What purpose does reduced major dia fastener serve? 2

[mechowie]

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?

 

[Metalguy]

Don't know anything about NAS6304, but the purpose of a reduced dia. is to allow it to stretch more, and reduce the load on the threaded area. Think of them as bolts that are made with a larger thread dia.

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

 

[unclesyd]

Aside from the above posted by Metalguybolts with 2 reduced sections are used for bolts that have to take shear along the shank, like in connecting rod bolts for large engines. The full size shank section is located in the shear plane where the cap and rod meet.

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.

 

[desertfox]

Hi mechowie

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

 

[Screwman]

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.

 

[mechowie]

Hi Screwman,

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.

 

[unclesyd]

All the ones I've seen and used have the reduced section at the minor diameter + a few thousand's, so we always used up to the maximum torque value based on the thread cross section. If possible we will check the elongation of the fasteners in an assembly. I don't think I've seen any with the reduced section smaller than the minor diameter of the fastener threads. If it was smaller you would have to see if you could develop the needed clamp force based on this diameter.

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.

 

[hondaknight1]

I might be being stupid here, but isn't this really about rolled threads? Instead of cutting threads into a shank, the threads are rolled from the surface. This creates threads with greater dia than the shank, the shank is sized to be the mid point between the major and minor diameters of the thread. This creates a stronger thread (because of cold work, it has a cleaner surface which has been compressed so you get greater fatigue resistance (which is where my knowledge of this comes in). The weakest point of a bolt is the minor dia, so as lnog as the shank is greater than this then in testing it will be just as strong as a cut thread bolt, if not stronger. And they are cheaper to produce. I have never really thought of them as "reduced cross section", this isn't really the point..

 

[Tmoose]

Some of the NAS series bolts are considered only for shear service in fittings and lugs. Low head heights, and very specific (and short) threaded sections. Not really intended to be torqued very high, or to be subjected to tensile loads, so cut vs rolled threads is not so big a deal. The shank diameter specs are just a few 0.000s of an inch under major diameter until they get over 0.5 inch or so.

 

[Screwman]

Moose,
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.

 

[mechowie]

Tmoose and screwman,
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!

 

[unclesyd]

Maybe not in your particular application but here is one approach to effective bolt length for reduced shank bolting for calculations where the fastener is also used in tension.

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.

 

[Tmoose]

We used some NAS bolts on a big Navy propulsion motor.
Forgot the number
The threads appeared to me to be ground, not rolled.

 

[Screwman]

Most of the NAS procurement specs require threads to be rolled (after heat treatment). A high quality aerospace grade thread will look like it has been ground.