Home |
Search |
Today's Posts |
|
#1
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
Hi Everyone:
How much pull can a 316 SS PH Machine Screw ¼ x 20 handle before it fails? Basically I want to know how many fasters I need to use to secure some deck hardware. After an afternoon searching the web I found lots of tech sites with the info but my math skills are limited to + - x / and don't know how to convert engineering units to simple old fashioned pounds of pull. So how many pounds pull can a ¼" x 20 x 2" 316 SS PHMS take before the nut rips off or the head pops off? Also, any good rigging/engineering sources for a non engineering type? Thanks, Bob |
#2
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
"Bob" wrote in message
ups.com... Hi Everyone: How much pull can a 316 SS PH Machine Screw ¼ x 20 handle before it fails? Basically I want to know how many fasters I need to use to secure some deck hardware. After an afternoon searching the web I found lots of tech sites with the info but my math skills are limited to + - x / and don't know how to convert engineering units to simple old fashioned pounds of pull. So how many pounds pull can a ¼" x 20 x 2" 316 SS PHMS take before the nut rips off or the head pops off? Also, any good rigging/engineering sources for a non engineering type? Thanks, Bob Bob, The reason you're having trouble finding your "one number" is that it's not really as simple as that. There's shear strength (failure due to lateral forces) and tensile strength (failure due to pulling forces) all of which have different types - yield strength (the force that it can take without permanent deformation), ultimate strength (the maximum it can stand) and breaking strength (how much stress it experienced at the point of rupture). You'd likely want to give yourself a factor of 3 or 4 as a safety margin for work hardening and galling through overtorquing (with all-stainless fasteners, too tight is nearly as bad as too loose.) Typically various fastener strengths are stated in megapascals (MPa) which you can convert to PSI by multiplying by 145. I don't know what deck fitting you're looking to secure but I'd suspect that any failure would occur elsewhere than the screws - heads or nuts pulling through the deck. -- Mike Harris Austin, TX |
#3
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
Mike Harris wrote: Bob, The reason you're having trouble finding your "one number" is that it's not really as simple as that. There's: shear strength (failure due to lateral forces) and tensile strength (failure due to pulling forces) all of which have different types - yield strength (the force that it can take without permanent deformation), ultimate strength (the maximum it can stand) and breaking strength (how much stress it experienced at the point of rupture). I guess its like asking how big is your boat? GRT, LWL, LOA, LBP, LOD, pounds, etc You'd likely want to give yourself a factor of 3 or 4 as a safety margin for work hardening and galling through overtorquing (with all-stainless fasteners, too tight is nearly as bad as too loose.) Yes, I found a site that mentioned 78 ft lbs is best touque for a 1/4" MS. Im able to understand that but........ Typically various fastener strengths are stated in megapascals (MPa) which you can convert to PSI by multiplying by 145. Yreka or was that Eureka....... Mpa was way over my head. There were a few othes but I can certainly multiply MPa by 145. What I would really like to know is how much pullig force can a 1/4 MS handle befor failing? But I a not sure how psi would give me a simple pounds of pull? Any suggestion for converting psi to a simple pounds pulling on the fastener? I don't know what deck fitting you're looking to secure but I'd suspect that any failure would occur elsewhere than the screws - heads or nuts pulling through the deck. Yes. Agreed on most cored GRP decks. I am going the epoxy potting and backing plate routeen. Should increase holding ability significantly. Thanks for the reply Mike. Sugestoins about that simple number in pounds or Kg? Bob -- Mike Harris Austin, TX |
#4
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
On 1 Sep 2006 09:10:02 -0700, "Bob" wrote:
Hi Everyone: How much pull can a 316 SS PH Machine Screw ¼ x 20 handle before it fails? Basically I want to know how many fasters I need to use to secure some deck hardware. /// Bob Annealed 316 wire yields at a pressure of 34,800 lb. [see Matweb] The cross section area of a 1/4 in X 20 tpi screw is about pi X r X r square inches where r is the effective radius. Using r = 0.06 inch gives a = 0.011 sq in so the force at yield is about 0.011 X 34800 lb That's about 390 lb. The bolt will break before the nut. You could give a X5 safety factor and take the maximum unit load as 75 lb each. You're welcome Brian Whatcott Altus OK |
#5
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
Brian Whatcott wrote: Annealed 316 wire yields at a pressure of 34,800 lb. [see Matweb] The cross section area of a 1/4 in X 20 tpi screw is about pi X r X r square inches where r is the effective radius. Using r = 0.06 inch gives a = 0.011 sq in so the force at yield is about 0.011 X 34800 lb That's about 390 lb. The bolt will break before the nut. You could give a X5 safety factor and take the maximum unit load as 75 lb each. You're welcome Brian Whatcott Altus OK Brian, Thanks for walking me through that. Seems straight forward with your explination. Now time for me to get back to the boat. Bob |
#6
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
Bob wrote:
Brian Whatcott wrote: Annealed 316 wire yields at a pressure of 34,800 lb. [see Matweb] The cross section area of a 1/4 in X 20 tpi screw is about pi X r X r square inches where r is the effective radius. Using r = 0.06 inch gives a = 0.011 sq in so the force at yield is about 0.011 X 34800 lb That's about 390 lb. The bolt will break before the nut. You could give a X5 safety factor and take the maximum unit load as 75 lb each. You're welcome Brian Whatcott Altus OK Brian, Thanks for walking me through that. Seems straight forward with your explination. Now time for me to get back to the boat. Bob I think this whole analysis, while very good, leaves out an important part: the threads. When screwed into fiberglass or wood (or perhaps even aluminum, depending on the length of thread engagement), the failure mode will be to rip the threads out of the hole, rather than to break the screw body in tension. Also, computing the max load on the assumption that it is indeed the body which fails (like for example, if the screw were to be installed into a stainless plate), fails to take into account the stress riser effect of the threads. Because of this it is likely that the screw will fail well before the calculated value of 390 lb. Thus the 5x safety factor based on the theoretical which Brian recommended... bob |
#7
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
The root diameter of a 1/4-20 screw is .201 inches and the min. yield
strength is (18-8 SS screw) 40,000 psi which, when combined with a safety factor of 5 gives 254 lb safe load (depending on what is screwed into). |
#8
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
On Tue, 5 Sep 2006 19:15:51 -0400, "Dave W"
wrote: The root diameter of a 1/4-20 screw is .201 inches ... Oh really? :-) Brian Whatcott Altus OK |
#9
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
Sure is.... according to my Starrett tap and drill guide. I was a little
surprised so I measured some and found it to be correct. Dave "Brian Whatcott" wrote in message ... On Tue, 5 Sep 2006 19:15:51 -0400, "Dave W" wrote: The root diameter of a 1/4-20 screw is .201 inches ... Oh really? :-) Brian Whatcott Altus OK |
#10
posted to rec.boats.building
|
|||
|
|||
Strength of a 316 PH Machine Screw 1/4x20 ?
On Wed, 6 Sep 2006 09:01:28 -0400, "Dave W"
wrote: Sure is.... according to my Starrett tap and drill guide. I was a little surprised so I measured some and found it to be correct. Dave "Brian Whatcott" wrote in message .. . On Tue, 5 Sep 2006 19:15:51 -0400, "Dave W" wrote: The root diameter of a 1/4-20 screw is .201 inches ... Oh really? :-) Brian Whatcott Altus OK Hmmm...I'm getting an uneasy feeling that I am being pulled in further than I want to go. Ah well: though machine screw series are designed to break at the bolt, the failure mechanism isn't quite via yield at some effective diameter as I suggested. A slightly better model is to visualize shear failure at the thread root, so this calls for computing an effective area held by the nut times the material shear ultimate - but this ignores (at least) two other factors: 1) you may have seen a model acrylic bolt section stressed at the threads, and viewed through polarizers - the shading highlights the X2 to X3 stress multiplication caused by the thread root form. This is where the cracks start. 2) fatigue life is dependent on a smooth transistion between threads and the shank of the fixing - this too can have a /2 factor on fatigue life. So, the carry home message is, nobody ever got jailed for misconduct for being either too conservative, or for knowing exactly what they are doing if they design right to the yield limit (as aero engineers often need to do.) But people do die if the engineering is skimped. The choice is yours, as always. I am of course joking about engineers being imprisoned for screwing up - it just doesn't happen - not in the US at least, as far as know. I think it would probably be better to pursue this kind of topic on a group like sci.engr.mech or sci.engr.civil if you want more Or, you can find perfectly respectable allowable or ultimate force tables for screws of many sizes and types in web-accessible form. Probably more than a sailor wants or needs? Respectfully Brian Whatcott Altus OK |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
`72 Evinrude V4 Problem | General | |||
Do not remove Shift Pivot Screw | General | |||
Strength of Steel Vs Carbon | ASA | |||
Fiberglass loss of strength | Cruising | |||
fixing screw heads pulled though surface ply | Boat Building |