vibrations translated from floor thru a Kinematic mount
vibrations translated from floor thru a Kinematic mount
(OP)
I have a very very sensitive x-ray microscope that weighs 4600 lbs. this device is designed to be move in and out of position. It is currently sitting on a 4 point kinematic mount with 4- 1" dia stainless steel balls on every corner. ( 3 corners drop into corresponding vee block and one corner sits on a flat plate) To move this device we manually crank up the ball mounts and the frame drops onto wheels so it can be moved.
The issue is that the device does not drop back into position accurately. It needs several raise and lower attempts to get it back into the vee blocks. I see there are tungsten carbide " canoe spheres" that might improve the repetitive accuracy.
but
A point was brought up that these "Canoe Spheres"
http://www.precisionballs.com/All_Vee_Blocks.php#c...
add a larger overall footprint ( contact surface) to the device and it could potentially amplify known vibration from the floor into this device. IS THIS POSSIBLE??? will a larger footprint potentially cause more vibration?
thanks for the input ...
The issue is that the device does not drop back into position accurately. It needs several raise and lower attempts to get it back into the vee blocks. I see there are tungsten carbide " canoe spheres" that might improve the repetitive accuracy.
but
A point was brought up that these "Canoe Spheres"
http://www.precisionballs.com/All_Vee_Blocks.php#c...
add a larger overall footprint ( contact surface) to the device and it could potentially amplify known vibration from the floor into this device. IS THIS POSSIBLE??? will a larger footprint potentially cause more vibration?
thanks for the input ...





RE: vibrations translated from floor thru a Kinematic mount
You have 7 points of constraint not 6. The 3 ball & V blocks give you all 6 constraints - the extra flat is superfluous or will stop one of the other mounts acting properly. Of course if your device isn't stiff then it may sag into all the points but the repeatably will be impacted.
Assuming your device is squareish, and the C of G is moderately symmetric then it seems your whole set up will tend to be unstable i.e. rocking on a very small level.
Also you may have issues with the balls sticking if your surfaces aren't smooth etc. and if the Herzian contact is high enough you may be damaging them.
The website you link has some excellent practical technical information such as:
https://www.precisionballs.com/Micro_Inch_Position...
I suggest reading over the technical info on that site very thoroughly and if you still aren't clear the last time I had a problem I was able to call them and get put in touch with their founder who has forgotten more about kinematic mounts than most of us will ever know. This was about 3 years ago though, not sure if he's still involved with the company as he was a little advanced in years.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: vibrations translated from floor thru a Kinematic mount
> we had a much lighter system, about 40 lb, on 3/8th inch balls and wound up dimpling the vees when we torqued down the bottom and top
> seems to me that if you're able to move the position by raising and lowering, then the mechanism that doing the raising and lowering might be driving the repeatability also.
> kenat's comment about sag should be taken seriously. Even steel bends, and keeping something that weights 4600 pounds stiff is challenging
> the canoe spheres seem to be a plausible way of getting a effect of larger balls without consuming the commensurate volume. wish we had thought of that 20 years ago.
TTFN
I can do absolutely anything. I'm an expert!
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RE: vibrations translated from floor thru a Kinematic mount
I will be moving the device in a few months to a new location and I want to put some thought into possibly putting the device on a rail system that can be locked down. Again the question comes up ...will locking the device to the floor add vibration?
thanks
RE: vibrations translated from floor thru a Kinematic mount
No, if the stiffness and damping properties of the floor does not change, then locking would not make a change to the vibrations in vertical direction. Lateral vibrations may change from locking, but that would depend whether the lateral floor vibrations are a potential source or not.
Walt
RE: vibrations translated from floor thru a Kinematic mount
Any potential for improving things such as moving to basement with thick foundations and or tied into bedrock or equivalent pedestal etc.?
How about changing the mount to true kinematic - even if not optimal - as you move?
I.e. split one of the V blocks so one side acts on on ball and the over side of the V acts on another. i.e. 2 'V's' and 2 opposed slopes. The force vectors may not play out quite as well but might be good enough - or at lease might be better than 7 point pseudo kinematic mount.
Would give you a chance to reconsider ball & groove size and finish. Also to play tricks with the material and even things like a 'gothic arch' rather than simple V.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: vibrations translated from floor thru a Kinematic mount
I have attached a photo of the vee block and ball.
thanks
RE: vibrations translated from floor thru a Kinematic mount
Hertzian contact stress isn't that complex - even I worked it out eventually.
Do you have facility for balls in all four corners?
If so then as I mentioned before you can split one of the V blocks into 2 'ramps' so that all 4 corners are supported. See below image 2 blue 'v blocks' 2 green 'ramps' giving total of 6 points of contact if you have 4 balls, one in each corner of mating part.
You may need to consider the geometry for optimizing the force vectors etc.
Do some research or at least Googling don't rely on us to give you all the answers.
http://www.precisionballs.com/tech_papers.php
Or if you have budget the below folks may be able to do the hard math for you (even if not budget they have some FAQs).
http://www.g2-engineering.com/technology-FAQ
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: vibrations translated from floor thru a Kinematic mount
http://www.precisionballs.com/split_vee_block.php
Points out that the spherical ball element(s) touching the split V need to be tangential to both surfaces for true kinematic/to avoid various issues. I.e ideally it would be one very big ball or truncated ball. Failing that portions of the nominally single ball.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?