440C Steel Bearing Dynamic Allowable 1

[dave2227]

Hi

I am updating an existing Hertzian contact stress calcualtion of a 440C Steel Ball Bearnig in contact with a steel plate. In the existing calculation a dynamic allowaable of 400,000 psi (at 10^7 revs) is used. Does anyone happen to know where I can find a reference for this allowable. A static allowable of 602,900 psi (4200 MPa) is also quoted. I can find this in most bearing catalogues as it is in the ISO 76:2006 Static Load Ratings standard. I have looked in the ISO 281_2007 Bearing Dynamic Load Rating standard but cannot find this stress level quoted.

Hardness of 440C is 58HRC.

Any help is very much appreciated.

Thanks

Dave2227

 

[TVP]

Look through some of the NASA Technical Reports by Zaretsky. Lots of published data on M-50 steel and silicon nitride, probably 440C as well.

http://ntrs.nasa.gov/search.jsp?Ntx=mode

matchallpartial|mode matchall&Ntk=All|Author-Name&N=0&Ntt=bearing contact stress|zaretsky

 

[Tmoose]

FWIW FAG considers 2000 MPa the :contact stress" endurance limit of 100Cr6 bearing steel, so "infinite" life is attainable if EHD lubrication, kappa > 2, and excellent cleanliness are maintained.

 

[boo1]

NASA-STD-5017, Table 1 (attached) shows:
Allowable Contact Stress for Bearing Materials Under Yield Loads
Bearing Material 440C Steel
Mean Hertzian Contact Stress Quiet Running 335 ksi (2310 MPa)
Mean Hertzian Contact Stress Non-Quiet Running 400 ksi (2760 MPa)

NHBB recommends that the load rating published for 52100 be reduced by 20% for 440C. This is a conservative approach to ensure that the bearing capacity is not exceeded under the most adverse conditions. This is incorporated in the reliability modifier (a2) as shown:

http://www.nhbb.com/reference/mini-instrument-bearings/load-ratings-bearing-life.aspx

Abbott Ball recomends TENSILE STRENGTH 285,000 PSI

http://www.abbottball.com/materials/stainless-steel-balls-440C.php

Cheers
B

 

[tbuelna]

dave2227-

Your OP describes a 58Rc 440C ball bearing in contact with a steel plate, and you ask whether 440C bearing material at Rc58 is adequate for a hertzian contact stress of 400ksi at 10^7 revs. Unfortunately, you need to provide more specific details.

First of all, where does the hertzian contact stress of 400ksi occur? For example, say your bearing system has a fixed inner race, a fixed load direction with respect to the inner race, and a complement of 20 balls, then there will be one spot on the inner race that is subject to 20 x 10^7 fatigue load cycles. Also, with the statistical fatigue life of rolling element bearings, the quality of the material can have a huge effect.

Regardless, in my opinion a hertzian contact stress of 400ksi seems a bit too high for 440C material at 10^7 cycles, even with an L10 reliability rate.

Hope that helps.
Terry

 

[dave2227]

Thanks to all who replied.

I had found the NASA-STD-5017, Table 1 information but wondered whether they assumed infinite life to be 10^7 revs. Seems a bit low to me.

The case inquestion is not your usual ball bearing case. It is part of a no-back device. There are 3 balls sitting in pockets between two plates. The pockets have ramps which the balls ride up when there is angular separation of the plates. This forces the two plates apart and applies pressure to a set of brake discs therefore stopping the device. Under normal forward running the balls transmit the full system torque.

The engineer perfomring the analysis some years ago assumed that anything less than 400,000 ksi contact stress (Hertzian calc) was good for 10^7 cycles. But he left no reference.

Thanks for your resposnes.

Dave