In what context?

One is a unit of torque, the other is a unit of pressure. The context will correlate them.

The pressure reading on the gauge is given in psi, but what determines this psi pressure reading, especially since we're torqing in Nm/ft-lb?

Hope you understand what i'm trying to get at?

"enineering yet logical explanation"

"Hope you understand what i'm trying to get at?"

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What do you mean by engineering, yet logical explanation - its sounds like you are inferring that most engineering explanantions are not logical.

In any case - no, we do not understand what you are getting at. WHAT EXACTLY ARE YOU DOING/USING/..............

You have used the word "most" to change the meaning of what i have said.....and yes i do mean that SOMETIMES engineering explanations are not logical (in the sense that we sometimes derive, calculate, make assumptions etc, for the questions which may require the simplest solutions. I'm not saying we are wrong by doing what we do, but the answer may lie right in front of us as simple as simple can get), although they my be factual.

Check the reply above to view my inquiry.

Nope.

Hello Guest,

Your question may have different answers. It depends on which bolting process you are referring to: Torquing or Tensioning (even when tensioners are used, people incorectly refer to "torque"). I'll first assume that you're asking this from a torquing perspective and, that the question relates to hydraulic torque wrenches.

The torque output of a hydraulic wrench is a function of its cylinder area and the hydraulic pressure of the pump. The supplier of the torque wrench should have provided you with a pressure-to-torque chart for your particular torque wrench; if you require x ft-lb of torque, the chart will indicate that the pump must be set at y psi. It should be clear that each hydraulic torque wrench model will have a unique pressure-to-torque relationship.

Which torque wrench is being used? I may have an appropriate chart for you.

Thanks. I was actually referring to torquing and not tensioning. I do have a pressure -to-torque chart for the torque wrench. The question is - do you know how they derived what pressure the pump must be set at for a particular torque value?

I would like to torque some bolts to 1650ft/lb, which means from the chart i have to set the pump at 8600 psi. What did they use to determine this 8600 psi?

Hello Guest,

I just read your most recent message which was posted before I finished my most recent message (!). When reading it, I think that it should be a bit more clear to you where this value comes from.

If you'd like, I can provide you with a spreadsheet that should make this even easier. In fact, if I can get it up onto the server, I'll attach a link so that you can access it from here.

An explanation and calculation for bolt preload can be found here. The preload divided by the bolt's cross-sectional area is stress (psi).

Hello Dac,

Indeed, you are quite correct. However, if the Guest's question refers to the relationship between a hydraulic wrench and a pump, your answer may be a bit confusing since most torque wrench pumps operate at a maximum of 10,000 psi. If, say, 60,000 psi bolt stress is required, most people would be left scratching their heads, wondering how to meet the spec .

To clarify this, we can take this discussion a bit further by going backwards :

Once the correct preload has been calculated, either in lbs force or psi bolt stress (or their metric equivalents), an engineer must then provide the folks out in the field with something that they can work with. Often, this is "torque" (in Nm or ft-lb). In order to define the required torque that would lead to the correct preload, the engineer has to make a wild-a**ed guess at what the K-factor might be. The K-factor refers to the many friction factors that will affect how much of the torque will actually result in preload. Unfortunately, since there are myriad uncontrollable friction variables, if the engineer guesses wrong, the joint may fail - either during hydro test (if it's a pressure vessel) or, in service. Here's a link that expands on this :Torque Misunderstanding.

Some may say that as long as calibrated torque wrenches are used, this shouldn't be a problem. Unfortunately, torque wrench calibration doesn't address this either. In fact, the use of calibrated torque wrenches often results in a dangerous false sense of security .

One cannot take for granted the fact that a certain "torque" will result in the correct fastener preload. On critical applications, this must be verified. There are a number of ways of doing this including special washers and the use of bolts which change colour depending on their clamp load (if they're loose, they're one colour. If they're tight, they're a different colour). Another method is elongation measurement: The amount that a bolt has stretched after being tightened (within elastic limits) is directly proportional to its clamp load.

<...if the Guest's question refers to the relationship between a hydraulic wrench and a pump...>

Well, Guest needs to state whether it does or does not for the benefit of any CR4 reader that is not clairvoyant!

Yes i would like to find out about the relation between the torque wrench and the pump, but more specifically, how is the pressure related to the moment/torque?

Again, I want to torque some bolts at 1650 ft-lbs and from the tables, i should set my pressure to 8600psi. How did the table derive that 8600 psi and not 10000 psi or 20000psi, etc etc....

Hello again Guest,

Sometimes I get carried away with too many technical details when an easy answer is all that's required. Oops. Let's try this:

The table "knows" what the tool's moment arm is (a measurement unique to each torque wrench model), it "knows" what the pump pressure is and, it "knows" what the tool's piston area is.

Torque equals force times distance

It's from the above factors that the torque-to-pressure chart is derived. "Calibration" is an individual tool's deviation from this plot.

Guest,

I admire your inquisitive mind. Assuming that you've got an acceptable answer to your initial question, don't stop here; the next question you should be asking is: "How does torque relate to bolt stress?" *. Many people don't even think about this. Hence, they wonder why some of their joints fail

* Reviewing the earlier posts in this string should provide you with this answer.

Here's the link to the spreadsheet:

http://www.heviitech.com/Downloads/HeviiBoltWorksheet264MetricImperialLocked.xls

Although it's one of our simplified worksheets, it does go into quite a bit of detail (likely more than you're looking for). It should be self-explanatory. If not, please let me know.

A very interesting component is the Torque Analysis at the lower part of the first worksheet. It shows the effect of the actual K-factor being inconsistent with that which the engineer had guessed when he applied it to the torque equation. It must be remembered that even if one gets lucky and the K-factor of one bolt is indeed the same as the one in the engineer's calculation, it's unlikely that the other bolts would share this value (unless the application is in a pristine, lab-like setting rather than being out in the dirty real world

Exemple of pump and hydraulic torque wrench.

In the picture the reaction arm is on the right.

Step No 1 - Define from load and assembly type preload, bolt dimensions and material, material

Step No2 - Define from bolt dimensions and contact sliding surfaces friction coefficients which could appear

Step No3 - Define required nominal torque and tolerances

Step No4 – Chose tool and connect with pump, pressure gage and torque transducer

Step No5 – Apply a pressure till the torque measuring device indicates the required torque for the application. Record this pressure value for further use.

As well pressure gage as torque transducer MUST be calibrated in order to assure traceability.

Remark:

1-As sketch shows between the "Th" value and the applied value there are losses in friction of seals and bearings and between applied torque and obtained preload (which is the ONLY parameter which counts for assembly function and life expectancy) there are above mentioned frictions at the bolt/nut sliding surfaces which are quite variable and have to be considered as probabilistic values.

2- As sketch shows the applied torque is NOT constant over the angle, deviations are in general around 3..5%. The nominal torque used for the tool rating is in general the maximal one obtained.