Help!

What DIDN'T Google tell you about them?

Part of the educational function of newsgroups is shorted out by requesting responses by email. Please feel free to discuss your project on the newsgroup.

One of the things you'll learn in your first job is that you can use vendors to provide a certain amount of technical information as well as product specifications. One good source for strain gauges is Omega Engineering. Their product catalog has a technical reference which should answer a lot of your basic questions.

You should also take a minute to look through the catalog to see what's out there.

Nice try on borrowing a strain gauge, but it won't happen. They're permanently affixed to the material being measured. Taking them off will damage them. The cost isn't that excessive -- you should be set on your project for $50 USD or so from Omega.

If that's too much, you might want to try eBay, or also you might want to throw yourself on the mercy of the apps people at the strain gauge manufacturers/distributors. They may have out-of-spec units or cosmetic QC returns which they could give you. I'd pay for shipping, though -- the big problem with samples is that you're dependent on the kindnesses of strangers, so you can't complain if they're like Uncle Joe on Petticoat Junction. Senior projects are always time-constrained, and you can't even get started without the sensor.

Good luck, and again, feel free to post if you need more information -- it looks like you might still need some help in specifying your strain gauge once you have a little knowledge. But it's mostly basic stuff -- if you take it a step at a time, it shouldn't be too hard.

Cheers Chris

Strain gauges need to be attached to the test piece so as to accurately transfer the strain (stretch) to the gauge. So the usual method is to glue them on. Thus you will likely not be able to borrow one.

Tension is going to affect the reading: the bending due to tension needs to be cancelled out. I'd suggest 2 gauges on opposite sides of the bar. If you wire them in series then you get cancellation of the bending strain.

M Walter

ambrosialea wrote:

You say you need to measure force. Strain gages measure strain (displacement), so you will need to calibrate your setup to get force from strain. I'm wondering what the short hollow bar is attached to, and if there might be a simpler way to measure deflection of the ends.

Another device that *does* measure force is a load cell, typically a metal ring with tabs on the ends to apply the load. The ring part includes a set of strain gages already attached, and the whole works is calibrated as a unit. This *is* the sort of thing you might be able to borrow (maybe from your Mechanical Engineering department?).

If you just need to get a load reading on one part to prove your design or something (as opposed to having a design that continuously monitors force and does something with the measured data), then you can get creative. You might consider an optical lever if you can arrange for a tiny mirror to be deflected, then measure the beam from a laser pointer as it moves on a distant wall.

Hope this helps.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

most electronic and maybe even mechanical types have a ZERO set, once you install the unit on the tube for example, you simple decide what is 0 strain at that point and zero the process display. in the electronic types you can set a gain differential and display offset to match a desired display that fits the current application along with output trip points etc... or even analog out to drive something else. it's almost like your looking for a load Cell ?

I would suggest that you look at BLH.

Keep in mind that many manufacturers will aid engineering students by providing some sample parts at no charge ... you might have to prove you are a student though. By all means contact the applications engineers for the most expert advise.

With no or little budget you will probably have to make the amplifier and indicator electronics. Look up Strain Gage Amplifiers for a start.

BTW ... someone mentioned placing two strain gauges on opposite sides of the tube to cancel out the bending effects. This will only tend to cancel them. You can also further reduce the bending effects by rotating the strain gage placement so they are located perpendicular to the bending.

One more suggestion, give a lot of thought to how you might reduce the bending forces on the cylinder. These can do nothing but detract from your ability to measure the tension.

Good Luck

Greg

Thank you for your responses, They were a lot of help. I will be contacting some companies and the websites that have been suggested.

To Explain: The rod was part of a bolt pre-loader I have to design without using hydrolics. There will be a little bending due to manually applying force will be slightly uneven. The tension in the bolt must be measured and I thought strain gauges could be used... if only I can figure them out... (if anyone can think of a better way to do it, I am open to suggestions)

I have looked into the loading cells, but they are expensive. If the school doesnt have one I can borrow, that is definately outside my price range.

I knew that strain gauges could only be used once, but I was asking if there was a place where you could rent or borrow a reader (indicator) because I only need it for a short time.

Once again, thank you!

Maybe you could adapt one of this manufacturer's products to your needs?

Or maybe contact the author of this paper to see if the method described can be practically demonstrated in your lab?

A CW ultrasonic bolt-strain monitor

Or get some other ideas here: