Bolts, screws and nuts modified by either undersizing of the bolt or screw thread as required by ISO 965-4 or oversize tapping of the nut thread in accordance with ISO 965-5, if manufactured to the full range of dimensional and mechanical property tolerances, may notachieve the expected assembly strength when combined with the specified mating component.
The reduction in assembly strength is due to the reduction in shear strength of the modified threads; refer to the work of Alexander1) on the strength of screw threads.
The following proposals indicate methods by which the full assembly strength of a 6g/6H thread tolerance combination may be achieved, when modified thread combinations are used.
Likewise, the removal of small particles of the coating by paring or whittling shall not constitute failure and therefore not be grounds for rejection.
a) Bolts and screws manufactured with thread tolerance class 6az in accordance with ISO 965-4The bolts or screws should not be manufactured to the minimum tensile strength, Rm, specified in ISO 898-1.
Care should be taken with bolts and screws of property class 8.8 not to exceed the maximum hardness of property class 8.8, because of the risk of hydrogen embrittlement.
b) Nuts manufactured with thread tolerance class 6AZ in accordance with ISO 965-5To achieve full assembly strength with nuts tapped oversize to thread tolerance class 6AZ.
two options may be considered:
1) Fit the bolts and screws with nuts of one property class higher, i.e. bolt or screw of
property class 8.8 with nut of property class 10, etc.
2) Fit the bolts and screws with nuts of the same property class but of style 2 nut height
instead of style 1.
For diameters greater than M10 in order to achieve the full assembly strength of nuts tapped oversize to thread tolerance class 6AX, nuts of an even higher property class than those used with thread tolerance class 6AZ should be used. In some countries, where this thread tolerance class is generally used, the national standards require the use of nuts of two property classes higher.