Oil pump gear material 3

[yoshimitsuspeed]

I started a thread in the Automotive section and got some good input but nothing that helps much in choosing an ideal alloy and hardness for this application.

http://www.eng-tips.com/viewthread.cfm?qid=402820

Details are in that thread but I am looking into making an upgraded oil pump gerotor gear set. I am looking for input on steel selection.
I don't know much about sintered steel properties or how the stock sintered gears compare to other steels. I assume that even quality mild steel should be stronger and more wear resistant but I am hoping to get input on what the best material might be. I am thinking something like 4150 because of it's high wear resistance. I am also thinking maybe hardened to somewhere in the middle of it's range. I don't know enough to have much science behind that but my thought is that it should be relatively hard but it might be good to have some conformability to debris and or be a little less brittle than it would be in it's max hardened state.
Just curious to see if anyone has any more input.

 

[EdStainless]

The high pressure industrial oil pumps that I have dealt with use a fairly mild alloy for the rotors, and then case hardened it.
The other option is to sinter PM steel ones in furnace with about 0.6% carbon potential, and thereby raise the carbon level and strength of the PM part. I have seen other alloys (such as 430 SS) used.
One of the big issues though is fit and alignment. Driving off of the as sintered ID is not great for getting good alignment and uniform loading. An ID fit that transfers the load more uniformly will help at high speed/pressure also.

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P.E. Metallurgy, Plymouth Tube

 

[yoshimitsuspeed]

I was thinking about case hardening but I believe that would mean machining it to rough just over dimension, hardening it and then doing final machining to ensure the post hardening geometry is perfect.
Is this worth the extra steps over something like 4140 or 4150 hardened to something like C45 and then machined in one process?

Is it possible to sinter a part that would be as strong, wear resistant and cavitation resistant as machined billet or formed stock like round bar? All of the sintered metal I am familiar with seems pretty soft and weak.

 

[tbuelna]

I was following the thread linked, and from I could see the problem was mostly due to factors other than lack of strength in the pump elements. The problem seemed to be caused by excessive operating misalignment in the pump assembly, and/or improper fit at the drive coupling interface. So changing the pump element material would not address the root cause of the problem.

Having said that, if you want pump elements with a surface highly resistant to abrasion, good dimension stability, excellent fatigue properties, and a tough core, then I would suggest using nitralloy N with a nitride case hardening. There is not much distortion produced from nitriding, so it would not require much material to be removed during finish grinding/honing.

Lastly, since your pump housing is made from cast aluminum, any housing surfaces the pump elements contact pose far more of a wear issue to the aluminum housing than the steel pump elements. As for concerns about cavitation damage to your pump elements, it would be more helpful to correct the flow condition causing the cavitation.

 

[yoshimitsuspeed]

Unfortunately I feel like there were too many overly vague and contradicting posts in the other thread for me to gain much from theories of cause or solution.
One person says the drive flats are too far apart causing just the edge of the crank to wear on the gear drive flat. I totally agree with this.
Then another person says they wouldn't make that gap any smaller for the same reasons the OEM didn't. I have concerns with doing this myself because of the risk of some unforeseen consequence.

The one thing I am sure of is that the Toda upgraded gears have a reputation for taking a beating and never failing so it's very tempting to go with the tried and true route over risking a new design that could be better but could lead to another issue. One thing I haven't inspected on the toda gears is the spacing of the gear drive flats. I would be interested to see how they compare to stock.

I mostly agree with you about the wear of the aluminum vs the gear material aside from the fact that the surface area between the gears and the housing is huge whereas the contact points on inner gear to outer gear contact area is much less. It also depends on how much I am able to sell them for. If they are going to sell for $400 each then a customer would be pretty happy if they went through a few housings per gear set. On the other hand if I can sell a complete new pump with upgraded gears for $100 then the gears may be considered more disposable.
I also agree with your point about cavitation as well and it will definitely be a focus to first try to verify if cavitation exists or could be likely and then next seeing what could be done to reduce it. I would still like a material that was decently resistant to it.

 

[tbuelna]

You need to understand the reasons for relieving the center section of the flats on the pump gear bore. This actually allows a better contact condition with the crank than having full width flats. The pump gear is always being driven by the crank in one direction, and due to the constraint conditions of this coupling there will usually only be contact at the two diametrically opposed outer flat edges. Having contact at the largest radial offset as practical helps reduce the normal force applied.

One thing that might help is adding a slight crown profile to the outer flank surfaces of the pump gear bore that contact the crank drive flats. This will give a more uniform contact stress under conditions of misalignment and clearance in the coupling.

 

[EdStainless]

You can get a lot stronger in PM than the OEM parts, but that takes volume.
Nitralloy would be a great option, but I would focus on the fit up issues.
I am sure that misalignment is the major cause of these failures.

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P.E. Metallurgy, Plymouth Tube