HeliCoil OnLine Tech Talk

Oct 99 - Screw Lock Insert - Prevailing TorqueScrew Lock Insert Prevailing TorqueHeli-Coil Screw-Lock inserts are manufactured in accordance with Military Specification Mil-I-8846. Mil-I-8846 requires each Screw-Lock insert to meet a 15 cycle prevailing torque test. Essentially, an insert must be installed into a properly prepared STI hole, the driving tang broken off and a torque test completed with a cadmium plated socket head cap screw. The screw is inserted into the insert and the maximum prevailing torque is recorded. This torque must be below a pre-determined maximum as determined by the specification. The screw is then cycled 15 cycles through the locking feature of the insert. The prevailing torque is then recorded on the 15th removal. This torque must be above the pre-determined minimum from the specification.Therefore, the strict number of cycles for a Screw-Lock insert is 15. Under certain conditions, it is possible to obtain an extended number of cycles. Screw hardness and quality as well as the type of lubricants used during the assembly process will dictate the possibility of extended cycles.For extended use of Heli-Coil Screw-Lock inserts, please contact the Applications Engineering Group to discuss possible lubricants and assembly procedures.Back