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Calculating PSI from Torque

Calculating PSI from Torque

Calculating PSI from Torque

(OP)
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
 

RE: Calculating PSI from Torque

(OP)
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).   

RE: Calculating PSI from Torque

looks reasonable.

RE: Calculating PSI from Torque

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

RE: Calculating PSI from Torque

(OP)
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.

RE: Calculating PSI from Torque

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

RE: Calculating PSI from Torque

(OP)
1 foot (close enough).

RE: Calculating PSI from Torque

Then I agree with your calculation.

You're gonna' need a bigger cylinder.

RE: Calculating PSI from Torque

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.

RE: Calculating PSI from Torque

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

RE: Calculating PSI from Torque

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 ?

RE: Calculating PSI from Torque

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.

 

RE: Calculating PSI from Torque

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.

RE: Calculating PSI from Torque

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 ?

RE: Calculating PSI from Torque

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.

RE: Calculating PSI from Torque

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

RE: Calculating PSI from Torque

(OP)
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?   

RE: Calculating PSI from Torque

moon161 - turn the jig upside down so the worker can push down rather than lift? I'd be looking for at least a 3 foot bar if I had to apply 250lb.ft repeatedly.

aldumoul - Ever used a socket set?  Ask your technicians if they have one you can look at. Perhaps you know them by some other name. http://www.amazon.co.uk/Socket-Sets-Hand-Tools/s?ie=UTF8&rh=n%3A114584031&page=1  has some good images.

" But it isn't going to be a problem using the end opposite of the ratchet side?" Yes. Hopefully the images +/or seeing these things in the flesh will help.

 

RE: Calculating PSI from Torque

(OP)
Not sure how you can use a torque wrench the more I think about it.  The bar is inserted into the part and is relatively loose fitting.  What i was referring to in regards to using the butt end was-inserting it into the ID of the part.  I will have to do some experimenting.   

RE: Calculating PSI from Torque

Take a 1" hex socket, 1/2" drive.

Weld the OD surface of it onto the end of your one foot bar in such an orientation that any ratchet or torque wrench when fitted to the socket acts as an extension to the length of your one foot bar, turning it into a 2/3/4 foot bar with a torque setting.

Then torque on your part = (torque setting on wrench/length of wrench) * (length of wrench + length of 'one foot bar')

RE: Calculating PSI from Torque

a torque is "just" a calibrated bar ... if you're going to do this with muscle power (as opposed to airpressure) i'd use a 2' bar (2 1' bars would work, double the torque wench reading)

RE: Calculating PSI from Torque

(OP)
rb1957: (2 1' bars would work, double the torque wench reading)

Wouldn't you want to divide the reading by 2?  Also, When you hold a normal (stock) torque wrench - How does the torque wrench take in account how far away you are from the object in which the force is being applied?   

RE: Calculating PSI from Torque

The torque setting in a torque wrench is based on a fixed length - centreline of the socket to mid point in the handle somewhere. You can apply force x at the handle or 2x at the half way point, it'll still 'break' at the same applied torque.

RE: Calculating PSI from Torque

i think the torque wrench works by knowing the torque arm (it's essentially a force gauge).  if it's calibrated for a 1' arm but you've added an extension piece, multiplying the torque by a factor of 2 then when the torque gauge is reading 1/2 your torque, you're applying the correct torque.

 

RE: Calculating PSI from Torque

(OP)
Looks like they took...

Force=50 PSI(0.75in^2/4)*pi
     =22.089#

Of course F=P/A
  They then calculated 22.089#/(0.75in^2/4)

That doesn't make any sense to me.  Considering they do not put the Torque equation in equillibrium with the pressure.  Trying to explain this to my boss who doesn't have an engineering degree hasn't been fun.   

RE: Calculating PSI from Torque

"F=P/A" ? ... F =P*A

"they effed up the calc" should be sufficient. or a simple demostration ... use your set up to drive a torque wrench, supply 50psi and get the torque reading.

RE: Calculating PSI from Torque

(OP)
My bad F=P*A (you can see I demonstrated this correctly).  If I start redesigning this I will attach a pic of the model.   

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