Shaft diameter Tolerance
Shaft diameter Tolerance
(OP)
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...
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...





RE: Shaft diameter Tolerance
RE: Shaft diameter Tolerance
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.
RE: Shaft diameter Tolerance
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?
RE: Shaft diameter Tolerance
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...
RE: Shaft diameter Tolerance
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RE: Shaft diameter Tolerance
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
RE: Shaft diameter Tolerance
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
RE: Shaft diameter Tolerance
RE: Shaft diameter Tolerance
respectfully
RE: Shaft diameter Tolerance
RE: Shaft diameter Tolerance
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
RE: Shaft diameter Tolerance
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...
RE: Shaft diameter Tolerance
respectfully
RE: Shaft diameter Tolerance
RE: Shaft diameter Tolerance
If you're having trouble getting it apart, you're not using enough Never-Seez at assembly.
Mike Halloran
Pembroke Pines, FL, USA
RE: Shaft diameter Tolerance