Shaft diameter Tolerance 3

[cokeguy]

What should the tolerance (difference between shaft outer diameter and fan inner diameter) be between a shaft and a fan impeller, for example, for a 4 inch nominal diameter shaft/fan assembly?

We had some trouble yesterday mounting an impeller in the shaft for a scrubber´s ID fan, and was wondering what´s the typical tolerance left in these applications. We measured before mounting with a not very trustworthy vernier gage, and there seemed to be only a 0.001 inch difference. Is this too small a tolerance? Thanks...

 

[imok2]

The types of fits have been given names. They range from an interference fit, where the parts are purposely made to be forced together. This fit can be further described as heavy through to light interference. Whereas a clearance fit is for parts made to have a space between them. This fit can be further described as tight through to loose. Between these two fits is the transition fit where interference may or may not occur. The amount of interference or clearance is achieved by specifying the tolerance range for the parts possible sizes.I would say 2-3 thousands is a good transition fit also you might consider heating the fan hub if it's still too tight and some lubrication could help

 

[Warpspeed]

I would use an automotive engine cylinder hone to expand the bore of the fan to something a bit greater than .001 clearance.

Expansion by heat may get it on easily enough, but getting it off again later will be a hell of a job, especially if there is corrosion.

I would just keep honing until it just slips on easily. It may be a slow job, but it may save someone a lot of grief in the future.

 

[MintJulep]

First off, the difference between ID and OD is not "tolerance", it is either clearance or interference, depending on which is bigger. Imok2 has done a nice job of summarizing this.

There is no "typical" value for such things, as the required value depends upon a bewildering number of factors, such as: How heavy is the fan? How fast is is spinning? What is the fan hub made of? What is the shaft made of? What is the temperature when the fan is in operation? What is the temperature when the fan is off? How well balanced does the assembly need to be? Are you designing for ease of manufacture, or ease of maintenance?

Did you consider that either the shaft, or the hub, or both might not be round?

 

[cokeguy]

Thanks for your replies, we managed to finally get the hub into the shaft after about 3-4 hours of honing manually (with sandpaper) both the shaft and the hub. From what I learned from your answers, obviously in this cases we need a clearance fit, but just how much clearance? From MintJulep´s comments, I realize that the nominal clearance calculation should be a well-engineered value, taking into account all the variables mentioned.

In this particular case, the new impleller we just mounted was made by a contractor that simply measured an existing fan, made some drawings, and manufactured the impeller only (not the shaft). They couldn´t measure the hub´s inner diameter because the impeller was already mounted on the shaft when they took the meaurements, they only measured the actual shaft diameter.

Anyway, and only as a ballpark figure, assuming this is a stainless steel fan used in a wet scrubber, with temperature swings of up to 80 degC and quite a bit of vibration from material deposited on hub and blades, which would be a typical clearance value between hub and shaft? Obviously if we order another impeller, we will look for a knowledgable and reputable company to determine exact dimensions, but now I simply want to know ballpark or typical figures used in these types of fans. Thanks...

 

[MintJulep]

I'm a big fan of taper-lock hubs myself.

http://www.dodge-pt.com/products/pt_components/bushing/bushings.html

 

[MikeHalloran]

I like taper- locks too.

One good reason is that as you increase the shaft to bore clearance in a typical primitive joint, you also increase the eccentricity induced in the hub when the setscrews are tightened. Which increases the vibration that loosens the setscrews...

As opposed to taper- locks, which remain concentric, and can be disassembled with simple tools and little effort.

Yeah, taper- locks cost a little extra. The first remove/ replace cycle more than makes up the difference.



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

If you're using taper locks in an application with a lot of heat cycling, be sure to retighten them after the first couple of heat cycles.
We finished the first kiln of a multiple lumber dry kiln installation. We put the first kiln in operation, instructed the owners operators (In writing and through proper channels) to retighten the taper locks after the first charge of lumber.
Then we headed home for Christmas. The instructions were not followed. When we returned in January to complete the other two kilns, we found a fan blade sticking through the roof.
I still like taper locks but I like to check them a time or two before I leave them.
respectfully

 

[MintJulep]

Good advice waross. But really that applies to just about any style of hub connection, or any mechanical joint for that matter.

 

[waross]

It was particularly bad with aluminum hubs and a temperature swing from below freezing, (Canada in the winter, loading a charge of frozen lumber) to over 230 deg. F on the fan deck during fan reversals. They are lucky to have only lost two fans.
respectfully

 

[cokeguy]

Thanks again, but normally when we start up a scrubber fan, we can´t (and don´t) stop it until after 2 or 3 months when maintenance is required, unless a critical pump or fan failure occurs. I can´t stop it just to retighten a taper lock, so I guess that rules out taper locks in my case, even if temperature swings during normal operation are not nearly as large as those that happen during start/stop cycles. Or, could a taper-lock-style fan hub withstand 2 or 3 months of unattended operation on its first run?

 

[waross]

In the dry kilns, the first temperature swing was by far the most severe. Once started the fans typically could not be accessed until the cycle was finished. Also, the fans reversed several times a day. At each reversal there was a temperature peak.
At the end of a drying cycle (about 3 days) the hubs were retightened. Industry feeling was that it was the initial temperature swing that caused the worst problems. The fans would survive the first drying cycle but if they weren't retightened after the first cycle the fans would start coming off. Grease the tapers.
Many taper locks are never retightened. The combination of 200 deg. F temperature swings and aluminum hubs is a special case.
respectfully

 

[cokeguy]

Thanks, just in case it rings a bell for anyone, our fans, including blades, hubs, casings, etc.. in fact the whole scrubber is 316L stainless steel, intial temperature swing upon startup is about 100 degF, and like I said, on average it runs for 2 to 3 months before a maintenance stop. Obviously the fan impellers last much longer, about 2-3 years, but its always the same story when we need to change the impeller, lots of sweat and tears to take it out and sometimes also to get it in, like this last time...

Anyway, we´ll explore the possibility of taper locks or other type of hubs in our future replacement fans, although it just occured to me that the scrubbed vapors sooner or later chew up and corrode anything that comes into contact with them, so probably a corroded 316L taper lock with corroded bolts would pose a pretty big challenge to get it out, probably much more than the simple hub we use right now.. or not? Anyway, thanks again everyone...

 

[waross]

If you are taking it out to replace it with a new one, it will be easier to disassemble with big hammers and wedges. Also, you have a good chance of cleaning the threads in the jack screw holes with a thread tap.
respectfully

 

[imok2]

I recall more then once, when we had a supply fan go down ,especially in a large computer room we would cut out the old shaft with the fan still attached and work all night to prepair for a new shaft and reconditioned fan by the millwrights who also worked all night because time was of the essence

 

[MikeHalloran]

I can understand having this problem once, maybe twice. Given that your crew is now very experienced at the same operation and not getting better, consider changing something. Anything. For starters...

If you're having trouble getting it apart, you're not using enough Never-Seez at assembly.



Mike Halloran
Pembroke Pines, FL, USA

 

[cokeguy]

You´re right waross, it should be easier to get out with a wedge, I didn´t think about it that way. And thanks for the advise Mike, in fact we use NO Never-Seez during fan assembly, we only grease or oil it to get the hub in. We do use it on bolts, flanges etc... but not when we mount a fan hub, or any hub for the matter, even a simple pulley or sprocket hub, so we have had our share of simple pulley hubs during dissasembly. I´ll have to advise my crew on using it for hubs also... Big ommission, me and some of my guys are still newbies at mechanical maintenance (the others are oldies with bad habits) but learning fast the hard way... thanks a lot...