Calculating solid fan shaft minimum diameter 1

[JB160]

Is there a formula for calculating the minimum fan shaft diameter for a fan that's being driven by an electric motor? Shaft is supported by 2 TAF pillow block bearings and is driven via belts & sheaves.

Fan HP Draw: 17.78
Fan RPM: 452
Electric Motor HP: 20
Electric Motor RPM: 1800
Shaft Material: CD1045 TG&P Bar

I've attached a drawing for further clarification of the drive assembly.

If I've left out any pertinent information just let me know.

Thanks in advance!

 

[drawoh]

JB160,

You have HP (horsepower?). You have RPM. You need torque, which is easily worked out from your information. Also, you need to know how you are attaching your sheave and your fan. These must withstand the torque too, and it might affect your shaft diameter.

--
JHG

 

[JB160]

The torque called out on the fan rating sheet is 465ft.lbs, That's using a factor of 2. As for attaching the fan & sheave to the shaft are you asking what type of bushing is used? Both ends are keyed also.

Thanks,

JB

 

[drawoh]

I assume you are sizing the shaft so that it does not break. Actually, a bunch of things have to not break.

[ol]
[li]Whatever holds the pinion sheave to the motor shaft must not fail.[/li]
[li]Whatever holds the driven sheave to the fan shaft must not fail. In the past, I have sized driven shafts to allow for a shaft key that would drive the torque.[/li]
[li]The shaft itself must not fail.[/li]
[li]Whatever holds the fan to the shaft must not fail. Again, your attachment mechanism may drive the shaft diameter.[/li]
[/ol]

This all is basic machine design you learned in college.

--
JHG

 

[Tmoose]

Don't forget critical speeds.
- First bending resonance of the fan shaft assembly.
- various resonances of the fan wheel itself.

That is a big ol' fan.
The belts are going to squeal on start up "across the lines."
That may tempt the millwrights into tightening up the belt tension a bit more.
The fan shaft will not like that.
Nor will the motor shaft, or the motor and fan bearings.
Smaller diameter sheaves/pulleys will make it all even worse.
Details like keyway ends and shaft shoulders will create stress concentrations right in pretty high moment areas.


Selecting which bearing will "float" to accommodate thermal expansion can be tough. A bearing with a bunch of radial belt load is not going to want to slide axially in its housing regardless the clearance. An engineered axially flexible bearing support can help.

A cylindrical bore bearing's fit on TG+P shafting will likely be loose. If the bearings are retained by set screws or eccentric collars etc, the constantly varying radial belt pull will have no difficulty working the drive end bearing loose in a few months.