Saturday, February 11, 2017

Stainless Steel Bolt Specifications

Whether bolting on some new project, or trying to understand why something failed, you need to know how much a fastener can hold.

Clamping Force. When torqued to ANSI specifications, this is how much a bolt will hold before it becomes loose due to stretch (bolts stretch under tension--everything does). If the bolt cycles past this load, it will move around, the hole will be damamged, and something will break. Additionally, this is beyond the working Load Limit (WLL), so we wshould never get here.

Sheer Strength vs. Tensile Strength. Only important so that we understand that they are slightly different. Since most loads are a combination, the WLL is conservatively based on sheer strength.

Working Load Limit (WLL). There is no universal standard for determining this figure. In safety critical applications and applications where forces are not well defined the safety factor is greater than low risk applications and those where the forces are static and well known. The total number of cycles and corrosion matter. For the purposes of this table I set it at 4:1 based on sheer strength.

Backing Plates. The need depends entirely on the strength of the base material. Obviously, a 1/4-inch bolt passing through a 1/2-inch steel plate does not need a backing plate. It does need a standard (bolting ) washer to provide a smooth bearing surface while torquing it down. However, when fastening to fiberglass, ANY SIZE steel bolt, even combined with an ANSI bolting washer, can exceed the compression strength of fiberglass, no mater how thick. The fiberglass will crumble and crack. I did a bunch of instrumented testing for articles using both thin cored laminates and solid glass, and several simple rules of thumb emerged:

  • Backing plate size: 5 bolt diameters minimum. 7 bolt diameters for thin skinned cores.
  • Backing plate thickness:
    • Stainless Steel--1/2 bolt diameter
    • Aluminum--1/2 bolt diameter. A bolting washer is also required. Not for damp locations; I've seen plates just crumble away in anchor lockers. Fine in a dry cabin.
    • Solid fiberglass--1 bolt diameter, unless bonded directly to the laminate. Then 1/2 bolt diameter. A bolting washer is also required.
    • Wood. Not recommended due to rot and splitting potential.
      • Plywood--2 bolt diameters. A bolting washer is also required.
      • Hard wood--3 bolt diameters. Not recommended, since all of he test samples failed by sudden splitting. A bolting washer is also required.
    • HDPE (Starboard). Not recommended. All samples gradually bowed and cracked under sustained load. Just don't do it.
  • Bond the bolting plate to the substrate if possible to insure good contact. Vital if the surface is very rough or curved. The bonding can be weak material, such as sealant. 
  • Threads. A single steel bolt will hold full tensile strength, even though it will strip before the bolt breaks when turned. The required thichness for the threads to hold full bolt tension varies with the material and how well the threads are cut, but is aproximatly:
    • Steel. 2/3 bolt diameter.
    • Aluminum. 1 bolt diameter in spar alloys.
    • Fiberglass. 1-2 bolt diameters, depending on the density of the lay-up. 1 bolt diameter in G10 and most pre-cast laminates. No standard value for cored laminates, HDPE  (creeps) or wood.
No guess work, just solid engineering principles, though even engineers go up a size when there is something they can't be sure of.

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