TM 1-1500-204-23-6
(5)
If correct torque is not obtained at required stud height, remove stud with driver and remover and
install another stud. Refer to applicable aircraft maintenance manual for correct torque values.
2-11.
Helical Thread Inserts. Helical inserts are precision coils of 18-8 stainless steel wire cold worked to a tensile
strength of 200,000 psi (Rockwell C 43-50), diamond shaped, and agree with AMS 7245D and MIL1-8846 specifications.
Each insert has a driving tang with a notch to facilitate removal of the tang after the insert is screwed into a helical
tapped hole They are used as screw thread bushings. In addition to being used to restore damaged threads, they are
used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to protect
and strengthen tapped threads in light materials, metals, and plastics, particularly in locations which require frequent
assembly and disassembly, and/or where a screw-locking action is desired.
a. Types. Helical standard and screw-lock inserts are identical except for a number of flats formed on the middle
coil or coils of the screw-lock insert Helical screw-lock inserts (dyed red) are used to secure screws and bolts against
loosening under impact and vibration in both ferrous and nonferrous materials, and conform to MIL-I-8846 They eliminate
lockwiring, locknuts, and other supplementary locking devices. Inserts are available in unified coarse and fine thread
sizes in screw thread and screw-lock types, stud-lock, oversize and twinsert, and tapered pipe thread sizes Helical thread
inserts can be identified by referring to tables 2-13, 2-14.
b
Selection of Helical Insert. Hole size determination and insert selection are outlined in the following paragraphs.
(1)
Hole size determination. Determine the original tapped hole thread size, whether national coarse
or fine, and whether a standard screw thread or screw lock type of insert is required Next, determine the length of
damaged or stripped threads to be replaced and whether the application is in a through hole or blind hole A through hole
requires that the driving tang be removed This is not necessary in a blind hole, provided the length of insert permits the
tang to clear the point of the screw when the parts are assembled Use only stainless steel inserts unless otherwise
instructed.
(2)
Insert selection. Use the original nominal thread size and length of tapped hole, as determined
in paragraph a above, to select the proper insert and insert tool from tables 2-13 through 2-15. The proper insert length
for blind holes will permit full screw engagement without interference if the driving tang is retained. Because all inserts
are notched for tang break-off, the tang should be removed if possible.
CAUTION
Use only bolts with correct grip lengths for thickness of work to be held
and with enough thread length to assemble with full thread engagement
into screw lock insert (e.g. , an AN single-digit bolt should not be used
with inserts of lengths equal to one and 1/2 to two times bolt size).
NOTE
An insert assembled in a blind hole, with the tang retained, should be
longer than the required bolt thread engagement by 1/2 the nominal
thread diameter or bolt size.
c. Insert Tolerance. The same insert is used for all classes of thread fits or thread tolerance ranges. Each variation
in fit is determined by the tolerance of the tapped thread used. The free insert, before assembly, has a larger diameter
than the tapped hole which receives the insert. The tapped hole reduces the insert in diameter as It is assembled. This
makes the insert conform to tapped hole tolerance or class of fit. This snug fit locks the insert in position.
d. Inspection Criteria for Helical Springs. Inspection criteria for all extension helical springs is as follows:
•
No breaks or cracks allowed Chafing in hook area shall not exceed 0.010 in depth.
•
Coils that have any corrosion pits shall be replaced.
•
Coils shall not be sprung; coils must be tight against each other.
2-56
