GB1590681A

GB1590681A - Method of welding a wheel nut body to a sheath and an apparatus for carrying out this method

Info

Publication number: GB1590681A
Application number: GB3287977A
Authority: GB
Original assignee: Towne Robinson Fastener Co
Current assignee: Towne Robinson Fastener Co
Priority date: 1977-08-05
Filing date: 1977-08-05
Publication date: 1981-06-03

Description

(54) A METHOD OF WELDING A WHEEL NUT BODY TO A SHEATH AND AN APPARATUS FOR CARRYING OUT THIS METHOD (71) We, TOWNE ROBINSON FAS TENER COMPANY, a company organized and existing under the state of Michigan, United States of America, of 4401 Wyoming, Dearborn, Michigan 48121, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of welding a wheel nut body to a sheath to produce a sheathed wheel nut, and to an apparatus for carrying out this method.

Most modern automotive wheels are removably attached to their axle by a ring of threaded studs which extend outwardly from frusto-conical depressions formed in the axle end. The wheel contains a similar ring of holes formed with frusto-conically dished edges which abut the depressions in the axle end so that the studs extend through the wheel holes. Wheel nuts are then threaded on the studs to secure the wheel to the axle.

The nuts and extending studs may be surrounded by a dish-shaped cover which attaches to the wheel to hide and protect the nuts and studs. Alternatively, wheel nuts having integral sheaths that enclose the volume about the axial end of the nut opposite to that which contacts the wheel may be employed instead of the wheel covers. The sheath protects the projecting end of the stud from damage due to weathering and impacts.

These sheathed nuts have often been formed from bar stock, on turning machines, with the sheaths being formed integrally with the nut body. Alternatively, sheathed nuts have been proposed having a conventional nut body fully covered by a stainless steel sheath which exposes one threaded opening of the nut body and forms a domed cover over the other opening. Our United States Patent Specification No. 3,364,806 discloses a sheathed wheel nut in which the nut body has a frustoconical section adapted to bear against a corresponding depression in a wheel and this frusto-conical section is left unsheathed, the sheath terminating at a land formed between one end of the wrench flats of the nut body and the adjacent section of the frusto-conical end.

On this nut body the free edges of the sheath are crimped against a shoulder formed at this land to retain the sheath to the nut body but wrenching forces and accidental impacts from stones and the like may loosen the sheath so that it tends to rattle when the vehicle is driven. Adhesives have been applied between the contacting surfaces of the sheath and nut body to prevent this loosening, but it is difficult and expensive to apply a quantity of adhesive which will securely retain the sheath to the nut body without any adhesive adhering on the threads.

It has also been proposed to weld the sheath to the nut body. This ensures rigid attachment and eliminates the danger of clogging the nut threads with the adhesive, but presents certain problems in application. In particular, the heat generated in the welding process may adversely affect the heat treatment of the nut body; for example, the nut body may be unacceptably softened, or the sheath may be discoloured or its oxidation resistance reduced.

The present invention provides a method of welding a wheel nut body to a sheath without causing substantial deleterious metallurgical changes in either member.

Accordingly the present invention provides a method of producing a sheathed wheel nut, from a wheel nut body having a polygonal outer surface portion extending axially from one end of the body and a frusto-conical surface portion extending axially from the other end of the body, and a stainless steel sheath having a polygonal portion for receiving the polygonal outer surface portion of the nut body and a closed end portion, said method comprising, fitting said sheath to said body with the closed end portion of the sheath covering said one end of the nut body and the polygonal outer surface portion of the nut body received within the polygonal portion of the sheath, the other end of the nut body projecting out of the sheath; connecting first electrode means to said other end of the nut body and second electrode means to said sheath in such a manner as to provide a relatively low electrical resistance between the first electrode means and the nut body and between the second electrode means and the sheath; pressing an area of the sheath against the nut body with sufficient force to reduce the interface electrical resistance between the sheath and the nut body at said area; and passing an electrical current having a peak value in excess of 50 KA between said electrode means for a duration of less than 30 microseconds, whereby the sheath is welded at said area to the nut body without causing substantial deleterious metallurgical changes to the sheath or the nut body.

The axis referred to herein of the nut or nut body used in the method of the invention is the axis of the central threaded aperture thereof through which the stud passes, and the term "axial" is to be construed accordingly.

It will be understood that because the contacts between the two electrode means and the nut body and sheath respectively are of relatively low electrical resistance the highest resistance portion of the electric circuit through which the electric pulse is passed is the interface area between the sheath and the nut body surface where the sheath is pressed against the nut body. The remainder of the confronting surfaces between the sheath and the nut body afford such an extremely high resistance that virtually no electric current passes therethrough. The current is thus concentrated into this interface area and a large conversion of electric power to heat occurs at that point. The method of the present invention thus bears resemblance to spot welding, although the welded area may take the form of a line.

The method of the present invention requires a sheath which leaves the frusto-conical end of the nut body exposed and is not applicable to other forms of sheathed wheel nuts where the sheath fully covers the nut body.

In the method of the present invention, preferably the forces exerted between the sheath and the nut body in the aforesaid area are not less than 1000 pounds per square inch, and to achieve this preferably the (or each if there is more than one-see below) second electrode exerts a pressure of not less than 1000 pounds per square inch on that part of the surface of the sheath with which it is in contact. Also, desirably, the peak current of the electric pulse is from 60,000 to 80,000 amperes and the duration of the pulse from 6 to 9 microseconds.

Conveniently, two second electrodes are used in the method of the invention, the two second electrodes being pressed with equal and opposite forces against diametrically opposed areas of the sheath. Each of the second electrodes is desirably provided with contacting surfaces capable of engaging each of a pair of adjacent wrench flats adjacent the line of intersection of said pair, the contacting surfaces of each second electrode preferably comprising two elongate surfaces each having their long axis parallel to the axis of the nut body.

The sheath used in the method of the invention is desirably provided with a portion extending normal to the axis of the nut body and the first electrode is pressed against the frusto-conical end of the nut body with a force having a component parallel to the axis of the nut body, the first and second electrodes imposing equal and opposite transverse forces on the nut body.

The passing of the electric pulse is desirably effected by discharging a capacitor through the first and second electrode means.

Naturally, the invention extends to a sheathed wheel nut welded by the method of the invention.

The present invention also provides apparatus for carrying out the method of the present invention, comprising: a press having two platens capable of relative movement toward and away from one another; said first electrode means mounted on one of the platens and capable of contacting said other end of a nut body to be welded; said second electrode means mounted on the other of the platens and capable of contacting a sheath to be welded; means for causing said second electrode means to press an area of the sheath against the nut body with sufficient force to reduce the interface electrical resistance between the sheath and the nut body at said area; a capacitor; and a current supply circuit including a switch and capable of discharging the capacitor through the first and second electrode means whilst said electrode means are connected with a nut body and a sheath respectively to produce a welding current of less than 30 microseconds duration and of peak value in excess of 50,000 amperes.

Preferred methods and apparatus of the invention will now be described in more detail, though by way or illustration only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a sheathed wheel nut welded by the method of the invention; Figure 2 is a section along the line 2-2 in Figure 1; Figure 3 is a section through an apparatus of the invention used for welding the sheathed wheel nut shown in Figures 1 and 2, and shows the apparatus with the sheathed wheel nut in position therein; Figure 4 is a section along the line 44 in Figure 3; Figure 5 is an exploded perspective view of the nut body and sheath of the sheathed nut shown in Figures 1 and 2, prior to welding; Figure 6 is a front elevation, partly in section, of a second apparatus of the invention, with a sheathed wheel nut being welded therein; Figure 7 shows the apparatus of Figure 6 in its welding position; Figure 8 is a section along the line 8-8 in Figure 7; Figure 9 is a section along the line 9-9 in Figure 6; and Figure 10 is a perspective view of a sheathed wheel nut welded in the apparatus shown in Figures 6 to 9.

The sheathed wheel nut shown in Figures 1 and 2, (generally designated 10) is generally similar to that disclosed in United States Patent Specification No. 3,364,806 and comprises a wheel nut body 12 having a central threaded aperture 14 and hexagonal wrench flats 16 which are parallel to the axis of the aperture i.e. the flats 16 form a polygonal outer surface portion of the body 12. At one axial end of the flats 16 there is a substantially flat end face 18, whilst at the opposed axial end of the flats 16 there is a frustoconical surface 20 adapted to mate with the frusto-conical depressions formed around the stud holes in a conventional automobile wheel. The ends of the wrench flats 16 adjacent to the surface 20 are formed with an inwardly tapering chamfer or shoulder 22 which meets a short cylindrical land 24 adjoining the surface 20. The land 24 has a diameter approximately the same as the distance between a pair of opposed wrench flats 16 so that the chamfer or shoulder 22 is present only adjacent the corners of the flats 16.

The nut body 12 is partially covered by a sheath formed of sheet metal, in fact 10-25 stainless steel i.e. stainless steel having a 10% nickel, 25 % chromium content. The sheath has flats 26 which form a polygon and, which extend over the wrench flats 16 and the free edge 28 of the sheath is turned over the chamfer or shoulder 22, so as to terminate adjacent the cylindrical land 24. This arrangement, disclosed in United States Patent Specification No. 3,364,806 provides a neat termination for the free edge 28 of the sheath, formed so that the edge will not interfere with the sceure joining of the sheathed nut to the wheel depression, and the forces exerted between the sheathed nut and the wheel will not tend to loosen the sheath from the nut body.

The axial end of the sheath remote from the edge 28 includes a flat, radially extending "ring" 30 which projects normal to the flats 26 of the sheath and has its inner surface in close abutment with the outer peripheral edge of the nut body end surface 18. This ring 30 extends around the whole perimeter of the sheath. The sheath also has a cylindrical domed end 32 extending upwardly from the nut body end surface 18; this domed end 32 is thus spaced from the adjacent axial end surface 18 of the nut body, thereby forming a chamber between the nut body and the sheath which can accommodate an end of a wheel stud projecting through the nut body.

The ring 30 overlies a section of the flat end surface 18 of the nut extending from the base of the domed section 32 to the edge of the ring connecting with the flats 26 of the sheath. The width of the ring varies along its perimeter and has its minimum at the centres of the flats 16; this minimum is not less then 0.025 inches.

The sheathed wheel nut 10 is substantially similar to that described in the aforementioned United States Patent Specification No.

3,364,806, except for the provision of the ring 30. In the nut 10, the contacting surfaces of the ring 30 and the nut body end surface 18 are resistance welded together at 34. This weld securely retains the sheath to the nut body 12 so that their engagement does not depend upon the engagement of the free edge 28 of the sheath with the chamfer 22 of the nut body 12. This weld securely retains the nut body 12 against forces that might tend to dislodge the sheath from the nut body during wrenching of the sheathed nut, and impact forces experienced whilst the sheathed nut is in service. As described below, the weld is formed in such a manner as to leave the finish of the sheath unimpaired.

The welding of the sheathed nut shown in Figures 1 and 2 is performed in the apparatus shown in Figures 3 and 4. This apparatus employs a press having a fixed lower platen 62 and an upper movable platen 64. The remainder of the press is conventional, and is not illustrated.

A lower steel die 66 is supported on the lower platen 62 and has an upwardly-extending annular support section 68. The internal diameter of the section 68 is equal to the internal diameter of the weld ring to be formed between the nut body and sheath. An annular retainer 70 formed of phenolic or similar thermo-setting synthetic resin is sup ported on the annular section 68 of the die.

The interior diameter of the retainer 70 is slightly larger than the cross dimension of the sheath between the corners of the wrench flats. Accordingly, when the loosely assembled nut body and sheath are inserted into the retainer 70, the domed section of the sheath extends within the annular section 68, with the outer surface of the sheath, at the ring 30 resting on the top of the annular section 68.

The sheath is held within the retainer 70, so that the unsheathed, chamfered end of the nut body projects upwardly thereabove.

A steel upper die member 72 is fixed to and projects downwardly from the upper platen 64. The die 72 has a central opening 74 and a chamfered edge 76 which extends at an angle complementary to the cone angle of the frusto-conical end of the nut body 12.

When the press platens 62 and 64 are brought together this chamfered edge 76 engages the frusto-conical end of the nut body and presses the nut body downwardly against the upper end of the annular die section 68, pressing the nut body 12 against the inner surface of the sheath at the ring 30. The press force is in the range of 3,000 to 4,000 pounds resulting in a pressure between the contacting surfaces of the sheath and nut body in excess of 1000 p.s.i.

A welding current is then applied to the nut body and sheath via a welding circuit which includes a transformed 78 having its secondary coil connected to the dies 66 and 72. The primary coil of the transformed 78 is connected in series with a contactor 80 across a bank of large electrolytic capacitors 82. The capacitors may be connected to a charging source by a second contactor 84. In operation, the contacts 80 are opened and the contacts 84 are closed to charge the capacitors 82. Then the contacts 84 are opened and the contacts 80 are closed, discharging the energy stored in the capacitors 82 through the primary coil of the transformer 78. This induces a current in the secondary coil which is passed between the nut body and the sheath at the ring 30.

The weld voltage varies between 2 and 5 volts and the peak weld current is in the range of 60,000 to 80,000 amperes. The weld time is from 6 to 9 microseconds. During welding heating occurs principally at the interface between the sheath and the nut body, in the area where the ring 30 is pressed into close contact with the nut body 12. The weld time is so short that the sheath does not oxidise and the heating action does not cause the carbon in the stainless steel sheath to migrate to the grain boundaries to lower corrosion resistance. The heat applied to the nut body is not sufficient to affect its hardness.

The welding circuit illustrated is a simplified version of a conventional capacitor discharge circuit, and it should be understood that any type of welding circuit which can apply an extremely short duration, high current pulse to the interface between the sheath and the nut body, while the nut body is being pressed into the sheath at the ring area, could be used in the method of the present invention.

An alternative form of apparatus for welding a sheath to a nut body is shown in Figures 6 to 8. This apparatus employs a fixed lower platen 100 and a movable upper platen 102. A lower die support 104 is affixed to the upper side of the lower platen 100. The support 104 has a recess 106 formed on its upper surface which is adapted to receive the domed section 32 of the stainless steel sheath. An insulated retainer 108 is affixed to the upper surface of the support 104 and rests beneath the ring 30 on the outer surface of the sheath, so that the domed section 32 projects into the recess 106.

A pair of second electrodes 110 and 112 are supported for horizontal sliding move ment along the upper edges of the support 104, at diametrically opposed points surround ing the cavity 106. The electrodes 110 and 112 are biassed toward their maximum sep aration from one another by a pair of coil springs 114 and 116. Their motion in this direction is limited by the contact between cam surfaces 118 and 120, formed on the radially outer sides of the electrodes 110 and 112 respectively, and contacting cam rollers 122 and 124 respectively which are affixed to downwardly projecting sections of a drive member 126 which is supported on the movable platen 102.

The radially inner parts of the electrodes 110 and 112 are each formed with a pair of surfaces 128. Grooves 130A are formed between each pair of extending surfaces 128 and the surfaces 128 are arranged to engage a pair of adjacent flats 26 of the sheath.

When the upper platen 102 is driven downwardly, the rollers 122 and 124 engage the cam surfaces 118 and 120 and force the elec trodes 110 and 112 radially inward. Each of the electrode surfaces 128 then contact the surfaces of the sheath 16 along lines on either side of one corner of the sheath. When the electrodes contact the sheath these lines extend along substantially the full length of the wrench flats.

At the same time that the electrodes 110 and 112 are moved into contact with the sheath the downward motion of the upper platen brings a cap 130 into contact with the exposed frusto-conical surface of nut body 12. The cap 130 is retained in a cavity 132 of the drive member 126 by a heavy coil spring 134. Thus when the cap 130 comes into contact with the frusto-conical part of the nut body 12 the continued downward motion of the drive member 126 compresses the spring 134.

The cap 130 acts as the first electrode.

The second electrodes 110 and 112 are connected to one terminal of a capacitative discharge power supply 136 while the cap 130 is connected to the other terminal. The power supply may be of the same type illustrated in Figure 3.

The electrodes 110 and 112 and the cap 130 exert forces on the sheath and nut body respectively, in excess of 1000 p.s.i. When the weld current is passed between the electrodes, it flows between the nut body and the sheath at the areas underlying the sheath surfaces contacted by the electrodes 110 and 112, welding these areas. The short duration of the electric pulse prevents a substantial propagation of heat from the weld site and accordingly the metallurgies of the nut body and the sheath are not substantially affected.

WHAT WE CLAIM IS: 1. A method of producing a sheathed wheel nut, from a wheel nut body having a polygonal outer surface portion extending axially from one end of the body and a frustoconical surface portion extending axially from the other end of the body, and a stainless steel sheath having a polygonal portion for receiving the polygonal outer surface portion of the nut body and a closed end portion, said method comprising, fitting said sheath to said body with the closed end portion of the sheath covering said one end of the nut body and the polygonal outer surface portion of the nut body received within the polygonal portion of the sheath, the other end of the nut body projecting out of the sheath; connecting first electrode means to said other end of the nut body and second electrode means to said sheath in such a manner as to provide a relatively low electrical resistance between the first electrode means and the nut body and between the second electrcde means and the sheath; pressing an area of the sheath against the nut body with sufficient force to reduce the interface electrical resistance between the sheath and the nut body at said area; and passing an electrical current having a peak value in excess of 50 KA between said electrode means for a duration of less than 30 microseconds, whereby the sheath is welded at said area to the nut body without causing substantial deleterious metallurgical changes to the sheath or the nut body.

2. A method as claimed in claim 1, in which the force exerted on the aforesaid area is not less than 1000 pounds per square inch.

3. A method as claimed in either of the preceding claims, in which the second electrode means comprises two electrodes which are pressed with equal and opposite force against diametrically opposed areas of the sheath to effect said reduction of interface electrical resistance thereat.

4. A method as claimed in claim 3, in which each of the second electrodes is provided with contacting surfaces engaging a pair of adjacent flats of the polygonal portion of the sheath adjacent the line of intersection of said pair.

5. A method as claimed in claim 4, in which the contacting surfaces of each second electrode comprise two elongate surfaces each having their long axis parallel to the axis of the nut body.

6. A method as claimed in claim 1 or claim 2, wherein said sheath has an annular portion extending normal to the axis of the nut body and interconnecting said polygonal portion and said closed end portion, said second electrode means is annular and is connected to said annular portion of the sheath, and said electrode means are urged together to press said annular portion of the sheath against the end face of the nut body at said one end thereof to effect the said reduction of inter face electrical resistance thereat.

7. A method as claimed in any one of the preceding claims in which the passing of the electric current is affected by discharging a capacitor through the first and second electrode means.

8. A method as claimed in any one of the preceding claims, in which the electric current has a peak value in the range of from 60,000 to 80,000 amperes.

9. A method as claimed in any one of the preceding claims, in which the electric current has a duration within the range 6 to 9 microseconds.

10. A method as claimed in claim 1 and substantially as herein described with reference to and as illustrated in Figs. 1 to 4 or Figs 5 to 10 of the accompanying drawings.

11. A sheathed wheel nut when produced by a method as claimed in any one of the preceding claims.

12. Apparatus for carrying out a method as claimed in any one of the preceding claims, the apparatus comprising: a press having two platens capable of relative movement toward and away from one another; said first electrode means mounted on one of the platens and capable of contacting said other end of a nut body to be welded; said second electrode means mounted on the other of the platens and capable of contacting a sheath to be welded; means for causing said second electrode means to press an area of the sheath against the nut body with sufficient force to reduce the interface electrical resistance between the sheath and the nut body at said area; a capacitor; and a current supply circuit including a switch and capable of discharging the capacitor

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The cap 130 acts as the first electrode. The second electrodes 110 and 112 are connected to one terminal of a capacitative discharge power supply 136 while the cap 130 is connected to the other terminal. The power supply may be of the same type illustrated in Figure 3. The electrodes 110 and 112 and the cap 130 exert forces on the sheath and nut body respectively, in excess of 1000 p.s.i. When the weld current is passed between the electrodes, it flows between the nut body and the sheath at the areas underlying the sheath surfaces contacted by the electrodes 110 and 112, welding these areas. The short duration of the electric pulse prevents a substantial propagation of heat from the weld site and accordingly the metallurgies of the nut body and the sheath are not substantially affected. WHAT WE CLAIM IS:

1. A method of producing a sheathed wheel nut, from a wheel nut body having a polygonal outer surface portion extending axially from one end of the body and a frustoconical surface portion extending axially from the other end of the body, and a stainless steel sheath having a polygonal portion for receiving the polygonal outer surface portion of the nut body and a closed end portion, said method comprising, fitting said sheath to said body with the closed end portion of the sheath covering said one end of the nut body and the polygonal outer surface portion of the nut body received within the polygonal portion of the sheath, the other end of the nut body projecting out of the sheath; connecting first electrode means to said other end of the nut body and second electrode means to said sheath in such a manner as to provide a relatively low electrical resistance between the first electrode means and the nut body and between the second electrcde means and the sheath; pressing an area of the sheath against the nut body with sufficient force to reduce the interface electrical resistance between the sheath and the nut body at said area; and passing an electrical current having a peak value in excess of 50 KA between said electrode means for a duration of less than 30 microseconds, whereby the sheath is welded at said area to the nut body without causing substantial deleterious metallurgical changes to the sheath or the nut body.

through the first and second electrode means whilst said electrode means are connected with a nut body and a sheath respectively to produce a welding current of less than 30 microseconds duration and of peak value in excess of 50,000 amperes.

13. Apparatus as claimed in claims 12 and substantially as herein described, with reference to and as illustrated in Figs. 3 and 4 or Figs. 6 to 9 of the accompanying drawings.