When in doubt

Let's look at a common scenario involving the use of a dimensional standard such as ISO 10512 which is for prevailing torque nuts with a non-metallic insert, in metric fine pitch thread and in property classes 6, 8 and 10.

Let's look at the information in this standard:
1. A drawing in Figure 1 of what the fastener should look like.
2. Dimensions (including tolerances) of the fastener.
3. It states that nuts have insufficient height and are fine pitched so there is a high risk of thread stripping so ISO 7041 nuts are preferably used. This is an interesting note in Paragraph 1 because it is basically saying - please don't use this standard if you can help it!
4. In Table 2 we are presented with the requirements for the standard. In this case:
a) Nut is to be made from steel.
b) Insert is made from non metal, polyamide (nylon) is suggested.
c) Property classes 6, 8 and 10 are possible and should comply with ISO 898-2 and ISO 2320.
d) There is a caveat that Class 10 fasteners can only be in sizes M16 and below. Larger diameter fasteners are not specified.

That seems like an awful lot of information. However this standard cannot be viewed in isolation. We have to consider what makes up a fastener:
1. General Requirements. ISO 8992
2. Thread. ISO 261, ISO 724, ISO 965-2
3. Mechanical Performance. ISO 898-2, ISO 2320
4. Tolerances. ISO 4759-1
5. Finish. ISO 4042, ISO 10683
6. Surface Integrity. ISO 6157-2
7. Acceptability. ISO 3269

As you can see there are a number of other standards which the dimensional standard (in this case ISO 10512) references. In all instances, the dimensional standard takes precedence.

For example, ISO 898-2 provides specifications for Class 12 fasteners. However, as ISO 10512 does not specify in Table 2 that class 12 fasteners are permitted, a fastener can be made dimensionally to ISO 10512 using class 12 material but it is considered a SPECIAL fastener because it is out of the scope of the dimensional standard.

Let's look at another fastener. ISO 4035 specifies thin nuts in style 0 in dimensions from M1.6 to M60. However, Table 3 in section 4 states that for fasteners between M3 and M39, they can be made from Classes 04 or 05 to ISO 898-2, anything outside that size range is as per agreement between manufacturer and customer. This also means that you can't make these fasteners in Class 10.

When in doubt - the Dimensional Standard takes precedence over the associated standards. Anything that is not to the dimensional standard is to be classified as a Special Fastener.

When in doubt - if a fastener dimensional standard has been withdrawn and a customer does not wish for an alternative, then whilst it can be made to the withdrawn fastener specification, the fastener will be classified as a Special Fastener.