Self locking nuts were introduced to the British aircraft industry in the 1930’s they were widely adopted and soon constituted a large proportion of the nuts used in aircraft construction.
Being proprietary articles, the nuts were subject to test and approval requirements. These were supervised by the Royal Aircraft Establishment (RAE) Farnborough. In 1942, the first issue of AD114, setting out design and test requirements, was published by the RAE and referenced ME9/7005/13/EB/55. The term “stiff nut” was chosen as the most suitable descriptive name for generic use: “self-locking nut” is now generally used.
The first issue of AD114 was in the initial step towards standardisation of stiff nuts, and specified certain general requirements and the minimum performance acceptable in respect of retaining properties. Manufacturers used proprietary drawings approved by the RAE and nuts were known to users by proprietary names and part numbers. In 1947 drawings were issued in the Society of British Aerospace Companies (SBAC) AGS series, providing common part numbers to which nuts of any make could be supplied. Maximum envelope dimensions were specified and other features affecting interchange ability for users’ information. Later, performance testing of production quantities was introduced to ensure that functional properties were maintained at or above the type testing of prototype nuts when submitted for approval.
Eventually, the Ministry of Defence (and its predecessors) became the custodian of AD114, responsibility being invested with AD/Stan (Air)* who also became SBAC authorised Design Authority, with AQD as the Quality Assurance Authority, for the AGS2000 series stiff nuts. However in 1982 a policy decision effectively reduced the visibility of AD/Stan (Air)* continuing as Design Authority. As a direct result of consultation with SBAC it was decided to convert AD114 to a SBAC Technical Specification (TS120) the authorised Design Authority is now the SBAC Sub-Committee on Airframe Standards.
In the 1970’s emphasis was placed on the need for a revision of BS2A125 through 2A216: 1958 – stiff nuts (unified threads) for aircraft. The appropriate BSI and SBAC committees considered it advisable to update AD114 on the lines to be adopted in the revision of BS, so that uniformity of requirements and the use of common test equipment could be achieved with respect to technical requirements and quality assurance aspects of the Unified and AGS series of stiff nuts.
The revised edition of BS2A125 through 2A216 was published in 1981: AD114 was revised and published as issue 6 in January 1982.
Issue 6 of AD114 superseded the partially modified issue 5, issued as an interim measure in July 1979. It is recognised; however, the stocks of stiff nuts manufactured to the requirements of issue 5 and 6 are still available from manufactures and stockists. Such stocks are acceptable for use until they are exhausted. It is essential, however that all stiff nuts manufactured to TS120 with effect from its publication must conform to the requirements stated therein.
Note: The title AD114 was previously “design requirements, qualification approval tests and acceptance tests for AGS2000 series stiff nuts (BA and BSF threads) *Previously AD/SAE, AD/RS(PE) and later SAE 1-4 and ADR(S)