Two steel hemispheres are diffusion welded in an atmosphere of 0.00002 torr at a temperature of 2130 degrees F for 4 hours with a pressure of 4 psi holding the hemispheres together. Weld is accomplished with only microdeformation

Moore, T. J.

English

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Al20 
Butt I •
Al20
Welding Jig
%1203 Tube 
/16" Wall 
1/16' Dia 
lollow Ball 
June 1970	 Brief 70-10331 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Fabrication of Hollow Ball Bearings by Diffusion Welding 
The problem: 
Although aircraft gas turbine engine rotor bearings 
currently operate at fairly high speeds, it is estimated 
that the engine designs of the next decade will require 
bearings to operate at twice such speeds. At these high 
velocities, the reduction in bearing fatigue life due to 
the high centrifugal forces developed between the 
rolling elements and outer race becomes prohibitive. 
To solve the problem of lower fatigue life in high-
speed ball bearings, various methods of reducing ball 
mass to reduce the centrifugal force are being consid-
ered. Spherically hollow balls, which are fabricated 
by welding two hemispherical shells together, offer a
possible solution. Experience to date with hollow balls 
joined by conventional welding techniques has, how-
ever, been disappointing. Excess weld metal and/or 
undercut at the root of the weld proved to be stress 
raisers. In service, cracking tends to start near the 
root of the weld, and flexure failures, which originate 
at the edge of the weld on the inside diameter of the 
hollow ball, have caused early spalling and even com-
plete fracture of the ball. 
It was, therefore, decided to fabricate spherically 
hollow balls for bearings by butt welding two steel 
hemispheres in such a manner that there would be no 
stress raisers, defects, upsets (too much or too little 
metal in the weld fillet), or unwelded areas. Earlier 
ball bearings produced by electron beam welding 
proved to be unsatisfactory and a better welding 
method had to be developed. 
The solution: 
Diffusion weld two steel (AISI-M-2) hemispheres 
in an atmosphere of 2 x 10 5 torr at a temperature 
of 2130T for 4 hours with a pressure of 4 psi (dead 
weight) holding the hemispheres together. Involved 
here are two main areas of application for diffusion 
welding: (1) for materials that cannot be successfully 
fusion welded by conventional processes; and (2) for 
welding applications where the weld must be accom-
plished with only microdeformation (i.e., produce 
weld joints with no upset material or other discon-
tinuity at the joint). 
How it's done: 
The faying surfaces of the two steel hemispheres 
were lapped to obtain flat surfaces. The hemispheres 
were assembled as shown in the figure and placed in a 
(continued overlcaI) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19700000319 2020-03-17T00:19:50+00:00Z
welding jig made of aluminum oxide (Al 20 3 ). A dead 
weight (tungsten rod) was placed on the upper hemi-
sphere to press the two hemispheres together in the 
jig and vacuum diffusion welding was accomplished. 
Metallographic examination of the weldment re-
vealed that complete welding had been achieved. No 
bond line was evident. Because of the extremely low 
pressure (4 psi) used, no macrodeformation was pro-
duced by the weldment. The ball was spherical in the 
as-welded condition and no excess material or under-
cut was present at the joint. 
Notes: 
1. Optimum conditions for this process have not been 
established, but it appears that good results can be 
achieved at lower temperatures and with less time 
at the welding temperature. 
2. Low pressure vacuum diffusion welding is an ex-
tremely promising joining method for fabricating 
hollow steel ball bearings, and seems well suited 
to other joining problems.
3. This diffusion welding method can be used easily 
with only a vacuum furnace; no loading rams are 
necessary (as would be necessary for a hot press) 
since a dead weight is a satisfactory load. 
4. Requests for further information may be directed 
to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: TSP70- 10331 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention may be made to NASA, Code 
GP, Washington, D. C. 20546.
Source: T. J. Moore
Lewis Research Center
(LEW-1 1026)
. 
Brief 70-10331	 Category 08


Fabrication of hollow ball bearings by diffusion welding

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