Hello everyone,

I would like to ask if anyone could please provide some feedback on the
following situation.

I have a 3/8" diameter steel rod that is threaded horizontally into a
vertical steel support at one end, and that has a moderate vertical
load applied to the rod at the other end, 5" or 6" from the support.
The load tends to deform, pry or bend the threads out of the support.

The threaded end of the 3/8" rod necks down to a 5/16 diameter by 24
pitch thread, and the necked down portion that the threads are applied
to, can only be .183" long Maximum. The actual threads won't quite be
..183" long because they probably cannot go completely flush to where
the rod necks down.

The threaded end of the rod is screwed into the steel support until it
bottoms out, right where the rod necks down...

(3/8 - 5/16 = .0625 / 2 = .03125) so there is a 1/32" wide portion of
the rod OD that bottoms out on the steel support.

I want the threaded connection to be as strong as possible so the rod
supports the largest load possible. Would I be better off going with an
extra fine 5/16 diameter X 32 pitch thread?

Are there any disadvantages of using extra fine threads that I should
be concerned with?

Since the rod is basically cantilever mounted horizontally, and the
loads are vertical, it seems that the amount of material that bottoms
out on the steel support may effect things.

For example, if I use a 3/16" diameter thread, a 10-32 thread, or a
1/4" diameter thread, then this produces more "neck down" on the 3/8"
OD rod than the 5/16" diameter thread, and this provides more surface
area to "bottom out" on the steel support.

My guess is that a 5/16-32 thread would stil provide the best holding
power.

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

Thanks
John

John2005 wrote:
Hello everyone,

I would like to ask if anyone could please provide some feedback on the
following situation.

I have a 3/8" diameter steel rod that is threaded horizontally into a
vertical steel support at one end, and that has a moderate vertical
load applied to the rod at the other end, 5" or 6" from the support.
The load tends to deform, pry or bend the threads out of the support.

The threaded end of the 3/8" rod necks down to a 5/16 diameter by 24
pitch thread, and the necked down portion that the threads are applied
to, can only be .183" long Maximum. The actual threads won't quite be
.183" long because they probably cannot go completely flush to where
the rod necks down.

The threaded end of the rod is screwed into the steel support until it
bottoms out, right where the rod necks down...

(3/8 - 5/16 = .0625 / 2 = .03125) so there is a 1/32" wide portion of
the rod OD that bottoms out on the steel support.

I want the threaded connection to be as strong as possible so the rod
supports the largest load possible. Would I be better off going with an
extra fine 5/16 diameter X 32 pitch thread?

Are there any disadvantages of using extra fine threads that I should
be concerned with?

Since the rod is basically cantilever mounted horizontally, and the
loads are vertical, it seems that the amount of material that bottoms
out on the steel support may effect things.

For example, if I use a 3/16" diameter thread, a 10-32 thread, or a
1/4" diameter thread, then this produces more "neck down" on the 3/8"
OD rod than the 5/16" diameter thread, and this provides more surface
area to "bottom out" on the steel support.

My guess is that a 5/16-32 thread would stil provide the best holding
power.

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

Thanks
John

Going to a smaller thread will only make it weaker. The threads will
pull out easier under the resulting tension component and the smaller
diameter where it meets the 3/8 diameter is where it will likely fail.
Extra fine threads will not help the pull resistance in the tension
component but the the larger minor diameter will help somewhat at the
junction with the 3/8 diameter. I would not go crazy with and extra fine
thread unless you happen to have the tap and die. Since you are limited
by the rod material the best thing you could do is to drill and tap the
rod as deep as possible and use a extra high strength set screw. The
wall will be only 1/32 but its material is the right place being
farthest away from the neutral axis. You might want drop down to a
1/4-28 screw to give you a little extra wall.

Hi Tomcas,

Thanks for your feedack,

A 1/4-28 thread would probably give me one more thread than a 5/16-24
thread. Also, as you point out, the 1/4-28 would also give a little
extra wall or contact area where the necked down portion of the rod
makes contact with the support.

The main reason I considered 32 threads per inch is just to get more
threads on the short length.

I hate to take the extra step of putting the set screw in (or possibly
a dowel pin) just becasue it's an extra manufacturing step.

Perhaps the 1/4-28 thread would be better than the 5/16-24, just
because of the extra thread ?

Thanks again for your feedback.
John

John2005 writes:

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

It sounds like you just want to believe somehow a thread choice will
make up for lack of material and/or tensile strength of that material.
It doesn't work that way.

http://www.engineersedge.com/thread_...ed-std-h28.htm

On 17 Jan 2006 16:28:33 -0800, "John2005"
wrote:

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

Re best pitch, I agree with tomcas. Put a nut on it. Read on...

Even if the "necked down" part is machined, you don't have much
bearing area in compression on the bottom of the rod and compression
force there is high for given moment because the radius is small. So
only a bit of yield there transfers all load to the thread.

Stress on the thread then varies with distance from the axial
horizontal plane with some parts stressed more than others.

Thread the rod a bit more and put a nut on it, then torque the nut
down tight against the mount. That gives you a larger bearing area in
compression resulting in more evenly distributed stress on the
engaged threads, thus reducing maximum stress on the thread.

John2005 wrote:
...snip
I would like to ask if anyone could please provide some feedback on the
following situation.

I have a 3/8" diameter steel rod that is threaded horizontally into a
vertical steel support at one end, and that has a moderate vertical
load applied to the rod at the other end, 5" or 6" from the support.
The load tends to deform, pry or bend the threads out of the support.

The threaded end of the 3/8" rod necks down to a 5/16 diameter by 24
pitch thread, and the necked down portion that the threads are applied
to, can only be .183" long Maximum. The actual threads won't quite be
.183" long because they probably cannot go completely flush to where
the rod necks down.

The threaded end of the rod is screwed into the steel support until it
bottoms out, right where the rod necks down...

(3/8 - 5/16 = .0625 / 2 = .03125) so there is a 1/32" wide portion of
the rod OD that bottoms out on the steel support.
...snip

If you look at the rod as a lever, the fulcrum is the bottom edge of the
3/8 section that bottoms onto the vertical steel support. When the
vertical load is applied to the end of the rod, say 5.5" from the
fulcrum, it is resisted by a horizontal axial load only 3/16" (.1875")
from the fulcrum (its not really 3/16", but you get the idea). Doing
the simple maths, the horizontal force is then 5.5/0.1875 = 29 times the
vertical load. If you weld a 1" diam steel collar to the end of the rod
where it buts against the steel support, the horizontal load is 0.5"
from the fulcrum and is therefore reduced to 5.5/0.5 = 11 times the
vertical load. This way the horizontal load on the thread is reduced
considerably.

--

Regards, Gary Wooding
(To reply by email, change feet to foot in my address)

In article .com, John2005
says...

The load tends to deform, pry or bend the threads out of the support.

You need to determine which one of those failure modes is happening.

If you cannot change the design to put a larger diameter where the
part mates to the vertical surface, then get rid of the turned
down section. Thread the part 3/8-24.

Jim


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You have a structural design problem. Knowing the load that will be
applied, you need to design a joint that will withstand the stress.
This calls for a little engineering, either by yourself or a
professional.
Bugs

I hate to take the extra step of putting the set screw in (or possibly
a dowel pin) just becasue it's an extra manufacturing step...


Thanks again for your feedback.
John


John, It really isn't an extra step, since you do not have to turn down and
thread the end of your rod anymore. Instead, you drill and tap for the set
screw, a very easy task. Trying to thread up to the shoulder, especially with
such a short stub to thread, is an exercise in frustration and won't produce
threads to the shoulder. And a grade 8 set screw will give you the highest
thread strength you can easily come up will.

Dennis

Hi everyone,

Thanks for your replies.

I cannot thread the rod through the support and/or put a nut & washer
on the other side of the support, or change the dimensions to larger
parts. The "support" is actually a round bearing housing with a flat
milled on it where the rod threads in. The threaded rod is basically a
lever that oscillates the housing back and forth. Everything is in a
very tight space, and the rod threads in right over a bearing, and
practically bottoms out on the bearing (this is why I cannot go deeper
and use a longer thread).

I figured I would just tolerance the thread length so it does not
bottom out on the bearing OD. Moving the needle roller bearing out from
underneath the rod does not help much since everything is cantilever
mounted, and there is still a shaft underneath the rod even if the
bearing is moved (you only get about a .05" addition to thread length).
Also, this would put the bearings closer together, and I want to keep
them spaced further apart if at all possible.

I also thought of forgetting about the thread length tolerance, and
threading the rod into the housing before the bearing bore is drilled.
This way, if a little thread extends into the bearing bore, it will be
machined away when the bearing bore is drilled. The problem I see here
is that if I have to take the rod out and put it back in, it will
probably not go back in right.

Welding the joint seems undesirable (it will deform the bearing bore) .
Heat would also deform the bore in the rod itself. I also don't have
the space to add any type of collar as garry suggested.

Based on all of the feedback I have gotten from this and other forums,
it seems the size of the lip where the rod necks down is fairly
unimportant, and I should keep the OD of the threads as large as
possible. It also seems a course pitch thread is better than a fine or
extra fine thread, even though there may be less threads over the .183"
length.

I misunderstood what tomcas suggested with the set screw, I can see it
would not be an extra step, and may be a better choice than machining
the shoulder since I would have threads along the full .183" length. I
would probably use loc-tite to keep everything from backing out or
loosening. I am not 100% sure it would really be stronger "overall"
though, becasue then the weak point looks like it would be the thin
wall left on the rod after it is bored through and tapped for the set
screw.

The way things are now, the existing 6mm OD hole in the center of the
rod does not go all the way through, and the drill point stops about
..12" from the shoulder where the threaded portion of the rod necks
down. Drilling the hole all the way through and using a set screw looks
like it could weaken things overall, (i.e., the threads are stronger,
but the weak point then becomes the thin wall of the rod where it
connects to the housing ) I don't know that for sure though. If a 1/4"
OD set screw is used, I would have a 1/6" wall remaining on the rod
where it connects to the housing, but it does not seem like much.

How close could I realistically get the threads to the shoulder where
the rod necks down? I guessed that with a 5/16-24 thread, I would loose
one thread at the shoulder and wind up having three useful threads.

If I have the space, I guess the best solution may be to do away with
the necked down portion, and try to thread the OD of some 3/8" drill
rod. The rod threads into a 1" OD round, but it is right at the edge,
i.e., the center of the 3/8" OD rod is only .236" from the edge or end
of the housing.

When I said before that the load tends to pry the rod out of the
support, I did not mean that the load actually caused the joint to
fail, but just that that is how the load acts on the system.

As I understand it, you can have cumulative stress on threads in a
cyclic application, so you can test it, it can work fine, but fail down
the road. I have no way to simulate a few years of use in a short time,
and that's why I made the post on the board to get feedback. The loads
should be fairly moderate, say 10 to 15 pounds but the thread is so
short I wanted to get some feedback so I can try to do the best thing
right from the start. Even though the loads are moderate, the short
thread length just worries me.

Thanks again for your help.
John

John2005 wrote:
Hi everyone,

Thanks for your replies.

Based on all of the feedback I have gotten from this and other forums,
it seems the size of the lip where the rod necks down is fairly
unimportant, and I should keep the OD of the threads as large as
possible.

Correct.

It also seems a course pitch thread is better than a fine or
extra fine thread, even though there may be less threads over the .183"
length.

Wrong. Finer is better. It has a larger root diameter.

jim rozen wrote:
In article .com, John2005
says...


The load tends to deform, pry or bend the threads out of the support.


You need to determine which one of those failure modes is happening.

If you cannot change the design to put a larger diameter where the
part mates to the vertical surface, then get rid of the turned
down section. Thread the part 3/8-24.

Jim


Now that's a good idea. Extending the thread way past what is needed
will move the stress concentration of the last tread out to a lower
stressed area and loctite will hold it in.

In article , tomcas says...

If you cannot change the design to put a larger diameter where the
part mates to the vertical surface, then get rid of the turned
down section. Thread the part 3/8-24.

Now that's a good idea. Extending the thread way past what is needed
will move the stress concentration of the last tread out to a lower
stressed area and loctite will hold it in.

there will still be a stress concentration at the point where the
wall of the vertical support coincides with the one thread root
of the rod that occurs at that surface. If it fails in shear, that
is the location where it will fail.

If you can, keep the threads completely inside the vertical support
and relieve them to the root diameter where it passes out of that
material. From what you say though it sounds as if you don't have
enough hole depth to do that.

In that case you will have to rely on the larger root diameter
strength alone. Remember cross section goes like the square of
the diameter. This might be enough.

If it fails by snapping at the point where the threads run out into
the exterior portion of the rod, then you can improve matters somewhat
by relieving that section down to the root diameter for a length
of about 5 or 10 threads - but you MUST put gentle radii on the
reliefs. I like do do this with a ball-nose end mill in the toolpost.

Jim


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jim rozen wrote:
In article , tomcas says...


If you cannot change the design to put a larger diameter where the
part mates to the vertical surface, then get rid of the turned
down section. Thread the part 3/8-24.


Now that's a good idea. Extending the thread way past what is needed
will move the stress concentration of the last tread out to a lower
stressed area and loctite will hold it in.


there will still be a stress concentration at the point where the
wall of the vertical support coincides with the one thread root
of the rod that occurs at that surface. If it fails in shear, that
is the location where it will fail.

If you can, keep the threads completely inside the vertical support
and relieve them to the root diameter where it passes out of that
material. From what you say though it sounds as if you don't have
enough hole depth to do that.

In that case you will have to rely on the larger root diameter
strength alone. Remember cross section goes like the square of
the diameter. This might be enough.

If it fails by snapping at the point where the threads run out into
the exterior portion of the rod, then you can improve matters somewhat
by relieving that section down to the root diameter for a length
of about 5 or 10 threads - but you MUST put gentle radii on the
reliefs. I like do do this with a ball-nose end mill in the toolpost.

Jim


Why not just cut a J thread.

In article , tomcas says...

Why not just cut a J thread.

He could do that. The main stress concentration
will still be at the thread root that lines up
with the vertical surface though.

Most folks will have trouble getting the specific
thread form like that, but it would probably increase
the strength by about a factor of two or so,
*if* it were a rolled thread. Lathe cut threads
are inherently not as strong so one has to keep
the overall assembly in perspective he

Most times very high strength fasteners are used
in engines where rotating mass has to be kept low,
or in aircraft where overall weight is a major
concern.

Basically any time a simple machine is designed such
that the extra strength of a special fastener is
deemed neccessary, one needs to re-evalute the design
and in most cases simply using a larger fastener
or re-designing the connection is the best way to go.

At one time bmw made their flywheels bolt onto the back
of their crankshafts using special M10 bolts. Waisted,
high strenght steel, relieved shanks and all. Special
torque values and all.

Those machines were *still* prone to spitting the flywheels
off the damn cranks. Their decision for the next
subsequent years models was simple: they went to an M12
bolt that was pretty plain jane. And the flywheels stayed
on from that point.

Mostly the folks who *need* high strength fasteners know
how to do the engineering so they can be used properly.
Those who don't are, honestly, better off 'using the next
size up.'

Jim


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