Originally Posted by
skridlov
It's been a few years since I had the temerity to post on PPRUNE but something I just read caused a double-take:
"The current situation is that the actuator has been modified by replacing the right hand thread at the input end of the control shaft with a left hand thread.
This removes the repetetive inspection on the nut for loss of torque."
I used to operate a couple of 1940s ex-US Army 6x6 Studebakers. All the wheel nuts on the L/H side wheels were left-hand thread. That's 1940s: wheels: on a truck...
These days I work on vintage watches, dating back to WW1 in some cases. Almost without exception the crown-wheels, which rotate counter-clockwise, have left-hand threads. There are some esoteric exceptions, but...
It boggles my easily boggled ageing mind to think that this strategy could have been ignored on a HELICOPTER! Blimey.
That was a popular solution on truck wheels and also a few vans and high performance cars in the 1960s. This method has died out now that truck manufacturers have found out how to do proper fastener design, tighten them properly, and use reliable tightening methods. Truck wheels are generally no longer user serviceable because of the high torques and specialist tools.
Specialist fastener design is troublesome whenever the format of the threaded components prevent the male threaded element taking the role of a spring. Every standard bolt is designed to act as a very stiff spring and effective reliable tightening requires that spring to be taken to near its limit. If the format of the threaded components, or the low compressive strength of the components being retained by the threaded components, prevents taking the spring to its limit then there is a serious problem. This serious problem is extremely common, even in automotive and aviation sectors where one might think things are all sorted! This explains all the split pins, locking wire, adhesives, and so on that we are familiar with in those industries. It also explains why in modern designs we can find fasteners abandoned and they just glue it if they can!
Aviation also has the Titanium problem. This is because Ti behaviour makes it probably the worst material for threaded fasteners that is in regular use. (S-92 oil housing, Cougar 91!!!)