9

A swing on the playset I built earlier this year had a loose bolt, so I went to tighten it and instead of the bolt tightening, I twisted it in half (wrench lever arm 6", bolt diameter 3/8", galvanized steel). Recalling that the bolts came with the swing hangers, and hearing rumors about inferior lag bolts being bundled with swings, I removed the other three bolts. All of them snapped instead of screwing out.

Is this normal -- that someone could twist a bolt in half without extreme effort -- or are the bolts defective?

If this is a defect, should I expect an identical galvanized steel lag bolt from my local hardware store will be of higher quality?

snapped 3/8" lag bolts

There are no markings on the heads.

Criggie
  • 11,950
  • 2
  • 26
  • 79
Sam
  • 295
  • 3
  • 8

6 Answers6

13

There are various grades of hardware. Standards, such as SAE J429and ASTM A307 in the United States, and ISO standards in most other locales, define bolt strength and marking. The lowest SAE J429 Grade, 1, has an unmarked bolt head and is about one quarter as strong as Grade 8, with six radial lines on the head.

For non-critical use in consumer items, it's not surprising that the bolt sheared easily. For that matter, with effort, I've accidentally sheared a 1/2" wheel stud, likely Grade 5 or stronger, with an ordinary tire iron (and had to drive to a garage with one stud missing).

That said, if it sheared easily with a 6" (15 cm) long wrench, likely that bolt was defective, and a new Grade 1 bolt should be stronger.

Use a torque wrench if this happens again.

DrMoishe Pippik
  • 10,706
  • 19
  • 32
7

I put a new Everbilt Grade 1 lag screw into the hardest scrap wood I have lying around and tightened it with the same wrench. I was definitely applying more torque, 40%-80% more, than what severed the cheap ones in the image, and the bolt survived.

So I feel confident saying that the initial bolts are inferior, though I don't know by how much. If I get my hands on some again I'll conduct a proper experiment and update this answer.

Sam
  • 295
  • 3
  • 8
3

In general, wrenches are designed to a length such that the average person gets about the right torque and doesn't break the bolt. Therefore if you break a bolt with a (non-adjustable) wrench, the bolt was sub-standard.

You can look up the recommended torque for all sorts of bolts in charts like this

There you can see that a steel 3/8" bolt is commonly torqued to about 240 inch-pounds, or 20 foot-pounds.

Bolts are commonly torqued to about 75% of their failure strength, so very roughly 30 foot-pounds. That would be 60 pounds of force with a 6" wrench. You could do that, but I think you'd know you were about to break something.

Having said that, this chart suggests 10 foot-pounds

Robin Bennett
  • 819
  • 5
  • 11
3

is it too soon to post a vid clip here, of the man everyone loves to hate, saying Chyna

https://www.onallcylinders.com/2014/03/13/diagnose-common-fastener-failures/

look up various articles on identifying torsion/shear/tension breakage of fasteners. that looks like breakage from torsion. and given the drawn putty look of the metal, very low grade soft metal.... i.e pot metal... chyna (not the wrestler chic)

ron
  • 2,049
  • 10
  • 14
2

Sorry, my english wording is not the best for this terminology.
Bolts are designed to withstand force along the pulling direction, but they are not intended to withstand forces to the side (Of course they withstand a bit).
Usually the the friction between two pieces which you mount with the bolts prevent your bolts to receive sideway forces.

So it can be that your bolts got damaged because they were loose and thus received too much sideway forces.
Edit: Just to be clear, I don't mean that you stressed the bolts in a wrong direction during tightening, I mean that the bolts might got damaged before, while they were loose.

Edit: I just noticed that not all bolts were loose but all bolts broke in the same way. So it is clear that stress to the wrong direction can't be the reason here.

Thibe
  • 121
  • 2
1

Hard to see with the angled picture, but it looks like the top two bolts have failed plastically with necking: https://en.wikipedia.org/wiki/Necking_(engineering)

This can only have occurred if there was a fixed end point at the other end of the bolt. Possible causes could include:

  • Bad/mismatched threads either in the fastener or in the threaded hole, these jammed during tighten/removal.
  • High corrosion of the fastener/receptacle, causing the fastener/receptacle pair to bind.
  • Over-tightening, causing deformation during install (rather than removal)
  • The body of the bolt was insufficiently strong to handle the expected forces from expected force levels between the thread thread and the receptacle (least likely - should run smoothly in normal operation).

Over-tightening/force shouldn't really apply when removing the bolts, so it would have to have happened during the initial assembly - someone else mentioned installing with a torque wrench, but this only works if you've an idea of a suitable torque.

I'd recommend using an anti-sieze compound during assembly next time, to reduce binding. I'd also make sure you use an SS fastener for high-corrosion applications. They are a bit more expensive, and are softer (Austenite vs ferrite), but if installed with an anti-seize compound, shouldn't seize.

If money is no limit you can use Silver-plated fasteners :) - no anti-seize necessary.

P.S. a good stainless should work harden, so necking should be minimal - which doesn't seem to be the case here, pending better side-on photos https://en.wikipedia.org/wiki/Work_hardening

SomeGuestPerson123
  • 49
  • 2