GB1564904A

GB1564904A - Self tapping screw

Info

Publication number: GB1564904A
Application number: GB4007475A
Authority: GB
Original assignee: GKN Screws and Fasteners Ltd
Current assignee: GKN Screws and Fasteners Ltd
Priority date: 1975-10-01
Filing date: 1975-10-01
Publication date: 1980-04-16

Description

(54) SELF TAPPING SCREW (71) We, G. K. N. SCREWS & FASTENERS LIMITED, a British Company of Heath Street, Smethwick, Warley, West Midlands, 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: This invention relates to a self tapping screw having a head and a shank with a thread extending over at least a portion thereof.

An object of the present invention is to provide a new and improved self tapping screw.

According to the present invention we provide a self tapping screw having a shank with a thread extending over at least a part thereof, the thread being of substantially triangular form and having a thread helix angle lying in the range 170 to 190.

Compared with a standard self tapping screw of the thread forming type according to BS 4174 a screw, of the thread forming type, embodying the present invention has an increased helix angle and this increases the stripping torque (i.e. the torque at which the threads formed in the material into which the screw is being driven are stripped) and the tapping torque (i.e. the torque required to rotate the screw to cause it to tap into the material into which it is being driven) compared with the stripping and tapping torques obtained with the standard self tapping screw, when mounted into a pilot hole of equal size.

Preferably the thread angle of the screw lies in the range 450 to 55 .

Compared to the standard self tapping screw a screw embodying the invention may have a reduced thread angle and this reduces the tapping torque (compared with the hereinbefore mentioned increased tapping torque which would otherwise prevail with a thread helix angle lying in the range 17 to 190), as the material into which it is being driven is displaced less as the thread angle decreases.

It is preferred that the thread is a two start thread as this gives balanced driving and reduces assembly time.

The major diameter of the screw may lie in the range .085 ins. to 0.195 ins.

For any given size of screw the major diameter of a screw embodying the invention should preferably be larger than the major diameter of a standard self tapping screw.

The ratio of major diameter to thread height may lie in the range 6.2-6.85:1.

Compared to the standard self tapping screw, the last two of the above features ensure, assuming the same size of pilot hole is provided as is normal to provide with the standard self tapping screw, that there is still obtained at least the normal extent of full thread engagemen whilst the core diameter of the screw is less than that of the standard self tapping screw and thus binding of the material into which the screw is being driven, on the core and which would lead to an increase in the tapping torque, is reduced or is eliminated.

A screw embodying the present invention is particularly suitable for use in synthetic pliable plastics material such as nylon, although the screw may be used in aI1 the other materials into which a standard self tapping screw can be driven.

When a standard self tapping screw is driven into relatively soft synthetic plastics material the following disadvantages arise.

There is relatively little difference between the stripping torque and the applica tion torque (i.e. the torque which is required to be applied to the fastener to fully drive it and tighten it in the material). This leads to frequent stripping of the hole tapped in the material both in cases of manual insertion where the operative misjudges the torque required to be applied and also when using a power driver with an automatic torque limiting clutch due to the inability, under normal production conditions, to achieve consistent torque limiting. Furthermore under such production conditions it is frequently the case that an operative is required to insert self tapping screws both into metal and into a synthetic plastics material using the same tool.A higher tapping torque is required to drive a standard self-tapping screw into a sheet metal work-piece than is required to drive the screw into the plastics material and thus the torque limiting clutch of the tool has to be set to permit driving of the screws into sheet metal leading to stripping of the formed thread when the fastener is driven into the plastics material.

A screw embodying the present invention overcomes this problem since, as explained hereinbefore, due to the relatively high helix angle, the suipping torque is increased.

Because of the fact that an increased helix angle requires a greater displacement of the material into which the screw is being driven for each rotation of the screw the tapping torque is increased. This could be maintained at an acceptable level by increasing the pilot hole size but this would lead to a reduction in the extent of thread engagement.

It is for this reason that desirably the thread angle of a screw embodying the invention is reduced as this reduces the tapping torque whilst permitting the screw to be inserted into a standard size pilot hole and still to achieve a nominal 70% full thread engagement.

In summary, a preferred example of a screw embodying the present invention has, compared to the standard self tapping screw, a higher stipping torque, a tapping torque sufficiently below the application torque so that less critical control of the application torque is required thus permitting of easy tapping without failure of the fastener (it being appreciated that the application torque is usually set at or below the minimum specified torsional strength of the fastener) an increased "pull-out" load and a decreased assembly time, and has these features when inserted into a pilot hole of conventional diameter for the size of screw concerned. In addition the pilot hole size is less critical.

Although the above discussion has been in relation to a thread forming type of screw the invention can be applied to a thread cutting type of screw having cutting flutes extending generally axially of the screw from adjacent the point.

The invention will now be described in more detail by way of example with reference to the accompanying drawings wherein: Figure 1 is a fragmentary side elevation of a screw embodying the invention; Figure Zis a diagrammatic illustration of the thread form of a standard nominal 6 gauge self tapping screw of the thread forming type, Figure 3 is a diagrammatic illustration of the thread form of a screw embodying the invention corresponding to a standard 6 gauge screw.

Referring now to Figure 1 of the drawings there is shown a self-tapping screw of the thread forming type having a head 10 and a shank 11. The head 10: may be of any desired type and may have a Pozidriv (Registered Trade Mark) recess therein, or may have some other form of driving recess therein or, if desired, may be provided with an external wrench engageable formation.

The shank 11 is generally cylindrical and has a tapered entering end portion, not shown. If desired the entering end portion may be in the form of a drill tip of known form. Also if desired, cutting flutes may be provided in the shank adjacent the point of a thread cutting type of screw is required.

The shank 11 has a two start thread 14 formed thereon over the whole of the length of the shank though if desired the thread may extend over only a portion of the shank.

The thread has a helix angle of 180. If desired, however, the helix angle may be any angle lying in the range 17 to 190. If the helix angle is less than 170 the desired increase in stripping torque is not achieved whilst if the angle is above 190 then the angle will approach relatively closely to the thread friction angle in certain materials and if this should occur then a very low axial pull out load would be required to remove the fastener.

In addition, problems would be encountered during manufacture of the screw by conventional thread rolling techniques due to a tendency of the thread to split into layers or leaves.

The thread is of essentially triangular form. The included angle, ,; of the thread is 500. The thread angle may, if desired, lie in the range 45 to 55 . If the thread angle is above 550 the desired reduction in tapping torque is not achieved whilst if the thread angle is below 450 there s3Lrl be a tes?d! ncy9 Er the threa ' f-g- bm layers or leaves due to the extent to which material has to be moved to form the thread having the designed increase in helix angle.

The ratio of the major diameter Y to the thread height h is 6.2 to 6.85:1 and the parameters of a screw embodying the present invention may be related by the following equation:

where D=Rolling Dia (Screw Blank Dia) P=Thread Pitch h=Tread Height Thread Angle d=1.05 XCor Diameter w.

Figure 2 shows the thread of a standard nominal 6 gauge self tapping-screw of the thread forming type whilst Figure 3 shows the thread of a screw embodying the invention corresponding to a standard 6 gauge screw with the thread parameters related to the thread parameters of the standard 6 gauge self-tapping screw shown in Figure size 2.

For other sizes of screw the relationship between the parameters is different.

Table 1 gives the values of the thread parameters of a screw embodying the invention for 4 screw sizes.

TABLE 1 Screw Gauge 4 6 8 10 T.P.I. 20 18 15 13 Major Dia .120/.116 .145/.141 .171/.167 .195/.189 Core Dia .084/.079 .102/.097 .1191.114 .136/.131 Major Dia (Mean) .118 .1414 .168 .192 Thread Depth .022/.016 .024/.0I75 .0295/.0225 .0335/.0265 Thread Depth (Mean)h .019 .0207 .026 .030 Pitch=+ Lead .050 .0555 .0667 .0769 Thread Helix Angle 180--58' 170--15' 170--40' 17 51' Rolling Dia.--Blank .095/.093 .115/.113 .135/.133 .154/.152 The pilot hole diameter is the same as the pilot hole diameter conventionally used for a standard self-tapping screw of the same size for insertion of the screw into pliable plastic materials.

The screw is preferably made, by conventional thread rolling techniques, of case hardenable medium carbon steel and after thread rolling the screws are case hardened.

Two samples of size 4 screws made in accordance with the diamensions set out in Table 1 and as shown in Figure 1 of the drawing were driven into a 0.094" diameter pilot hole in nylon 66 and the following results were obtained: Tapping Torque 4.5 Ibs.in.max 4 Ibs.in.min Stripping Torque 40 Ibs.in.max 30 Ibs.in.min (Bit broken) (recess reamed) Axial Pull Out Load 765 lbs.max 740 lbs.min (5/16" penetration).

By way of comparison a standard size 4 self-tapping, of the thread forming type, screw was tested for pull out in the same material in the same size of pilot hole and with the same penetration and a pull out load of 380 Ibs. was measured.

A further test performed on a standard self-tapping of the thread forming type screw of size 4 was to drive the screw into a pliable glass filled nylon material, this was not identical to nylon 66 but it was found that the tapping torque required for a standard self tapping screw was the same when inserted into the pliable glass filled nylon material and nylon 66 and it is thought reasonable to assume that the characteristics of the materials for the purposes of these tests are similar.

The table below sets out the values of tapping torque, stripping torque and the minimum torsional strength of the screws in respect of a standard self tapping screw of the thread forming type according to B.S.S. 4174 when inserted into the glass filled nylon material and a screw embodying the invention when inserted into nylon 66. Figures quoted are average over 5 samples.

TABLE 2 Standard Self Tapping Screw Invented Screw Tapping Torque Ibs.in. 5 4.25 Stripping Torque lbs.in. 13.5 35 Minimum Torsional Strength Ibs.in. 13 13 From the above it will be seen that in the case of the standard self tapping screw the tapping torque is 38.4% of the minimum torsional strength whilst with the invented screw the tapping torque is 32.6% of the minimum torsional strength. This means that less of a given "application torque" would be utilised in performing the tanning operation and a higher ratio between application torque and tapping torque is achieved with the invented screw compared with the standard self tapping screw.

In addition, with the standard self tapping screw the stripping torque is substantially equal to the application torque and 2.7xthe tapping torque whilst with the invented screw the stripping torque is 2.69the application torque and 8.24Xthe tapping torque. This means that the safety factor between the tapping and stripping torques is approximately 3 times greater with the invented screw compared with a standard self tapping screw.

These two features mean that torque control of the driving means becomes less critical for the invented screw than is necessary for inserting a standard self tapping screw.

These two features together with the high pull out load obtained with the invented screw demonstrates the advantages of a screw embodying the present invention compared with a standard self-tapping screw. Further tests have been carried out to compare a screw embodying the present invention with a standard "TWINfast" (Registered Trade Mark) fastener which has in its 4 gauge size 24 threads per inch and a nominal diameter of .112 ins.

In these tests size 4 screws were driven into medium density faced chip board and a pilot hole size of 1.8 mm was used. The results obtained are set out below: TABLE 4 Tapping Stripping Axial Torque Torque Pull-out Invented Screw 4 lbs.ins. 15 lbs.ins. 135 lbs.

TWINfast 4 Ibs.ins. 10 lbs.ins. 112 Ibs.

It will be seen that with the invented screw the stripping torque is 3.75 times the tapping torque whilst with the TWINfast screw the stripping torque is 2.5the tapping torque. This means that the safety factor beween tapping and stripping torques is 1.5 X greater with the invented screw compared with a TWINfast screw.

It will also be seen that there is a 20 ', increase in axial pull-out load using similar insertion depths of 5/16th".

A screw embodying the invention may be used in conjunction with an unthreaded nut having a plain cylindrical bore made for example of synthetic plastics material such as nylon and having a conventional hexagonal outer configuration. Comparative tests were carried out using 6 and 8 gauge standard self tapping screws of the thread forming type. The results of these tests are set out in Table 5.

TABLE 5 Stripping Application Tapping Torque (Ib./ins.) Torque (lb./ins.) Torque (lb./ins.) 6 gauge 19.4 18 5 Invented Screw 8 gauge 31.75 30 8 Standard Self 6 gauge 12.25 18 5 Tapping Screw 8 gauge 20.38 30 8 The application torque for a self-tapping screw is usually 75-80% of the torsional strength of the screw and the minimum torsional strength of a 6 gauge screw is 24 Ib./ins. and of an 8 gauge screw is 30 Ib./ins.

WHAT WE CLAIM IS: 1. A self tapping screw having a shank with a thread extending over at least part thereof, the thread being of substantially triangular form and having a thread helix angle lying in the range 170- 190.

2. A screw according to Claim 1 wherein the thread angle of the screw lies in the range 450 to 55 .

3. A screw according to Claim 1 or Claim 2 wherein the thread is a two start thread.

4. A screw according to any one of the preceding claims wherein the major diameter of the screw lies in the range 0.085 ins. to 0.195 ins.

5. A screw according to any one of the preceding claims wherein the ratio of the major diameter to thread height lies in the range 6.2-6.85:1.

6. A screw according to any one of the preceding claims wherein the thread helix angle is 180.

7. A screw according to any one of the preceding claims wherein the thread angle is 500.

8. A screw according to any one of the preceding claims wherein the rolling diameter, D; thread pitch, p; thread height, h; thread angle, a; and the core diameter x 1.05, d; are related by the expression

9. A screw according to any one of the preceding claims wherein the screw is of the thread forming type.

10. A screw according to any one of Claims 1 to 9 wherein the screw is of the thread cutting type.

11. A screw according to Claim 1 substantially as hereinbefore described with reference to and as shown in Figures 1 and 3 of the accompanying drawings.

12. In combination a self-tapping screw according to any one of the preceding claims driven into a workpiece wherein there is a nominal 70% of full thread engagement between the screw thread and the workpiece.

13. In combination, a self-tapping screw according to any one of Claims 1 to 11 and an unthreaded nut having a plain cylindrical bore.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The application torque for a self-tapping screw is usually 75-80% of the torsional strength of the screw and the minimum torsional strength of a 6 gauge screw is 24 Ib./ins. and of an 8 gauge screw is 30 Ib./ins. WHAT WE CLAIM IS:

1. A self tapping screw having a shank with a thread extending over at least part thereof, the thread being of substantially triangular form and having a thread helix angle lying in the range 170- 190.