Often our readers have faced the phenomenon of seizure, of which we have already discussed extensively in our blog. This phenomenon is often due to the reckless use of self-braking stainless steel nuts. The numerous reports of similar incidents, led our expert to make some considerations on this product.
What international regulation should be considered?
The international standards to consider are :
Both regulations seem to exclude this type of nuts from their application.
In these points:
- ISO 3506-2 (in establishing the mechanical properties of stainless steel nuts)
- ISO 2320 (in establishing and mechanical properties and performance of brake nuts).
This is quoted verbatim in ISO 3506-2 regulation (fig.1),“It does not apply to nut requiring properties such as locking abilities and weldability”, while remaining the rule of reference for most of the products. In fact 3.1 certificates according to ISO 10204, ofter refer to this rule for post-production tests,
ISO 2320 regulation (fig.2), instead, refers only to carbon steels, so no stainless steel.
In practice, DIN 985-982-980-986 self-locking nuts are excluded from application.
So what are the tests that are usually carried out on austenitic stainless steel nuts?
The tests usually carried out on austenitic stainless steel nuts are:
- Chemical composition
- Mechanical Property
- Load test
In the absence of a regulatory reference, either the agreement between purchasers and producers is maintained, or we adhere to the normative references inserted in the single norms:
- DIN 982 refers to DIN EN ISO 898 and DIN 267-15
- DIN 985 refers to DIN267-15
- DIN 986 refers to DIN EN ISO 898-2 and DIN 267-15
- DIN 980 refers to DIN EN ISO 898-2 and 267-4
Also DIN 985-982-986 regulations, which has been withdrawn in favor of the ISO 10511-10512-10513, refer exclusively to carbon steel and have no reference to stainless steel. In both versions, the old Din versions and the new ISO versions.
Moreover, looking at the tables, in ISO 898 (standard establishing “the mechanical properties of the carbon steel and alloy steel fasteners”) the data of the second part are not those of austenitic stainless steel.
Why is it difficult to find in the standards, specific references to self-braking stainless steel nuts?
This is probably due to the thermal coefficient K of this material.
STAINLESS STEEL AND HEAT
Stainless steel is a bad heat conductor: it warms up very quickly and e gives its heat very slowly (this is one of the reasons that makes it very appreciated in kitchen utensils).
This factor causes the fillets to run the risk of overheating, deforming and welding together giving rise to the seizure phenomenon (intrinsic phenomenon to the nature of threaded connections of this material).
WHAT ARE THE CONSEQUENCES OF SELF-BRAKING NUTS?
Self-braking nuts (due to their anti-unscrewing function) increase friction on threads and, therefore, heat.
In the case of nuts that are not entirely metallic, instead, the heat is retained inside the thread, because the rings are made of polyamide (an insulating material).
Finally, we know that the thermal expansion of stainless steel is greater than carbon steel, so you can easily guess why the self-braking nuts in stainless steel can run into the phenomenon of seizure.
What precautions must we have, then, in using these dice?
– the use of these nuts on long threads and on connections that need to be unscrewed for maitenance
- prevention of the phenomenon with special treatments (es. Gleitmo)
- using A4 nuts and A2 screws
- with the use of anti-gripping products (like Based on molybdenum bisulphide products).
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