SCREW PILE

Abstract

The present invention relates to a screw pile comprising a shaft and at least two blades extending outwardly from the shaft. Each blade has a leading edge that contacts earth as the screw pile is screwed into the ground. The leading edge includes a swept back portion adapted to deflect rocks that come into contact with the swept back portion of the leading edge during insertion of the screw pile into the ground. In one embodiment, the leading edge of each blade includes two or more straight edge portions extending at different angles to each other. The straight edge portions may include an inner edge portion extending from the shaft and the swept back portion extending at an angle from the inner edge portion.

Description

FIELD OF THE INVENTION

The present invention relates to an improved screw pile.

BACKGROUND TO THE INVENTION

Screw piles are used in the construction of buildings and other structures. A typical screw pile comprises a shaft, normally made from mild steel or a higher strength steel. A helical screw or blade is attached to the shaft. In order to insert the screw pile into the ground, the screw pile is rotated and pressed downwardly which causes the helical blade to bite into the ground and to screw into the ground. Once the screw pile has been properly inserted into the ground, the weight borne by the screw pile is distributed from the helical blade into the earth that lies underneath the helical blade. Further, the earth positioned above the helical blade assists in resisting any lifting forces applied to the screw pile and thereby assists in maintaining the screw pile in the ground.

Conventional screw piles comprise a single helical blade. The blade has a leading edge that moves through and breaks the earth as the screw pile is screwed into the ground. Conventional screw piles have a leading edge on their blade that extends generally perpendicularly to the outer periphery of the blade (when viewed from above). As the shaft is normally cylindrical in shape, the leading edge of the blade may be considered to extend outwardly from the shaft in the radial direction.

Throughout this specification, the term “comprising” and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise.

The applicant does not concede that any prior art to cutting the specification forms part of the common general knowledge in Australia or elsewhere.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide an alternative screw pile.

In a first aspect, the present invention provides a screw pile comprising a shaft, two blades in the form of opposing flat plates extending outwardly from the shaft, each blade having a leading edge that contacts earth as the screw pile is screwed into the ground, each leading edge including a first straight edge portion extending from the shaft and a second straight edge portion forming an obtuse angle with said first straight edge portion.

In a second aspect, the present invention provides a screw pile comprising a shaft, two blades in the form of opposing flat plates extending outwardly from the shaft, each blade having a leading edge that contacts earth as the screw pile is screwed into the ground, each leading edge including a first straight edge portion extending from the shaft and a swept back straight edge portion forming an obtuse angle with said first straight edge portion, the swept back straight edge portion being adapted to deflect rocks during insertion of the screw pile into the ground.

In a third aspect, the present invention provides a blade for a screw pile, the blade having a leading edge that contacts earth as the screw pile is screwed into the ground, the leading edge including a first straight edge portion for extending from a shaft of the screw pile and a second straight edge portion forming an obtuse angle with said first straight edge portion.

In one embodiment, a third straight edge portion forms an acute angle with the second straight edge portion

The screw pile of the present invention may comprise two blades in the form of angled plates. The angled plates may be mounted to the shaft. The angled plates may be mounted to the shaft, for example, by welding. Alternatively, the angled plates may be integrally formed with the shaft. The angled plates may be generally flat angled plates. The angled plates may have opposite pitch to each other. For example, when viewed from side on, one angled plate may extend downwardly from left to right while the other angled plate may extend downwardly from right to left.

In another embodiment, the two blades are in the form of helical blades.

In some embodiments, the blades are positioned on opposite sides of the shaft. As most shafts used in screw piles are cylindrical, the blades may be positioned on diametrically opposite sides of the shaft.

The lowermost edge of the blades or the angled plates is suitably positioned above a lowermost part of the shaft. In this fashion, the shaft extends below the blades and the lower end of the shaft contacts the ground before the blades contact the ground. Suitably, the lower end of the shaft may be fitted with or formed with an attack bit. The attack bit engages with the ground when the screw pile is being screwed into the ground and this acts to stabilise the screw pile as it is being screwed into the ground. The attack bit may take the form of two diametrically opposed points formed on the lower end of the shaft.

In some embodiments, the leading edge of the blades may include a portion that extends at an angle to a perpendicular line extending from the shaft (when viewed from above).

The leading edge of the blades may comprise two or more discrete portions extending at different angles to each other. For example, the leading edge may have an innermost portion that is adjacent to the shaft that extends in a direction that is generally perpendicular to the shaft. A second portion may extend from the first portion, with the second portion extending at an angle to the first portion. The second portion is desirably a swept back portion. The leading edge may also comprise further portions. For example, the leading edge may comprise a third portion extending at an angle to the second portion and, in some embodiments, even a fourth portion extending an angle to the third portion. Desirably, the third portion sweeps back from the second portion and the fourth portion sweeps back from the third portion. In this embodiment, the portions of the leading edge may be generally straight edge portions.

In other embodiments, the leading edge may smoothly sweep back from the innermost portion thereof located adjacent to the shaft. In this embodiment, the leading edge may take the form of a curved leading edge that progressively sweeps back as it extends away from the shaft.

The leading edge of the blades of the screw pile in accordance with the present invention has at least one portion that sweeps back from the direction of rotation of the blade during insertion of the screw pile into the ground. The at least one portion that sweeps back from the direction of rotation of the blade assists in deflecting any rocks that may be encountered during insertion of the screw pile into the ground. In this way, impact loads on the leading edge of the blade during insertion of the blade into the ground are lowered.

In some embodiments, the rear part of the blades may comprise a generally square or rectangular part.

In some embodiments, the trailing edge of the blades may include a portion that extends at an angle to a perpendicular line extending from the shaft (when viewed from above). The trailing edge may include a first portion and a second portion extending at an angle to the first portion. Alternatively, the trailing edge may smoothly sweep back towards the shaft. In these embodiments, the trailing edge also sweeps back towards the shaft. This also assists in deflecting rocks or other hard material away from the trailing edge of the blades during insertion of the screw pile into the ground. Further, the swept back trailing edge forms a “widened opening” between the blades, thereby reducing the tendency for rocks or other hard material to become jammed between the blades.

In some embodiments, the tailing edge may mirror the shape of the leading edge.

The screw pile in accordance with the present invention, in having two blades, also provides for greater stability during the insertion phase of the screw pile into the ground. In this regard, each blade engages with the ground as the screw pile is being screwed in. Therefore, the forces applied to each blade by engagement with the ground tend to cancel out with each other to thereby result in a more even distribution of forces on the screw pile during insertion.

The use of two blades in the screw pile of the present invention provides a further benefit. It will be understood that as the screw pile is screwed into the ground, the earth is disturbed as the blades of the screw pile are rotated through the earth. Once the screw pile has reached its installed depth, there is a section of earth located near the leading edges of the blades that has been undisturbed because the blades have not passed through that section of earth. Those skilled in the art will readily understand that undisturbed earth typically demonstrates a greater capacity for bearing load than disturbed earth. As there is a region of undisturbed earth near the leading edge of two blades in the screw pile of the present invention, the load bearing capacity of the installed screw pile in accordance with the present invention should be enhanced when compared with the load bearing capacity of conventional screw piles having a single helical blade.

A further advantage arises in that earth is positioned above both blades of the screw pile of the present invention when the screw pile is installed in the ground. Thus, resistance to lifting forces applied to the installed screw pile should be enhanced when compared to conventional screw piles having a single helical blade.

The behaviour of the screw pile under both compression and tension may be further enhanced in embodiments where the rear part of the blades comprises a generally square or rectangular part.

In embodiments where the blades are made from generally flat plates, fabrication of the screw pile may be greatly simplified. For example, elliptical cutouts or recesses may be formed in the blade, with the shape of the elliptical cutout or recess following the shape of the outer periphery of the shaft along the angle at which the blade is to be mounted to the shaft. The blade may then be simply cut or stamped and subsequently welded to the shaft. This is a simpler fabrication route than with conventional single helical bladed screw piles, which typically involve forming a semi-circular plate and shaping that plate into a helical flight for subsequent attachment to the shaft.

In embodiments where the blades are in the form of generally flat plates, the leading edge may be formed to the desired shape by stamping or cutting the leading edge.

The shape and/or size of the blades may vary in accordance with the proposed use of the screw pile. For example, for screw piles intended to be used in sandy soils or easily friable soils, a wide blade may be used. A wide blade is appropriate because the screw pile can be easily screwed into the sandy or friable soil. Further, the wide blade assist in spreading compression and tension loads applied to the screw pile into the soil around the screw pile. For screw piles intended to be used in clay sites, a less wide blade may be used. For screw piles intended to be used in sites having rocky ground, even less wide blades may be used to facilitate penetration of the blade through the earth during installation. The blades used on the screw piles may have further swept back portions to enhance the deflection of rocks during installation.

The thickness of the blade may also be increased if the screw pile is to be used in sites that require greater force for installation.

In some embodiments, the leading edge of the blade may be bevelled or sharpened in order to enhance penetration of the leading edge through the earth as the screw pile is screwed into the ground and to enhance the ability of the blade to deflect rocks or other hard material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a screw pile in accordance with one embodiment of the present invention;

FIG. 2 shows a plan view of the screw pile shown in FIG. 1, but only showing one blade for clarity purposes;

FIG. 3 is a plan view of the screw pile as shown in FIG. 1;

FIG. 4 is an underneath view of the screw pile shown in FIG. 1;

FIG. 5 is a perspective view of a screw pile in accordance with another embodiment of the present invention;

FIG. 6 shows a plan view of the screw pile shown in FIG. 5, but only showing one blade for clarity purposes;

FIG. 7 is a plan view of the screw pile as shown in FIG. 5;

FIG. 8 is an underneath view of the screw pile shown in FIG. 5;

FIG. 9 is a perspective view of a screw pile in accordance with another embodiment of the present invention;

FIG. 10 is a side view of one of the blades used in the screw pile shown in FIG. 9;

FIG. 11 shows a plan view of the screw pile shown in FIG. 9, but only showing one blade for clarity purposes;

FIG. 12 is a plan view of the screw pile as shown in FIG. 9;

FIG. 13 is an underneath view of the screw pile shown in FIG. 5;

FIG. 14 shows a plan view of a screw pile in accordance with a further embodiment of the present invention;

FIG. 15 is a view of a plate that has been cut to shape for use as a blade on a screw pile in accordance with an embodiment of the present invention; and

FIG. 16 is a plan view of a blade for a screw pile in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

It will be understood that the drawings have been provided for the purpose of illustrating embodiments of the present invention. Therefore, it will be appreciated that the present invention should not be considered to be limited solely to the features as shown in the drawings.

FIGS. 1 to 4 show various views of a screw pile in accordance with an embodiment of the present invention. The screw pile shown in FIGS. 1 to 4 is designed for use in sandy soils or other soils that are easily friable. The screw pile 10 shown in FIGS. 1 to 4 comprises a shaft 12. Shaft 12 is a generally cylindrical shaft that may be made from mild steel or a higher strength steel. The shaft 12 may be a hollow shaft. The shaft 12 has an upper end 14 and a lower end 16. Two driving lugs 18, 20 (see FIG. 3) are attached to the shaft near the upper end 16. The driving lugs 18, 20 engage with a driving mechanism to facilitate screwing of the screw pile 10 into the ground.

The screw pile 10 also includes two blades 22, 24. As best shown in FIG. 1, the blades 22, 24 mounted to the outer periphery of the shaft 12, with each blade being mounted at opposed angles to the longitudinal axis of the shaft 12. Each of the blades 22, 24 comprises a flat plate.

The lower end 16 of the shaft includes an attack bit formed by points 26, 28. During installation of the screw pile 10 into the ground, the attack bit contacts the ground before any other part of the screw pile. The attack bit bites into the ground and assists in maintaining the screw pile in the desired location of installation.

Once the attack bit has penetrated a short way into the ground, the blades 22, 24 of the screw pile 10 then engage with the ground and also dig into the ground. This causes the screw pile 10 to screw into the ground. Rotation of the screw pile 10 continues until the screw pile has been screwed to the desired depth.

FIG. 2 shows a plan view of the shaft 12 and the blade 24. The blade 24 has a leading edge, generally indicated by reference numeral 30. The direction of rotation of the blade 24 is shown by the arrow in FIG. 2. The leading edge 30 is the part of the blade 24 that contacts the earth as the blade 24 is rotated during insertion of the screw pile 10 into the ground. Therefore, the leading edge 30 engages with and breaks the earth as the screw pile is screwed into the ground. The leading edge 30 includes a first portion 32, a second portion 34 and a third portion 36. First portion 32 is located adjacent to the shaft 12. First portion 32 extends generally perpendicularly to the outer periphery of the shaft 12. In this regard, first portion 32 of the leading edge of blade 24 may be considered to extend in the radial direction from the shaft 12.

The second portion 34 of the leading edge 30 sweeps back from the first portion 32. Similarly, the third portion 36 sweeps back from second portion 34. In this way, if the second portion or the third portion contact rocks or other detritus or hard material during screwing in of the screw pile 10, the swept back angle of those portions of the leading edge assist in removing or deflecting the rocks or hard material, thereby facilitating insertion of the screw pile 10 into the ground and minimising the likelihood of damage to the screw pile or to the equipment used to screw in the screw pile.

The first portion 32 of the leading edge 30 may be considered to comprise a small and strong initial straight blade attack region. The second portion 34 may be considered to comprise a medium follow-on, swept blade attack region. The third portion 36 may be considered to comprise the largest follow-on, increased sweep blade attack region.

It also can be seen from FIG. 2 that the rear part 38 of blade 24 is of generally square or rectangular shape.

The other blade 22 of screw pile 10 has a generally similar arrangement at its leading edge and rear part.

FIGS. 5 to 8 show another embodiment of a screw pile 50 in accordance with the present invention. The embodiment shown in FIGS. 5 to 8 is very similar to the screw pile 10 shown in FIGS. 1 to 4. For convenience, similar features will be referred to using similar reference numerals, but with the attachment of a ′. These features need not be described further. The screw pile 50 shown in FIGS. 5 to 8 is designed primarily for use in clay soils.

The screw pile 50 as shown in FIGS. 5 to 8 differs from the screw pile 10 as shown in FIGS. 1 to 4 in that the width of the blades is less. In particular, the blade 24′ shown in FIGS. 5 to 8 has a width W that is less than the width W of the blade 24 shown in FIGS. 1 to 4. The reason for this is that the screw pile 10 is designed for use in sandy soil sites. As these soils are easily broken, the additional width of blade 24 does not cause difficulties in screwing the screw pile 10 into the soil. Further, the additional width of blade 24 (and the corresponding blade 22 that is also mounted to the shaft 12) assists in spreading the load from the screw pile into the friable soil. In contrast, the narrower blade 24′ shown in FIGS. 5 to 8 provides good penetration into firm to hard clay soils. Firm to hard the clay soils provide good geotechnical support and the blades 22′ and 24′ are optimised to provide the best load characteristics in those soils. Although not shown in any of FIGS. 1 to 8, the blades on screw pile 10 may also be thinner than the blades on screw pile 50.

FIGS. 9 to 13 shown various views of a screw pile 60 in accordance with another embodiment of the present invention. The screw pile 60 has a number of features in common with the screw pile 10 as shown in FIG. 1 and, for convenience, those similar features are denoted by a similar reference numeral is but with the addition of a ″. These features need not be described any further.

The screw pile 60 is intended for use in rocky soils. Accordingly, compared to screw piles 10 and 50, as shown in FIGS. 1 to 8, the screw pile 60 has blades of lesser width. The blades 22″ and 24″ are also thicker than the blades 22, 24 and 22′, 24′. As shown in FIG. 10, the blade 22″ also has a bevelled leading edge 23, which assists in breaking or penetrating the ground in advance of the leading edge during screwing in of the pile 60, as well as enhancing the deflection capabilities of the blades 22″ and 24″.

A further difference is that the front part of the blade 24″ includes first portion 32″, second portion 34″, third portion 36″ and fourth portion 37. The first portion 32″ comprises a small and strongest initial straight blade attack region, the second portion 34″ provides a medium follow-on, swept blade attack region, the third portion 36″ provides a larger follow-on, increased sweep blade attack region and the fourth region 37 provides a trailing edge blade to help final deflection of rocky material. The blade 22″ has a similar leading edge region.

The rear part 38″ also has a trailing blade region 39 for load distribution. The trailing blade region 39 may be expediently formed by simply cutting off a corner of the rear part of the blade.

The embodiment of the screw pile 60 shown in FIGS. 9 to 13 has the smallest and thickest blade with added cutting and deflection in the blades. This version of the screw pile is designed to penetrate the most difficult soil that contains rocks, debris and cobbles. This version is less reliant on blade load capacity than the versions shown in FIGS. 1 to 8 since it is designed to chase and engage bedrock.

FIG. 14 shows a plan view of a screw pile in accordance with a further embodiment of the present invention. The screw pile 100 shown in FIG. 14 has a shaft 102 and two blades, designated in FIG. 14 as Blade A and Blade B. Blade A has a leading edge 104 that includes first attack edge portion 106, second attack edge portion 108 and third attack edge portion 110. Similarly, blade B has a leading edge 112 that includes first attack edge portion 114, second attack edge portion 116 and third attack edge portion 118.

Blade A has a trailing edge 120 having edge portion 122 and swept back edge portion 124. The trailing edge 120 joins to the shaft 102 via a perpendicular portion that is located adjacent to edge portion 114 of leading edge 112 of Blade B, in the view shown in FIG. 14. Blade B has a trailing edge 130 having edge portion 132 and swept back edge portion 134. The trailing edge 130 joins to the shaft 102 via a perpendicular portion that is located adjacent to edge portion 106 of leading edge 104 of Blade A, in the view shown in FIG. 14. It will be appreciated that the trailing edge of one blade is at a different vertical height to the leading edge of the other blade.

As can be seen from FIG. 14, the trailing edge of each blade mirrors the leading edge of the other blade to create a “wide and opening between the blades to prevent rocks and to debris jamming therebetween. These white and openings are shown by reference numerals 140 and 142. The swept trailing blade edges also aid in deflecting any rocks or debris that attempt to jam between the blades.

FIG. 15 shows a steel plate 150 that has been cut to a shape that is suitable for use as a blade on a screw pile in accordance with the embodiment of the present invention shown in FIG. 14. The plate 150 is made from 8 mm thick, 350 grade steel, although it will be appreciated that the plate may be made from different grades and/or thickness of steel or indeed it may be made from other alloys or materials. The plate 150 has leading edge portions 152, 154 and 156, and trailing edge portions 158, 160 and 162. The plate has a side edge 164. A half oval cut 166 is made to allow the blade to closely conform to the outer periphery of the shaft of the screw pile, it being appreciated that the plate is mounted to the shaft at an angle to the horizontal (thereby necessitating a half oval cut). As can be seen from FIG. 15, the trailing edge is a mirror of the leading edge.

According to another aspect of the present invention ,there is provided a screw pile blade 200 as shown in FIG. 16. The blade 200 has a leading edge 202 that contacts earth as the screw pile is screwed into the ground. The leading edge 202 includes a first straight edge 204 portion for radially extending from a shaft of the screw pile, and a second straight edge portion 206 forming an obtuse angle 207 with said first straight edge portion 204. A third straight edge portion 208 forms an acute angle 209 with the second straight edge portion 206. As can be seen from FIG. 16, the trailing edge 210 mirrors the leading edge 202. The blade 200 is effective in working clay and cobbled soils.

Screw piles in accordance with embodiments of the present invention provide robust screw piles having enhanced installation capabilities. The use of twin blades evens out the forces that are applied to the blade during installation of the blade into the ground. Further, loads borne by the screw pile are evenly distributed through the twin blades into the ground. In embodiments where the blades are formed from flat plates, the blades are easily made by cutting or stamping and the blades can be very simply joined to the shaft of the screw pile by welding. Some embodiments of the screw pile include attack points or an attack bit on the end of the shaft, which also assists in keeping the pile aligned during installation and minimises oscillation of the screw pile during installation.

Screw piles in accordance with various embodiments of the persent invention may also provide the following benefits:

• • Unlike normal screw piles, the twin blade pile has 2 main sets of attack/embedment blades' for increased & balanced load distribution during and after installation, effectively doubling the engaged footprint
  • The pitched flat twin blades, provide counter balanced penetration into soil during installation
  • Each blades cutting edge provides 3 progressive stages of cut/embedment during ground penetration
  • Each blade provides swept progression to combat resistance for improved penetration,
  • As each blade rotates to engage—the swept angle increases relative to the rotation
  • The twin flat blades provide a square ‘dual combined’ end bearing plate for compression & tension loads
  • The trailing edge of each blade mirrors the leading edge of the opposite blade to create a ‘widened opening’ between the blades to prevent rocks and debris jamming. The swept trailing blade edges also aid in deflecting any rocks or debris that attempt to jam between the blades.
  • The pile components and design, provide a simple & cost effective manufacturing process

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. One possible variation of the screw pile may involve the addition of a further set of twin blades above and spaced from the lower twin blades that are shown in FIGS. 1 to 13. These further twin blades are suitably mounted in similar fashion to the shaft of the screw pile. In this embodiment, the earth that is present between the lower set of twin blades and the upper set of twin blades is trapped between the two sets of blades. As a result, the screw piles in accordance with that embodiment act in a fashion that is similar to friction piles.

In other embodiments, the flat plate twin blades, as shown in FIGS. 1 to 13, may be replaced by helical twin blades or twin helical flights.

It will be understood that the present invention encompasses all variations and modifications that fall within its spirit and scope.

Claims

1. A screw pile comprising a shaft, two blades in the form of opposing flat plates extending outwardly from the shaft, each blade having a leading edge that contacts earth as the screw pile is screwed into the ground, each leading edge including a first straight edge portion extending from the shaft and a second straight edge portion forming an obtuse angle with said first straight edge portion.

2. A screw pile as claimed in claim 2, wherein the plates have opposite pitch to each other.

3. A screw pile as claimed in claim 1, wherein a lowermost part of each plate is positioned above a lowermost part of the shaft so that the shaft extends below the plates and can contact the ground prior to the plates.

4. A screw pile as claimed in claim 3, wherein a lower end of the shaft includes an attack bit having two diametrically opposed points at its end.

5. A screw pile as claimed in claim 1 wherein, when viewed from above, the first straight edge portion extends radially from the shaft.

6. A screw pile as claimed in claim 1, wherein said straight edge portions include an inner edge portion being the first straight edge portion extending from the shaft, an intermediate swept back edge portion being the second straight edge portion and that sweeps back against the direction of rotation of the blade, and an outer edge portion forming an obtuse angle with the intermediate swept back edge portion.

7. A screw pile as claimed in claim 6, wherein the leading edge of each blade is bevelled or sharpened.

8. A screw pile as claimed in claim 6, wherein each blade is symmetric and a trailing edge of each blade mirrors the leading edge.

9. A screw pile comprising a shaft, two blades in the form of opposing flat plates extending outwardly from the shaft, each blade having a leading edge that contacts earth as the screw pile is screwed into the ground, each leading edge including a first straight edge portion extending from the shaft and a swept back straight edge portion forming an obtuse angle with said first straight edge portion, the swept back straight edge portion being adapted to deflect rocks during insertion of the screw pile into the ground.

10. A screw pile as claimed in claim 9, wherein said two blades have opposite pitch to each other.

11. A screw pile as claimed in claim 10, wherein a lowermost part of each plate is positioned above a lowermost part of the shaft so that the shaft extends below the plates and can contact the ground prior to the plates, a lower end of the shaft including an attack bit having two diametrically opposed points at its end.

12. A screw pile as claimed in claim 9 wherein, when viewed from above, the first straight edge portion extends radially from the shaft.

13. A screw pile as claimed in claim 9, wherein a straight outer edge portion forms and obtuse angle with the swept back straight edge portion.

14. A screw pile as claimed in claim 13, wherein the leading edge of each blade is bevelled or sharpened, each blade being symmetric whereby a trailing edge of each blade mirrors the leading edge.

15. A blade for a screw pile, the blade having a leading edge that contacts earth as the screw pile is screwed into the ground, the leading edge including a first straight edge portion for extending from a shaft of the screw pile and a second straight edge portion forming an obtuse angle with said first straight edge portion.

16. A blade as claimed in claim 15, wherein a third straight edge portion forms an obtuse angle with the second straight edge portion.

17. A blade as claimed in claim 15, wherein the blade is symmetric and a trailing edge of the blade mirrors the leading edge.

18. A blade as claimed in claim 15, wherein a third straight edge portion forms an acute angle with the second straight edge portion.