Strain transducer HBM SLB-700A

[Pina87]

Hi
I was wondering if this sensor could be used also to measure bending deformation. In the data-sheet no direction of loading is specified.
Can it be bend in both directions or only into the tensile direction?

Is this calculation correct:

 

[Pina87]

Anyone?

http://www.hbm.com/en/menu/products/transducers-sensors/strain/slb700a/?geoip_cn=2

 

[BrianE22]

I looked at their website. They say that for bending you need to mount another transducer on the other side of the beam and wire them in parallel. I'm guessing the small amount of curvature you get from bending messes up the strain reading.

 

[MikeHalloran]

It's not intended to be bent at all.
It bolts to a large member and indicates the tensile strain in that member, pretty much like a strain gage would; it's just easier to mount than a strain gage.

Call the manufacturer for further help with your application.


Mike Halloran
Pembroke Pines, FL, USA

 

[Pina87]

What kind of sensor would you suggest? Load cells need to have the force input at the mounting hole, I suppose that a bending moment could damage the load cell. Isn't it so?

 

[MikeHalloran]

I won't suggest anything, mostly because you haven't told us squat about what you're actually trying to do.

The extant product seems to comprise a strain gage bridge premounted to a simple bar to simplify field installation. Mounting two on opposite faces of a(n assumed symmetrical) larger bar or beam and connecting their outputs in parallel strongly implies that is the case. That's how you connect strain gages on opposite faces of a bar to cancel out any bending strain, when you are only interested in the tensile force.

... which in turn suggests that you could connect them differently is you wanted to see the bending strain. But you really really really should talk to an HBM applications engineer.


Mike Halloran
Pembroke Pines, FL, USA

 

[Pina87]

So the sensor will not be distroyed if it is bend. So I can use it for measuring the force F that acts on the top of the lever.

 

[MikeHalloran]

Yes, the sensor WILL be destroyed if it is bent.
I wish you would stop asking leading questions; it's a bad habit.

Now, the obvious reply:
WHAT FRICKIN' LEVER?


Mike Halloran
Pembroke Pines, FL, USA

 

[Pina87]

Sorry...beam .
Why will it be destroyed? Reason...

 

[MikeHalloran]

Okay, ignore my last comment. I have looked at your calculation sheet, and at the installation instructions for the transducer.

The transducer is rated for only 500 microstrain.
If you load it to 705 microstrain, it will be permanently damaged, i.e. both the zero and the span will probably change, or maybe one of the elements will break.

I'm guessing that's why you put the 5mm step in the bar, but then you have to use the section dimensions at the transducer location to recalculate the transducer response. Off the top of my head, the resulting asymmetry makes me unconfortable, but it may be ok; check with HBM.

You should get a signal from the transducer when you apply the force at the end of the cantilever beam. If the force is applied at a corner as shown, not the center of the appropriate edge, the transducer output may be misleading because of the twisting moment on the beam, for which I don't think the transducer will compensate.

The transducer is optimized and compensated for steel beams. On an aluminum beam, the zero is going to drift with temperature, probably enough to bother whatever you are trying to do.

Again, please call HBM.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pina87]

Hmm. Ok...the section must be corrected to 500 and steel needs to be used because of the temperature change. The section calculation is on the save side because I don't count in the section of the sensor (or not?). The force will be applied on the center so there will be no torsion. Perfect..that's all I wanted to know.

Thnx again.

 

[MikeHalloran]

To tell you the truth, I wouldn't use the sensor above half its nominal rating.
That provides some safety factor against overload. Note carefully the cautions that there is no overload protection within the sensor. As opposed to commercial load cells, some of which have internal overtravel stops.

Additionally, ISTR in the really fine print that strain gages are a little bit inelastic in the high end of the range where they are normally rated, and I think I worked out once that the Wheatstone bridge, despite its many fine qualities, masks any strain asymmetry from accidental asymmetrical loading. I.e., the signal doesn't tell you that an individual gage was stretched too far, but the gage is still damaged, and you can't trust the bridge anymore.

I am concerned about asymmetry here, because the sensor has no protections for it, and your intent is to bolt it to a very slender bar, whereas HBM contemplates bolting it to big-ass beams or columns. I repeat, call HBM.



Mike Halloran
Pembroke Pines, FL, USA