What purpose does reduced major dia fastener serve?
What purpose does reduced major dia fastener serve?
(OP)
I have a question about NAS6304 bolts. Along with reducing the head height, these bolts have a reduced major diameter. I have a foggy recollection that these may be intended for internal UNJF threads. However, since the thread contact area (and hence friction) is less, applying a 'standard' torque calculation will result in a higher preload. Anyone have any good references to help me use these fasteners safely?





RE: What purpose does reduced major dia fastener serve?
Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling
RE: What purpose does reduced major dia fastener serve?
With a reduce section you can effectively get a longer fastener without having to stretch the fasteners at the major diameter. Under certain conditions this can greatly improve fatigue life.
We use reduced cross section fasteners on our high speed equipment for several reasons, like the above mentioned shear load, lowering the rotating mass, ease of assembly in close tolerance holes.
RE: What purpose does reduced major dia fastener serve?
The reduced major diameter serves to reduce the stress on the threaded part when the bolt is subject to shock loading.
If a bolt of the usual form having a full sized shank and threaded end is used to support a tensile load then the threaded part will have a higher stress than the plain shank.
If a tensile load is suddenly applied, the energy absorbed by each unit volume of the bolt will be proportional to the square of its stress at that location; hence a large part of the energy will absorbed at the threaded part.If the shank of the bolt is turned down, the shank will undergo a higher stress and hence will absorb a greater proportion of the energy, thus relieving the material at the section near the thread.
regards
desertfox
RE: What purpose does reduced major dia fastener serve?
I think most of the replies have concerned a reduced grip and they are correct regarding that section of the fastener.
The reduced major is intended to assure that the ID of the hole in the structure is not damaged by the threads during the installation of the fastener in the hole, possibly setting up a crack initiation site. A full sized major would possibly allow a zero clearance condition to exist at MMC, so the majors are reduced by .001 to assure clearance.
RE: What purpose does reduced major dia fastener serve?
Thanks very much. I think you're right that I may have mislead folks into thinking the shank or grip was the reduced part. However, this particular spec basically defines a full diameter shank and grip lengths out to 6+ inches.
I think combining the thoughts of unclesyd regarding shear loading of the shank with your comments about clearance through a close fit ID (possibly in two or more clamped members) makes perfect sense.
I still have the issue of the right torque value to achieve a proper preload, but I've resolved that this will have to be arrived at experimentally using load washers.
RE: What purpose does reduced major dia fastener serve?
The relatively longs bolt I mentioned for the turbine/air compress/recovery turbine are based some value derived by testing, but works out to be very close to a value based on thread diameter. These bolts were designed by air craft designers.
We have used a reduced cross section fastener and a Spiralock nut to resolve a very nagging fastener failure problem on some high speed equipment.
RE: What purpose does reduced major dia fastener serve?
RE: What purpose does reduced major dia fastener serve?
RE: What purpose does reduced major dia fastener serve?
Exactly. For these parts there are no cut threads allowed (and no manufacturer that I know would do that). Most majors are required to be a minimum of .001 under the grip diameter and then there are those parts which require much more reduction in major diameter for clearance.
RE: What purpose does reduced major dia fastener serve?
I've come to the same conclusion. These bolts are really meant to be shear pins; the threads are turned down to provide clearance through tight MMC holes that one would want for a shear application. To allow for misalignment of two or more members would either require 1) opening up the hole diameters or 2) turning down the bolt major diameter. For an application with the goal of high clamping forces (tensile), option 1) is the way to go. For a pinning application (shear), the winner is option 2) and hence bolts like these were created.
This particular NAS (0.2500-28) specifies a shank diameter of 0.2490" ±.0005" and thread major diameter of 0.2425" ±.0015". That's a 7.5 mil reduction off the nominal major diameter!
RE: What purpose does reduced major dia fastener serve?
http://www.riverhawk.com/boltloading.php
Where a fastener has a reduced section for weight reduction the reduced section is sometimes replaced by a drilled hole in the shank. This approach requires extensive engineering and testing.
RE: What purpose does reduced major dia fastener serve?
Forgot the number
The threads appeared to me to be ground, not rolled.
RE: What purpose does reduced major dia fastener serve?