Calculating PSI from Torque

[aldumoul]

The customer's print calls out 250 ft-lbs of torque mininum on a weld. This part is cylindrical via a progressive die. And this part is welded to complete the cylinder. A round bar (1 foot long, close enough) is inserted into the inside diameter of the part.... on the other end of the bar there is a 0.75" air cylinder pushing up on the bar. The cylinder radius block attached to it (to easily make the conversion) The part is constrained in a fixture. There was a gage made sometime ago... In which someone calculated 50 PSI=250 ft-lbs. I Disagree. I calculated appx 565 PSI. Obviously there are other factors such as the friction created by seals, rings, etc in the cylinder. Let's consider them negligable.

Torque=P(A)Dist
Cylinder area @0.75"=Appx 0.442..... pi*(.375^2)

So 250 ft-lbs=P(0.442)*1 ft
Divide both sides by 1 ft (ft cancels out)
Next, divide both sides (0.442)...=565 PSI

 

[aldumoul]

Also, the manufacturer of the air cyliner (Bimba) states that a 0.75" Dia bore will exert a force of 0.4 times the air line pressure (approximately).

 

[rb1957]

looks reasonable.

 

[rb1957]

if in doubt, why not test it, with a torque wrench ?

 

[aldumoul]

This is a "prying" torque. The bar is inserted into the inside diameter and on the other end the cylinder pushes up. And simply the part cannot fail before the 250 ft-lbs is reached. I question the torque call out. Perhaps impact testing is the only way to test it.

 

[MintJulep]

What is the distance from the weld to the air cylinder?

 

[aldumoul]

1 foot (close enough).

 

[MintJulep]

Then I agree with your calculation.

You're gonna' need a bigger cylinder.

 

[DuncanM]

Or weld a socket in a suitable orientation on the end of your 1 foot bar to allow the use of a torque wrench, set to reflect the additional extension.
250ft.lb is not a huge amount of torque really.

 

[aldumoul]

How does this weld socket work? Apparently I'm missing your idea altogether haha.

 

[aldumoul]

Here are some pictures showing the part and gage (not in commision till I put it back together).

 

[aldumoul]

Here is a picture of the part (notice the weld). Previous post was showing the gage/part.

 

[rb1957]

the idea is to prove the strength of the weld, right ? looks like the air pressure is applied vertically (up from the table) ?

if you don't have a 500psi airsupply, why not apply the torque mechanically ?

 

[DuncanM]

I may have mis-interpreted your process: this would be for low volume stuff.

1" square hole to match a standard torque wrench's drive square (designed for hex sockets) in a piece of material suitable for welding or other connection to your 'foot long bar'. Torque wrench then acts like an extension along the axis of your bar and can be set to any torque required. Modified to take the foot long bar into account.

 

[DuncanM]

Should always refresh before posting!

You could do the job with a 1/2" drive torque wrench. Something like:

http://www.sears.com/craftsman-digitork-reg-torque-wrench-25-250-ft-lbs-1-2-in/p-00944597000P

 

[moon161]

Good calc. 50 PSI is off by a factor of 11.3. Are you sure the torque spec. is not 250 in-lb? Seems that's what the tool was designed to develop, on purpose of by accident.

Just looking at it, you can tell there's no way you're getting 250 ft-lb out of that @ 50 PSI.

Another possible flaw is that the stroke looks pretty short. You might be pushing the cylinder to the stop w/o twisting the part enough to develop 250 ft-lb of torque.

Do you have a clue by four at your desk? You may need to use it.

 

[rb1957]

is the requirement expressed in terms of torque only missing something. your set-up creates shear and torque at the interface ... double the moment arm, 1/2 the shear ?

 

[moon161]

RE: Torque wrench.
250 ft-lb, applied with a 1-1/2 - 2 ft. bar would be a tough lift to do too frequently throughout the day, also call for a big counterweight on the table.

 

[rb1957]

that's what linemen are good for ...
they might even pay you for the work-out

 

[aldumoul]

Duncan,

I love your idea. very simple and cheap. I realize this seems like a very fundamental/simple idea. But it isn't going to be a problem using the end opposite of the ratchet side? I would assume not. And does the Square head on the wratchet drive have to be press fit into the square (fixed) hole?