Leighton,

 

I am an engineer and, while I do not play one on TV, I have played the part in US court (even stayed at a Holiday Inn Express a couple of times). You are certainly correct in your assertion that using a lock washer or nylock nut will alter the effective torque on the fastener, but I would stop short of agreeing that using a torque wrench with them is a waste of time. The effects of the locking mechanism just have to be accounted for in the torque value used. In the end, the torque wrench will still be far more accurate and consistent than your 'calibrated' hand.

 

With regard to the lack of lock nuts/washers on rod bolts and case half bolts, the design of these joints typically makes them unnecessary. The clamp length of these fasteners is typically quite long with respect to their diameter, resulting in more elastic stretch when drawn up to full stress. This makes it less likely that the fastener can be sufficiently unloaded by vibration or thermal effects to the point that it will rotate on its own. Additionally, theses fasteners are generally fashioned from high strength steel, which allows higher bolt stress and increased static friction to resist loosening once properly tightened.

 

Torque wrenches are far from perfect for determining fastener preload anyway. Even when all other variables are held constant, a good torque wrench will produce a significant variance in preload from one fastener to the next. In addition, thread fit, lubrication and the surface hardness of the head, or nut, thrust interface all have a major effect on the resulting preload for a given torque. 

 

When the preload requirements are truly serious, the change in bolt length can be measured to accurately determine the preload. Another approach is to continue torquing until the fastener reaches its plastic strain range. This can be observed on the torque wrench when the torque increase per degree of rotation starts to decline or a given rotation beyond snug can be used. Of course, fasteners subjected to plastic strain should be considered single use components.

 

It has been my experience that fasteners over 3/8" frequently are significantly under torqued by both professional and shade tree wrench turners who rely on feel. When the bolt size reaches 1" it is very rare that it will the adequately tightened by feel. While I have never studied studied them, I suspect that fasteners  1/4" and less are more often over torqued when tightened by feel.

 

Finally, I have observed that it is quite common for fasteners to experience creep strain in the first hours after installation at maximum allowable torque. Checktorquing fasteners the following day is sometimes usefull.

 

Rob