How does one calculate the load capacity of a particular screw ?
I will be mounting my 37" plasma tv to the wall. It weighs 67 lbs. I have
purchased an appropriate wall mount bracket. In a perfect world the screw
holes for in the bracket would line up with the studs, but that isn't likely
so I will first attach a piece of 3/4" plywood to the wall so it will span
two studs. The mounting bracket will then be screwed to the plywood.
The mounting bracket will be secured to the plywood in 4 places. The TV is
67 lbs (plus the bracket itself is 12 lbs) That's about 20 lbs per screw.
Will 4 screws in 3/4" plywood be strong enough?
How does one calculate the load capacity of a particular screw (probably
will get 1" lag screws with 1/4" diameter)?
Each lag bolt of that size can take hundreds of pounds if properly
installed. Most common bolts and screws in the hardware store will not have
a rating on them. When you get into machine bolds for auto, aircraft, etc,
you'll see markings on the bolt heads designating the class or grade they
The bolts will easily take the weight. Be more concerned about drilling the
right sized pilot hole and then not stripping the wood by over tightening
There are too many variables to quantitatively determine screw load for such
a simple installation, as it will be drastically affected by screw material,
guage, strength of wood stud, and dozens of other factors.
You haven't said how you will attach the plywood to the wall studs. That is
a critical attachment, as the safety of the mounting is as good as its
weakest point. I would prefer to attach the bracket directly to the wall
studs using whatever screw length will penetrate the stud wood about 2
inches, after leaving the sheetrock. First get the bracket in the house and
see if by chance your stud spacing will accomodate the screw spacing on the
bracket - it might be already designed to mount on the standard 16" stud
Then, if you must go the plywood route, try to get screw length that will
also penetrate the stud about 2 inches. Your plan to use 1 inch screws into
the plywood, net of bracket width, would likely work fine. As the other
poster said, see that you predrill small enough pilot holes so that screw
drives tight, but not so tight it splits the wood, with care not to
overtighten. Same with the stud pilot holes - there you might just use self
tapping wood screws, without any pilot hole.
The tensile strength of a screw equals the strength of the steel in the
screw (about 30 000 psi) multiplied by the cross sectional area of the
Don't trust one calculation though, the weak link might be something
else. The screws could pull through the plywood or pull out of the
stud. Put it together and test it by trying to pull it off the wall.
Some mounts, like mine can be screwed (2" lag bolts ?) up to 30"
spaced studs. The mount has horizontal holes - not just a single hole.
Then move and adjust before the final torque. Holds up to 180 pounds.
"My doctor says I have a malformed public-duty gland
and a natural deficiency in moral fiber, and that I am therefore
excused from saving Universes."
Use lag bolts to attach the plywood to the wall. Use a stud finder to
locate the center of the studs and pre-drill a pilot holt. The lag bolts
should be long enough to extend 2 to 2 1/2 inches into the studs. If you
1.2" drywall and use 3/4" plywood, that comes out to be about 3 1/2" length.
I would not trust four screws. They won't have all that much to grip on with
just a 3/3" plywood backer. Either use more screws or ue carriage bolts
inserted from the back of the plywood before you attach it to the wall.
Well its not the strength of the screw but the strength of the join as
you know since you decided to mount it in plywood and not drywall. So a
screw in plywood is pretty darn strong and doubt it will shear off.
Is there no better brack you can buy that is designed to mount to the
studs? Its hard to believe the bracket is designed to mount in the
Also, how many pounds per screw will depend on the bracket design and
how they are attached to the bracket. Lots of screws for hanging things
have ratings on their package for what they will hold. These are for
the screw and the anchor to gether.
"Then said I, Wisdom [is] better than strength: nevertheless the poor
I would think the bracket is at least 16" long and can grab a couple of
studs. I think the OP is either moving it to a location off center or just
afraid of the setup. a couple of 2" screws into the studs would easily hold
80 pounds. Just look at most wall mounted cabinets.
Follow up on installation...
I didn't need the plywood after all, the mounting bracket's 4" horizontal
slots allowed me enough left/right play to attached directly into 2 studs
while keeping the center of the TV exactly where I wanted it on the wall.
Used 1/4" x 3" long lag screws (1" was just to get through the plaster and
lath). When the bracket was on the wall, I grabbed hold of it, put my feet
on the wall and hung my weight from it. I'd rather if it were to fall, it
only be dropping me off the wall instead of the $$$ TV.
It held quite securely. :)
Thanks to all for your replies.
I would use some long #10 pan head screws to attach the plywood to the wall,
probably 4 or 5 on each of at least 2 studs.
To attach the bracket to plywood, I would use some bolts with "T" nuts
inserted into the plywood from behind, and the bolts threaded into them from
the front through the backet.
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