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Posted: 6/3/2008 4:12:57 PM EST
[Last Edit: 6/3/2008 4:13:35 PM EST by zeekh]
I have will be screwing a 3/16" plate to a block using a 1/4-20 18-8 stainless steel cap screw. The screw isn't hardened ( if thats possible) or special in any way. Anyone have any idea how much weight can that bolt support before it shears off? Thanks zeekh
Link Posted: 6/3/2008 4:16:49 PM EST

Originally Posted By zeekh:
I have will be screwing a 3/16" plate to a block using a 1/4-20 18-8 stainless steel cap screw. The screw isn't hardened ( if thats possible) or special in any way. Anyone have any idea how much weight can that bolt support before it shears off? Thanks zeekh


What application are you using it for?
Link Posted: 6/3/2008 4:18:07 PM EST
Does it have numbers or markings on the bolt head? You can usually just do a google search and find out what the markings mean for all kinds of bolts.
Link Posted: 6/3/2008 4:18:08 PM EST
How is weight being applied to the bolt? In a shearing direction across the centerline or a pulling along the centerline of the bolt?

Link Posted: 6/3/2008 4:25:56 PM EST
Single shear or double? What grade bolt?

A single 1/4x20 bolt ranges from about 160-370 pounds. But that applies to two smooth surfaces of similar material.

IT really depends on a whole bunch of factors.
Link Posted: 6/3/2008 4:30:07 PM EST
IIRC, all cap screws are grade 8. I don't have my Machinery's Handbook handy so I would suggest google.
Link Posted: 6/3/2008 4:36:16 PM EST
A grade 8 1/4-20 will support around 5400lbs in shear.

For an 18-8 bolt you need to multiply the shear area by the tensile strength and a constant to relate the shear to tensile strength. For steels this number is .5-.6 for a steady load and .3 for undulating loads.

You then decide your factor of safety and whether or not you want to add a stress riser correction for the root of the thread.

The root dia of a 1/4-20 bolt is .185 and the tensile strength of 18-8/A2 used in a SHCS is around 100ksi.

SO .185*100,000*.5 ~ 9250lbf. Using a factor of safety of 3 yields a shear loading of 3083lbf per fastener.
Link Posted: 6/3/2008 4:40:38 PM EST

Originally Posted By Nmbmxer:
A grade 8 1/4-20 will support around 5400lbs in shear.

For an 18-8 bolt you need to multiply the shear area by the tensile strength and a constant to relate the shear to tensile strength. For steels this number is .5-.6 for a steady load and .3 for undulating loads.

You then decide your factor of safety and whether or not you want to add a stress riser correction for the root of the thread.

The root dia of a 1/4-20 bolt is .185 and the tensile strength of 18-8/A2 used in a SHCS is around 100ksi.

SO .185*100,000*.5 ~ 9250lbf. Using a factor of safety of 3 yields a shear loading of 3083lbf per fastener.



I would recommend the part in red if it's going to have a significant load on it.
Link Posted: 6/3/2008 4:41:34 PM EST
[Last Edit: 6/3/2008 4:42:23 PM EST by Kuraki]
ETA nope, nm
Link Posted: 6/3/2008 5:01:44 PM EST
[Last Edit: 6/3/2008 5:03:16 PM EST by zeekh]
Thanks for the replies. Basically it is a Stainless steel block with a stainless steel plate mounted to the side of it. The bolt in question will hold the plate to the side of the block. The plate will have a load of 50 lbs on it. The shear will be perpendicular to the center line of the bolt. Actuall there is a pltae on each side of the block. Both of the plates ( 3"long x 3/4" wide x 3/16" thick) will be bolted onto the ends of a 1" square piece of Stainless steel that is 4 1/2" long. They will form an upside down "U". The 2 legs of the U will be pinned ( 1/4" Stainless steel pins that are removable ) into a piece for steel to lift it. there will be a small gap betwwen the legs of the "U" and the piece being lifted ...maybe .040"total. It will only be lifted once a week or so. When not in use the "U" braket will not be mounted to the piece if steel being lifted. On paper the bolt looks a little small but it seems I have plenty of room to spare. Thanks zeekh
Link Posted: 6/3/2008 5:06:00 PM EST
Failure analysis? What will happen if it fails?

Is the only possible loading perpendicular to the axis?

Torque?

Link Posted: 6/3/2008 5:12:28 PM EST

Originally Posted By Keith_J:
Failure analysis? What will happen if it fails?

Is the only possible loading perpendicular to the axis?

Torque?



Big problems if it fails. No torque just verticle lift gently by hand.
Link Posted: 6/3/2008 5:53:57 PM EST
50 pound load? I'd say you don't have anything to worry about.
Link Posted: 6/3/2008 6:08:01 PM EST
man, I get enough of this stuff at work

what material is the plate so we can do a bearing check. always want fasteners to be bearing critical
Link Posted: 6/4/2008 2:31:53 AM EST
[Last Edit: 6/4/2008 2:33:32 AM EST by Merlin]

Originally Posted By Nmbmxer:
A grade 8 1/4-20 will support around 5400lbs in shear.

For an 18-8 bolt you need to multiply the shear area by the tensile strength and a constant to relate the shear to tensile strength. For steels this number is .5-.6 for a steady load and .3 for undulating loads.

You then decide your factor of safety and whether or not you want to add a stress riser correction for the root of the thread.

The root dia of a 1/4-20 bolt is .185 and the tensile strength of 18-8/A2 used in a SHCS is around 100ksi.

SO .185*100,000*.5 ~ 9250lbf. Using a factor of safety of 3 yields a shear loading of 3083lbf per fastener.


I'm not familiar with "18-8/A2" SS bolts. According to McMaster-Carr (page 3094), SS 18-8 bolts only have 70ksi tensile strength (which is Grade 2 territory), vs. 150ksi for a standard Grade 8 bolt.

McMaster has 17-4 High Strength SS bolts, at 140kSI TS, a 1/4-20x2" costs about $6 each (page 3096).

McMaster (enter the page # in the search bar)


Good luck.

Merlin
Link Posted: 6/4/2008 2:44:23 AM EST

Originally Posted By Nmbmxer:
A grade 8 1/4-20 will support around 5400lbs in shear.

For an 18-8 bolt you need to multiply the shear area by the tensile strength and a constant to relate the shear to tensile strength. For steels this number is .5-.6 for a steady load and .3 for undulating loads.

You then decide your factor of safety and whether or not you want to add a stress riser correction for the root of the thread.

The root dia of a 1/4-20 bolt is .185 and the tensile strength of 18-8/A2 used in a SHCS is around 100ksi.

SO .185*100,000*.5 ~ 9250lbf. Using a factor of safety of 3 yields a shear loading of 3083lbf per fastener.


I have no idea what the fuck you just said, but it sure sounds cool!
Link Posted: 6/4/2008 2:49:33 AM EST
Garden variety stainless fasteners are NOT grade 8.

I looked into this once, and I believe they are only grade 5 at best,
unless you specifically buy better bolts.

I think you will find that most are only grade 2.

Link Posted: 6/4/2008 3:03:55 AM EST
[Last Edit: 6/4/2008 3:22:25 AM EST by AeroE]
Link Posted: 6/4/2008 3:05:49 AM EST
[Last Edit: 6/4/2008 3:09:12 AM EST by AeroE]
Link Posted: 6/4/2008 3:06:58 AM EST
[Last Edit: 6/4/2008 5:31:31 AM EST by AeroE]
Link Posted: 6/4/2008 4:20:52 AM EST
It is also good design practice to use a shoulder bolt in a situation like this. Drill and ream the hole ~3 times as deep as the fastener diameter, this will let the larger shoulder portion deal with the bending moment on the bolt. You also have a larger shear area with no stress riser so it is much better with an adulating load.

Or use a shoulder bolt thru with a nut on the backside.
Link Posted: 6/4/2008 5:35:56 AM EST
Link Posted: 6/4/2008 5:38:16 AM EST

Originally Posted By machinisttx:
IIRC, all cap screws are grade 8. I don't have my Machinery's Handbook handy so I would suggest google.


Soft stainless is far from grade 8.
Link Posted: 6/4/2008 5:50:49 AM EST

Originally Posted By AeroE:

Originally Posted By The_Reaper:
Garden variety stainless fasteners are NOT grade 8.

I looked into this once, and I believe they are only grade 5 at best,
unless you specifically buy better bolts.

I think you will find that most are only grade 2.




18-8 fasteners should not be categorized as "Grade 2", "Grade 5", and so on, lumping them with carbond steel fasteners. 18-8 steel is strengthened by work hardening as it does not respond to heat treatment for strengthening.

The stock will either be labeled 1/4 Hard, 1/2 Hard, or some other desination up to Full Hard. Equivalent letters (A, B, C, etc.) may also be used, a step backwards in communication in my opinion.


I think that 18-8 which is falls into the specs for 304 stainless is work hardened while being formed, and then annealed down to the desired hardness and strength charteristics.

As a general guidline, for connecting 3/16 plate, I would reccommend you drill and tap for 3/8 fasteners.

At this point I would also like to see a ms paint just for the heck of it.
Link Posted: 6/4/2008 5:56:12 AM EST

Originally Posted By Nmbmxer:
It is also good design practice to use a shoulder bolt in a situation like this. Drill and ream the hole ~3 times as deep as the fastener diameter, this will let the larger shoulder portion deal with the bending moment on the bolt. You also have a larger shear area with no stress riser so it is much better with an adulating load.

Or use a shoulder bolt thru with a nut on the backside.

Agreed with the shoulder bolt. Other pluses include them being hard as fuck, and ground round for nice even stress distribution.
Link Posted: 6/4/2008 6:26:26 AM EST
Link Posted: 6/4/2008 6:32:26 AM EST

Originally Posted By Nmbmxer:
A grade 8 1/4-20 will support around 5400lbs in shear.

For an 18-8 bolt you need to multiply the shear area by the tensile strength and a constant to relate the shear to tensile strength. For steels this number is .5-.6 for a steady load and .3 for undulating loads.

You then decide your factor of safety and whether or not you want to add a stress riser correction for the root of the thread.

The root dia of a 1/4-20 bolt is .185 and the tensile strength of 18-8/A2 used in a SHCS is around 100ksi.

SO .185*100,000*.5 ~ 9250lbf. Using a factor of safety of 3 yields a shear loading of 3083lbf per fastener.



Hubba hubba......

Props...
Link Posted: 6/4/2008 11:55:50 AM EST
[Last Edit: 6/5/2008 4:31:19 AM EST by AeroE]
Link Posted: 6/4/2008 2:41:14 PM EST

Originally Posted By AeroE:

Originally Posted By Rock_Hind:

Originally Posted By AeroE:

Originally Posted By The_Reaper:
Garden variety stainless fasteners are NOT grade 8.

I looked into this once, and I believe they are only grade 5 at best,
unless you specifically buy better bolts.

I think you will find that most are only grade 2.




18-8 fasteners should not be categorized as "Grade 2", "Grade 5", and so on, lumping them with carbond steel fasteners. 18-8 steel is strengthened by work hardening as it does not respond to heat treatment for strengthening.

The stock will either be labeled 1/4 Hard, 1/2 Hard, or some other desination up to Full Hard. Equivalent letters (A, B, C, etc.) may also be used, a step backwards in communication in my opinion.


I think that 18-8 which is falls into the specs for 304 stainless is work hardened while being formed, and then annealed down to the desired hardness and strength charteristics.

As a general guidline, for connecting 3/16 plate, I would reccommend you drill and tap for 3/8 fasteners.

At this point I would also like to see a ms paint just for the heck of it.


Nope. Annealing is just that, there are no intermediate stages, or at least not in use by any reputable maker.

I sure as heck wouldn't trust a "partially" annealed fastener as it's metallurgy would be non-uniform.

Any of the 300 series alloys could be formed into a forged bolt blank, or machined from bar stock. That's not to say all of them would be the best choice for one or the other process, just that it's possible.



I do like a good fastener discussion.

I found one quick reference to a partial annealing process here:

http://www.specialbolts.net/fastenerterms.htm

Anneal also refers to the heat treatment given 400 series stainless fasteners after their manufacture (also called hardening and tempering) to lower hardness and increase toughness. For example, fasteners of 410 stainless may score over 200,000 psi after manufacture and be too brittle. By annealing at 1000 degrees F., tensile strength would reduce to 125,000-150,000 psi, while annealing the same material to 500 degrees F. would bring tensile to 160,000-190,000 psi.

Of course that is not an 18-8 stainless.

Here is a pretty pic of some hi-tech fastneres to keep everyone entertained.

Link Posted: 6/4/2008 3:25:42 PM EST
[Last Edit: 6/4/2008 3:28:25 PM EST by zeekh]
Thanks for all the repiles. Some of them are a bit too technical for my feeble mind . I decided to use the shoulder bolt and counter bore the threaded side a bit so the shoulder will be at the shear point. As mentioned before, the load is hand lifted with no jerking or sudden take up of slack.
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