Modal Test Cantilever Beam - Stinger Attachment
Modal Test Cantilever Beam - Stinger Attachment
(OP)
Hi there,
I'm attempting to perform a modal test on a cantilever beam. The idea is to measure the strain at the constrained end using strain gauges, with output to a datalogger. A signal generator and shaker will provide the sinusoidal applied force at the tip. My question relates to how best to transmit this force to the beam.
Currently ball bearings transmit the force to the tip end but the clamp around the ball bearings is fairly massive and I'm worried this will produce inaccurate results. It appears that thin "stingers" might be the solution but I'm not sure how these are attached to the beam. I realise the object is to keep the applied transverse forces to a minimum so that the driving force is purely axial.
Would a tapped plug in the beam be a solution and screwed into the stinger? Or a hole right through the beam and clamped at either end? Or would an epoxy adhesive be more suitable?
Appreciate any advice you can provide. Many thanks in advance.
I'm attempting to perform a modal test on a cantilever beam. The idea is to measure the strain at the constrained end using strain gauges, with output to a datalogger. A signal generator and shaker will provide the sinusoidal applied force at the tip. My question relates to how best to transmit this force to the beam.
Currently ball bearings transmit the force to the tip end but the clamp around the ball bearings is fairly massive and I'm worried this will produce inaccurate results. It appears that thin "stingers" might be the solution but I'm not sure how these are attached to the beam. I realise the object is to keep the applied transverse forces to a minimum so that the driving force is purely axial.
Would a tapped plug in the beam be a solution and screwed into the stinger? Or a hole right through the beam and clamped at either end? Or would an epoxy adhesive be more suitable?
Appreciate any advice you can provide. Many thanks in advance.





RE: Modal Test Cantilever Beam - Stinger Attachment
The details depend on the size of the beam.
The ball bearing idea sounds awful.
A photo of your test setup may inspire useful comment.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Modal Test Cantilever Beam - Stinger Attachment
I'm worried if I start putting holes through the beam and clamping then it will constrain the beam and apply an unwanted (and unmeasured) moment.
Photos of current setup are included below. Thanks again.
RE: Modal Test Cantilever Beam - Stinger Attachment
Tobalcane
"If you avoid failure, you also avoid success."
RE: Modal Test Cantilever Beam - Stinger Attachment
I think with one end fixed and exiting the other end will give you different results than exciting just at the fixed end. With this set up, I don't think you can call this a cantilever beam modal test, but one fixed end and one pinned end study.
Tobalcane
"If you avoid failure, you also avoid success."
RE: Modal Test Cantilever Beam - Stinger Attachment
The standard stinger we use is brass rod, 5/32" OD, thinned to about 1/8" or less in two places at each end. Again length is 6-8".
Your current setup is ingenious but unlike anything I have ever used and I would expect it to affect your results in many odd ways.
As Twoballcane implies, exciting the tip of a cantilever will produce a response dominated by the first mode. In theory you should be OK.
Incidentally Blevins says the first four modal frequencies are in the ratio of the squares of these numbers
1.8 4.6 7.8 11
for a free/clamped beam.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Modal Test Cantilever Beam - Stinger Attachment
hardly a free-response test of a cantilever.
what level of precision are you seeking?