Stainless steel fasteners
Stainless steel fasteners
(OP)
Hi!
I am having a few problems being reported to me about the stainless steel fasteners used on a machine currently being built. The fasteners are of A4-70 grade and use nyloc nuts. When they are being tightened up by the fitters some of the fasteners are suffering from thread stripping (on the screw),some have sheared off completely. The ones that have tightened up ok generally shear off when trying to undo them! I specified a torque value given by the fastener supplier and the fitters are saying that the nylon patch in the nut is "jamming" up the threads on the nuts. I'm not sure as to wether the torque value is wrong or the fitters are right. Has anyone experienced a similer problem with locking nuts? Which are the best to use on stainless steel fasteners? Ive contacted the supplier about this but they are busy and can't get an engineer over until next week and I need an answer sooner (management!)
The screws are M8x1.25-6g A4-70, Nyloc style full nuts, 25Nm torque value (for reusable fastener),washer under screw head and nut. Hope someone can help!
Thanks.
I am having a few problems being reported to me about the stainless steel fasteners used on a machine currently being built. The fasteners are of A4-70 grade and use nyloc nuts. When they are being tightened up by the fitters some of the fasteners are suffering from thread stripping (on the screw),some have sheared off completely. The ones that have tightened up ok generally shear off when trying to undo them! I specified a torque value given by the fastener supplier and the fitters are saying that the nylon patch in the nut is "jamming" up the threads on the nuts. I'm not sure as to wether the torque value is wrong or the fitters are right. Has anyone experienced a similer problem with locking nuts? Which are the best to use on stainless steel fasteners? Ive contacted the supplier about this but they are busy and can't get an engineer over until next week and I need an answer sooner (management!)
The screws are M8x1.25-6g A4-70, Nyloc style full nuts, 25Nm torque value (for reusable fastener),washer under screw head and nut. Hope someone can help!
Thanks.





RE: Stainless steel fasteners
RE: Stainless steel fasteners
Rod
RE: Stainless steel fasteners
I have done some investigation today. A lot of people i talked to mentioned lubricants and also the problem of galling with stainless steel fasteners. However, no one has yet given me an explanation of why this is a particular problem with stainless steel. I've had inspection measure the fasteners and they are within their tolerance grade. I've checked the standards and it seems that A4-70 grade has similar tensile strength and hardness to metric grade 8.8 steel fasteners. If the fasteners are nearly the same as 8.8's in tolerances, tensile strength and hardness then why do the A4's experience galling and not the 8.8's? I have not experienced these problems with softer/weaker fasteners like brass or aluminium.
Can anyone explain why stainless steel fasteners have this problem? Is it something to do with its composition? Friction co-efficient?
I know this probably seems like a minor problem but i'll be working with stainless steel (and stainless fastening systems) alot more in the coming months and i would like to get a full picture of its problems.
Thanks.
RE: Stainless steel fasteners
By the way, aluminium and zinc surfaces can also be very prone to galling.
RE: Stainless steel fasteners
A www.goggle.com (or other internet) search for "stainless steel fastener galling" will yield a wealth of info...
RE: Stainless steel fasteners
RE: Stainless steel fasteners
NOT a 'Pat A'nswer!!(Pun intended).
Anti seize is NOT a good lubricant for SS fasteners in ALL cases. Quite often its use is restricted in certain applications (electrical, explosives, etc.). However it is acceptable where its use is not restricted (as is just about ANY lube).
Rod
RE: Stainless steel fasteners
Maybe wrong torque recommendations you have?
RE: Stainless steel fasteners
Anyone any experience of this working?
RE: Stainless steel fasteners
Using dissimilar spec SS has NOT proven benificial in my experience.
Rod
RE: Stainless steel fasteners
Womble
RE: Stainless steel fasteners
Anti seize is a good product to use in most applications. If you don't have a can of this in your shop, I have been recommending 2 things for years now to customers and have yet to have the problem reappear.
First, don't use power tools. The faster the revolutions, the greater the chance of stainless galling. If you must use power tools, turn the speed down as low as possible.
Second, and more importantly, wax is a basic coating that will help eliminate the galling. Any wax will work, even floor wax. Dip the nut, let it dry, and use it. If you have large quantities to use, have your supplier get the product wax plated. Also, if you need a bio degradable coating, this is available on the market.
Galling is common, especially with stainless nylon lock nuts, but it can be eliminated.
David Long
Marine Fasteners
RE: Stainless steel fasteners
Thread galling is also referred to as “Cold Welding” as during fastener tightening, pressure builds between the contacting thread surfaces and protective oxides are broken. The interface metal high points then shear or lock together. This cumulative clogging action causes additional adhesion and increases the severity of the galling.
In extreme cases the galling effect leads to complete thread seizing and if tightening is continued the fastener can be twisted off or its thread stripped.
To help reduce the amount of galling produced, slowing down the RPM of screw tools will frequently reduce and in some cases solve the problem completely. As the screw tool RPM increases, the heat generated during tightening increases. As the speed and inevitably the heat increases so does the tendency for the occurrence of thread galling.
If corrosion resistance is not an issue then changing the alloy grades of stainless or material type of the bolts that you are using could also reduce the problem of galling.
Lubricating the internal and/or external threads frequently eliminates thread galling. The proposed lubricant should contain substantial amounts of Molybdenum disulfide (Moly), graphite, mica or talc. Some proprietary, extreme pressure waxes may also be effective. Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating. Several chemical companies offer anti-galling lubricants.
RE: Stainless steel fasteners
If you can use very hard nuts (17-4PH or 17-7PH) it will help.
For all around use I don't like the compounds that contain metals, you can get into problems. Look for ones based on synthetic lubricants with tungsten disulfide. I have made my own by mixing WS2 with wax, but it isn't worth the hassle.
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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: Stainless steel fasteners
AWILD, you appear to know SS fasteners well enough but, there are just too many applications that have "special" lube requirements to generalize, eg --- "The proposed lubricant should contain substantial amounts of Molybdenum disulfide (Moly), graphite, mica or talc."
EdStainless, your experiences seem to be similar to some of mine. Many applications such as the food industry have restrictions on lube types. Indeed, some places (eg Golden West Foods) we were instructed NOT to use lubes of any type. A problem I refer to at another gov't. job in a post above from a couple of years ago. This restriction, using lubricants on SS hardware, is still prevelant in some of todays gov't. jobs, unfortunately.
Biosafe waxes are so common that it makes one wonder what hidden agendas go into the makeup of the specs for some of these jobs.
Oh, yes. There is a flipside to this. At a "rendering" plant on Bandini Blvd. in L.A. back in the early 80's while installing a piece of machinery, the 'lube' problem was that there was so much (all over the floor, walls, tools etc) that it was extremely difficult just handling the SS fasteners...! ;o)
Rod
RE: Stainless steel fasteners
RE: Stainless steel fasteners