Rotating shaft with high alternating side loads

[Dougt115]

I need to find a solution.

I have a small shaft ID about 0.3 inches that needs to rotate 15 degrees and carry an alternating side load of 800 to 1600 lbs for 2 million cycles. OD cannot exceed 0.4 inches. Width is constrained to 0.25 inches. Frequency is 1Hz.

Bushings are wearing out before 100K cycles and causing slop.

I cannot find bearings that can handle the loads.

I was thinking of trying a straight steel on steel with a DLC coating. Basically make my own bushings.
Friction is not an issue...yet. Do to the forces.

 

[3DDave]

Always interesting to see the calculations at the end of problem.

.05 wall X .25 long bushing? Polish the shaft to 4-8microinch, and add a couple of oil-soaked felt washers.

The shaft is sitting at almost 4000psi contact pressure on the bushing, so if you go with a really hard material coating in the bushing, it better be extremely smooth also. How much wear can you accept?

You're up to 23ksi in shear load alone; what's the fatigue limit for the shaft material?

 

[tbuelna]

Dougt115-

Can you provide a sketch and more specific details of your shaft/bushing arrangement? For example, how is the load applied to the shaft? How is the shaft supported/constrained by the bushing(s)? What is the fit/clearance between the bushing and shaft?

 

[Dougt115]

The shaft is a short rod, 0.5 inches long diameter of 0.325, of 17-4PH H900. Press fit at one end and the bushing in a plate at the other . Life expectancy of the rod is infinite cycles. There is a symmetrical configuration that goes into another plate and maintains the alignment, like a sandwich.

The unit is used by people so the slop is what people are able to feel or hear as the case may be.

The shaft has proven to be able to handle the loads but the bushing is wearing out. This is why I am looking at DLC coating the rod and eliminating the bushing entirely. I can close the tolerances to 0.0005 which is better than the bushing break-in wear specified at 0.003.

But will DLC work as a sliding surface? Or are there other options I could look at?

 

[tbuelna]

It's still not entirely clear from your description what your shaft and bushing arrangement looks like. A simple sketch would be very helpful.

Based on a simple P/A with a .325" dia x .50" L shaft/bushing contact and a 1600lbf radial load, the bearing stress does not seem too excessive for common high-strength plain bearing materials like alum bronze. But if the shaft load is cantilevered, you likely will have edge loading in the shaft/bushing contact and the bearing stresses will be far greater.

You can certainly increase the abrasion resistance of the bushing by making the surface harder using a coating like DLC, but this approach just addresses the symptom rather than the underlying problem. If you indeed have an edge loading problem, then you should first try to resolve it by modifying the contact geometry of the shaft journal and bushing to normalize the contact under operating loads.

 

[Tmoose]

Hi DougT115,

Did you say what your bushing is made of?

switching materials, hardness, etc sometimes can increase wear life 2 or 3 X.

Lubrication can often easily increase wear life 10X or more.