Set Screws 2

[CHAUD99]

I'm doing an analysis for an aluminum dock. The legs are square tubing inside square tubing. A 1/2 inch by 13 bolt is threaded through the outside tubing and pushes up against the inside tubing. In a set bolt kind of fashion. I need to calculate how much force can be applied to the inside (moveable) square tubing before it slips. I'm looking for a way to calculate how much torque applied to apply to the bolt.and the resorting force on the inside tubing. any ideas?

Cheers

 

[dvd]

Sounds like a matter of displacement. You can deflect the tubing wall with the screw. 1/13" of screw displacement for each revolution of the screw. You don't need to calculate torque, just calculate your stresses based on a displacement.

By the way, if there is any chance for uplift, it would be a lot better to through bolt. Thermal expansion may be a game changer for you. Are your screws steel?

 

[Jboggs]

A dock is not something that you want to suddenly slip with no warning. With set screws under load, its not a matter of if, its a matter of when they will slip. Because they will slip - guaranteed. You can take it to the bank. I would STRONGLY recommend adding holes to the tubes and pinning them at the chosen height. You can change the height later by just pulling the pin and selecting a different hole set. You can by tube with holes in it from several sources. Let me know if you can't find any.

 

[rrguy]

If you don't have access to a load cell etc... Easiest in my mind would be a cylinder with known area. Build a simple jig or using some slings to keep cylinder alligned. With either a simple hand pump or powered moving a simple cylinder against leg til it slips. You can record the pressures. You can caluculate the force required to create slip & record force required to coninue movement, with common formula. F= PxA you will probably have to subtract any pressure needed to move cylinder freely.

 

[Screwman1]

The big unknown in this design will be the force at which the inner tube will cripple after it has been gradually deformed by the pressure of the set screw over time. I agree with Boggs that this is a design that will slip under at some unknown and random load due to variables outside of the designers control. Cross drilling and pinning is the way to go. You could design a locking one way clutch like on pipe clamps that would allow the user to drop the leg until it hits the bottom and then grips the OD of the leg.

 

[MikeHalloran]

>>> ... A 1/2 inch by 13 bolt is threaded through the outside tubing and pushes up against the inside tubing. In a set bolt kind of fashion. ... <<<

I have fired people for coming up with crap details like that.

You should, too.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi
You can estimate the axial force on the bolt from the formula:-

F = T/(0.2xd)

F= axial bolt force

T = torque applied to bolt

d = bolt diameter

O.2 is a friction coefficient.

All that said you need to check the strength of the threads in the aluminium which I doubt will be very good and liable to strip, also if you tighten the bolt to any degree you need to consider whether or not it will permanently dent the tube or not.

Desertfox

 

[kingnero]

Is permanent deformation of the inner tube permitted?
If not, you can forget the whole idea of a set screw arrangement.

clamping (with a slitted outer tube) or cross bolting are the better options here, my .02$

 

[FACS]

It looks like only rrguy gave you any real insight into your question. (Typical Eng-tips responses)

Because of the variables, I would agree to building a fixture (not a jig) and run some experiments for your analysis.
I would use the original tubing expected in the design, and remember to not use pieces you deformed.
It seems simple enough but will probably have to be two part.

First, use a torque wrench to measure torque, and measure the resulting force on a load cell.
Then, take that torque reading to another fixture where you have the tube assembly set up.
Apply the same torque, and then use an air cylinder to make the tube slip.
Measure the pressure that made it slip.

Run a sample lot and take an average.

Just an idea




Charlie

http://www.facsco.com

 

[Tmoose]

Looks like at least one company might do something similar on an adjustable leg, at least using a bolt, although what it is bearing against is not at all clear.

http://www.boatliftanddock.com/images/PRODUCT/large/556_2_.jpg

 

[CHAUD99]

Thank you.

 

[MikeHalloran]

Ah.
You would be hard pressed to find a stronger bad example to use as a baseline.


I will caution you about that.

In Real Life, your baseline for improvements will be someone else's best effort, and someone else's sacred cow. ... and it may in fact be better than your best effort. When you evaluate alternatives, be brutally honest with yourself, and/or get help from a neutral party.

... and be very persistent and pushy and neutral about finding all the bounds, limits, and parameters associated with the problem before you start solving it.

... and double check your results, again. Mostly, when you think you have a brilliant, simple, elegant solution, it only seems so because you are not seeing an important part of the problem space. Look again.




Mike Halloran
Pembroke Pines, FL, USA