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JuddJones

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Reply with quote  #1 
A good friend gave me a call as he knows I do vibration for a living. He is a really talented car/engine builder and was finishing up a 63 Ford Galaxie for a customer. He explained that after building a 429 to replace the stock engine he had a bad vibration in the car and was concerned that he was going to have to pull the engine back out because the crankshaft was out of balance. We talked about where he feels the vibration and the severity. He claimed that he felt a vibration in the seat anytime the engine RPM was near/above 2000 Rpm (it persisted through the RPM range to 5.5K). We talked a little bit about the whole physics of unbalance and the square of the speed rule. Once he knew the vibration would be 4x times as severe at 4k vs 2k RPM we ruled that out. I told him it was probably a resonance in the exhaust and setup a time to look at it for him. I showed up with my data collector and performed a bump test on the exhaust. Low and behold there were 3 distinct peaks in the spectrum around 2000 CPM. The smaller two were around 1300 and 2500 CPM, and the largest was 4x the amplitude of the other 2 at 1797CPM. I made a few measurements to make a best guess at the best place to add more exhaust hangers. He welded one hanger on and a new bump test showed a 3x reduction in amplitude with all 3 peaks being similar in amplitude. I spoke with him after he took it for test drive the next day and he said the vibration can only be felt now for a small 200RPM band that is outside any RPM range where the car would spend any large amount of time cruising. It was a fun to apply what I do for a living on a hobby I enjoy (cars).
OLi

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Reply with quote  #2 
I once did a cardan shaft balancing in the car in the yard (in the summer) as the owner had adapted a new gearbox and the shaft was not so centered and he was driving to Norway for a meeting and didn't want to damage the seals etc. in the new gearbox and it worked fine it actually may have been a similar brand and for sure vintage. I was also discussing a similar problem in a bit newer Chevy where he put as large engine that could fit and also modified the drive train but all was balanced so I suspected the suspension in that case as he had added a few Kg hardware in the front end but it could be the exhaust also. This was during a balancing session of a couple of pump impellers in his lathe in the garage :-) I also once or almost twice found a critical speed in a factory original truck with a load swapping thingy where the OEM swapped gear ratio in the final reduction in the wheels to reduce consumption and did not test where the cardan shaft speed ended up relative shaft critical a very nice run up curve, really classic, one was a cargo truck brand new with valid warranty where the operators vibration limit was the cig pack dropping to the floor from dashboard when speeding out of city limits and the other was a horse transporter where the horses refused to board the vehicle due to tickly feet.  
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Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
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Walt Strong

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Reply with quote  #3 
Good job Big J!

I grew up in my father's auto repair shop when I started walking. Hard to believe all of the dangers I survived! I have done a few diagnostics and corrective actions on cars/Jeeps. Glad to see that you and Olov gets their hands greasy now and then!

Walt
fburgos

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Reply with quote  #4 
Well done Big J, its allways satisfactory to help a friend and better if you can show what you do for living
RustyCas

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Reply with quote  #5 
When people find out what I do, they almost always ask if I can come balance their ceiling fan.  I always say, "Nope!  That's pretty much the one thing in the world I can't balance." (I'm sure there are other things, but none so far... except ceiling fans.)
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fburgos

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Reply with quote  #6 
Quote:
Originally Posted by RustyCas
When people find out what I do, they almost always ask if I can come balance their ceiling fan.  I always say, "Nope!  That's pretty much the one thing in the world I can't balance." (I'm sure there are other things, but none so far... except ceiling fans.)


that thin line between want and can
Vibe-Rater

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Reply with quote  #7 
I have a friend who had a Nissan XTrail front wheel drive and there was and audible noise coming from the front end.  Mechanic (also a friend) thought it was a wheel bearing (which it was) and wanted to replace both sides to be sure. But friend owner was selling and said to me daringly - can you pick which side? I could see their thinking.  They still don't quite believe what I do is a "proper job".

Anyway. Front end up on stands, tacho tape on drive shaft, set up some points and told mechanic to hold it steady at a certain decent rpm.

Compared both sides and it was clearly the drivers side which is on the right hand side in Australia. Showed clear non synch harmonics in the ball park you would expect for typical wheel bearings. (with sidebands in this case indicates outer race)

Replaced that one and sound went away.

They asked me what that would cost if it wasn't for friendship. Answer, you would have been better of replacing both sides.... rgds

It was fun though to show friends what can be done with vibration "black magic".


OLi

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Reply with quote  #8 
I retrieved a rear hub unit as they are called now on now, Chinese owned local car brand here that used to be related to a 3-letter bearing brand also "local" after changing it for the umpteen time as they bought it from another bearing company and did not specify a seal to keep salt water and gravel out.... modern design.... and in that time it was still Swedish. Anyway I spun the hub unit in a lathe and it passed 3 times as high numbers as when I suggest swapping bearings on an industrial application so when you hear it in the operators position in a car it is bad. Indication here were my sons in the backseat complaining that now is the time daddy, bearings are gone again and they could tell what side it was.
I had a customer that did the same on the front wheels of his SAAB but he did it with a CSI box and cables driving around the block.

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JuddJones

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Reply with quote  #9 
Good times!!! Love all the stories.
RustyCas

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Reply with quote  #10 
We all have a story like this:  Sitting in church a few years ago, and all I could hear during the sermon was the low rumble from a bad bearing in a blower behind the stage, about 75 feet away behind several walls and mounted up fairly high.  I asked the pastor after the service, "How do you concentrate with that rumble in the background?"  He said, "Rumble? What rumble?  What are you talking about?"

As Mr. Monk would say, "It's a blessing... and a curse."


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JuddJones

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Reply with quote  #11 
Our sawmill has a vibrating conveyor that shakes the whole building at 420CPM. I find it hard to concentrate in meetings with the 420CPM so prevalent in every office/conference room. I wear noise cancelling headphones in my office (Bose....one of the best purchases I have ever made) so I can concentrate on the task at hand while working at the computer.
OLi

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Reply with quote  #12 
Put it on isolators? Sometimes it works.
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Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
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JuddJones

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Reply with quote  #13 
Its a resonance issue with the slope sheeting that funnels the wood into the conveyor (vibrating conveyor 500 foot long with slope sheeting most of the length). The whole system is my arch nemesis. I may get the problem resolved before I retire (20+ years). It is kind of nice to have a standing issue to test my knowledge as I learn. It is like having a standing Lab experiment anytime I need to apply my training.
RustyCas

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Reply with quote  #14 
Quote:
kind of nice to have a standing issue to test my knowledge


And you can blame all kinds of stuff on the slope sheeting....

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