Shaft unbalance caused by oversize key
Shaft unbalance caused by oversize key
(OP)
Hello,
I am in need of assistance. I am very new at this. I am told a larger than normal key on a shaft must be balanced with some mass added on the other side of the shaft. That's fine. But we are supposed to use a vibration test to verify the shaft is balanced properly. I assume the location of the extra mass would be opposite the key (180 degrees). Someone suggested the 4 run method can be used to verify balance. Can we attach some mass and just look at vibration in/sec in the frequency domain and assume the equipment is balanced if the overall vibration is below, say .11 in/sec- 0 to peak? I don't understand the units of permissible residual unbalance as in Appendix C of ANSI/AMCA 204. I do have a Commtest vb1000-T analyzer available to use.
This test is Monday. What do you suggest? I can't become an expert in a day so I'm looking for practical suggestions if possible.
Nisqually
I am in need of assistance. I am very new at this. I am told a larger than normal key on a shaft must be balanced with some mass added on the other side of the shaft. That's fine. But we are supposed to use a vibration test to verify the shaft is balanced properly. I assume the location of the extra mass would be opposite the key (180 degrees). Someone suggested the 4 run method can be used to verify balance. Can we attach some mass and just look at vibration in/sec in the frequency domain and assume the equipment is balanced if the overall vibration is below, say .11 in/sec- 0 to peak? I don't understand the units of permissible residual unbalance as in Appendix C of ANSI/AMCA 204. I do have a Commtest vb1000-T analyzer available to use.
This test is Monday. What do you suggest? I can't become an expert in a day so I'm looking for practical suggestions if possible.
Nisqually





RE: Shaft unbalance caused by oversize key
If I am following the logic, you are saying
that the key pulls whatever is being keyed off
center causing a slight shift or eccentricity.
if it possible to just file a flat on the high
side untill it balances? No extra stresses on
the shaft this way. I once read that if you
are using two keys, the second should be about
120 degrees from the other rather than at 90
degrees. I assume you are using a setscrew to
hold the key in place or are you using a tapered
key? If you are using a setscrew, it would tend
to pull the mass off center, but the setscrew hole
above the end of the setscrew to the od would reduce
the mass also, so maybe you could c'drill the set
screw hole. Sounds like an interesting problem.
Keep us posted. There is nothing better than
experience to help us on our way.
RE: Shaft unbalance caused by oversize key
A couple of things here. The first is an easy one...profile the key to match the shaft and the coupling symmetry. This is much easier than trying to compensate for a key that is not profiled. The second thing is that these type of balances are largely dependent upon the individual circumstance, ie. it would be much more important to balance a 3600 rpm centrifugal pump assembly say 100 hp because of a large key as opposed to a 450 rpm vertical pump because of the obvious speed differences and the overall mass ratios of the keys compared to the rotating assembly. One other thing to remember here is that misalignment can also be a major contributor to your overall vibration and can influence your 1X reading, so always deal with that first before balancing to justify the balance operation in the first place. Just a few thoughts.
J MacK
RE: Shaft unbalance caused by oversize key
RE: Shaft unbalance caused by oversize key
First and foremost with 3600 rpm equipment, alignment is always a big issue even with belt drive.You MUST align the sheaves properly first before you undertake any kind of bqalancing operation. Misalignment on belt drives can produce peaks predominately at 1X, so it is especially hard to discern alignment issues from balance issues from the FFT. You could make an alignment check with a relative phase measurement across the IB drive and IB fan bearings, you are looking for a phase difference of approx. 180 degrees to verify misalignment(I would assume that you would have reflective tape or a strobe if you are going to continue with a balance operation). If your fan is supported on both sides by bearings (not overhung) and the key is not symmetric at about the center position of the fan, and this is your only source of unbalance (which is most unlikely) then yes add the unbalance weight in one plane on the opposite side. I would always suggest a two plane balance, because No.1 ...the static unbalance case is rare, and No.2 ...you are dealing with high speed equipment (unbalance force is proportional to the SQUARE of speed). If the fan is overhung, you must balance in two planes (dynamic balancing).
J MacK
RE: Shaft unbalance caused by oversize key
I have a technical article concerning keyed shafts and balance. If I can find it I will send it to you via e-mail if you provide me with an address.
Roy Gariepy
Maintenance and Reliability Dept.
Bayer Corporation Dorlastan Fibers Div.
Goose Creek, South Carolina USA
RE: Shaft unbalance caused by oversize key
When the coupling is initially balanced the void is calculated and an appropriate mass is added with the intent of arriving at a condition where it would be the same as if there was NO key-way at all. This can get complicated if the coupling is of a different material and some worry about the mass missing because of the gap (clearance) over the key. In the big picture this is insignifigant
Good luck
Ralph
RE: Shaft unbalance caused by oversize key
RE: Shaft unbalance caused by oversize key
Here's a quick summary of the minimum set of measurements one would make. Vibration is recorded at each pedestal for all runs.
1) Initial condition.
2) Trial imbalance at zero degrees on plane 1
3) Trial imbalance at zero degrees on plane 2
The trial imbalance should be big enough that the vibration on the nearset bearing should change by at least 45 degrees and/or 30% in magnitude (roughly) for each test. You don't have to use the same trial mass for each plane, but it is simpler.
This will give you a two plane solution, if all the emasurements are accurate. More practically you might like to test with two levels of imbalance, and mount the trial masses at 0, 90, 180 and 270 degrees. This makes plotting the vector diagram much more accurate, and may reveal linearity problems or the presence of other sources of first order vibration.
Before asking more questions, please read up on balancing, we are happy to help but you need to be confident of the maths and concepts.
Cheers
Greg Locock
RE: Shaft unbalance caused by oversize key
The force of the missing mass in the key seat can be calculated, but chances are that the forces created by the runout of your coupling will aproach that of the missing key. where its influence may add or subtract the net force.
If your missing mass was say 1/2 x 1" then you would have some 16 gram inches missing. This, at 3600 RPM would exert roughly a 13 pound radial pull. If your coupling was say 20 pounds and had a tir of .003" (.0015" runout) it would create some 13 gram inches of unbalance with a resulting radial pull of 10 pounds.
This unbalance force will show up as a dynamic unbalance on your fan. If it is possible to balance your fan by adding (removing) weight to the coupling then by all means do it, you may be lucky
Good luck
Ralph
RE: Shaft unbalance caused by oversize key
- pulley
fan - -------------|||---shaft (with unfilled keyway
- /\ /\ | on this side of pulley)
- brg4 brg3 | belt
|
motor|||pulley
OK. Tell me. Is a picture really worth a thousand words?
Not sure why the major concern with the empty keyway Ralph2. This particular work place is hard to describe in a few words. Does this picture shed any light at all?--Nisqually--
RE: Shaft unbalance caused by oversize key
Yes a picture is worth a thousand words, and you will have to dynamically balance this type of overhung setup if you want to be effective in reducing vibration due to imbalance, it is basically the same for overhung pumps as well. Is it possible to cut the shaft off to eliminate your problem?
J MacK.
RE: Shaft unbalance caused by oversize key
Cheers
Greg Locock
RE: Shaft unbalance caused by oversize key
I am guessing you are trying to balance at the sheave because the vibration amplitudes are highest here. It may be unbalance, but it might also be sheave eccentricity. Perhaps you should remove the belts and use a dial indicator to check for sheave run-out.
Another cause can be a bent shaft, some folks like to tighten belts TOO tight. Yes, belts CAN bend shafts.
Something I would suggest is to look closely at the vibration amplitudes of the fan versus the motor. If the motor is shaking at fan speed AND at amplitudes higher than on the fan, you have an eccentric problem. (Picture a cam instead of a pulley.)
If you determine that it really is unbalance due to the keyway (0 or 180 deg from keyway), I would advise your customer to use the proper key. The rule of thumb for key lengths is 1/2 the length of keyway.
And a few words from experience ... please don't use those lead weights with double stick tape, they are a serious hazard when used on sheaves.
RE: Shaft unbalance caused by oversize key
It would seem that some of the more recent issues to you are insignificant, or should be "ignored". I can see why we have petrochemical plants having major disasters with this type of attitude towards precision. My advice would be to read some of the advice that Locock is writing...you may learn something. Sorry if this offends, but it should be brought to your attention, as people rely on this advice and might make decisions with serious consequences based upon sub-optimal information.
Just an observation,
J MacK
RE: Shaft unbalance caused by oversize key
The likely scenario here is some technology student noticed a bump on the shaft next to the coupling and is getting excited for nothing. Insignificant, without a better idea of the big picture who knows?
The accepted process for balancing a fan in a balance machine is to dynamically balance without the coupling using a fully formed half key to completely eliminate the void of the key seat. Then (if one has it) install the coupling with a key that both fills the void in the coupling and fills the excess key seat in the shaft. Recheck the balance, if it is not in tolerance then make the correction on the coupling. Most times one does not have the coupling.... the balancer now must relly on the trades person to assemble the coupling with a correct key. Here he has a choice. He can make a formed key (full but with the excess removed or he can add a full size key and have it fill 1/2 of the length of the exposed (unused) key seat. Both methods will suffice, but the latter will leave a "lump" where someone could get concerned.
The reason for having a longer keyseat than required (typical of a lot of small fan assemblies)is to allow axial movement of the coupling to align the belts to the motor.
And.. To dynamically balance that fan one needs two correction planes. If you want to use the coupling as one then the other should be on the center of gravity of the assembly (typically just inside the back of the fan. Or use both sides of the fan, the outside (suction) and the back face, these are the planes that would have been used when originally balanced.
Cheers
Ralph
RE: Shaft unbalance caused by oversize key
RE: Shaft unbalance caused by oversize key
Have 2 balance rings made from a piece of plate that can be split and bolted back together. A disk some 8 inches in diameter by .875 wide would likely suffice. Counter bore for (4) socket head screws (3/8) grade 8 or better two in each side with the depth of both sets equal (and opposite) to equalize the mass removed (two bolts will go in rightside up and two will go in upside down) The disks must be bored to a interface fit (on the shaft) while clamped together. Have as many 5/8 holes drilled and tapped radially into the periphery as can be evenly spaced. Have the disks clamped on to a mandrel the same size as the shaft and statically balanced on a balance machine.
Now install the disks on your fan, torque the bolts to the designed specifications and proceed to field balance the assembly making corrections by adding 5/8 set screws in the balance rings. If your balance requires adding a substantial amount of weight (i.e. several holes are completely full) you might be advised to rotate the disk so that the split is in line with the heavy / light side. This will minimize the stress on the 4 bolts holding the ring together. When complete and satisfied stake the setscrews to prevent them turning out.
BUT....... I reiterate...... if the vibration has increased over what was the "norm" SOMETHING has changed. And hot or not, if a blade has come off the fan; if a buildup on the fan has come off un evenly; if your shaft is bent; your balance attempts will be an exercise in frustration and the results will not last long.
Good Luck
Ralph
RE: Shaft unbalance caused by oversize key
Ralph
RE: Shaft unbalance caused by oversize key
Cheers
Greg Locock
RE: Shaft unbalance caused by oversize key
Bearing#3 .09H .04V .08A
Bearing#4 .12H .03V .08A
These are in/sec 0-to-peak.
No chance of bringing in someone. Months of training hoops to get thru just to be able to walk into that place. They are having a reorg or mission re-alignment (misalignment for short). I believe they decided to let everything run to failure and bag predictive maintenance.
The management (although paid in triple figures) isn't too smart just adept politicians. There are more managers than electricians and millwrights. Very top heavy and very dumb.
RE: Shaft unbalance caused by oversize key
Cheers
Greg Locock
RE: Shaft unbalance caused by oversize key
Bearing#3 .09H .04V .08A
Bearing#4 .12H .03V .08A
Original -
Bearing#3 .18H .13V
Bearing#4 .31H .04V
These are all in/sec 0-to-peak.
No axial recored in original.
RE: Shaft unbalance caused by oversize key
Managers read eng-tips as well. Just for future reference, you might want to use words such as sub-optimal, instead of top heavy and dumb. Being right does not gaurantee success,... skilled negotiation will. 15% technical,... 110% interpersonal.
J MacK
RE: Shaft unbalance caused by oversize key
RE: Shaft unbalance caused by oversize key
RE: Shaft unbalance caused by oversize key
After all this I would like to know how you accomplished your balance as well. What masses did you add / remove and where.
Ralph
RE: Shaft unbalance caused by oversize key
JMack- thanks for the tip on interpersonal relations. i need all the help i can get in that area. have you ever worked for a company that has more v.p's than electricians? have you ever worked at a company where education is not valued?
DanT, Ralph2 - Don't try this at home. I don't think you can call what we did balancing but we did lower the vibe readings. We took a hose clamp and clamped a weight on the end of the shaft (all the way to the right in my diagram). We put it at 0, 120 and 240. 0 being the top of the shaft at the keyway. The lowest vibe readings were at 120 (I think it was 120. I don't have my paperwork with me, it's at home). We weighed the hose clamp (20 grams). We weighed the mass (8 grams). We made 28 gram mass and drilled and tapped a hole and bolted the mass on with a lock washer at the 120 position and took final readings. I have had time to read up since then. Our instrument can measure phase so next time I would like to do it right possibly using double plane balancing with phase. If there is a next time.
RE: Shaft unbalance caused by oversize key
I always used 0,90,180, 270, a little bit of redundancy is no bad thing. Find the optimum between the two lowest out of those four. When you've found the best position fit another hose clamp in the same location. If that makes it better fit another one. If it is worse move the two apart in 10 degree stages until they are opposite (ie at (best angle+90) and (best angle -90)) Somewhere in there should be the correct solution!
The trick is getting a measurable reduction. Once you've got that you know you ar eon the right track and trial and error will let you home in on a solution.
Cheers
Greg Locock