As observe things, the use of lock rings is fairly unusual in steel conduit systems - typically limited to situations like a running thread joint (where you can't turn either length of conduit into a coupler), or occasionally where conduit is taken into a thin walled box (lock ring each side and a female bush) - but the more usual arrangement is a male bush and a coupler, so no lock ring needed.Generally where the threaded conduit is taken tight into a threaded coupler or threaded conduit box, nothing more is needed.

   - Andy.

I agree, however, in machinery with vibration, or fixed installations with reasonable thermal cycling, brass nuts do seem to work loose, or at least over the years I have found quite a few that have done. And glands for SWA which in terms of the hole mounting method are similar construction to conduit mountings,  seem particularly bad, or perhaps some 'skilled installers' have not mastered the use of a spanner.  Hence my preference for  pirahnas, especially on plastic boxes, where I am not comfortable to achieve the gorilla torque required for it not to shake loose, without risking the box.
note that the male bush/ coupler method is tighter to thread wire into than the male tube and 2 nuts in a tight spot where the bore is smooth right into the box, though I agree it is more pleasing to the eye and gives a bit of friggle room for installation adjustment. Have used something similar on pyro wire as well where the wire stops a short distance above the box, and the cores are led in the last 3-4  inches in conduit with coupler, reduces the risk of a snap-off at the glands, and allows some room for working slack.
Mike

AJJewsbury said:

typically limited to situations like a running thread joint

That is the only place where I have used them. It seems to me that if you do not use a lock ring (or nut), electrical continuity may be impaired. I have also heard of the use of two rings - customary in Scotland?

AJJewsbury said:

typically limited to situations like a running thread joint

That is the only place where I have used them. It seems to me that if you do not use a lock ring (or nut), electrical continuity may be impaired. I have also heard of the use of two rings - customary in Scotland?