I’m mounting some antique street lights in my garage. The lights are about 20 pounds, and are at the end of 24" “masts” made out of 1-1/4" galvanized pipe. I used four 1/4 inch lag screws all the way through the 2X8 1960s vintage pine joists. Is this a reasonable screw size for the job? I know that the straight pull-out force is something ridiculous, like over a hundred pounds per screw, but don’t know how to calculate the torque being generated. I’m guessing almost all the pull-out force is on the top two screws, and the shear force is on all four?
I don’t know what the pull-out force would be or how it would be calculated, but I would expect, if the screw threads were well-designed, the limiting load would be the tensile strength of the screw.
At 0.25" diameter, assuming a strength of 30,000PSI (conservative for mild steel), that would be 1500 Lbs per screw. You don’t want to load the screw more than 50% of it’s strength, and you probably want one screw to be able to carry the whole load, if necessary, so the load on the two top screws need to be limited to 375 lbs (half of the 700 Lb 50% limit).
Now, you don’t give us the dimensions of the base, which is important. Assuming the top screw holes are 6" higher than the pipe, and the lower ones are 6" below the pipe, you essentially have a 2:1 lever on the top screws. Now 1-1/4" Schedule 40 galvanized pipe weighs 2.27 Lbs/ft, so you have 20 + 4.5 or 24.5 Lbs, assume all weight is concentrated at the end of the pipe and you still only have 50 Lbs loading on the upper screws. So, you are about 7.5 times stronger than the calculated limit.
Make sure the screw are tight, since it will be friction, not shear, that holds the fixture to the wall (if you load those screws in shear, they will likely fail).
The base is a standard 1-1/4" pipe flange, the screw holes are about 2" from the center.
That would increase the load. Instead of a 2:1 lever, you have a 6:1, so instead of a calculated load of 50 lbs, it works out to be 150 lbs. Still well under the 375 limit.
If going all the way through, why not bolts?
If using wood screws of any size, try to be sure to pre-drill the hole in such a manner that the screw is cutting threads and use lube to make it cut rather than tear and leave a bit of room at the bottom of the hole if you are not going all the way through.
Excellent advice.
The box the lag screws come in should have a size for the pilot hole. You want just a little smaller than the root diameter of the screw, so for 1/4", if the box doesn’t have a size, I’d go with 7/32".
As for lubricant, the recommendation is often paraffin, but that can be a bit hard to find. I have a brother who spent over 30 years working as a carpenter and he always had a bar of soap in his toolbox, said that soap did just as good and was much easier to find. Plus, you could use it to clean your hands when you finished the job!
My granddad always lubricated his handsaws with soap. Worked great.
I feel this thread title deserves a sexual pun of some sort, but I am not clever enough to come up with anything. Can someone help me out?
25 lbs of shear is going to cause 4 screws to fail?
Lag screw quality seems to be hit or miss. At least the standard ones with the hex head. I’ve had a number break while driving them
Into Douglass fir with the recommended pilot hole and candle wax on threads.
The ones with the torx drive heads seem to be very good. Properly tempered high quality steel and an improved thread form.
Works great for many things…
Eventually, yes.
Bolt fasteners are not intended to be loaded in shear. If you load a lag screw in shear, it will act to loosen the hole it is in. The looser it gets, the worse it gets, and eventually the screw will no longer stay in the hole.
These fasteners work by supplying a clamping load to clamp the fixture to the wall. It is the friction between the wall and the fixture that keeps the fixture in place. If you load the screw in shear, then you are not supplying the clamping load and it is much more difficult to keep the fixture from moving. Once the fixture starts moving, it’s just a matter of time.
If I heard cracking when I put the screws in, does that mean the pilot hole wasn’t big enough and the installation is no good?
I was nervous about the length of the arms, so I shortened them, replacing a 18" pipe with a 6", cutting the overall length in half.
Here’s a picture.
http://imgur.com/WcEClgB