Main Rotor Retaining Nut Lock

Original Post 8/27/2018

I was contacted by a mechanic who in the process of removing a main rotor hub assembly after removing the main rotor found that the retaining nut lock could turn the nut by hand. (not torqued)

This could have been this way for some time.

This would allow the hub to work back and forth on the split cones and splines and slight up and down against the nut and mast thread damaging them all.

This could have happened even following the manual I am enclosing TM 55-1520-210-23-1 page 1-104 in the special inspection section AFTER INSTALLATION OF MAIN ROTOR HUB.

Notice the delete paragraph & a. Deleted. This was the instruction to re-torque the main rotor nut after 10 hours of flight. Th is was deleted after the required the use of a power wrench that is described in the manual after change 24 in Nov. 1992.

This re-torque was required because hub were torqued with break or dial type wrenches many used with mechanical torque multipliers.

This torque is an innateness torque thus requiring a follow-up re-torque. Which could be the reason the above hub was not torqued.

After the introduced the power wrench commonly called a Power-Dyne depicted on TM 55-1520-210-23-1 page 5-12 figure 5-7.

There are different types of these power wrenches but they work by dappling torque through hydraulic pressure.

When torqued with this type no re-torque is required. Following the highlighted instructions from TM 55-1520-210-23-1 page 5-12, page 5-13, page 5-14 and 5-15