Torque Test Explained

KATO Fastening Systems certifies Tangless® and tanged self-locking CoilThread® screw thread inserts to NAS1130 and NASM21209 respectively. The 15-cycle torque test is a particular test outlined across multiple National Aerospace Specifications. The thread preparation of the aluminum test block is outlined in NASM33537. The insert dimensions are detailed in NAS1130 and/or NASM21209. Finally, the test bolt requirements and torque measurement procedure are explained in NASM8846. For reference, the coverage of the specifications mentioned in this article can be found on our website.

Thread Preparation Requirements

NASM8846 is the procurement specification for all of KATO CoilThread inserts. In this specification, the thread preparation requirements (found in NASM33537) are listed.

The internal threads must be:

  • Aluminum alloy 2024-T4 or T351.
  • Class 3B (Aerospace class of fit) - For gaging procedure to confirm conformance to Class 3B, see (Gaging procedure article). Dimensions for internal threads found in NASM33537.

Bolt Properties

Stated in NASM8846, the bolt must be:

  • Cadmium plated.
  • Class 3A (Aerospace class of fit). It must meet class 3A after cadmium plating.
  • Bolts must be of sufficient length so the thread runout does not enter the insert and that a minimum of one full thread extends past the end of the insert.
  • A new bolt must be used for each full 15 cycle torque test.

Torque Measurement Procedure

Using internal and external threads outline in the above sections, measuring the prevailing torque of the installed insert should be done per the following procedure:

  • Thread the cadmium plated bolt, by hand, into the insert until it contacts the locking coil. You will know it engages the locking coil, because even with a 2-56 insert, additional significant effort is needed to thread a bolt past a locking coil by hand.
  • Measure the maximum torque needed for the bolt to pass into and past the locking coil(s) for three complete turns. This value is to be compared to the table supplied in NASM8846 (link to locking torque table).
  • Avoid adding any axial load to the insert during measurement. This can skew results high.
  • Avoid heating of the bolt and/or insert during measurement. Slow and steady turns to yield dependable measurements.
  • To obtain the minimum breakaway torque, measure the maximum torque required to remove the bolt the same three turns from the locking coil. Compare this number to the same locking torque table as above. This completes one cycle.
  • Repeat for a total of 15 cycles. The values of all cycles recorded must conform to the torque table referenced in NASM8846.

So, the maximum locking (prevailing) torque going into the bolt is measured, as is the minimum torque required to remove the bolt. Semantically speaking, the torque value table measures the maximum torque for installation, and the minimum torque for removal. Not, a range for installation. The minimum breakaway torque is the important number for design engineers.

Differing Torque Values

When requested by the customer, KATO supplies the 15-cycle torque test performed at the manufacturing facility. However, it is always advised that customer perform their own torque test to verify they get the desired values in their application. It is unlikely the customer will get the exact torque values measured at the manufacturing facility. Cadmium plated bolts are rarely used in the field anymore, and different frictional coefficients can yield different torque results. There should be minimal difference between measured torque values assuming galling does not occur. Soft, uncoated stainless steel bolts are very popular in the aerospace industry, but are not allowed for testing per NASM8846. Stainless steel bolts tend to gall. For galling protection, please see our technical article.

If you have any questions, please contact us.

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