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Ooh - let me guess... It's a stainless screw in a stainless thread and has suffered spalling? Spalling is a really wierd effect that only happens with screws

Ooh - let me guess... It's a stainless screw in a stainless thread and has suffered spalling? Spalling is a really wierd effect that only happens with screws and threads made of stainless steel. You can tighten a screw with your fingers (no tools, just finger tightened), and then try to undo it and it'll suddenly be stuck. You try with tools, and it's still stuck. You use a lot of force, and it snaps off! It happens because stainless steel can act a little like animal fur - rub in one direction and it slides easily, while when you change direction the fur meshes and refuses to move at all. Doesn't happen reliably either - you can have 100 identical screws in 100 identical holes, and perhaps 5% will get stuck. Normally the fix is to put vaseline or some other lubricant on the screws (before it gets stuck - after it's stuck you're sol), but I figure the NASA team doesn't want that lubricant contaminating their sample.

deleted 3y ago
[deleted]
Brajeshwar 3y ago
I'd really love that, perhaps, perhaps, we get an update to this comment -- “Hey guys, NASA just contacted me.” :-)
pibefision 3y ago
I like this forum because this kind of explanations. Thank you
xattt 3y ago
Is there a way to anticipate the “direction” of the “grain” of stainless steel?
ben_w 3y ago
I'd be a bit surprised if they have two moving parts, made of the same metal, in contact with each other: https://en.wikipedia.org/wiki/Cold_welding https://en.wikipedia.org/wiki/Cold_welding
cuSetanta 3y ago
From what I can see the TAGSAM head did use stainless steel screws, but the majority of the structure is aluminium [1]. So its possible there is a galvanic corrosion issue that caused the screws to bind, but I would be a little surprised at that based on how the TAGSAM has been handled throughout its life. Still does feel like the most likely issue though. Regarding lubrication on screws in space, it is done quite regularly with a variety of lubricants (as well as thread lockers) [2]. I can imagine that none were used in a system that is meant to be kept as free of contaminants as the TAGSAM is though. Hard to know with any amount of confidence. From experience with spacecraft integration, when the article mentions that they didn't have a suitable tool for removal of two screws, my head immediately jumps to an access issue. Its amazing the number of times I have heard 'it fits in the CAD' from the designers, when you have no access at all for getting a screwdriver to the screw, let alone a torque wrench. So it could be as simple as that. [1] - https://link.springer.com/article/10.1007/s11214-018-0521-6 https://link.springer.com/article/10.1007/s11214-018-0521-6 [2] - https://ecss.nl/hbstms/ecss-e-hb-32-23a-threaded-fasteners-handbook/ https://ecss.nl/hbstms/ecss-e-hb-32-23a-threaded-fasteners-h...
- MilStdJunkie 3y ago
  There's a lot of push in the defense world to get to so-called "Digital Twin", where cabling, conduit, maintainability and other late-stage product work is done purely in CAD and in simulation. I'm very open to the idea . . however . . when I get a pile of geometry with no fastener material data, no soft bodies of any kind (tape, string, wire, cloth, rope, etc), broke fillets . . and then someone tells me "there you go, you can figure out the maintenance breakdown now. DIGITAL TWIN!" . . I'm not filled with confidence. Someone, somewhere, needs to spec out the minimum required for each stage of this kind of work . . they might find that doing things like wire channels in new product isn't fantastically easier than doing it in a prototype. You can do it in CAD< but you still have to get the design right. Tool clearance doesn't magically appear in the right place by screaming DIGITAL TWIN. This is all aerospace. In another industry, with less demanding performance requirements, the easy solution to all problems is always "increase the tolerances". Then you can jam anything in there. Another problem: logistics systems want to know the sparing level for what's being supported, not what's being designed. This, incredibly, isn't addressed so far as I can see; adopters are just sort of pretending that bleeding edge is the same as the as-built, supported configuration. When I bring it up, the general response is that LSA can be batch-create from the design, totally ignoring the fact that a lot of changes come the other way, from field modifications. The solution for this - for NGAD and Raider - is to just not do logistics as it's been practiced for the last forty years. Spoiler: this is the correct answer. 30 year sustainment programs are hilarious fictions that do nothing but generate high margin work for primes.
johnwalkr 3y ago
I work in the industry and I’ve never heard of spalling with relation to stainless steel fasteners. The effect that happens with stainless steel fasteners is galling, when there’s enough friction in 2 mating fasteners of similar material to break through the surface oxide layers and the mating threads weld together. Lubrication helps but in aerospace there’s typically requirements to use dissimilar materials for screws and female threads. The dissimilar material can be a coating but I think they’d probably avoid unnecessary materials in this case and use (for example) titanium screws and inconel threaded inserts. Any mechanical engineer at NASA and its vendors knows this and it’s covered in detail in the NASA fastener handbook.
- xtiansimon 3y ago
  > “…when there’s enough friction in 2 mating fasteners of similar material…and the mating threads weld together.” I’m not in industry, but i wrench my motorcycle. I’m learning so much here. I’m usually worried about “galvanic corrosion” when aluminum and stainless steel corrode and making removing the steel screws all but impossible.
  - T3OU-736 3y ago
    (At the risk of leading you down a rabbit hole): consider that most torque specs are for dry torque, but in many instances, anti-corrosion measures and anti-seize compounds are quite fine to use, but the torque values have to be adjusted waaaay lower to avoid over-torquing. Consider, by way of example, the oil pan bleed screw. It and its threads are well lubed, and while granted, it is not holding much of a clamping load, it is still subject to heat cycles, vibration, etc, and yet it stays happily in. (Not an Mechanical Engineer, just a fellow occasional wrench-spinner who has battled rust) When a clamping load is involved, it starts to matter if there is an expectation of vibration, at which point something like Nord-Lock is called for. I am sure my much more knowledgeable peers here will point out the glaring mistakes and omissions I have made, causing learning to have occurred. :)
    - johnwalkr 3y ago
      This is something I’ve seen several times. often in aerospace the threads are supposed to be lubricated but if you get lubricant under the head you can get way more preload than you expect. Same for changing the washer material for a thermal or other reason and not realizing it has much lower friction. For unique applications in aerospace, it’s common to do testing to determine a non-standard torque-preload curve. In some industries, preload is measured more directly using hollow bolts (so you can measure the before and after length therefore determine preload), ultrasonic measuring devices (same reason) or preload sensing/indicating washers (there are many types).
analog31 3y ago
I think, as mentioned, the term is "galling."
- ralferoo 3y ago
  This is kind of funny as the "normal" usage of the word (by which I mean non-engineer usage) would mean causing annoyance or resentment. That seems pretty apt for the current situation!
amelius 3y ago
Maybe the atoms in the screw and the atoms in the capsule decided that they now form a single piece of metal.
FrustratedMonky 3y ago
Cool, never knew this, now all those home improvement projects where I got a screw stuck and had to drill it out make more sense.
- biorach 3y ago
  You're unlikely to be using stainless steel screws very often in home DIY
  - FrustratedMonky 3y ago
    Fancy mirrors and fixtures. You'd be surprised.
rainbowzootsuit 3y ago
I worked with ultra high vacuum equipment and am pretty familiar with this. The term I've heard is "galling." UHV is not compatible with the use of greases either. We would use silver plated screws to avoid stuck fasteners, and to avoid galling for small and delicate fasteners inside of the chamber where silver couldn't be used as it might be a contamination source, Trichloroethylene would work as a lubricant that evaporated without residue.
- Workaccount2 3y ago
  The NASA fastener handbook specifically calls for using silver plated nuts on steel bolts for aerospace. Very cool. >Silver plating is cost prohibitive for most fastener applications. The big exception is in the aerospace industry, where silver-plated nuts are used on stairdess steel bolts. The silver serves both as a corrosion deterrent and a dry lubricant. Silver plating can be used to 1600 “F, and thus it is a good high- temperature lubricant. https://ntrs.nasa.gov/api/citations/19900009424/downloads/19900009424.pdf https://ntrs.nasa.gov/api/citations/19900009424/downloads/19...

deleted 3y ago

[deleted]

Brajeshwar 3y ago

I'd really love that, perhaps, perhaps, we get an update to this comment -- “Hey guys, NASA just contacted me.” :-)

pibefision 3y ago

I like this forum because this kind of explanations. Thank you

xattt 3y ago

Is there a way to anticipate the “direction” of the “grain” of stainless steel?

ben_w 3y ago

I'd be a bit surprised if they have two moving parts, made of the same metal, in contact with each other: https://en.wikipedia.org/wiki/Cold_welding https://en.wikipedia.org/wiki/Cold_welding

cuSetanta 3y ago

From what I can see the TAGSAM head did use stainless steel screws, but the majority of the structure is aluminium [1]. So its possible there is a galvanic corrosion issue that caused the screws to bind, but I would be a little surprised at that based on how the TAGSAM has been handled throughout its life. Still does feel like the most likely issue though. Regarding lubrication on screws in space, it is done quite regularly with a variety of lubricants (as well as thread lockers) [2]. I can imagine that none were used in a system that is meant to be kept as free of contaminants as the TAGSAM is though. Hard to know with any amount of confidence. From experience with spacecraft integration, when the article mentions that they didn't have a suitable tool for removal of two screws, my head immediately jumps to an access issue. Its amazing the number of times I have heard 'it fits in the CAD' from the designers, when you have no access at all for getting a screwdriver to the screw, let alone a torque wrench. So it could be as simple as that. [1] - https://link.springer.com/article/10.1007/s11214-018-0521-6 https://link.springer.com/article/10.1007/s11214-018-0521-6 [2] - https://ecss.nl/hbstms/ecss-e-hb-32-23a-threaded-fasteners-handbook/ https://ecss.nl/hbstms/ecss-e-hb-32-23a-threaded-fasteners-h...

MilStdJunkie 3y ago
There's a lot of push in the defense world to get to so-called "Digital Twin", where cabling, conduit, maintainability and other late-stage product work is done purely in CAD and in simulation. I'm very open to the idea . . however . . when I get a pile of geometry with no fastener material data, no soft bodies of any kind (tape, string, wire, cloth, rope, etc), broke fillets . . and then someone tells me "there you go, you can figure out the maintenance breakdown now. DIGITAL TWIN!" . . I'm not filled with confidence. Someone, somewhere, needs to spec out the minimum required for each stage of this kind of work . . they might find that doing things like wire channels in new product isn't fantastically easier than doing it in a prototype. You can do it in CAD< but you still have to get the design right. Tool clearance doesn't magically appear in the right place by screaming DIGITAL TWIN. This is all aerospace. In another industry, with less demanding performance requirements, the easy solution to all problems is always "increase the tolerances". Then you can jam anything in there. Another problem: logistics systems want to know the sparing level for what's being supported, not what's being designed. This, incredibly, isn't addressed so far as I can see; adopters are just sort of pretending that bleeding edge is the same as the as-built, supported configuration. When I bring it up, the general response is that LSA can be batch-create from the design, totally ignoring the fact that a lot of changes come the other way, from field modifications. The solution for this - for NGAD and Raider - is to just not do logistics as it's been practiced for the last forty years. Spoiler: this is the correct answer. 30 year sustainment programs are hilarious fictions that do nothing but generate high margin work for primes.

johnwalkr 3y ago

I work in the industry and I’ve never heard of spalling with relation to stainless steel fasteners. The effect that happens with stainless steel fasteners is galling, when there’s enough friction in 2 mating fasteners of similar material to break through the surface oxide layers and the mating threads weld together. Lubrication helps but in aerospace there’s typically requirements to use dissimilar materials for screws and female threads. The dissimilar material can be a coating but I think they’d probably avoid unnecessary materials in this case and use (for example) titanium screws and inconel threaded inserts. Any mechanical engineer at NASA and its vendors knows this and it’s covered in detail in the NASA fastener handbook.

xtiansimon 3y ago
> “…when there’s enough friction in 2 mating fasteners of similar material…and the mating threads weld together.” I’m not in industry, but i wrench my motorcycle. I’m learning so much here. I’m usually worried about “galvanic corrosion” when aluminum and stainless steel corrode and making removing the steel screws all but impossible.
- T3OU-736 3y ago
  (At the risk of leading you down a rabbit hole): consider that most torque specs are for dry torque, but in many instances, anti-corrosion measures and anti-seize compounds are quite fine to use, but the torque values have to be adjusted waaaay lower to avoid over-torquing. Consider, by way of example, the oil pan bleed screw. It and its threads are well lubed, and while granted, it is not holding much of a clamping load, it is still subject to heat cycles, vibration, etc, and yet it stays happily in. (Not an Mechanical Engineer, just a fellow occasional wrench-spinner who has battled rust) When a clamping load is involved, it starts to matter if there is an expectation of vibration, at which point something like Nord-Lock is called for. I am sure my much more knowledgeable peers here will point out the glaring mistakes and omissions I have made, causing learning to have occurred. :)
  - johnwalkr 3y ago
    This is something I’ve seen several times. often in aerospace the threads are supposed to be lubricated but if you get lubricant under the head you can get way more preload than you expect. Same for changing the washer material for a thermal or other reason and not realizing it has much lower friction. For unique applications in aerospace, it’s common to do testing to determine a non-standard torque-preload curve. In some industries, preload is measured more directly using hollow bolts (so you can measure the before and after length therefore determine preload), ultrasonic measuring devices (same reason) or preload sensing/indicating washers (there are many types).

analog31 3y ago

I think, as mentioned, the term is "galling."

ralferoo 3y ago
This is kind of funny as the "normal" usage of the word (by which I mean non-engineer usage) would mean causing annoyance or resentment. That seems pretty apt for the current situation!

amelius 3y ago

Maybe the atoms in the screw and the atoms in the capsule decided that they now form a single piece of metal.

FrustratedMonky 3y ago

Cool, never knew this, now all those home improvement projects where I got a screw stuck and had to drill it out make more sense.

biorach 3y ago
You're unlikely to be using stainless steel screws very often in home DIY
- FrustratedMonky 3y ago
  Fancy mirrors and fixtures. You'd be surprised.

rainbowzootsuit 3y ago

I worked with ultra high vacuum equipment and am pretty familiar with this. The term I've heard is "galling." UHV is not compatible with the use of greases either. We would use silver plated screws to avoid stuck fasteners, and to avoid galling for small and delicate fasteners inside of the chamber where silver couldn't be used as it might be a contamination source, Trichloroethylene would work as a lubricant that evaporated without residue.

Workaccount2 3y ago
The NASA fastener handbook specifically calls for using silver plated nuts on steel bolts for aerospace. Very cool. >Silver plating is cost prohibitive for most fastener applications. The big exception is in the aerospace industry, where silver-plated nuts are used on stairdess steel bolts. The silver serves both as a corrosion deterrent and a dry lubricant. Silver plating can be used to 1600 “F, and thus it is a good high- temperature lubricant. https://ntrs.nasa.gov/api/citations/19900009424/downloads/19900009424.pdf https://ntrs.nasa.gov/api/citations/19900009424/downloads/19...