Solid fan shaft replacing a hollow fan shaft

[DavidGeorge]

At work, we have an old Carrier air handling unit and are replacing the fan shaft. The original steel fan shaft was hollow where the (2) fan cages attached. Our replacement steel fan shaft is solid steel and consequently weighs much more than the original shaft. Perhaps 80 lbs. compared to 15 lbs. Does this increase in shaft weight present any problems with operation? The bearings are flange ball bearings mounted to a reinforced steel cabinet wall.

[PinkertonD]

Hi David,

{EDIT}
David, skip over these musings and go straight to Bob's. The man obviously has first-hand knowledge of the equipment.

ummm, probably not. I would guess the bearings were not sized that closely that 65-lbs would make a difference.

There will probably be a little slower run up to full-speed time, but I think that would be marginal, but you may want to have an Amp-meter on the motor for a few cold starts to make sure that it is within the motor's scope.

Having said that, I would still get the bearing numbers (usually on the side of inner or outer race) and just check the total loads involved, shaft, impeller et al.

Gotta ask, why are you replacing the shaft?

[RWOLFEJR]

I'd give it more consideration before I'd throw it in there.

The extra weight will have some effect. Whether or not it will matter much I couldn't say for certain without more information. The bearing selection on the original unit was based on the weight of the hollow foam filled shaft and the weight of the fans and the speed it was designed to run among some other things. My recollection is they really crank those units up. Should the unit speed happen to put this heavier solid shaft near first critical, you could have... "a whole lotta shakin' goin' on..." with all the extra weight. Also your factory shaft was internally balance to their specs after all machining to ensure smooth operation and simpler installation. It was also most likely urethane foam filled to dampen any resonant frequency noise potential of the hollow design. There's also the cabinet design and its ability to carry the added load of the machined solid bar to consider.

If this is one of their 4" shafts (most of their 2 or more fan units were I think) you might be looking at as much as 34 lbs. a foot heavier for this part. That's pretty significant...especially if it's one of their longer bigger shafts...?

I'm 100% sure you can still get the exact same part you had / need. nudge... nudge... wink... wink. PM me if you'd like.

Far as replacement of these shafts goes... It's usually bearing failure due to over-tightening the drive belts that will eventually break the drive journal. That or simply running the life of the bearing. Some of those units have been around for maybe 60+ years? Hopefully somebody heard it squealing up there before it blew and messed up your fans.

Good luck,
Bob

[DavidGeorge]

Thanks for the replies. Here's a little more information. The shaft is 70 in. long, 1-3/16 in. diam. at each end, stays that way for 9 in. and then expands to the hollow portion which is 3 in. diameter which is in the center 62 in. of the shaft. Two squirrel cage fans mount towards the center of the shaft. The unit is indeed over 50 years old, and I think some techs may have been overzealous in removing the previous bearings and damaged the shaft. And we weighed the new and old shafts: old - 20 lbs. new - 115 lbs. RPM will be 1062. If we install it, we will definitely check the amps and any evidence of extra shaking.

[RWOLFEJR]

Your numbers don't add up (9" + 62" + free end journal length of about 2-7/8") but regardless you'll be fine so long as all is straight and balanced. I forgot that the first critical of the shaft itself pays no mind to shaft weight. Only matters what load on the shaft and that'll be the same (same fans) so long as not out of balance. Bearing loads are close enough to same / same.

One little note... Anytime you install new self-aligning flange bearings. Take a broom stick or something and put it throught the bearing and rotate the self-aligning feature a few times to make sure it's moving before installing the shaft. Should be a note in the box about that but if not play it safe and do that. Sometimes they're very tight out of the box and on the smaller sizes could actually deflect your journal some at assembly.

[DavidGeorge]

Thanks again, and you were rather kind about that less than stellar math or typing. The shaft is actually 9" long on each side of 1-3/16" diam. and 52" in the middle of 4" diam. which I'm hoping adds up to a total length of 70".