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Alex

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Dear experts, I have a question about hard bearing balancing machine sensors. I know both soft and hard bearing machine principles and difference. Here is a question:

Why does a hard bearing machine use force sensors? I know they are built in a gap on both supports and measure dynamic force between two gap surfaces.

But isn't the force proportional with acceleration just looking the 2nd Newton law? So why there couldn't be accelerometers mounted?

Thank you

  
RustyCas

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Reply with quote  #2 
I have no firsthand knowledge (Walt will probably know) but I suspect it could be because they’ve always used them? Don’t force sensors pre-date accelerometers?  I suspect if you buy a new IRD soft-bearing machine, it won’t use accelerometers either.  When I bought a used B50 I didn’t pay much attention to the transducers - I just removed them and replaced them with 793V velocity sensors (Wilcoxon, an accel with onboard integration) which I understand.  A lot of things are done a certain way “because we’ve always done it that way” like using Plotdata in MHM for instance. 
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Alex

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Vibration sensors are common for soft bearing machines. But I've looked at hard bearing machine providers....no one uses vibration sensors.
OLi

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Well it it is more complicated than that sort of.
1. I think soft bearing balancing machines were the first ones. They are soft as they operate above frame resonance and sensors measure distance that can be argued to be the distance btw. rotation center and center of gravity. Sensors may look as MCV velocity probes but almost never are, they have the magnet mounted in the frame and the coil moved by the bearing assy. No spec. machine mount required.
This apply to Schenck, IRD and even our design from 1972 etc. These may be used with accels and modern systems or existing sensors that almost never die and systems for that. System do suppress disturbing vibration from surrounding and that may not be as good using std. accels direct mounted. 
You need the conventional testrun vector balancing procedure so for a one off rotor balancing you need to start 3-5 times on average.
2. Stiff machines are run lower than the frame resonance and are by that limited in top speed.
Calculations are made by the measurement of the pure force in the bearings with whatever force measurement device you can find, piezo, mechanical, strain-gauge etc. Should be stiff mounted on foundation, but that seems to be more like forgotten these days?
Software are just calculating how to minimize that force and you enter physical data of the rotor, start once and get some weights calculated so if lucky it is just 1 start, cost effective and what not. Anybody have those calc's described? Vector balancing procedures are all over the place, not same for stiff machines, and by that the price of them. So I think in my world you need some know how to run a soft machine but you know when your job is good.

I have argued previously for soft machines that I guess still can be found if you search. Lately I have by market demand deviated to upgrading also stiff machines and my arguments still stand for the soft but if you like a fast balancing in production with quality only as good as you barely need, then you know what is ruling these days.
Since for no obvious reason, stiff machine measurement systems are insanely expensive I have also converted or applied normal vector balancing technology to stiff machines with normal accels (or MCV's) and since we applied that since 1972 and it works well, but takes more starts and work to balance it is more likely to be used in service workshops, on the other hand you can use the same toolkit as fieldbalancing tool and that as far as I know is not really possible with a measurement system for a stiff machine or not at least with the same software. In theory you can run a stiff machine if you can read a ruler and have the rotor weight but you have no idea if the machine made the magic or not......

So this is the short version. I am happy to be corrected or informed :-). You never know everything.

If I read the new ISO standard version 21940-11 I can get the idea that it is leaning toward stiff machines and making soft machine operators balancing to like 20% better than required, indicated or am I wrong? It is a discussion on if the acceptance residual unbalance should be calculated at the bearing or the balancing plane or wherever else. Is this "in the balancing plane" more conforming to API view on things?


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OLi

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Reply with quote  #5 
 Vendor:
Swapping hats, I can't resist informing about our latest system for field balancing and upgrading balancing machines MEBalancer.
1 and 2-plane wireless btw. measurement box and PC with calculation and presentation software in windows.
Static, dynamic calc, vector plot with tracking, tool compensation, very basic FFT and vib level.
Can be run in demo mode for training and on any number of pc's but only operating with the box connected.

http://www.vtab.se/?lang=en
 

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Alex

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Reply with quote  #6 
Oli, why do you think there are more starts needed when choosing accelerometers instead of force sensors? Is that related with sensor's resolution/sensitivity/precision.....?
Vibe-Rater

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Reply with quote  #7 
I only ever balanced paper machine rolls on a soft stand.  I am no balancing guru as some of you are.  Worked well. And after vibe measurements confirmed the good outcome. Pre and post. Sometimes rolls were so out that they would botrtom out the soft spring mechanism. We would replace the old balance weights and start from scratch. Worked well.  
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Reply with quote  #8 

Soft/stiff. It is the principle. In a soft you use the standard vector balancing method with test weights that calibrate the sensitivity. There are a obsolete IRD method to fix this using a defined shaker motor to shake the bearings with a defined force. Only seen that in 1 machine so far.
OEM built balancing machines never use accel's as fa as I know, only refurbed by people like us.
OEM built still use a modified MCV sensor as described above and or distance sensors of some kind, eddy probes, potentiometers etc. They all measure the distance one way or another not acceleration. Some brands still use the modified MCV sensor for stiff machines as they build a force sensor out of that so it is a little messy.

Stiff machine that measure force you do measure the unbalance induced force. Software ask you to enter rotor weight, physical dimensions of the rotor so I think it basically calculate the distribution of the weights from the force data. You do a calibration of the force compared to unbalance weight with a calibration rotor doing a zero run and a calibration run. It will be done in industry every 30 years and in aero industry every 3 months.......

So it is no technical or sensitivity difference btw. a stiff and a soft machine but as a operator I find a correct setup soft machine giving me better dynamic performance and the chance to get a optimal balancing. In a stiff as it is stiff you don't "feel" it getting better or worse you need to rely on the instrumentation and if that goes crazy you don't know. One normal problem is the pretention of the piezo sensors for force measurement, if you "dunk" the rotor in to the bearings they may get loosened and calibration is totally off.

So oldtimer rule apply, check balancing when done by applying a known weight and see the reading indicate it, special in a stiff machine and a expesive rotor.
By the way in a soft machine the bearings are mounted on springs one way or another, they may have screws you can tighten if you get overload or as in our machine we change the vertical blade springs for thicker with a heavy rotor, you calibrate anyway every time with testweights so it is no problem.

Also in a soft machine and modern instrument you can save the calibration and reuse it if you have many identical rotors so the you only need 1 start and weights are calculated.

Then there are the discussion what machine represent the reality best? A papermachine is most often very soft and a aero jet engine also, a steam turbine? Depend on size I would say. I have IRL no indication that one principle is better than the other when the part is mounted on site.


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fburgos

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Reply with quote  #9 
Quote:
Originally Posted by OLi

So oldtimer rule apply, check balancing when done by applying a known weight and see the reading indicate it, special in a stiff machine and a expesive rotor.



this procedure was done in my my old job as residual unbalance check, add a know weight in a known location and should be represented on the next run, as i remember was done 3 times in diferent locations and ploted

Quote:
Originally Posted by OLi
By the way in a soft machine the bearings are mounted on springs one way or another, they may have screws you can tighten if you get overload or as in our machine we change the vertical blade springs for thicker with a heavy rotor, you calibrate anyway every time with testweights so it is no problem.


I only have seen an old IRD machine dont remember it had springs/blade springs, as I remember the rollers were mounted on a "rail system"

I've heard of ussing rubber of certain "hardness" in softbearing machines, i guess is the same principle of the springs to support the rollers assembly, right?

OLi

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Reply with quote  #10 

IRD design is a unique "grandfatherclock" friction less device hidden in a black box that is a square pendulum with bearings in each corner, that is the optimal way to get a soft machine but cost extra. I even made a scale size model out of aluminium beams as it looks so nice. :-) 

Schenck in old times used like over-sized piano-wire beams vertically in soft machines I just use flat iron verticals as I only want low stiffness horizontally even saw-blades for small machines for like laser prisms small fans and other toys.

I in principle don't like rubber elements in "my" machines as I think rubber as a spring is a more nonlinear such compared to bending a piece of steel.
They also get much stiffer over time until they stop working.
I know there are brands or re-sellers that use things like that but I am not one of those if I can, there are one machine I upgraded from a 1960 strobe solution that they
still use and that require so much vibration to trigger so they used rubber mounts and then do the final touch with a vector system so everybody happy........
Customer is always right, right?


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RustyCas

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Reply with quote  #11 
By the way, I have an IRD B50 I need to sell. If anyone is interested I’ll post details in the For Sale section.
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