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Instrumented Fasteners
ROV ENG   12/1/2013 11:15:13 AM
It's not difficult to put a strain gauge into the center of fasteners as small as 1/4" in diameter, we were doing this in the early 80's using proprietary techniques. The sensors were sealed inside the fastener and were suitable for use underwater. The two issues that limited widespread use were the cost and the reduction in strength of the fastener for smaller sizes. This only works for bolted fasteners unless you have a very well defined parting line between the two joined structural elements, which is where the sensor must be located. Accuracies of 0.1% were routinely achieved. Most customers were not so interested in measuring the tension in the fastener during installation, although that was useful information, they were more interested in measuring forces during operation of the machine.

Per ASME B18.2.1, a bolt is designed to be inserted through a hole and secured with a nut, while a screw is designed to be used in a threaded hole—sometimes along with a nut.

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Re: reliability?
RJHonders   11/29/2013 4:59:44 PM
CookieJar, you hit the nail on the head. Hard to put sensors in small screws and get the signals out.

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Re: torque
vapats   11/28/2013 8:39:39 PM
Torque is not a proper measure of screw/bolt tension;  you must measure the actual elongation (stretch) of the fastener.

Torque is a mere (and often highly inaccurate) approximation.


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prabhakar_deosthali   11/28/2013 6:50:02 AM
To ascertain that the screws are not overstressed while tightening them,  one can use the torque screw drivers where you can set the maximum torque that will be applied to the screw while it is tightened.

That will eliminated the possibility of the built-in sensor getting damaged by the screw tightening action.

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Re: reliability?
vvc0   11/27/2013 6:07:51 PM
Hmmm, many bolts don't have nuts, such as lag bolts (self tapping), and head bolts (inserted into a tapped hole in an engine head), to name a couple. Also, screws can have nuts, such as machine screws, and even sheet metal screws with tinnerman nuts. The differentiation is more based on the diameter than the fact it is used with/without a nut, but even this is somewhat nebulous.

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Re: reliability?
Stephen_M   11/27/2013 2:35:11 PM
A bolt is used with a nut to hold things together. A screw can be used without a nut; ex: sheet metal and wood screws, self tapping, self drilling screws.

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cookiejar   11/27/2013 12:48:47 PM
I assume the idea is to locate a load cell in the tip of a screw to measure force. 

Having worked with load cells, I found the major design challenge using load cells was to keep them from being overloaded and going into plastic deformation.  Just the thought of a applying a tightening torque to a screw makes me cringe as that would twist the delicate load cell structure.  If you build in a mechanism to stop rotation between the two ends of the load cell, the stop friction will affect your readings. 

Trying to do all this in the extremely limited room at the end of a screw would be a challenge as I've always thought of screws being less than 1/4" in diameter and bolts being 1/4" and larger.  But I suppose that may have been lost in translation.

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