Converting pressure to torque

[craigclick]

In the manufacture of laminated timber structures the pressure should be 0.6-1.0 N/mm². In attempting to convert to 'foot pounds', I'm just all over the place with ridiculously high figures (325 ft pd) to incredibly low (4.7). The latter coming from 1 N/mm2 = 1 MPa.

Can anyone shed some light? Thanks

 

[klaus.]

well...we do it 1000x every day

 

[BridgeSmith]

klaus, is your application similar? Do you calibrate the torque, or do you have some means to calibrate the system to provide the required pressure? In the situation at hand, even if the torque is calibrated and correct, the the pressure can vary widely depending on the conditions of the clamps (lubrication, prying or wedging actions, etc.). While the theoretical pressure can be calculated based on torque in a perfect system, it cannot be done with good accuracy in the real world.

"I suspect 'turn of nut' is probably the best way to control the applied load."

I'm not convinced that turn of the nut will work with the setup shown. There are several layers of wood, which in itself has a highly variable modulus of elasticity, and the lumber is not likely to be flat, either. Gaps between boards will require some pressure to close, but how much is again highly variable. Calculating pressure based on compression strain in this situation would be highly inaccurate.

 

[klaus.]

we have to tiden different bolts on structures...
The torque wrench need o be calibrated before work starts and in between....
this is standard procedure in steel construction industry

sometimes it is necessary to do a test before to find out the correlation between torque and force
e.g. using not standard materials of bolts

 

[BridgeSmith]

This situation would be similar to one where you would have to do a test to determine the relationship between torque and force, klaus, but in this case it seems that by the time the test is done, he'd have the final answer. He still needs a load cell or some other means to directly measure the force exerted.

 

[klaus.]

yes of course....depends on the size of the project

 

[rb1957]

just wondering, how effective are sand bags (or shot bags) for your application ? gives you pretty good control over load. is this a production application, where time is critical ? (so you don't want to lug heavy bags around) is there a local H.S. with O Line who need training ??

another day in paradise, or is paradise one day closer ?

 

[craigclick]

I cannot tell whether the photo I thought I had attached in my reply above was actually attached, so I'll try again.
We built this press to laminate non-structural members that go into the fabrication of doors and windows. It is a work-in-progress. We wanted some consistency in our applying pressure to clamps, hence the idea of using a torque wrench. As you can see, we're applying pressure in two axes. In the 'Y', we're using part of a common glue clamp that is used in shops, and tightened by hand.
We are using a torque wrench, set 'intuitively' at 25 ft lbs, which is about a half-turn more than is possible by hand. This 'appears' to be a good setting; I don't think we are 'starving' the members of glue with this pressure, and further milling of the members shows good adhesion.
As you can see, with this pressure we've problems on the 'X' axis, with distortion of the 2" square Cal. 11 tubing, which is being corrected.
Thanks all for your comments; they have been very educational. Cheers

 

[BridgeSmith]

Looks like maybe the clamps for "X axis" need clamp screws at the 1/4 points instead of, or in addition to, the one at the midpoint. Perhaps some heavy duty spreader clamps could supplement the center bolt.

 

[LittleInch]

I agree, those vertical bars need at least two screw jacks. One will always be unstable and in the middle of the beam is clearly overloading the beam which is being bent as you say.

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