Is This Strong Enough?

[ExpertNoob]

We have some equipment that just came in with removable fixtures. The fixtures are locked in place by pins the majority of the time. During a jig change, the pins are released and the fixture rolls out. My question is about the plates that are mounted on the fixture carrier. There are two 12mm thick plates, one on each side of the fixture, that are there to keep the fixture from rolling out unintentionally. These plates are bolted on with an 8mm wing nut and shoulder bolt (refer to illustration). The shoulder of the shoulder bolt bottoms out just before the head does. If the fixture were to roll against the plates, it would contact about 70mm from the bolts. I would guess the fixture is 800# and typically sits at a 45 degree angle. It seems like the bolts would not support the full weight of the fixture because of the large mechanical advantage. The fixture is sitting on rollers. Unfortunately I do not have information on materials, but everything is steel.

 

[Tmoose]

So the pink 800 lb fixture would love to roll downhill in the direction of the "LOAD" arrow, if the dark blue plate were not there ?

 

[ExpertNoob]

Tmoose, yes promptly plummeting to its demise.

 

[EngineerTex]

Perform a free-body diagram on the system, using the properties of the correct shoulder bolt from here:

https://www.mcmaster.com/shoulder-bolts

If you don't have a good safety factor for it in the static condition, then you are probably correct to be concerned about it getting up to speed and imparting an impact load.

Alternatively, any safety system should be able to be tested. You may consider fixing a sling to your fixture raising the crane hook far enough to leave just enough slack to let the fixture hit the retainer plate at the 45 degree angle. I think that you can fill in the rest, but if you don't know where I'm going with this and you don't know how to do this safely and be confident of your crane and sling size, then I wouldn't recommend this method. If you do try this and it passes, then I would be sure to replace any fasteners in the load path. Keep an easy-out on hand.

Engineering is not the science behind building. It is the science behind not building.

 

[Jboggs]

After looking at your picture I share your concern 100%. In fact, it looks to me like someone designed something with the explicit intention that it fail. First, I HATE (all caps, right) shoulder bolts in any application that applies anything other than PURE shear loading, and I don't like them much for that. In my almost 45 year experience shoulder bolts are usually just a failure waiting to happen. This application applies both bending and tension, but no shear. Was this system even tested before it was shipped? In my opinion a complete redesign is recommended.

 

[ornerynorsk]

Shoulder bolts do very well in the application that they are most used for, which is punch strippers, hence the other moniker Stripper Bolts. Pure tension, heavy cyclical shock loading, have never had much trouble with them.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[SwinnyGG]

What did the manufacturer of the equipment say when you asked this question?

This arrangement looks to me like it's designed to use that shoulder bolt as a hinge; note the opening for the wing nut stud to slide out so the part can rotate out of the way of the fixture, allowing it to move.

What size is the shoulder bolt?

Is it fitted into a machined hole on the other side (so that the full shaft diameter handles the bending load, not just the smaller threaded end) or is it very short with just the thread engaged in resistance to the bending load applied by the fixtures?

Are these two assemblies the only mechanism that prevents the fixtures from ejecting themselves?

 

[EdStainless]

I don't mind seeing shoulder bolts used for other loading, but you have to size them based on the threads and not the shanks.
That said, this sure looks weak to me also.
Beef this up.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[ExpertNoob]

EngineerTex, the load on these plates would be static as there is very movement of the fixture when constrained. I will look up the bolt data and calculate.

Jboggs & jgKRI, the shoulder bolt is in fact used as a hinge point for the plate to swing out of the way without completely removing it. So I suppose one bolt will take the grunt of the load until it makes it to the head of the other. The threads of the shoulder bolt are M8. I didn't measure the shank because it doesn't matter. The shank is slightly longer than the plate so to work as a pivot point. Only the threads are engaged, there is no counter bore for the shoulder bolt. There are four location pins that restrain the fixture. Once the pins are removed, these plates are the only thing left. When they change over the fixture is horizontal. Perhaps they're only to keep the fixture located while in that orientation. I haven't asked the manufacturer, from my understanding they built this equipment from designs originating from our facility. Even our old equipment is this way.

 

[SwinnyGG]

So this plate is:

1) Not the only device which keeps the fixture located and

2) typically only in danger of being loaded when the fixture is being removed horizontally (and thus never subjected to a significant portion of the fixture's weight)?

I mean.. ultimately it's your conscience here but I guess, based on your latest description, I don't see what the fuss is about.

 

[ExpertNoob]

The only reason I ask is that when the machine is in home position the fixture is at a 45. If the pins were to come out in this position (which they have years ago, before the plates were installed, and the fixture fell on the floor), it would be left to the stopper plates to keep the fixture from falling out. I feel they have a false security now that there are "stopper plates."

 

[Sparweb]

In my almost 45 year experience shoulder bolts are usually just a failure waiting to happen.

You'll be pleased to hear that there are shoulder-bolts used in certain attachments in some products from a certain company known as "Boeing". Enjoy your flight.


No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[Jboggs]

"You'll be pleased to hear that there are shoulder-bolts used in certain attachments in some products from a certain company known as "Boeing". Enjoy your flight."

I should explain that my disdain for shoulder bolts does not come from the design of the bolts themselves, but from the tendency of so many designers to misuse them. I have used them myself in many applications. I agree that they work extremely well in die type stripper applications and obviously on Boeing aircraft. But so many engineers try to use them for purposes for which they were never intended, such as trying to use them in place of a pressed dowel pin. Sorry, the shoulder diameter and the thread centerline will never perfectly match.

And thank you for the example of the Boeing aircraft. You are right. I am confident in a Boeing aircraft. But its not because I trust those bolts. Its because I trust the process that resulted in the proper use of those bolts on that aircraft.

 

[12345abc6ttyui67]

If the 'stopper plates' are installed as some means of 'secondary retention' for which credit is taken in a risk assessment (for example, it's ok to work under this because it's held on pins, and if they fail, we have a plate to stop it falling) then the plates must be designed to resist the applied load in this case.