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grip length calculation

grip length calculation

grip length calculation

Do you include the nut as part of the material thickness when determining grip length?

RE: grip length calculation

No, the nut/collar is not included.

RE: grip length calculation

shairfair... hope this makes sense....

NO and YES....regarding "nut as part of the GL"

NO. Grip length is generally material stack-up dependent. However stack-up can vary greatly due to variations in thickness tolerance; and thickness of a specified washer (always use a washer). In general 1/2-thread in the stack-up is OK... ideally the bolt grip should terminate the washer thickness. [more to come on this]

The fastener grip is more precisely controlled than the material stack-up. Grip length being the "usable bearing area" from underneath the bolt head to the end of the shank... just where the threads start. Here is where "YES" for nut considerations in GL have to be considered...

There MUST be at least 1-bolt thread fully protruding beyond the end of the Nut when the nut has been fully "torqued". Some specs say 1-1/2 others say 2-threads. This indicates the nut is fully engaged on the threads. Note: since most nuts have a small thread chamfer, 1-visible thread length protrusion beyond the [absolute] end of the nut is actually slightly more than 1-thread... usually 1/4-to-1/2. Likewise the nut MUST NEVER bottom-out on the bolt-threads! If this occurs, the fastener installation will not attain intended stack-up preload... and will be essentially "loose".

Nuts are funny creatures. They come in all sizes from extremely thin [low height, shear only], to extremely deep in the threaded section [full shear and tension capable]. They can have counterbores to accept a small amount of shank protrusion into them safely... or be threaded the full length with just "clean-up chamfers in the threads. Some are "fat" across the span of the washer-base and wrenching area [hex, double-hex, spline, etc]. while some maintain a broad "washer-base" and taper-way-down in width across the wrenching area for weight control. Alloy/temper/strength also makes a big difference, too. Also most threads in Nuts are machined [cut]; while most aerospace bolts [the narrower part of the threaded installation] are now roll-formed for fatigue durability.

Here is where it gets dicey! Bolt threads can also have a large variation in length: short threads [typ 1.2--1.4D length] to medium threads [typ 1.4--1.8D length] to long threads [Typ 1.8--2.2D length] to very long-threads [typ 2.5--3.0D+ length]. The bolt thread-length chosen, typically drives nut selection, IE:  
low height Nuts [shear, low tension] on short threads; regular height Nuts [shear or full tension] on long threads, etc.

Where it gets ugly is when the wrong nut is selected for the bolt threads; and/or the intended structural purpose. I have had guys try to put a regular height nut on a short thread bolt... and whine cause grip is too long or too short. Short thread bolts demand precision grip measurement and a matching low height nut [shear or shear-tension rated]. A regular height nut probably wont have proper thread protrusion even when "bottomed" on the short bolt threads. When installing thousands of fasteners, this precision fit saves a lot of weight and interior protrusion. However, long thread bolts with LH or regular nuts, plus washers, allow some breathing room for grip adjustment and imprecise fits.... As long as the added weight and protrusion are tolerable!  

When a nut is ~80% (or more) of the thread length, there is very little room for grip-length error/variation. Nuts ~50% of the thread length have lots of flexibility for grip error and adjustment, especially with washers. A worst case comedy is when a long thread bolt has a low-height castellated nu, secured by a cotter-pin [thru the drilled-hole in the end of the bolt threads], in a long-grip scenario. The stack-up of washers to adjust the nut so the castellation cuts align with the thread-hole for cotter-pinning can be huge. Due weight and stiffness issues, most "specs limit "hard" [steel, ti, etc] washers to no more than 4 (1-under head, 3-under nut or 4-under nut); and soft washers [aluminum, etc] to no more than 3 (1-or none under head, 2-under nut or none-under head and 3-under nut.

Regards, Wil Taylor

RE: grip length calculation


wktaylor did an excellent job of explaining the issue.  In summary, use the correct type and grip length of fastener for the load (tension or shear), never put threads in bearing, use the correct type of nut, use the correct type and number of washers both under head and under nut, provide a clear and explicit description of the fastener installation on your engineering documentation, and most importantly do a careful dimensional tolerance stack-up at both MMC and LMC conditions.

Good luck.

RE: grip length calculation

"Likewise the nut MUST NEVER bottom-out on the bolt-threads! If this occurs, the fastener installation will not attain intended stack-up preload... and will be essentially "loose"."

and if "shanked out" with enough enthusiasm, can be a bitch to remove.

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