Building a Dynamic Balancing Set

Hi, James,

That's certainly a niche market you've got. Sorry I can't help (haven't got the foggiest about balancing gear) - but I hope someone here can.

Good luck,

John

All over the USA we have used equipment dealers some don't have an online presence. Businesses like motor re builders increase and shrink with the economy over here. When people have extra money they blow it on cars.

That's certainly an excellent answer if ever there was one.

Thanks for this. I have an old manual form a Schenk H5 machine and the principals seem pretty straight forward. However the higher the degree of accuracy the more components clearly need to be refined. Their accelerometers are basically large permanent magnet devices with fixed coils. As the magnet moves with vibration the coils are cut by the moving lines of the magnetic field and hence a small voltage is induced. This is then amplified and filtered, depending on the degree of sensitivity required, and interfaced with an input from a photo tachometer recording the rotor position. Their old display units used a light beam bounced off three mirrors two of which were controlled by field coils at 90 Degrees and projected on to a fixed graticule which effectively gave a display of position and degree of imbalance. Needless to say it sounds simple but only because I hugely over simplify. Unfortunately this is the sort of thing I need and even second hand it costs the earth to someone running a small business. However I'm determined to acquire this facility and hence why I'm relying on the input of more experienced engineers in this field. I need both direction and practical input if I am to embark on building such a set up.

Best wishes James

You obviously have an extremely good handle on the necessary means of achieving success. I must admit that the technical requirements, more especially the electronic side of things, is beyond my knowledge. I know I require two plane as some of the blowers I deal with have multiple impellers on a single shaft and even the small blowers with rotor mounted impellers tend to develope a couple imbalance which requires two plane correction. The real question is can such a system of accelerometers, photo tachometer, amplifier and filters be built as a one off by someone with good mechanical skills but no direct electronic back ground? Is sufficient information available to support such a project? The only other alternative is the expediant of a portable system integrated with a fixed hard bearing set up. This alternative still means a reasonable capital investment. My researches so far would suggest £5000.00 which is not insignificant.

Hi jamescz121,

what quality of balancing do you need:

coarse (easily seen vibration and felt by hand if housing is touched)

(hand-tools, motor-bikes, bad cars)

fine: not to be seen but felt by very feeble touching with fingertips or finger-nails (near 1µm)

precision grinding, best motor cars, precision machine tools,

very fine: not resolvable by human feeling (flywheels)

ultrafine: (gyros)

This will decide how much amplifying and noise immunity you will need. I assume you have an asynchronous motor?

So your balancing has to be immune to large vibrations caused by the motor, or the motors balanced separately? Or the coupling is not transmitting motor vibrations?

If you can answer these questions and requirements are not too stringent I can suggest to you an experimental setup that will be below 2000$, likely near 500$.

(Not counting your time nor your gain in experience.)

We once had to do this for a very high quality system used in scanning photos for manipulation and then printing these dot-wise on films. Scanners and printers need incredibly good balancing. This was not an easy experience but later we sold 10 such systems to other users.

RHABE

Sorry for the lack of response terribly pushed at work. The industry standard is G6.5 but ideally I would like G3. The thing is the more vibration the more noise tends to get generated especially once the bearings become worn. All the best blowers have sleeve bearings so life expectancy is very good but I often come across blowers where either the front bearing has been knocked out by vibration or the rear and I understand this is due to original balancing having been carried out single plane using the impeller only rather than balancing two plane and taking into account any couple. I really would be most grateful for any help you can offer.

Best wishes

James

Can you supply a photo of a typical blower assembly?

Hi,

is your G6.5 in g.cm or g.mm?

RHABE

ISO specifications.

http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=27092

I am fairly certain, a contradiction in terms I know, that this is gram.mm.

James

Hi, I don't know the geometry of an Organ Blower, and I suspect it may be large diam, low-speed, but if it happens to be the opposite, then there is a company in the UK which makes balancing machines for Automotive Turbo-charger wheels, which might be worth taking a look at.

Company is TURBO-TECNICS, at Brackmills, Northampton, UK - find it on Google

Good Hunting!

Needless to say the geometry is usually large diameter overhung impeller say 12 to 24 inches in diameter with single impeller blowers or upto five large diameter impellers on a single shaft each around 36 inches in diameter. Generally operating speeds are 1500 with large units sometimes 3000 with the smallest blowers. However thanks for the advice I will have a look at turbo-tecnics.

Thanks James

James --

Let's proceed slowly here. If you were to go to a balancing machine producer with a serious intention to buy a product from them the first thing they would want to see is a detailed description of the balancing problem you are trying to solve since they are in the business of engineering specialized balancing equipment as well as "off the shelf" machines.

In spite of that here you are asking for advice and have gotten the attention of several experienced engineers (and at a pretty fair price, I might add) yet you have told us little of the details of your application. In addition you have given us very little information on your ability to create and fabricate specialized tools for your business except that you are probably not ready to make a do-it-yourself project out of building an analog electronic circuit that includes things like op amps and filters (neither am I; maybe RHABE is? As a mechanical engineer I know it is one thing to talk about filters; it's another thing entirely to know how to design them, poles and zeros being largely a mystery to me).

So tell us a bit more about the size and weight of these impellers, what kinds of shafts they are mounted on, how they are supported in the machines you are dealing with, etc. How bad is the balance problem you are dealing with? Is there any way to lead us to illustrations or best, cross-sectional drawings of these machines? Why the original manufacturers of these large fans may not have done the impeller balancing well enough to suit your customers or yourself?

Ed Weldon, Los Gatos, CA, USA

Ok, most organ blowers consist of an impeller direct driven from a motor i.e. mounted on the motor shaft. In the very smallest of cases the impellers are around 300mm(12") in diameter the largest 900mm(36") diameter. They are constructed from two plates with radial blades between and a central hub. The average thickness of an impeller, I say average because some can be tappered in section from the outside to the centre, is from 25mm(1") to 75mm(3") depending on diameter. The relative thickness of impellers means that the majority have been single plane balanced in production most probably using a mandrel and many of the older machines are likely to only be statically balanced. With smaller units the impellers are often run on two pole motors(approx. 3000 RPM).Larger single impellers are always run on four pole motors(approx. 1500 RPM). With single units weight can be anything from 2 to 10 Kg. 4.4 to 22 Lbs. Bearing in mind that I restore old blowers, as part of that process, I have these types of impellers two plane balanced as an assembly with their respective rotors. At the more major end of the blower market I deal with multi stage blowers where upto five large impellers are mounted on a single shaft which is close coupled to a motor. The main difference here is the weight, upto 70Kg, again these units are balanced two plane as an assembly.

Needless to say I am involved both as an organ builder an engineer in the practical solution of problems. I work in a specialist field where the potential for profit is small but the drive for perfection is great. Unfortunately there is always a wall where, without capital investment, no improvement can be made and this area is just such a wall for me. Previously I have conquered the matter of motor rewinding and rebuilding due to the inconvenience of using outside contractors in this area and the fact that I deal with motors long since obsolete. Also few rewinders are prepared to do basket windings on short throw unequal pole motors as it takes time to achieve neatly. What I'm trying to say is I'm not afraid to put in the work and research on a project like this but I know my limitations and I know in this case without practical input from experienced engineers with the specialist knowledge I'm probably sunk.

So any input is gratefully received and very much appreciated.

Best wishes

James

James,

A few more questions: After balancing, how is the rotor attached to the motor?

What exactly is a 're-build'? are you building new rotors, fitting old to new motors, or, constructing completly new air sources (design irrelevant; function matters)?

I use the term rebuild in the same sense as one might rebuild an engine. As far as is practical the machine is returned to as new condition with the exception of improving the quality of balancing. We take blowers that have burned out motors, decayed casings, broken impellers and worn out bearings or any combination of factors. Re-manufacture bearings and damaged components, rewind motors, often adapt components to take modern equivalent bearings etc etc. All castings, metal fabrications etc in serviceable order are stripped to bare metal, primed and painted with a finishing coat of abrasive resistant lacquer. I wish I could manage to post some photos as one particular rebuild of a 1913 tandem blower started with little more than a broken heap of rusty cast iron and bits of sheet steel. Most blowers we deal with are 50+ years old built in the days when obsolescence was a dirty word our aim is always to ensure that once rebuilt a blower can be expected to deliver the same trouble free life span before requiring any further work.

Oh yes and impellers are normally attached by a hub on the impeller, usually refered to as a top hat. This may be keyed to the shaft but is often retained with grub screws or bolts pinching into recesses in the shaft to give a positive location.

That may be one of the issues that has been plagueing you.

If the tophat does not not fit the motor in exactly the same manner as your balancing arbor, the runout will change and offset the mass.

Would you consider up-rating the tophat?

Are you trying to achieve historic accuracy?

If not, let me propose an alternate path:

Permanently fit rotor to a mandrel (re-fitting usually alters balance), balance on knife edges.

Make up elastomer cushioned bearings for both ends of mandrel.

Connect to motor with a doubly flexible coupling (my preference at this time is the flex-disc style)

Allow the elastomer to damp motions.

P.S. The earliest blower I rebuilt was made in 1906. It was a monster cased in timber and about the size of a small car in a pit under an organ, the organ clearly having built over the top after the blower went in. No one else in the field would consider rebuilding it and the church were facing costs of £10,000 to £15,000 to replace it. Needless to say they did not have the money as in England many churches are not well subscribed. I rebuilt the unit for £3500 to a condition where it should easily do another hundred years including having the four impeller assembly, 38 inches in diameter, balanced way beyond its original specification. I do not subscribe to the idea of built in obsolescence or doing as little as you can get away with for as much money as possible hence I will not become a millionaire but I will rest easy at night.

Hi,

do you have the possibility to measure low AC voltage?, down to which level?

Do you have an electronics partner to build some basic amplifier and filter (analog)? or will you do this yourself (not really complicated.) <this is fastest and most easy way>

Do you want to measure phase at the two measurement planes by oscilloscope or by computer including software development, input card. (Phase and amplitude of the two unbalance signals compared to reference phase at arbitrary but rotor-fixed position.) <this is modern approach but requires considerable working time>.

RHABE

All the developement of this will have to be by me with some help from my new found guidance engineers. By the way I have seen two old GR Vibration Meters 1553A up for grabs. They do not however come with any vibration pick ups and there is no guarantee of condition. Might they still be a possible starting point. Alas I can find no specification for these units and GR is no longer in existence. They are American and it would appear that GR specialised in laboratory testing equipment. I realise they could be a pig in poke but might it be worth the chance especially if manuals and specs can be located?

James

If the impellers have a short length/diam ratio (say, ~1/5, or so), & are separable in the case of multi-stage blowers - perhaps static balancing would suffice?

Yes, I think so too,

only one major difficulty that remounting is requiring extreme accuracy to maintain the achieved balance level.

So I assume that only precision ground conical matching surfaces will be adequate - thus making two-plane balancing more economic.

RHABE

How much do you rely on the motor bearings to define the rotational axis?

What #/yr are you contemplating?

Yes, certainly,

motor bearings are one cause of unbalance in near perfect rotors.

very good ball bearings have concentricity errors between outer or inner mounting surfaces and raceways near or above 1µm, bad ball-bearings more likely around 5 to 10µm.

So if balance has to be stable then rings (rotating ones) have to have a tight fit to prevent any rotation!

RHABE

1- If the parts are short with respect to their diameter then it is enough to measure at one end. If parts are long measure has to be done at both ends since the dis-balance will generate a torque due to its axial position.

2- The dis-balance generates a centrifugal force the higher the rpm the bigger it will be. It is possible to use also force transducers (S-type 2/support allowing a force measurement in x &y) to quantify it. This will simplify the design of the rig but it will reduce the relative signal. It is necessary to have a correct value of the rotating speed since without it the time affected at an operation can be too long.

3- You do not need to get involved in the electronic design as long as you do not want to make a series production. There are acquisition boards with several channels you can use with the sensors and many of them are already equipped with filters and even ADC so that you only have to couple them with a PC either via a series or USB connection. Most of them are delivered with the soft for acquisition and programming of the acquisition and data processing in the PC. A supplier is for instance National Instruments.

4- If you place somewhere a magnetic pickup you get a signal reference at every turn and the angle where the correction in +or- has to be done can be computed by the PC as value (in g x cm) and as angle with respect to this reference.

5- What is important is the mechanics of the rig which should be as perfect as possible in order to avoid parasitic accelerations when the parts rotate.

It is not a difficult project but it is one where details are important and have to be analysed very carefully.

If you need any further support use personal messages for your questions

Easy to balance, what speeds do the blowers run and what weight are they. Be glad to help you. I am a EE Electronic Engineer only in the Balancing field forr 35 years.

www.ghbalancer.com E-mail is info@ghbalancer.com

Regards, Gary L. Hildreth, Service Engineer-EE

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