Why are/were device screws flat head instead of pan head?

[John Hawkinson]

Hello. I hope this is a reasonable question to post here, it's been kicking around the back of my head for a long time, and I spent way too much time today trying to find an answer to no avail, although it's really just trivia.

Classically and historically, the screws to attach a device to a box were 6-32 flat head. And by "flat head" I mean true flat head screws with conical undersides and flat tops that are designed to sit flush in countersunk holes, I don't mean the straight "slotted" drive style (versus Phillips, Robertson, etc.), although device screws were usually that too. That is, style (e) in this diagram from Wikipedia:


Nowadays, device screws that come with a receptacle are all 6-32 pan head (maybe extra-low profile compared to a normal pan head). That is, underside of the screw head is flat, and the top is slightly rounded. (Also they are Phillips. Or a variant like combination). Style (a) in the diagram [or maybe style (d)].

To the engineer in me, it's obvious that these screws should be pan head. The slot/hole in the yoke means that a flat head screw has just a line of contact with the device yoke; and since the hole is usually slotted, it's not even a full 360° of contact. Of course, a pan head screw has nearly the full surface area of the screw head in contact with the yoke, rather than the thin line (still, not 100% contact because of the slot, but still a lot more than the flat head).

And although modern receptacles now come with pan head screws, it appears that electrical prosumer manufacturers still seem to sell flat head screws in, e.g., screw kit assortments designed for devices (e.g. Gardner-Bender SK-632T; Southwire makes one too). Weirdly both market them as a "slotted flat head design" when they clearly use Phillips drive flat head screws (perhaps they did not used to and the marketing copy is old?).

Did outlet cover plates used to sit tighter against receptacles so there wasn't room for the added height of the pan head? When and why did it change?

I'm not sure if perhaps this is addressed in the NEMA OS 1, OS 2, or OS 3 standards (those are, respectively: Metallic Outlet Boxes; Nonmetallic Outlet Boxes, Device Boxes, Covers and Box Supports; and Selection and Installation Guidelines for Electrical Outlet Boxes), or maybe in their older versions?

Thanks for the history lesson!

 

[ptonsparky]

I take it that you haven’t found a job yet and have a lot of spare time.

 

[Dennis Alwon]

I guess that flat heads sit closer to the device which may have prevented the older plates from sticking out a bit. The real question is why did it take so long to get to phillips heads or square head

 

[James L]

I take it that you haven’t found a job yet and have a lot of spare time.

He said "the engineer in me"

I'd bet this stuff keeps him awake at night.

 

[James L]

I don't think there's substantially more surface contact on the screws nowadays. The slot in the yoke is large - not only sideways

 

[tom baker]

A very short post would get more responses.
Pan head screws have a more finished look..Device covers date from the beginning of the electrical jndustry, the reason is buried in history

 

[John Hawkinson]

Pan head screws have a more finished look..Device covers date from the beginning of the electrical jndustry, the reason is buried in history

A more "finished look" under the cover plate? We're talking device-to-box here, not coverplate-to-device.
p.s.: ptonsparky: doing fine, although I'm sure the pandemic does produce a bit of instability in us all!

 

[GoldDigger]

When you do not tighten the screw all the way, to get the device far enough out from the box to match the cover plate, the flat head has a wedging action against the sides of the slot, which a pan head would not have.
The tulip head (f) would too, but it has a thicker head and is used mainly for cosmetic applications, of often with a dress washer.

 

[kwired]

When you do not tighten the screw all the way, to get the device far enough out from the box to match the cover plate, the flat head has a wedging action against the sides of the slot, which a pan head would not have.

Which you used to see a lot of in older installations. Metal faceplates was more common at one time and would give some support from plugging in cords as well. Some the old heavier brass faceplates likely had little room to allow for a mounting screw that wasn't flush/nearly flush with the device yoke.

 

[zbang]

I suspect this is another lost to the mists of antiquity things. That said, I've wondered about it too, and why they're still slotted heads with a skinny slot.

 

[tom baker]

A more "finished look" under the cover plate? We're talking device-to-box here, not coverplate-to-device.
p.s.: ptonsparky: doing fine, although I'm sure the pandemic does produce a bit of instability in us all!

Sorry missed it was device strap not cover, was a late reply.

 

[texie]

The real question is why did it take so long to get to phillips heads or square head

I wish I could remember when they changed from slotted flat head to pan head phillips/combo. Kind of quaint that we actually used a slotted head.

 

[James L]

I wish I could remember when they changed from slotted flat head to pan head phillips/combo. Kind of quaint that we actually used a slotted head.

We were still using slotted when I started in 1992
I remember buying my first Phillips head speedy screwdriver in 1994

 

[Dennis Alwon]

Gosh, the days before cordless drills and installing 125-150 devices with slotted head screws. I bought on of those twirly screwdrivers but I slipped with that every once in a while and left a mark on the wall so I did it by hand

 

[jim dungar]

Gosh, the days before cordless drills and installing 125-150 devices with slotted head screws. I bought on of those twirly screwdrivers but I slipped with that every once in a while and left a mark on the wall so I did it by hand

I found they worked okay, if you used one hand to hold it in alignment with the screw. One handed operation was always a toss up.

 

[kwired]

Gosh, the days before cordless drills and installing 125-150 devices with slotted head screws. I bought on of those twirly screwdrivers but I slipped with that every once in a while and left a mark on the wall so I did it by hand

Wasn't in trade very long before we started using cordless drills to drive screws, but did use "Yankee screwdriver" to mount devices a few times early on in my career.

 

[Dennis Alwon]

I found they worked okay, if you used one hand to hold it in alignment with the screw. One handed operation was always a toss up.

They do unless you get wild with it and try to go too fast.. LOL

 

[Russs57]

Could it be they tended to "self center" the wiring device? They pretty much pre-dated the whole grounding/bonding clip.

 

[LarryFine]

I got the best one-hand results if I looked just below the lower bend, i.e., the top of the shaft, and controlled it with my wrist to keep it in one place, as it would have if I were holding it stationary with my other hand; i.e., spin the shaft without twirling it.

If that part of the shaft is moving in a circle, then you're making too large or too small a circle with your wrist motion.

 

[ptonsparky]

I had help that used that twirly screwdriver to plug & switch a house for me. Trouble was he used it for the screws to hold the conductors too. Plates weren’t on when I did a walk through and I could see the loose connections on one, Four of four failed when I pulled them to check. He reworked all of them, and I still have that screwdriver on the workbench where he thew it. I went back over each one again.