Jam nut ???

[Marco Annaratone]

Hi everybody,

I recently bought through EBay a Schneider Symmar-S 360mm which came with a retaining ring. I just saw on Ebay a similar 360mm Symmar-S which comes however with a "retaining ring and a jam nut". Can some kind soul tell me what a jam nut is, what is used for, and whether I need to get a hold of one for my 360mm Symmar-S? (I assume that the latter is mission impossible, maybe?)

The Large Format Photography site talks in the introduction to large format lenses about a "flange" (which I assume is the retaining ring) but not about a "jam nut". BTW, I also saw the two terms --- retaining ring and jam nut --- used interchangeably, so ... I am totally confused.

Thanks!!!!!!!!!

[Jonathan_5775]

Wellllll.... I BELIEVE the term "jam nut" is an abbreviation/stupidification of "jamb nut" (see also: duck tape). And - I BELIEVE the original word is "jamb" as this piece tends to function exactly like a door "jamb" - which is, to stop the door. Like the door jamb, the jamb nut holds the lens fast against it's board. Further - I believe this is exactly the same thing as a retaining ring - and the person in question made an error in the copy.

[Ling Z]

The Schneider Symmar 360 is mounted on a Copal #3 shutter, which usually comes with a retaining ring and a flange (like a washer or spacer, a Copal #0 or #1 shutter comes with a retaining ring only). So I guess the "jam nut" might be the "flange".

[David A. Goldfarb]

It's definitely "jam nut" and not "jamb nut" according to Random House Webster's Unabridged. A jam nut for a shutter is the same thing as a retaining ring, which fastens the shutter to the board from behind the lensboard. A flange is screwed onto the front of the lens board. There are some flanges that can also be used behind the lensboard as a retaining rings. If you've got a small board and a big shutter, use it as a flange for extra clearance. If you've got a large lensboard, use it as a retaining ring so you can easily swap the lens with another one on the same board or put it on another type of lensboard, if you need to.

Perhaps the seller is just confused about the terminology. I wouldn't really worry about it, as long as you have something to hold the lens in place.

[Dean Tomasula]

A jam nut usually is used in context with an enlarger lens. It has the same purpose as a retaining ring, to hold the lens to the board. As David said, perhaps the seller is getting his terminology confused.

[Oren Grad]

Your 360 Symmar-S probably came in a Copal 3 shutter. Every Copal 3 I've ever seen needs both a spacer ring and a retainer ring. The retainer ring may be what was referred to as a "jam nut", it's also what L Zhu is calling the "flange".

As it happens, just this morning I called Calumet to order a retainer ring for a 360 Caltar S-II in Copal 3 which I bought used but which came with only the spacer ring. This is exactly the same lens as you have, just under a different label. You should be able to get the retainer ring from stock at either Calumet or Schneider, or by special order through B&H.

[Oren Grad]

Oops - just reread your message, and it's not clear whether it's the retainer or the spacer that you're missing. Either way, you should be able to get the missing part easily.

[Marco Annaratone]

Thanks to all of you but there is still fog in my head...

The guy selling the 360mm Symmar-S shows indeed two separate rings in the pictures. I have only one ring.

The one I have works just fine: I use a Rodenstock wrench on it to tighten the lens+shutter to the board. So my question is: what should I do with the second ring? What is its reason to exist? I do not mean to be a pain in the butt, just curious.

Tnx!

[Ernest Purdum]

I don't think you need it, but if there are sufficient protruding threads behind the retaining ring, you can, if you like, thread the second ring behind the retaining ring. this will keep the retaining ring from the unlikely possibility of loosening. This is the function of a "jam nut". I am, surprised at your question. I have never encountered a jam nut being provided as part of a lens installation. This seems to be proof that however long you live (I am 76 )there are still things to be learned.

[Harold_4074]

OK, this just in from the Department of Obscure Information: the proper use of a jam (or lock) nut is for it to be placed between the main nut and the bearing surface. Both nuts are turned simultaneously to take up the tension, the main nut moving down and the lock (jam) nut moving backwards against it. If, as is commonly done, the jam nut is tightened down on top of the main nut, its effect is to _loosen_ the main nut (think about which side of the thread is carrying the load).

I hope that I can remember this when I'm 76, although what it has to do with photography is beyond me....

[Andrew_4548]

Harold -you're on the right tracks :-)

A jam nut is usually a half height nut used to lock a main nut in place on a thread / against a washer, component etc.

By tightening the jam nut aginst the "outside" (non load carrying side) of the main nut, you take up any slack in the thread pitch between the nut and bolt and remove any free play which would enable just the main nut to work loose over time with the right vibration etc...