Fastener engineering question

How much/many threads do you need to before overkill? IOW, if you have a 3/8" bolt, how many threads, or how deep do you need to get to maximum "clamp" force.

Looking to tap an aluminum head but a machine shop said they would want to weld it up (add a boss) because there is not enough material. The clamping force mandated would be ~35 ft*lbs. I haven't mic'd the head yet, so I'm not sure exactly how much meat there is.

I don't want to misquote anyone, but I thought someone said that 1/2 the diameter of the bolt was what you needed.

Looking for a formula...

Ryan

To reach 75-85% of the fasteners tensile yield strength, you would like to have 1-1.5x bolt diameter in engagement length for non ferrous materials... brass, aluminum, etc..

For steel, cast iron and other ferrous metals, .75-1.0x bolt diameter.

For formulas, bookmark this page as there is a lot of good engineering information throughout this site.... I've also found that the "Machinery Handbook" is an invaluable source of information. Pick one up when you get the chance.

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

-Mindgame

Thank you, Mindgame :).

Ryan

You're always welcome Ryan. :)

-MG

Since I know the "why", I set mine up on a floating drill press table, leveled and drilled/tapped. Didnt pay much attention to thickness of where I was drilling. Its been apart several times, and the threads show no signs of wear/pulling out.

Read the ARP book as sometimes you want more threads than you might think engaged for fatique reasons also.

Read the section in Machinery's Handbook. That is a good starter and has fairly thorough equations.

The largest disparity in length of engagement comes from the mismatch in the material properties (particulartly the ultimate tensile strength) between the hole and the bolt. Also, the governing equations change based on whether the hole is stronger than the stud or visa versa.

In addition, it's a good rule of thumb to add two extra threads to your final length of engagement number. That comes from the fact that the studs & bolts can be damaged during manufacturing. It's really just another safety net.

HTH,


Steve.

Using a "safe" 1.5(dia of bolt) rule (not calculating with the actual tensile strengths), the flange of the head isn't going to have the thickness I'm looking for. Even if I Helicoiled it, it is my opinion that it really needs a thicker flange (more meat) to get the necessary thread engagement.

This might be a real stupid idea, but what the heck...

What about using a bolt and a nut on the bottom side of the head flange? I'd have to remove the guidplates to access it, but I don't see why it wouldn't work. Assuming no pushrod interference, and obviously, a "re-do" if you drop a nut into the lifter valley, it seems like it could work.

Any thoughts?
Ryan

If ya put a helicoil in the hole and get a long thread stud,screw it in then put a nut with locktite behind the flange that will be double strong.

If ya put a helicoil in the hole and get a long thread stud,screw it in then put a nut with locktite behind the flange that will be double strong.

My plan exactly. Thanks, Mr. 'Dude :)

Ryan

My plan exactly. Thanks, Mr. 'Dude :)

Ryan

Anytime

Those calculations are for a tight spec bolt. if the hole/bolt doesnt fit well, those specs need to be a lot deeper.

We use .75*D depth for mil spec stuff in steel, with no failures