Fan Hub Cast Iron

[triumph993]

Folks,

I'm trying to figure out what material my fan hub is made of so that I can know it's yield point. It is either cast or ductile iron.

The application is a hydraulic fan drive on a bus. The fan motor shaft is a 1:8 taper with a 5/8-18 UNF thread for the nut. The shaft can take a torque of up to 135 ft-lbs. The taper on the fan hub is 1.00 inch long with the small end bore at 0.75" and the large end bore at 0.875". The hub thickness at the taper bore is 0.5".

I torqued the nut to 90ft-lb and installed the fan along with the radiator, charge air cooler, etc. IOW's it's a huge effort to remove the fan hub at this point. Everything must come out.

I ran thru some calculations using a material yield point of 18855 psi and I find that the tangential stress on the hub is right at the yield point (18113 psi). In my calculations I'm using a steel-to-cast iron dynamic coefficient of friction of 0.23, and a nut clamping force of 8640 lbs.

My numbers and calculations look something like:
Hub inside dia: 0.8125" (ave)
Hub outside dia: 1.7125"
Hub length: 1.00"
taper angle: 0.0624 radians
friction angle: 0.2261 (u = 0.23)
bolt clamp force: 8640 lbs (u = .2)
hub bore contact pressure: 12052 psi
grey cast iron yield point: 18855 psi
inside tangential stress: 19054 psi
outside tangential stress: 7002 psi

My question is, how can I tell if the hub is made of cast iron or ductile iron, and how would it fail if over stressed in either case? If it's cast iron I think it would have shattered given the loads exerted on it, especially since during pre-assembly I torqued as high as 120 ft-lbs, which is well above the ultimate strength of grey cast iron. This leads me to believe that I'm actually working with ductile iron.

 

[metengr]

triumph993;
Some thoughts to consider;

1. Since this is a tapered fit with a locknut, your tangential bore stresses in the hub region regardless of material of construction will rapidly decline as you progress away from the bore region. So even though you believe you have exceeded the elastic limit of cast iron, it should not fail in a brittle manner unless one or more stress risers were specifically present on the bore surface (highest tensile stress location) or an impact load was applied to the entire hub in service.

2. Is this a new (replacement) hub or an existing hub?

3. The only way to really tell if you have gray cast iron versus ductile iron after installation is to polish an area on the surface of the hub, chemically etch this location and examine the microstructure with a portable microscope. Yes, this requires some level of effort with proper equipment but this is the most definitive method.

4. If the answer to 2 above is an existing hub and it was removed and re-installed, what is your concern?

5. If this is a new hub, I would do what I can to locate material information.

 

[arunmrao]

Take a hammer and strike the hub casting. If it gives a bell sound,it is ductile iron,it is thud sound,then it is cast iron. Hope it helps.

 

[triumph993]

Guys, Thanks for your help.

This is new replacement hub. I have the old hub, so doing a material analysis on the old one is possible. I bought the part from BlueBird so I don't know the vendor, but I have a call into BlueBird to see if they're willing to share that information with me.

I did give the old hub the ring test. I have two references to compare the ring to. Reference (1) is definitely grey cast iron and it produces a real thud, absolutely zero ring. Reference (2) is an old hydraulic fan motor tapered shaft that I removed from the old fan motor. This shaft is ductile iron with a gear milled into the end. Reference (2)rings like a bell. When I strike my old hub, it rings significantly more than Reference (1), but significantly less than Reference (2).

Metengr, I can take the old hub and polish and etch it. What kind of power microscope do I need to use to see the micro structure?

Thanks

 

[arunmrao]

If it is an old one,perhaps it could be malleable iron. however,replacing with ductile iron in both the instances should not be an issue. A 100x microscope will be more than adequate. You only need to look at the graphite morphology,if it is rounded or flaky. Do not use any etchant.

 

[triumph993]

Arunmrao,

I'll see if I can gain access to a 100x microscope. In the mean time, since I have the old hub in my hand, are there any destructive tests that I perform to narrow down it's composition. For instance, the hub has a 5" diameter and 0.5" thick flange with a 3.5" bolt circle (six bolts). Can I hit it with a center punch, or take a hammer to chip it's edge to gain any insight between grey and malleable cast iron?

 

[arunmrao]

Perhaps the drilling chips can give an insight into the type of material.It has been a very,very long time since I have seen a malleable iron casting .

Also the fractured surface can give indications of the type of iron.

 

[btrueblood]

Are you sure it isn't a cast steel part?

 

[triumph993]

Metengr brought up the notion of stress risers and impact loads. Fortunately this is a very easy life for the hub. The fan exhibits no impact loads and the start up is gradual, thanks to a pulse width modulation valve; however, stress risers are present. The shaft and hub bore do have a keyway. On the hub bore, the keyway is 0.25" wide and 0.1375" deep. This means that the hub material thickness is only 0.3125" at the keyway, which again leads me to believe that if it was grey it would have failed already.

In addition, because it does ring with a steady sustain, I'm thinking that grey cast is not likely, this leaves cast steel, malleable, or ductile iron. Anyone know of a service where I can send my old hub to have it analyzed?

 

[unclesyd]

Aside from ringing the part there is another practical test to use.
One the old part take a very small sharp chisel and try to make a small chip. If you can make a chip it is either Ductile, Malleable or Nodular Iron. if the chip flakes off you have a cast iron.

 

[triumph993]

The chip test takes a bit of experience. After trying it on what I know to be grey cast iron I can say that my old hub peels more than it chips. I'd say that both the hub and the cast iron sample appear to be about the same hardness, but that's just my gut feel after chiseling and filing both items.

One thing that I didn't mention before is that the hub has a 3.5" bolt circle with six holes and the bolt holes are tapped. The threads are a good clean cut. Can grey cast iron be tapped with good thread results?

 

[triumph993]

If it rings, and it does, can I assume Malleable Iron and at a minimum a yield strength of 30,000 psi?