Lock washers

I was wondering if using a lock washer with a bolt would be a good way to keep the nut from loosening, hanger from spinning. The Metolius spring force hanger provides this type of effect. Why don't bolts come with a flat washer and a lock washer? Any reasons not to add a lock washer?

A cursory google search indicates that lock washers are likely ineffective and possibly detrimental to keeping a nut from loosening.

some developers use a dab of loctite. with good results apparently 

Loctite is great on Wedge/Stud Bolts, I use the blue medium strength which allows for easy retightening without the worry of the bond being so strong it causes the whole bolt to spin upon future tightening.

For Stainless 5 piece bolts we've found that going back a second time to tighten down to spec again is needed in softer sandstones where the rock seems to relax a little after initial installation. 

Figuring out why you're getting spinners after initial installation would be a good place to start, if you're cleaning the hole well, doing good surface prep, torqueing down to spec, aligning the hanger well, doing a second tightening after initial install, etc... and still getting spinners then it's probably a better option to use an epoxy bolt.

As counterintuitive as it sounds, lock washers generally contribute to more loosening. Loctite and  a regular washer will likely help. I suspect that much of the loosening of hangers on threaded studs comes from the back-and-forth side-to-side pivoting movement that they experience as they’re loaded a little more to the left and right. A flat washer could minimize how much of that back-and-forth pivoting is transferred to the nut.

Note that regular loctite is less effective with stainless hardware. I’ve used Loctite 248 quite a bit professionally on stainless and had good results. Also really convenient that it’s in a stick form kind of like chap stick and is much less messy in tricky situations. It’s also still generally not too hard to remove later.

AvE - Nord-lock washers (YT)