOREIGN PATENTS OR APPLICATIONS Feb. 26, 1974 134,301 10/1919 Great Britain 52/157 3,442 11/1868 Great Britain.... 52/157 163,571 7/1904 Austria 52/157 869,531 5/1961 Great Britain 52/157 Primary ExaminerJohn E. Murtagh Attorney, Agent, or Firm-Mason, Kolehmainen, Rathburn & Wyss [57] ABSTRACT A screw anchor for penetration and holding in soft soil comprising an elongated shank rotatable about its 1ongitudinal axis and having a lower end adapted to penetrate into the earth upon rotation. A helical flight is secured to the shank for holding engagement in the earth and includes a crowned upper surface extending outwardly of the shank and sloping downwardly adjacent the outer edge thereof, thereby increasing the holding strength of the flight by providing a larger, frustoconical, shear area in the soil rather than a smaller cylindrical shear area, aswith conventional anchors.

3 Claims, 5 Drawing Figures IICII PATENTEBFEB26 I974 INVENTOR. EARL M. JAHNKE 1 SCREW ANCHOR the anchor for connection to brace and support a utility pole, pipeline or other structure. Relatively high pulling forces are encountered in typical installations for an- .choring utility poles because of wind and weather forces on the poles and components carried thereby. Screw anchors, after being power driven into the soil, derive their holding strength from the weight of the earth column above the screw flight as well as from the shear forces which the soil is able to withstand acting along a generally cylindrical area having a diameter equal to that of the helical flight of the screw anchor. In relatively soft soils and sandy loams which act somewhat more like fluids than solids, the holding strength of a conventional helical flight screw anchor is reduced because of the reduced ability of soft soils, and the like, to withstand shearing forces without displacement of the soil or pullout.

Prior art screw anchors when used in relatively soft soil have been known to pull out'when subjected to high stresses caused by wind storms, hail and the like, acting on the supported structures and one method of avoiding this problem has'been to drive the anchors down in the soil far enough until the helical flight penetrates into a more solid soil, which soil has better holding strength than the soil thereabove. However, this solution results in greatly increased costs in initially setting the screw anchors and in some cases it is impossible to drive the screw anchors down far enough to reach a soil layer with sufficient holding strength.

The present invention provides a novel screw anchor having substantially increased holding capability even in soft soils, sandy loams, and the like. The screw anchor of the invention distributes the pullout force over a greater area in the coil for a given diameter flight, so that a screw anchor, in accordance with the present invention, having a flight diameter equal to a conventional prior art screw anchor, will have a greatly increased holding strength in similar-type soils. The screw anchor of the invention takes advantage of the phenomenon of soft soils wherein the soils act like flu-- ids and the pullout forces are directed normal to the outer edge of the screw anchor flighting or helix.

It is therefore an object of the present invention to provide a new and improved screw anchor for penetration and holding in the earths surface.

Another object of the present invention is to provide a new and improved screw anchor for improved holding strength in soft soils, sand loams, and the like.

Another object of the present invention is to provide a new and improved screw anchor which has increased holding strength or pullout resistance in comparison with a conventional screw anchor having equal diameter flighting.

The foregoing andother objects and advantages of the present invention are'accomplished by a new and improved screw anchor comprising an elongated shank rotatable around its longitudinal axis and having a pointed lower end adapted to penetrate into the earth as the shank is rotated. A helical flight is secured to a lower portion of the shank for penetrating and holding engagement in the earth, especially in soft soils and the like. The helical flighting is formed with a crowned upper surface andcurved cross section which extends outwardly from the shank and slopes downward to the outer peripheral edge of the flight. Because soft soils act in a manner somewhat similar to fluids in that they show little ability to withstand shear stresses, when a pull is exerted on the shank of the screw anchor the forces resisting pullout are pressure forces directed normal to the surface of the flight and hence act within an upwardly enlarging, frustoconical volume of earth above the flight. The volume of this frustoconical earth segment is considerably greater than that of a cylindrical column of earth above the flight having a diameter equal to the flight and accordingly, the screw anchor of the present invention provides greatly increased holding or pullout strength in comparison to conventional screw anchors having flights of equal diameter.

For a better understanding of the present invention, reference should be had to the following detailed description when taken in conjunction with the drawings, in which:

FIG. 1 is a vertical sectional view illustrating a new and improved screw anchor constructed in accordance with the features of the present invention and shown as it is installed in the earth;

FIG. 2 is a transverse, sectional view taken substantially along line 22 of FIG. 1;

FIG. 3 is an enlarged, side elevational view of the hub of t e screw anchor shank before it is formed into finals ape;

FIG. 4 isia side elevational view of the hub" taken substantially along line 44 of FIG. 3; and

FIG. 5 is a fragmentary, longitudinal sectional view of the screw anchor taken substantially along line 5--5 of FIG. 2, showing the downwardly curved or crowned helical flighting.

Referring now more particularly to the drawings,

.therein is illustrated a new and improved screw anchor constructed in accordance with the features of the present invention and referred to generally by the reference numeral 10. The screw anchor 10 is especially adapted for use in counterbalancing longitudinal, transverse anduplift forces on utility poles, pipelines and other upright structures and the upper end of the anchor is normally connected by means of guy wires or structural members. The screw anchor 10 includes an elongated shank 12 having an attachment eye 14 at the upper end or other suitable connector means adapted for connection to a'guy wire, brace or other structural member. The lower end of the shank 12 is formed with a threaded portion 12a which is threadedly received in a threaded axial bore 16a provided in the upper end of a lower shank hub 16 of the square cross section.

As best shown in FIG. 2, the lower hub 16 is adapted to be power driven by the application or torsional forces to screw the anchor into the ground. A hollow tubular wrench 18 is telescoped onto the upper end of the hub and is power rotated by a suitable source of motive power (for example, the power takeoff on the back of a line truck). The'lowe'r hub is preferably constructed from square stock material and,as shown in FIGS. 3 and 4, is subsequently flattened in a forming operation until the square cross section of the lower half is changed to a relatively thin, flat, rectangular cross section having the dimensions W and T, as shown. Round or hexagonal stock could also be used and subsequently flattened as shown. After flattening, the lower end of the hub is trimmed to a point 20, as shown in FIG. 3. After this treatment, the upper half of the hub 16 is secured in a chuck or other holding device and the pointed end is likewise secured in coaxially aligned relation in a chuck or other device. One of the chucks is rotated relative to the other in order to produce a spiral, twist drill shaped lower end portion as shown in FIG. 1. The arrow A of FIG. 4 indicates how rotational twisting of the lower end of the hub 16 relative to the upper half forms the spiral twist drill shape lower end portion. Preferably, the twisting torque is applied until approximately one and a half or two complete revolutions have been attained to provide the desired length of spiral twist drill shape, as shown. During the flattening and twisting operation, the hub 16 is normally at elevated temperature and may be subjected to longitudinal tension during the twisting in order to control the pitch of the screw point portion. It has been found that the spiral, twist drill shaped, pointed lower end section of the hub prevents walking of the screw anchor during initial penetration and is much easier "to control in starting into the ground, at a desired angle, than prior art anchors.

In accordance with the present invention, the screw anchor is provided with a crowned, or upwardly concave, helical flight of at least one turn, indicated generally by the reference numeral 20. The axial pitch of the helical flight 20 is approximately equal to the pitch of the twisted screw point of the lower hub. The helical flight extends outwardly of the hub 16 at approximately right angles or slightly less than 90, relative to the longitudirial axis of fiafifihcifihfia'is crowned so as to curve downwardly toward the outer edge of the flight. The outer edge of the flight is sloped downwardly and forms an angle a relative to a line perpendicular the axis of the shank as shown in FIG. 5. In soft soils which behave, from a physical standpoint, somewhat similar to fluids or liquids, the resistance to pullout is based on pressure or compression forces rather than shear stresses and develops in a direction normal to the upper surface of the flight. As shown in FIG. 1, the resistance to pullout of the anchor 10 (for example, as indicated graphically by a pulling force P exerted on the attachment eye 14 of the shank rod 12) is distributed by the crowned flighting 20 over a frustoconical volume, bonded by the lines C, which intersect the outer edge of the flight at approximately 90 or normal theretoThe weight of soil in the frustoconical volume is considerably greater tlfin the weight of soil in a cylindrical volume above the flight as indicated by the lines DD. Accordingly, the screw anchor 10 of the present invention has greatly increased holding strength in soft soils in comparison to conventional screw anchors of equal flight diameters.

It has been found that when driving the screw anchor 10, a reduced downward pressure is required for driving than with conventional screw anchors. The surface area of the frustoconical volume defined by the lines C is considerably larger than the surface area of the cylinder of earth column bounded by lines D and accordingly, in addition to increased holding strength due to increased weight of thesoil against which the pullout force is distributed some increased holding strength is provided by a greater shear area affected by the frustoconical surface in comparison to the cylindrical surface area generated by the lines D which represents the stress distribution area in the soil of a conventional anchor. This is true because even soft soils, sandy loams, and the like, which are capable of withstanding some shear stresses and the crowning or curving of the flight 20 in cross section as shown, distributes this stress over the much larger frustoconical area. The anchor 10 is especially well suited for use in soft soils, sandy loams, and the like, but is also useful in high stress applications in regular soils. A much greater holding strength is thus achieved with the screw anchor 10 in accordance with the present invention, than with conventional screw anchors having equal diameter flights.

While there has been illustrated and described a single embodiment of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in they art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention. 4

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A screw anchor for penetration and holding in the earth comprisingan elongated shank rotatable about its longitudinal axis and having a lower end portion of po lygonal cross-section adapted to penetrate into the earth and a helical flight secured to said lower end portion of said shank for holding engagement in the earth, said helical flight having a diameter substantially greater than the transverse dimension of said shank and having a crowned upper surface extending outwardly of said shank and curved to slope downwardly adjacent the outer edge thereof, said lower end portion including an upper segment above .said flight for driving engagement with a tool telescoped thereon, said lower end portion including a lower segment below said flight twisted about said longitudinal axis forming a spiral, twist drill shaped pointed lower end for penetration into the earth.

2. The screw anchor of claim 1 wherein the axial pitch of said spiral lower segment is approximately equal to that of said flight.

3. The screw anchor of claim 1 wherein said spiral, twist drill shaped lower segment comprises between one and one half and two turns of twist about said shank axis.

Claims

1. A screw anchor for penetration and holding in the earth comprising an elongated shank rotatable about its longitudinal axis and having a lower end portion of polygonal cross-section adapted to penetrate into the earth and a helical flight secured to said lower end portion of said shank for holding engagement in the earth, said helical flight having a diameter substantially greater than the transverse dimension of said shank and having a crowned upper surface extending outwardly of said shank and curved to slope downwardly adjacent the outer edge thereof, said lower end portion including an upper segment above said flight for driving engagement with a tool telescoped thereon, said lower end portion including a lower segment below said flight twisted about said longitudinal axis forming a spiral, twist drill shaped pointed lower end for penetration into the earth.