3 ms·

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 ga

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).

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).