Announcement

Collapse
No announcement yet.

SHCS and material thickness?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • J Tiers
    replied
    Originally posted by Bented View Post

    Please explain this.
    Why? Fake parts, you should already know.

    Leave a comment:


  • Bented
    replied
    Originally posted by J Tiers View Post

    "sabotage steel".
    Please explain this.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by challenger View Post
    What about the material? So many SHCS come from China and aren't of the quality that they once were. I wonder if American companies still produce general purpose fasteners?
    If it's made of chinese butter-soft "sabotage steel", that would change the thickness that matches the screw strength..... but the same principle applies. I'd still make it for the standard part though, presumably the reason is for strength, and for that you would not be using the "sabotage steel".

    Leave a comment:


  • challenger
    replied
    What about the material? So many SHCS come from China and aren't of the quality that they once were. I wonder if American companies still produce general purpose fasteners?

    Leave a comment:


  • oxford
    replied
    Originally posted by rickyb View Post
    I have to side with Lew. Very little information to start with and not enough afterwards to say any thing other than generalities. No surprise only 15% are on topic. The rest are what if’s, which is a natural progression of a conversation with little specific info.
    Yes first post wasn’t very clear. I did clear it up on the first page though.

    From what I see, Lew just likes to come into threads and bitch about what’s going (or not going on). So Lew, do you want to add anything useful to this thread? You can pick either the topic I was looking for info on or you can go with thread engagement where some in this thread went. It’s your choice. Others in here have already posted a lot of useful info, what do you have?

    Leave a comment:


  • rickyb
    replied
    I have to side with Lew. Very little information to start with and not enough afterwards to say any thing other than generalities. No surprise only 15% are on topic. The rest are what if’s, which is a natural progression of a conversation with little specific info.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by Lew Hartswick View Post
    See what happens when you don't describe the question adequately ?? Two pages of BLATHER that have nothing to do with the question. . :-(
    ...lew...
    Well, that's rather nasty....

    I think that out of the 29 responses, 3 or 4 were actually on-topic. Of course, yours was one that was totally off-topic, and presumably counts as "BLATHER" (so does this response, I suppose)

    Leave a comment:


  • Lew Hartswick
    replied
    See what happens when you don't describe the question adequately ?? Two pages of BLATHER that have nothing to do with the question. . :-(
    ...lew...

    Leave a comment:


  • rickyb
    replied
    Originally posted by dian View Post

    you cant achieve this by engagement. if you put an 12.9 screw in regular aluminum you can make it 10 meters long and i will still come out in one piece.

    also, if looking at similar materials you cant just increase the length of engagement proportionally to the strain, as the stress distribution on the threads is very progressive (degressive?). from memory: first thread carries 30% of load.

    edit: i wonder if another way of looking at how much force the part can support would be contact pressure. here things get interesting. for a m10 8.8 head cap screw (hole 11 mm) contact pressure is 331 n/mm2. for 12.9 its 567 n/mm2.

    "recommended" max. pressures:

    mild steel 260 n/mm2
    1045 700
    4340 850
    ci 600-1100
    alu 140-370*

    *450 mpa

    interestingly contact pressures for screws smaller than 10mm are around 300 n/mm2 and for larger than 10mm around 500 n/mm (for 8.8 that is).

    so the shear strength might not be the only parameter to consider.
    We are talking about a general rule to follow and some reasons for it. Yes there are many more variables to consider to complete the design of a joint but no rule of thumb considers the variables.

    Leave a comment:


  • dian
    replied
    Originally posted by rickyb View Post

    It was a long wait to see the correct answer. Engagement must be enough so that the fastener fails not the part. Whatever that takes in engagement is the right amount of engagement.
    you cant achieve this by engagement. if you put an 12.9 screw in regular aluminum you can make it 10 meters long and i will still come out in one piece.

    also, if looking at similar materials you cant just increase the length of engagement proportionally to the strain, as the stress distribution on the threads is very progressive (degressive?). from memory: first thread carries 30% of load.

    edit: i wonder if another way of looking at how much force the part can support would be contact pressure. here things get interesting. for a m10 8.8 head cap screw (hole 11 mm) contact pressure is 331 n/mm2. for 12.9 its 567 n/mm2.

    "recommended" max. pressures:

    mild steel 260 n/mm2
    1045 700
    4340 850
    ci 600-1100
    alu 140-370*

    *450 mpa

    interestingly contact pressures for screws smaller than 10mm are around 300 n/mm2 and for larger than 10mm around 500 n/mm (for 8.8 that is).

    so the shear strength might not be the only parameter to consider.
    Last edited by dian; 02-01-2021, 01:16 PM.

    Leave a comment:


  • rickyb
    replied
    Originally posted by oxford View Post





    So if one was going to go for less strength for whatever reason, would you go with a standard height fastener head and less material under it or go to a low profile head and more material.

    Fwiw, McMaster states standard head at 170ksi and low profile at 140ksi.

    Jerry thanks for the post where this quoted came from, good info in it.
    Or you can use stainless cap screws which are about 75ksi

    Leave a comment:


  • J Tiers
    replied
    Sizing for a 170ksi SHCS, and actually using a low profile type, would err on the side of safety. It also allows the use of more material under the head without anything protruding, so that can be a bolt failure guarantee of sorts if the design is done that way.

    I understand the idea of the fastener failing first, as the more easily replaced part. Obviously that does involve the part with the hole being stronger than "required". Probably few situations in which "we" find ourselves in our shops will be such that we are likely to break fasteners, parts, or anything, really.

    I believe it has been suggested that nuts should fail before bolts, and now we have the idea that the bolt should fail before the larger part. I would suppose that the nut failing would be equivalent to the bolt failing, and if the bolt fails before the plate, and the nut before the bolt, then surely the nut is the "fuse" here.

    Now, what I did however, considered the ultimate point of failure. That is where the distorted metal finally ruptures. It is not including any factor of safety, and does not mean that the plate will not be distorted when the bolt fails.

    I am sure that the bolt failing first is a great idea if you intend to reuse the plate, but the reality likely is that very few connections are going to do that. In anything but a straight pull situation, the plate is likely to be bent just due to general geometry of the forces, long before the bolt fails or the plate fails by shearing out the remaining material in a recessed bolt hole.

    If one intends reuse after failure, then it will come down to a more complex issue of the yield points. One would want the bolt to yield before the plate..... I believe that is just a rabbithole that is not worth pursuing unless you are doing rather specialized design. If there is a need for that, you will already know it, know how to handle it, and will not need any advice from the rest of us.

    I am not a mechanical engineer, I am electrical. I did take a fair bit of structures and related courses in distribution, but not even a minor in it. There are a few here who are MEs, and if you want more, I expect they will chime in and correct our mistakes.

    On a related subject, that is likely relevant, I did, a while back, some testing of screws in aluminum, using the screw itself as the force applicator, by torquing it down. This was to see if threads or heads failed first, and where. You may be able to find it with the "search" on the site. The screws were SHCS.
    Last edited by J Tiers; 01-31-2021, 12:27 AM.

    Leave a comment:


  • oxford
    replied
    Originally posted by J Tiers View Post
    , for a 170ksi SHCS

    For mild steel, it should be about 75% of the fastener nominal diameter, for the same range.

    Obviously, for less than maximum strength, you can go thinner.

    Originally posted by rickyb View Post
    Engagement must be enough so that the fastener fails not the part.

    So if one was going to go for less strength for whatever reason, would you go with a standard height fastener head and less material under it or go to a low profile head and more material.

    Fwiw, McMaster states standard head at 170ksi and low profile at 140ksi.

    Jerry thanks for the post where this quoted came from, good info in it.

    Leave a comment:


  • rickyb
    replied
    Originally posted by tom_d View Post



    The 1-1/2 times the diameter works well for length of engagement. In general terms anything over 1-1/2 times the diameter, the screw will break before the threads strip. Of course this is an approximation based on common sense things like using course threads on soft materials and fine threads on harder materials, and subject to mathematical fine tuning that the shade tree mechanic and machinist might not be privy to.
    It was a long wait to see the correct answer. Engagement must be enough so that the fastener fails not the part. Whatever that takes in engagement is the right amount of engagement.

    Leave a comment:


  • J Tiers
    replied
    Doing some rough calculations, what I get is the following, for a 170ksi SHCS, and assuming that maximum strength is wanted, so the screw and the remaining plate thickness should be the same strength.

    The under-head thickness for aluminum 6061 should be about 85% of the fastener nominal diameter, for fasteners in the 1/4" to 3/8" range

    For mild steel, it should be about 75% of the fastener nominal diameter, for the same range.

    For smaller fasteners, the core area goes down fast, so the thickness can be less.

    Obviously, for less than maximum strength, you can go thinner. And since I used a conservative strength for the screw, going a bit thicker would not be a bad thing, for max strength. Of course, for maximum strength where that matters, it will all be calculated out....


    The work:
    Ok, assume a 170ksi screw.

    For 1/4-28 SHCS, head is 3/8" diameter, circumference is 1.17" minor dia is 0.2"
    For 5/16" -24screw, head is 0.468 diameter, circumference is 1.472" minor dia is 0.26"
    For 3/8"-24 screw, head is 0.5625 diameter, circumference is 1.767" Minor dia is 0.32

    So for these, core area of 1/4-28 (conservative) is 0.03 sq in (5.27 ksi). Core area of 5-16-24 is .05 sq in (8.5 ksi) and core area of 3/8-24 is .08 sq in (13.6 ksi)

    Aluminum 6061 is around 45 ksi, mild steel is around 50 ksi, So shear for these would be 22.5 ksi for aluminum 6061, and 25 ksi for mild steel

    Aluminum 6061 needs then 0.23 square inch for 1/4-28, or a thickness under the head of about 0.212"
    Mild steel needs 0.210 square inch for 1/4-28 or thickness under the head of 0.18"

    Aluminum 6061 needs then 0.377 square inch for 5/16-24 or a thickness under the head of 0.25"
    Mild steel needs then 0.34 square inch for 5/16-24 or a thickness under the head of 0.23"


    Aluminum 6061 needs then 0.60 square inch for 3/8-24 or a thickness under the head of 0.34"
    Mild steel needs then 0.54 square inch for 3/8-24 or a thickness under the head of 0.31"

    The rule of thumb would be then that for this general range, the under-head thickness for 6061 aluminum should be about 85% of the fastener nominal diameter.

    For mild steel, it should be about 75% of the fastener nominal diameter.
    Last edited by J Tiers; 01-30-2021, 07:27 PM.

    Leave a comment:

Working...
X