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Screw to use

Stainless Steel Self-Tapping Screws are manufactured from an Austenitic Steel which falls within the 18 per cent Chromium/10 per cent Nickel range which is considered suitable for general corrosive conditions. The actual specification is 305 S19 or A.I.S.I. Type 305, with a tensile strength of about 35/45 tonf/ins2(549-695 N/mm2).

These screws are NOT as hard as the more widely used carbon steel Self-Tapping Screws. They are ideal for use with aluminium alloy sheets but are in some cases unsuitable for use in other than the thinner Austenitic Stainless Steel sheets due to the galling tendency of this material. They should be used with care and tapping hole sizes are critical. We strongly recommend that any proposed application be tried out in practice and we are always prepared to advise on this subject.

Choice of the correct type of screw depends on several factors:
  • Material – malleable, brittle, thickness of section?
  • Method of preparing hole – punched, drilled rim, extruded, moulded?
  • Method of driving – hand or power screw-driver?

The obvious aim is to achieve as much engagement as possible between fastener and mating material with reasonable driving conditions to avoid over-stressing on the screw, operator fatigue (if hand driven) or difficult conditions for power screw-driver operation.

Ideally, the stripping torque for the application should equal the minimum torsional load of the screw (see table) thus fully utilising the product. Under these conditions, we recommend a tightening torque of 66 per cent of the stripping torque, to provide an adequate safety margin, since it is not always possible to ensure that power screwdrivers deliver an exact torque.

The second recommendation is that the tapping torque should be 33 per cent of the stripping torque and these two parameters – tightening torque and tapping - determine the type of self-tapping screw to use and the pilot hole size for the particular application.

It is not always possible to obtain these ideal conditions, for when using self-tapping screws in very thin material, soft or brittle plastics, a tightening torque of 65 per cent of the torsional load of the screw would be more than sufficient to strip the thread formed in the material. It is then necessary to consider a smaller pilot hole or rim extruded in this material and similarly a similar hole or deeper penetration in plastic. This in turn can lead to difficulties in starting, or failure due to splitting because of radial stresses induced on the surrounding material. It is a matter of balancing one factor against another to obtain optimum results.