High contact load on silver plated bearing 1

[horstr1]

Has anyone done high-load bearings with silver plating? I hear and read things with words like 'high load' in them, but nothing quantitative. Obviously the material strength of silver is way less than structural metals. But in a thin film on a wide surface, do numbers like yield strength mean less? I've got a load that may go as high as 100+ ksi statically, but much much less when in motion. Any experiences/thoughts, anyone?

 

[Cockroach]

I've seen some impact damage in the valve industry, predominently mainline block and bleed Ball & Gate Valves. In both cases the Ball/Gate are degreased, pickled and then flash coated with electroless nickel to a depth of 0.0005/0.0008 inches. The piece is then baked in a furnace and allowed to cool slowly back to ambient in a controlled setting.

The hardnesses expected are about HRc 45-50, sometimes harder depending on whether a Chrome coat is flashed over top of the ENC layer. With debris coming down the pipeline, gas line transmission being the worst, partially closed valves usually took a lot of surface degradation over pressure drops of Class 600, 1460 psi.

I did drop steel ball bearings from various heights onto the surface of Balls and Gates, trying to determine some sort of activation energy for coating damage, I would need to look for the datum since I never published. But the results were interesting, at least for me, and the data did provide some sort of grasp as to the magnitude of mass verses velocity in natural gas flows that would damage the valving.

Compressor station firing up for example, welding rod fragments left in the pipeline would literally trash a valve downstream. I did see extensive damage from a screw driver travelling about 20 km down the line and hitting a gate, stabbed right into the steel about a good quarter inch.

Static load of 100 ksi? I guess it would depend on how the load is spread out on the silver surface, if the member would deflect under the load, what sort of adherence the silver has to the steel substrate, flaking characteristics of the coating, etc. The problem entails a lot of metallurgical-chemical analysis to figure out how the silver behaves at the interface of the parent steel, is there porosity giving the silver coat something to hang on to? What is the surface finish of steel and condition prior to the silvering process? Did you bake the piece? How was it cooled? Did you remove the hydrogen from the ionization during the process? The list goes on and on.

But good luck with the problem.

Regards,
Cockroach

 

[horstr1]

Thanks for responding, Cockroach. Those are indeed some interesting experiences. Did you find out whose screwdriver it was and kick their butt for a while?

My application is a flat surface against a flat surface. Basically a large thrust washer. The bearing itself will be Stellite, or possibly beryllium copper. Either one should work fine without the silver, it's just an enhancement we're thinking about.

I did find an old textbook that had an in-depth analysis of how a wide plate behaves under a compressive load. There are about 9 pages of math involved, but the simple distillation is that as the thickness gets small, the friction resisting the metal squeezing out gets more and more important. At very small thicknesses such as a plated coating, I calculate the load can be hundreds of times the tensile yield strength of the material. I had a gut feel that would be the case, now I have some theory to back it up.

We're going to plate some bearings and test them, so I'll post the results when we do. The tentative plan is to run some plated and some not, and find out whether it holds up and whether it helps.