Slow (145RPM) balance specifications

[Ralph2]

I am trying to find some "slow" speed balance specifications.. It "seems" to me that the standard ISO or API specs are perhaps not that valid for rotors in the 100 RPM or lower speed ranges.

One can "validate" (to a degree)by doing some math and calculating what kind of actual forces are developed at operating speeds at various balance specifications. However the formula I use .062 x (N/1000)^2 x W (in grams) x distance (in inches)= a result in pounds of force.. is not totally valid (I suspect) for ALL speed ranges. When used for a rotor rotating at 5 RPM for example the value does not make that much sense.

On a horizontal rotor I use as a minimum a balance that will prevent the rotor from finding its own heavy spot.. typically on our Schenck HB5 setup this works out to a static API 4W/N calculation... however this is more of a safety generated specification than a operational one.

The rotor that has initiated this question is a vertical application, 16 foot shaft turning a impeller / agitator at 145 RPM with combined weight of 3600 pounds (1600 & 2000). So.. my "safety" induced spec is not required and need to justify an operational one... Anyone with any thoughts??

This is a bit of an ongoing job.. and so far we (I) have picked a ISO 1940 spec of G16 for the impeller and G6.3 for the shaft. Still, that leaves 38KG.in of unbalance in the impeller when I do the standard math

Thanks for your time

 

[TPL]

I did a quick check of your numbers and 38kg-in for the impeller looks about right.

This corresponds to a radial force of 52lbf at a speed of 145 rpm. Since the shaft and impeller form a single rotating unit, why have you chosen different G values? Apply G6.3 to the impeller and the radial force reduces to just over 20lbf.

I would suggest you get these components onto the balancing machine and get the residual unbalance value down down to the lowest resolution that the machine can deal with.

 

[Tmoose]

if the 16 foot shaft is mounted to a face at the upper end, the accuracy of the face (axial runout) and the pilot diameter (radial runout) WILL combine with the accuracy of the shaft features to move the impeller off center.

ISO G 6.3 @ 145 rpm equates to less than 0.015 inch eccentricity, but is also equal to a change in centering. That could be moving from 0.007 offset at 0 degrees to 0.007 offet at 180 degrees. If the installed runout and clearance of the impeller centering feature of the shaft are not much better than .007 " TIR, then the tag that says "balanced to G 6.3 (when centered in the balance machine)" should disappear like smoke.

 

[Ralph2]

Thanks to both... The different ISO specs were picked for purely practical reasons.. The "shaft" is actually a 12" diam pipe with ordinary pipe flanges (16" OD) (gusseted) It is "easy" to balance by welding weights inside the pipe.

Both faces and pilots are machined to be true. This was one of the initial problems.. as the fab shop that made these actually thought they were sections of pipe.

The impeller.. looks very much like the juicer you use to get your orange juice from an orange with.. 5 vanes cast into a top plate (44" diam).. with openings from the center.. A large quantity of air is blown down the pipe / shaft.. this coupled with the rotation helps separate the sand from the liquid (froth flotation a tar sands operation).

To balance I can cut off a chord (from the top plate) but must stay ~2 inches from the vanes to prevent the air from short circuiting. Thus I am limited to taking ~3kg per segment.. The spec ISO G16 was picked more to enable a "balanced" status than an engineered balance.

Some of these castings are very badly centered and the worst one so far has required a 6.47KG correction at 22 inches. This one required grinding 3 chords back almost to the maximum to achieve an "in tolerance" even though it still required a correction of 900 grams.

Because of the "large" corrections yet still withing the tolerance I was hoping to find some engineering data / specs on relatively slow speed rotors. An area where the traditional formulas do not apply as readily.. in my opinion..

 

[TPL]

It "seems" to me that the standard ISO or API specs are perhaps not that valid for rotors in the 100 RPM or lower speed ranges
Could you explain why? ISO 1940 is as good as anything you will find, since it takes account of operating speed.

this works out to a static API 4W/N calculation... however this is more of a safety generated specification than a operational one
4W/N works out at about 67% of G1 allowance so it is a very good balance spec.

The spec ISO G16 was picked more to enable a "balanced" status than an engineered balance
Whats the difference? You could have picked ISO G4000 and made the same statement. I suspect that you are not doing this regularly and would suggest that you take the opportunity to get the balance as good as you possibly can before the machine is built up.

TMoose comments are very valid: how are the shaft and impeller joined together. I would suggest that the best results will be obtained by balancing as a built unit, possibly dowelled to maintain relative positions if you have to disassemble.

I am not sure what the concerns are here - balancing is carried out to reduce forces acting (principally) on the bearings - large slow moving machines can have larger allowable unbalance since the resulting force is lower.

Some of these castings are very badly centered and the worst one so far has required a 6.47KG correction at 22 inches. This one required grinding 3 chords back almost to the maximum to achieve an "in tolerance" even though it still required a correction of 900 grams
Surely thats a problem with the casting - could you consider changing the process so that the castings were better centred or change some of the casting dimensions to allow better scope for balancing?

 

[Ralph2]

Thanks for your comments TPL.. Let me respond to each of your points.

I am not so sure that the ISO 1940 specs adequately deal with slow speed rotors.. and specifically do not deal with slow vertical rotors. One can certainly use them.. but.. I am questioning there validity.. And, as my original post asked.. was any one aware of any "other" standards that one could consider.

API 4W/N is over kill for many jobs.. and is addressed in the API 610 (and others) as being essentially non repeatable and thus somewhat a waste of time. Meeting a customers spec of ISO G2.5 will leave enough imbalance in a large bull gear rotating at 30 RPM to find its own heavy spot and thus becomes a safety issue (my opinion). My own personal spec.. if possible and time and conditions allow... is to balance all rotors that run at less than 1000 RPM so that at least statically they meet 4W/N. The balance report states ISO G2.5 but I have done the math and know statically it meets 4W/N.

True.. But in the applications listed in the ISO 1940 charts this application comes closer to ISO G16 than the crankshaft of a steam propelled frigate. I have been balancing on Schenck Cab 690 on a frame 5HB machine since 1993 full time for a large jobbing shop. I consider myself to be quite knowledgeable on the technical end of balancing.. self taught but with an inquiring mind... Thus my request for any additional thoughts from my peers on slow speed vertical specifications. But, to get back to why I quit where I do (at ISO G16).. strictly practical.. there is just not the material available to go all the way. Picking ISO G16 allows me to balance the impeller / agitator and show the customer a balance report that shows "in tolerance".

How the assembly is assembled and subsequent runouts are beyond our control.I would be very surprised if the runouts (tir) are less than .125" at the impeller mounting flange in a free vertical state. Once the rotor is influenced by the hydraulic forces it is any ones guess. If.. balanced perfectly, I calculated that once a runout of .041 is reached (.082 tir)the rotor has induced 38kg.in of imbalance.. So much for the balance!

Back to my original question.. There is a difference in the forces generated (and restrained by the bearings) between a vertical and horizontal slow speed application. Consider a large disk turning at, say 10 RPM with a large imbalance, vertically the unbalance weight will want to bend the shaft or cause it to not be vertical, this will apply a side load on the bearing but the torque required to turn this rotor will be constant. Now consider this same rotor turning horizontally.. the load on the bearings (overhung) will essentially be equal as the shaft is rotated. the torque required to turn this rotor will vary considerably. At 10 RPM centrifugal forces are not sufficient to influence either case yet the horizontal rotor clearly needs balancing while the vertical one does not. At "some" speed clearly the centrifugal forces become a issue.. I was hoping to find some information / rational behind "that" point.

Yes.. the casting issue needs addressing... hard to tell a customer though that the $20,000 dollar casting he had shipped from Brazil is junk..

 

[Tmoose]

For my money, acceptable bearing loads are the first criteria, followed by how the machine "feels" when running. The variable torque of a horizontal shaft is first rank too, but, as you said, normally removed on higher speed machines.

If more impellers are in production perhaps a mass centering operation could be added before machining the locating features.

Could you introduce an eccentric "adapter" between the impeller and the "shaft" to shift the impeller CG closer to the rotating center?
Similarly, if the "high spot" of a circular feature is marked on the impeller, comparison of the as-installed high spot would help determine where you really are, balance wise.

If the installed centerline runout of a beautifully balanced 125 rpm rotor is 0.125" (0.062" eccentric) you are back up over G40

 

[TPL]

I am not so sure that the ISO 1940 specs adequately deal with slow speed rotors.. and specifically do not deal with slow vertical rotors. One can certainly use them.. but.. I am questioning there validity.. And, as my original post asked.. was any one aware of any "other" standards that one could consider.
Lets get right back to basics – all you are trying to achieve when balancing a machine is to get the centre of mass to coincide with the centre of rotation. You cannot achieve perfect balance for any machine so there are guidelines such as ISO1940 which suggest acceptable levels of unbalance for machines based on their mass, speed and duty. If you choose not to believe the guidelines, then you must surely accept that unbalance is measured in units of mass-distance (gram-mm or oz-in or even kg-in) so , if ISO 1940 doesn’t float your boat, then you can invent your own standard but ultimately, you have to go for the smallest value of kg-in that fits your scenario.

API 4W/N is over kill for many jobs.and is addressed in the API 610 (and others) as being essentially non repeatable and thus somewhat a waste of time.

This is what API 610 actually says:
API Standard 610--Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries
5.9.4.4 If specified, impellers, balancing drums and similar rotating components shall be dynamically balanced to ISO 1940 -1 grade G1 (equivalent to “4W/n” in US Customary terminology).
…
…..
……
With modern balancing machines, it is feasible to balance components mounted on their arbors to U = 4W/n (nominally equivalent to ISO grade G1), or even lower depending upon the weight of the assembly, and to verify the unbalance of the assembly with a residual unbalance check. However, the mass eccentricity, e, associated with unbalance less than U = 8W/n (nominally equivalent to ISO grade G2.5) is so small (e.g. U = 4W/n gives e = 0,000 070 in for an assembly intended to run at 3600 r/min) that it cannot be maintained if the assembly is dismantled and remade. Balance grades below G2.5 (8W/n) are, therefore, not repeatable for components.

API610 does not say that 4W/N is overkill and neither does it say that it is a waste of time. What it does say and explain is that Balance Grades below G2.5 are not repeatable for components or assemblies that are balanced and then disassembled for rebuild. What this is telling you and what you are ignoring is that you should be either balancing your impeller and shaft as a single assembled unit or improving the build.

But, to get back to why I quit where I do (at ISO G16).. strictly practical.there is just not the material available to go all the way. Picking ISO G16 allows me to balance the impeller / agitator and show the customer a balance report that shows "in tolerance".
Isn’t this somewhat dishonest? You are simply selecting the G value to meet your capability and then using this value taking advantage of the customers naivety.

I would be very surprised if the runouts (tir) are less than .125" at the impeller mounting flange in a free vertical state. Once the rotor is influenced by the hydraulic forces it is any ones guess. If.balanced perfectly, I calculated that once a runout of .041 is reached (.082 tir)the rotor has induced 38kg.in of imbalance.. So much for the balance!
What has this got to do with the balance? If you incorrectly assemble already balanced components what on earth do you expect? This is really bad engineering.

There is a difference in the forces generated (and restrained by the bearings) between a vertical and horizontal slow speed application. Consider a large disk turning at, say 10 RPM with a large imbalance, vertically the unbalance weight will want to bend the shaft or cause it to not be vertical, this will apply a side load on the bearing but the torque required to turn this rotor will be constant. Now consider this same rotor turning horizontally.. the load on the bearings (overhung) will essentially be equal as the shaft is rotated. the torque required to turn this rotor will vary considerably. At 10 RPM centrifugal forces are not sufficient to influence either case yet the horizontal rotor clearly needs balancing while the vertical one does not. At "some" speed clearly the centrifugal forces become a issue
This is just techno-babble - there is no difference in the force generated by a horizontal unbalance or a vertical one - the equation is quite simple force = mass*radius*angular velocity^2. The bearings of a horizontally mounted machine have to take account of gravity (i.e. weight).

Lets look at your 2000lb impeller at 145rpm: its static weight is 8909N
You have suggested 38kg-in as the residual unbalance – this corresponds to 950kg-mm or 0.950kg-m (this is the m*r in force = m*r*w^2 - need to keep this in consistent units)
Doing the sums shows that at 145rpm, the impeller unbalance produces a radial force of 215N (49lbf), which is a tiny fraction of the static weight – at 10rpm, the unbalance force would be around 0.25 lbf.

hard to tell a customer though that the $20,000 dollar casting he had shipped from Brazil is junk
Of course it isn’t – maybe you should try working with your customer so that he can request better castings from his supplier so that you and he can work together to get a better final product.

 

[Ralph2]

Thanks to both for your thoughts..This application is "new technology" and the customer is having numerous issues and ongoing design changes. However, for the moment we have what we have and must try to accommodate the customer as best as we can.
Lets get right back to basics – all you are trying to achieve when balancing a machine is to get the center of mass to coincide with the center of rotation.
This is true and to determine the location of this axis the balance machine relies on the forces generated by centrifugal force. So.. in the absence of sufficient centrifugal forces.. how then to balance? Given a horizontal application one can "at least" do a static balance without even running the balance machine. In a vertical application there is no "static". Given that difference I was hoping there were some "other" standards.

API610 does not say that 4W/N is overkill and neither does it say that it is a waste of time.
I agree.. it does not. Overkill and "waste of time" are not the kind of words one would find in an API manual.

What this is telling you and what you are ignoring is that you should be either balancing your impeller and shaft as a single assembled unit or improving the build.
I agree..if, we were trying to achieve better than ISO grade G.2 which we are not

Isn't this somewhat dishonest? You are simply selecting the G value to meet your capability and then using this value taking advantage of the customers naivety.
No.. I selected a G value based on what is achievable from a practical perspective, coupled with the application and general rotor types listed in the ISO 1940 nomogram. However.. these do not address low speed (chart starts at 100) or vertical applications.

This is just techno-babble - there is no difference in the force generated by a horizontal unbalance or a vertical one - the equation is quite simple force = mass*radius*angular velocity^2. The bearings of a horizontally mounted machine have to take account of gravity (i.e. weight).
I agree.. but only when the speed of rotation generates enough centrifugal forces to mask the forces of gravity. That speed is where a vertical application versus a horizontal one would see a difference.
Consider a rotor, horizontal between bearings, with a large unbalance.. the weight on each bearing will be constant due to gravity. Consider then this same rotor vertical, gravity will attempt to move the axis from vertical thus putting a side load on the bearings, as the rotor turns this force on the bearings also rotates. This is a force not seen in the horizontal application so long as the RPM is "low".

Doing the sums shows that at 145rpm, the impeller unbalance produces a radial force of 215N (49lbf), which is a tiny fraction of the static weight – at 10rpm, the unbalance force would be around 0.25 lbf.
Interesting.. given that the ultimate goal of balancing is to keep centrifugal forces within a manageable amount for the bearings to restrain. Even with the crude ISO G16 spec your calculations at 49lbf is only 2.49% of the static journal load which seems reasonably acceptable