GB2153033A

GB2153033A - Driving formations of screws

Info

Publication number: GB2153033A
Application number: GB08500942A
Authority: GB
Inventors: Andrea Bazzurro
Original assignee: BARGE BULLONERIA; BULLONERIA BARGE SpA
Current assignee: BARGE BULLONERIA; BULLONERIA BARGE SpA
Priority date: 1984-01-17
Filing date: 1985-01-15
Publication date: 1985-08-14
Also published as: SE8500173D0; NL8500096A; DE8501071U1; GB8500942D0; IT8452845V0; GB2153033B; FR2558220A1; DE3501414A1; BE901515A; SE8500173L; FR2558220B1

Abstract

The head of a screw has a driving formation which has a multilobed profile (7) in a plane perpendicular to the axis of the screw, in which the two sides (8) of each lobe lie at least partially in radial planes, and the sections (10) which join the radially outer ends of pairs of the radial sides (8) lie on a single imaginary conical or pyramidal surface coaxial with the screw. The driving formation can be the inside of a recess or the outside of a projection on the bolt head. The sections (11) joining the radically inner ends of the radial sides (8) lie on the surfaces of a polygonal prism coaxial with the screw x, or on a conial or pyramidal surface. Precise setting of tightening torque by a complementary tool is possible. <IMAGE>

Description

SPECIFICATION Screws suitable for precision tightening torques The present invention relates to screws and particularly to screws adapted for use with precision tightening torques, having a head with an engagement surface for a tightening key. According to the prior art, such an engagement surface may be constituted by the outer surface of the head of the screw or by the surface of an axial recess formed in the head of the screw.The main types of head which are currently used are the hexagonal head, square head, double-hexagonal head (that is having a profile in a plane perpendicular to the axis of the screw which is defined by the superposition of two concentric hexagons of the same size staggered relative to each other by an angle of 30 ), double-square head (a profile defined by two squares staggered by 45 ), head with a square recess, head with a hexagonal recess, head with a double-square recess, head with a doublehexagonal recess, head with a double-triangular recess (in which the profile of the recess, in a plane perpendicular to the axis of the screw, is defined by the superposition of two equilateral triangles of equal size staggered relative to each other by an angle of 60 ) and head with a cross-shaped recess.

In all known types of screw specified above, the contact and reaction surfaces which are engaged by the tightening key in use of the screw are not radial planes containing the axis of the screw. Consequently the forces generated in correspondence with the said bearing surfaces upon tightening of the screw are not normal to these surfaces. The components of the said forces which do not act normally to the bearing surfaces give rise to a deformation of the contact surfaces between the screw and the tightening key and prevent the magnitude of the tightening torque being determined precisely.In order to reduce this disadvantage one is forced to adopt manufacturing tolerances which are very narrow both with regard to the engagement surface of the head, or the recess formed in the head, and the engagement surface of the tightening key so as to render the area of the contact surface as large as possible and limit the specific deformation load. In other words, the minimum and maximum play which arises from the difference between the dimensions of the tightening key and the head of the screw, or of the recess formed in the head, must be reduced as far as possible. The said limitation on the manufacturing tolerances on the other hand makes it difficult to engage the tightening key with the head of the screw, particularly when automatic screwing systems are used.Furthermore, the impossibility mentioned above of determining the tightening torque precisely prevents the use of the screws in assemblies with axial preloading close to the yield point.

In this case, in fact, any resulting permanent deformation of the bearing surfaces of the tightening key and of the screw would make both the screw and the tightening key unservicable for correct use in a short period of time.

It has also been proposed (for example in the case of screws with heads with crossshaped recesses) to form surfaces for engagement with the tightening key as part of a conical surface coaxial with the screw in order to facilitate the insertion of the tightening key without the necessity for very narrow manufacturing tolerances. In this case however, the disadvantages mentioned above resulting from the fact that the bearing and reaction surfaces of the screw and of the tightening key are not subjected to forces which are purely normal to the surfaces themselves during operation of the screw still remain.

Finally it has also been proposed (see U.K.

patent no. 1,032,144) to form a screw head with a circumferential series of radial projections each of which has two sides contained in radial planes passing through the axis of the screw. Even this known application is not, however, completely satisfactory because of the need to use very narrow tolerances in the manufacture of the screw and of the tightening key, with the disadvantages consequent thereon already mentioned above. Furthermore, this latter type of screw is of a relatively complex and expensive construction.

The object of the present invention is to provide a screw which is able to obviate all the disadvantages explained above.

In order to achieve this object, the invention provides a screw the main characteristic of which lies in the combination of the following two characteristics: a) the head of the screw has an engagement surface for a tightening key, which has a multilobed profile in a plane perpendicular to the axis of the scew including a circumferential series of equi-angularly spaced lobes projecting radially outwardly, in which each lobe includes two radial sides which lie at least partially in two planes passing through the axis of the screw, and a central section which joins the radially outer ends of the two sides of each lobe, b) the portions of the said surface for engagement with the tightening key which define the said central sections of the lobes of the multilobed profile lie on a single imaginary conical or pyramidal surface coaxial with the screw.

The said surface for engaging the tightening key may be constituted by the side surface of an axial recess formed in the head of the screw or by the side surface of an axial appendage with which the screw head is provided. Correspondingly the tightening key will have an engagement surface of comple mentary form to the engagement surface of the screw.

By virtue of these characteristics, the bearing and reaction surfaces of the screw head and of the tightening key are constituted at least partially by radial planes passing through the axis of the screw whereby the forces generated during tigthening are normal to these planes, with the advantage of allowing the disadvantages explained above which occur with the known forms of screw to be overcome. At the same time, the cone shape of the mutual engagement surfaces of the screw and of the tightening key avoids the need for narrow tolerances in the manufacture of the screw and of the tightening key.

Preferably, moreover, the sections connecting one lobe with the next of the said multilobed profile are disposed along the sides of a regular polygon having its centre on the axis of the screw and the portions of the said engagement surface which define these sections are planes parallel to the axis of the screw.

By virtue of this characteristic, the screw can be operated in emergency even with a conventional key as will be described in greater detail below.

Further characteristics and advantages of the present invention will become apparent from the description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure 1 is a partially sectioned perspective view of a screw according to the present invention, Figure 2 is a perspective view of a variant of Fig. 1, Figure 3 is a section taken on the line Ill-Ill of Fig. 1, Figure 4 is a perspective view of a tightening key usable for turning the screw of Fig. 1, and Figure 5 is a schematic sectional view in a plane perpendicular to the axis of the screw illustrating the tightening key 4 in its condition of engagement with the screw.

With reference to Fig. 1, reference numeral 1 generally indicates a screw having a threaded shank 2 and a head 3 which has an axial recess 5 in its end surface 4 for engagement by a tightening key 6 (see Fig. 4).

As illustrated in Fig. 3, the side surface of the recess 5, which constitutes an engagement surface for the tigthening key 6, has a multilobed profile 7 including a circumferential series of equi-angularly spaced radial lobes each of which includes two radial sides 8 contained in two planes passing through the axis 9 of the screw and a central section 10 which joins the radially outer ends of the two sides 8.

Reference numeral 11 in Fig. 3 indicates the sections connecting one lobe of the multilobed profile 7 with the next.

With reference to Fig. 1, the lobes of the said multilobed profile are defined by radial grooves 1 2 formed in the recess 5. Each groove 1 2 has a base wall 1 3 corresponding to the central section 10 mentioned above and two sides 14 corresponding to the sides 8 of the section profile, lying at least partially, as stated above, in radial planes passing through the axis 9 of the screw.

The base walls 1 3 of the groove 1 2 of the recess 5 all lie on a single imaginary conical or alternatively pyramidal surface coaxial with the screw.

Reference numerals 1 5 indicate the faces of the recess disposed between the grooves 12, corresponding to the sections 11 of the profile of the recess in a section perpendicular to the axis of the screw.

According to the preferred embodiment illustrated in Fig. 1, the faces 1 5 lie in planes parallel to the axis of the screw and are all equidistant from the latter. In other words, according to this embodiment, the sections 11 (see Fig. 3) of the multilobed profile 7 correspond to the sides of a regular polygon having its centre on the axis 9 of the screw.

By virtue of this characteristic, the screw can be turned in emergency even with a conventional tightening key (in the case illustrated, a hexagonal key for screws with a hexagonal recess in the head) adapted to engage only the inner faces 1 5 of the recess 5.

Alternatively, the faces 1 5 may form part of a single, imaginary conical or pyramidal surface coaxial with the screw.

Adjacent the end surface 4 of the head, the side surface of the recess has a bevel 5a extending around the entire perimeter of the recess.

The tightening key 6 illustrated in Fig. 4 intended for insertion in the recess 5 to turn the screw, has a side surface with a form complementary to the lateral surface of the recess 5. Consequently, the active part of the tightening key includes a shaft 1 6 having a series of radial teeth 1 7 intended to fit within the radial grooves 1 2 of the recess in the head of the screw. Each radial tooth 1 7 has two radial sides 1 8 lying in planes passing through the axis 1 9 of the shaft 1 6 and an outer surface 20. The outer surfaces 20 of the radial teeth 1 7 lie on a single, imaginary conical or pyramidal surface coaxial with the shaft 1 6 and identical to the imaginary surface containing the base walls 1 3 of the radial grooves 1 2 of the recess in the screw head.

Reference numeral 21 indicates the portions of the surface of the shaft 1 6 between the teeth 17.

In a section lying in a plane perpendicular to the axis of the shaft 1 6 (see Fig. 5) the key 6 also has a multilobed profile including a circumferential series of equi-angularly spaced lobes defined by the radial teeth 1 7. As seen in Fig. 5, the angle between two sides 18 of ech radial tooth 1 7 (indicated G) is less than the angle between two sides 14 of each radial groove 12, indicated B. Furthermore, the distance P (see Fig. 5) between two diametrally opposite faces 21 is less than the distance between two opposing faces 1 5 of the recess 5. Consequently, when the tightening key 6 is inserted in the recess 5 of the screw head, there are clearances indicated g and fin Fig.

5 between the facing surfaces of the recess and of the key.

As seen in Fig. 4, each radial tooth 1 7 has two lateral bevels 1 7a adjacent the smaller diameter end of the key.

When it is necessary to tighten the screw, the tightening key 6 is inserted in the recess 5 formed in the screw head and the desired tightening torque is applied to the screw by means of the key 6. Upon engagement of the shaft 1 6 of the key in the recess 5, the shaft is advanced axially until the outer faces 20 of the key are brought into contact with the base walls 1 3 of the grooves of the screw recess.

As is clearly seen in Fig. 5, this may be achieved without the need to use strict manufacturing tolerances for the screw and for the key. Once the axial engagement of the key in the recess of the screw head has been achieved, the key is rotated so as to bring one of the sides 1 8 of each radial tooth 1 7 into contact with the facing radial side 1 4 of the corresponding groove 1 2. The force generated by the tightening torque in correspondence with these surfaces is normal thereto, affording the advantages mentioned above.

The invention has been described in the appended drawings which reference to an example of a recess having six equi-angularly spaced grooves. Clearly, however, there may be any number of grooves and a corresponding number of radial teeth on the tightening key. Six- or four-grooved solutions are, however, preferred.

The angle of inclination of the base wall 1 3 of the grooves 1 2 to the axis 9 of the screw may also be of any magnitude even though the angle is preferably between 5t and 7 in order not to limit excessively the extent of the radial sides 1 4 which act as bearing and reaction surfaces during the tightening. The solution in which the base walls 1 3 are portions of a conical surface is also preferred to that in which the base walls form part of the faces of a pyramid with a regular base coaxial with the axis of the screw.

The angles A and B which define the width and the spacing of the grooves 1 2 of the recess of the screw may also be different from each other depending upon the desired clearances and the strength characteristics which are to be achieved for the teeth of the recess and for the teeth of the tightening key.

The diameter of the smaller end of the tightening key must be chosen so as to prevent this end of the key coming into contact with the bottom of the recess in the screw head.

The "conical" coupling between the tightening key and the screw head during engagement of the key with the head allows perfect centering of the key to be achieved.

Fig. 2 illustrates a variant in which the head of the screw has, instead of a recess, an axial appendage 22 of a form complementary to that of the recess 5 illustrated in Fig. 1.

Obviously in this case the tightening key must have an engagement surface complementary to that of the axial appendage 22.

In Figs. 1 and 2, parts having equivalent functions are indicated by the same reference numerals.

By virtue of the characteristics which are described above, the following advantages are, in conclusion obtained: a) it is not necessary to adopt strict manufacturing tolerances for the screw and the tightening key, b) the engagement of the tightening key with the head of the screw is facilitated by the possibility of the greater clearances "g" and "f' illustrated in Fig. 5, by the entrance bevels 5a, 1 7a and by the conical form of the engagement surfaces on the head of the screw and the tightening key, c) once the tightening key has been engaged with the head of the screw, the mutual conical coupling allows perfect centering of the key relative to the screw to be achieved, d) at the beginning and during screwing, the bearing and reaction surfaces on which the forces generated by the tightening torque act are radial planes passing through the axis of the screw. The forces are thus normal to these surfaces and do not give rise to deformation beyond the limits of elastic deformation; consequently it is also possible to determine the magnitude of the tightening torque with great precision, e) the possibility of undoing the tightened screw is ensured, both by means of the specific key described above and with conventional keys adapted to engage the inner faces 1 5 of the recess 4 or of the axial appendage 22. In the first case an ordinary hexagonal key suffices while in the second case a known key with articulted teeth may be used.

Naturally the present invention also extends to other models which achieve equal utility by using the same innovative concept.

Claims

1. Screw, characterised by the combination of the following characteristics: a) the head (3) of the screw (1) has an engagement surface (5, 22) for a tightening key, which has a multilobed profile (7) in a plane perpendicular to the axis of the screw including a circumferential series of equi-an gularly spaced lobes projecting radially outwardly, in which each lobe includes two radial sides (8) which lie at least partially in two planes passing through the axis (9) of the screw, and a central section (10) which joins the radially outer ends of the two sides (8) of each lobe, b) the portions (13) of the said surface for engagement with the tightening key which define the said central sections of the lobes of the multi-lobed profile lie on a single, imaginary conical or pyramidal surface coaxial with the screw.

2. Screw according to Claim 1, characterised in that the said engagement surface of the tightening key is the side surface of an axial recess (5) formed in the head (3) of the screw (1).

3. Screw according to Claim 1, characterised in that the engagement surface of the tightening key is the side surface of an axial appendage (22) of the head (3) of the screw (1).

4. Screw according to Claim 1, characterised in that the sections (11) interconnecting one lobe and the next of the said multilobed profile (7) are disposed along the sides of a regular polygon having its centre on the axis (9) of the screw.

5. Screw according to claim 4, characterised in that the portions (15) of the said engagement surface (5) of the tightening key which define the sections (11) interconnecting one lobe and the next of the multilobed profile (7) are planes parallel to the axis of the screw.

6. Screw according to claim 1, characterised in that the portions (15) of the engagement surface (5) of the tightening key which define the sections (11) interconnecting one lobe and the next of the multilobed progile (7) lie on a single, imaginary conical or pyramidal surface coaxial with the screw.

7. Tightening key for operating a screw according to any one of the preceding claims, characterised in that it has an engagement surface complementary with the corresponding engagement surface of the screw.

8. A screw substantially as hereinbefore described with reference to, and as shown in, Fig. 1 of the accompanying drawings.

9. A screw substantially as hereinbefore described with reference to, and as shown in, Figs. 2 and 3 of the accompanying drawings.

1 0. A tightening key substantially as hereinbefore described with reference to, and as shown in, Fig. 4 of the accompanying drawings.

11. Any novel feature or combination of features described herein.