Does the Use of Stover Type Lock Nuts Change Torque Requirements???
This past weekend I was changing out my TB bolts with Grade 8 9/16" bolts, Grade 8 flat washers and Grade 8 Stover lock nuts and a question arose in my mind.
Stover lock nuts work by providing resistant friction via uniquely shaped threads and it takes extra ft/lbs of force just to put them on before you actually tighten and torque them.
My question is, does using Stover type lock nuts change the torque requirements when tightening these nuts down? For example, if it requires an additional 10 ft/lbs of force just to turn a Stover lock nut on a bolt, should my final torque on my TB bolts be 135 ft/lbs instead of 125 ft/lbs?
Do we have any structural engineers on here who can answer this question? Thanks.
These types of nuts are in a group of lock nuts called "prevailing torque nuts" (also called Autolock nuts, Unitorque nuts, Crownlock nuts, and automation lock nuts). Short answer: no. If the torque specification for a hex nut is 10 ft-lb, the Stover nut will not need additional torque to fully deform the threads (same for a Nylock nut). The additional resistance they have as you tighten or loosen it, called prevailing torque, due to the deformed top threads (or Nylon insert for the Nylock nut) is so small, that you'll be looking at a difference of a few in-lb, or fraction of an in-lb.
I'm a technical writer so I deal with this stuff all day, but didn't actually know the answer. So I ran upstairs and bounced it off a few system and product engineers ($100M+ in sales company).
2012 CRUSH JK Rubicon 6-Speed, Black Leather, Heated Seats, 430N, Connectivity, Auto AC, Power Convenience, Deep Tint, Trailer Tow, Dual Top
stover nut = a type of prevailing torque nut; provides resistance to loosening.
prevailing torque = the torque required to run the nut down the bolt before clamping force is made.
preload = clamping force of the tightened nut and bolt.
Acceptable torque will nearly always be a range despite values often being specified as a single value.
The prevailing torque of a stover nut will affect the resultant torque and preload as compared to a standard nut though probably not enough to fall outside, or far outside, of the required torque range.
In your application of replacing suspension nuts and bolts the question must be asked, were the original nuts a type of locking nut? The manufacturers torque specification would have accounted for this either way for the original fasteners.
Look at it with extreme numbers. If the prevailing torque of the nut were 100 ft/lbs and the required torque was 125 ft/lbs then your torque wrench would indicate 125 ft/lbs when preload reached a value to add just 25 ft/lbs of additional torque. In other words not tight enough. Again, this is an extreme example but shows how the prevailing torque will affect the resultant effective torque.
If replacing a standard nut with a prevailing torque nut a method of measuring the prevailing torque is to install the nut to near its last turn then measure the prevailing torque using a deflection scale torque wrench. This value can then be added to the torque specification. e.g. if the specification is 125 ft/lbs and it takes 6 ft/lbs to turn the nut, torquing the nut to 131 ft/lbs would give a similar clamping force as a standard nut torqued to 125 ft/lbs.
There are other considerations. Just a couple are: friction coefficient differences due to different coatings, manufacturing lubricants, etc. Unlikely to be significant but measurable variables. Of course the thread pitch must be equivalent as well.
Please read the first two paragraphs again.
I'm not an engineer but I've been doing some reading on the subject. Comments?
2012 JKU Rubicon: Flame Red, Manual, Max Tow, Dual Top, Supl. Air Bags, Heated Seats, ACE Rock Sliders.