Mechanical design - pin/boss selection

[shorion]

Hi all,

I'm currently designing a heavy duty pole. The pole has to have a pin in it and I've been asked to detail a boss for the hole/pin that goes through it.

Does anyone have any pointers or could put me in the direction of a text book that works out how to select the correct boss size.

Currently I'm using a DN40 SCH80 as the main member. About 500mm down the member is a hole for a approx 30mm boss where a pin has to go thorough.

I checked the bar in bending and shear without the boss and it passes. With the holes cut out, the piece fails. With the boss then inserted... no idea I can use standard bending and shear formula but I was curios if there was any kind of design books/material on how to calculated forces/stresses in a boss.

Thanks in advance.

 

[moon161]

How will the boss be connected to the pipe?
What material is the boss?

If it's not welded, riveted, glued or otherwise well connected, my concern is that the bar/pipe would fail instead of effectively transferring appropriate load to the boss.

 

[drawoh]

shorion,

You are showing us a narrow window into a somewhat complicated design problem. We cannot answer your question intelligently. If there are any safety issues, you are in this over your head.

The bigger the hole in your pole, the weaker your pole is. The smallest hole happens when there is no boss, just your pin.

What is your boss accomplishing? Is it made of a stronger material than your pole, to spread out side loads from your pin? If the boss made of low friction material? Is the boss expendable and replaceable?

--
JHG

 

[Tmoose]

Is there one enforced direction relative to the boss that the bending will occur?

Must the boss be flush with the pole OD, or can it protrude?

http://i.ebayimg.com/images/g/JBcAAOSwv-NWZw30/s-l1600.jpg

 

[shorion]

Moon161
Boss is fully butt welded to the pipe.
Boss is made from the same material.

drawoh
The bigger the hole the weaker the pole. Reference? Codes/books?
Boss is made from same material.
Boss is welded in place and protrudes.

The hole is 30mm D the pole is 48mm OD

Tmoose
No bending can occur in any direction. Over 15 load combinations putting the CHS in tension compression torsion.
The boss can protrude.

 

[drawoh]

...

drawoh
The bigger the hole the weaker the pole. Reference? Codes/books?
Boss is made from same material.
Boss is welded in place and protrudes.
...

Reference? How about any mechanics of materials book you have ever used. You did not mention the welding. The rhyme was an accident.

What is the material, and what is the boss for?

--
JHG

 

[Tmoose]

The link I posted before showed (poorly) a fairly typical detail used when using a through bolt type arrangement. It was the swing arm (rear suspension) of a high performance off road motorcycle. The spring/damper attaches about half way along the swing arm, so bending is at a maximum right there.
This may help explain.

http://hva-factory.com/sites/default/files/IMG_0550.JPG.crop_display.jpg

Rather than adding bosses to each side of the tube, a thick walled tube is inserted in the hole through the main tube, and welded on each side.
This adds material (although not in the best location for bending resistance), and allows the thru bolt to be fully tightened and not crush the main tube, as would happen if a reinforcing boss was simply added to each side to provide added bearing for the bolt/pin.

Note a similar detail is used where the two bolts secure the front engine mount plate to the down tube.

 

[moon161]

Omer Blodgett, design of weldments. You should be able to pick it up from a lincoln welding dealer for about $25.

I'd consider the stresses at a section going through the center of the hole, and develop the moment of inertia as a composite of positive and negative bodies at that section, and consider the bending stress. This will give you an idea of the stress that a junior or senior ME student would give. IDK if it's sufficient for your purposes.

I'd use a heavy pipe going through instead of a boss as shown in the picture and welded all around.

IDK about applicable codes and safety factors, definatelly up to you.

 

[drawoh]

...
Rather than adding bosses to each side of the tube, a thick walled tube is inserted in the hole through the main tube, and welded on each side.
...

I have done something similar, although in my case, it was a frame going inside an aircraft. We drilled out the main square tube.
We welded in a smaller round tube and we drilled that out. This allowed us to insert bolts and clamp them down hard. Also, we welded another, un-drilled square tube next to the first one to take the primary load.

--
JHG

 

[flash3780]

I'm really having a hard time trying to envision this application. Do we have any idea of the function of this boss/pole/pin combination? What sort of loading will it see? Is the pin pressed in? Are there bolts as well? What constitutes a failure of the design? Is there fluid in the pipe? Are thermal stresses a concern?

Anyhow, I tried to sketch up what you're talking about below. If the boss is part of a bolted flange, it will be loaded in shear. Most likely, your limiting location will be your weld joint. Shigley's Mechanical Engineering Design has a good basic discussion of sizing and analysis of fillet welds. That would be a good place to start.

<p align=center><img src="

https://files.engineering.com/downl...d79-297b8fe51cf1&file=pole_boss_weld_pin.png"

width=300></p>

Of course, there are several areas where you could overstress your joint: Pin shear, weld loading, pin crush stress, etc. You should outline all of the ways that your joint could fail, arrange them from most severe to least severe failures and evaluate them in order.

If there's a specific failure mode that you cannot figure out how to analyze for a given loading, post back and I'm sure someone can point you in the right direction.

 

[chicopee]

You should know that the practice to reinforce a hole within a member is to weld a ring within that hole. a boss will do that but you will need to ensure that a certain weld radius at the junction between the boss and the member is maintained so as to minimize sharp discontinuity.
A rough rule of thumb is that the amount of metal removed has to be put back into the reinforcement. Ring reinforcement is the way to cope with holes made in members so do an E search under that topic. A properly designed boss should fit that topic. For stress analysis, get books on machine design in order to determine stress factors in discontinuities when you do bending, tensile/compressive stress calculations. I would also look at increasing the pipe thickness where the hole is to be made instead of welding a boss