Anybody have experience with investment cast gears? 1

[AllAngles]

We have a device we're working on that contains a pair of bevel gears that are about 8:1 ratio (pinion is 0.6"OD, ring is 3.5"OD) and 22DP. Machining these from 303 stainless is expensive, even at reasonable volumes (1,000-10,000/year). Our gear set is very low RPM (<200rpm on the pinion) and will likely run less than 1 hour total in it's life (short bursts of running with long stretches of sitting still). It will see pretty substantial torque when it does run, however.

The cost of investment casting the gears, even with the large up-front tooling cost, is about half of the price of hobbing. The catch is, of course, they can only hit +-0.005 tolerances. I think (hope?) that because our application is low RPM and low hours life expectancy, we might be able to get away with those tolerances. We'll have to do some machining to clean up the bores, etc., but I don't want to have to retouch the gear teeth.

We would probably need to set up a rig that could run in ring/pinion pairs before they went into units, but that would be relatively easy to set up.

Thoughts?

 

[Spurs]

Ever think of Powder Metal Gears?

Sounds like a better solution for you

If interested let me know

 

[AllAngles]

Yep, that was my first choice. Surface area of the larger gear is apparently large enough (about 5.5in^2) that it's not very practical, at least with stainless.

Thanks!

 

[tripleZ]

The use of stainless PM gears is really going to depend on what dimensional tolerances you want combined with your strength requirements. At 5.5 sq inches, you're probably going to need a 550-ton mechanical press or equivalent hydraulic, depending on the # of gear levels. You can coin for added strength & dimensional stability depending upon the geometry.

Bevel gears can be tricky if the geometry requires a steep bevel as you can't get even fill and motion of the powder column over the teeth. So sometimes you end up with strength issues. A picture is worth a thousand words here! I personally hate coining into pockets as you tend to break dies quickly.

Would you expect your annual volume to be over 10,000 or typically under? The tooling costs for PM for these will be high...likely in the ballpark of $15K. A lot of shops won't touch them at such a low volume. MPIF lists the shops specializing in SS. A few coming to mind are: SSI Technologies, Alpha Sintered, Metaldyne. Shops with good gear experience would be Chicago Powdered Metal, either of the Burgess Norton shops, and the Capstan family. There are others out there who might be more willing to look at the smaller run sizes as some of the aforementioned shops might not quote at such a small volume.

 

[AllAngles]

I expect 10,000 to be the high-end of annual volume. Our product is offerend in various sizes and configurations, each particular gear set fitting only one combination thereof - 10k would be what we expect to be our most popular (budget) model

That said, $15k in tooling wouldn't be the end of the world - it could easily pay back within a couple years, especially if we could avoid having to do secondary operations. IC tooling is about $8k, FWIW.

Our bevel is a decent fit because the ring-gear bevel is very shallow due to the high ratio. I'm looking at some other assembly options that would let me reduce the area of the ring-gear part to make it a little easier to press.

Thanks for the heads-up on the PM shops - only one of those is somebody I've contacted already.

 

[Spurs]

AllAngles

If you are interested in PM gear, I could help you with specs and sourcing

http://www.webgearservices.com

 

[tbuelna]

AllAngles,

A 22dp gear tooth is very small (about .071 thick). So even a +/-.005" tolerance is probably unacceptable for your gears.

If your gear stresses are low enough for a softer (austenitic) grade of stainless steel, you might be able to roll form the gear teeth. This is commonly done with production spline teeth on external shafts. The tooling would not be too expensive and it would give a fairly accurate finished tooth in a rapid, single step operation. The only thing you might need to modify would be your gear tooth pressure angle. Roll forming would work best with a very high pressure angle, usually around 40deg or so.

Hope that helps.
Terry

 

[monkeydog]

I had a supplier switch from cut gears to PM gears without telling me, result was fractured gears during test.
Glad it did not happen when operational.

 

[AllAngles]

Terry, thanks for the suggestion about roll-forming, I had not seen that option presented elsewhere. We may be able to get away with a softer stainless, roll forming would probably work harden the teeth a bit anyway.

Monkeydog - was this recently? Everything I've read and been told says that PM physical properties are almost identical to wrought parts. Was your applicaiton unusual, or were the PM gears just that much weaker?

So far in my investigation, PM is well in the lead with the lowest up-front tooling costs and (by far) lowest per-part cost at our volumes. Of course, if they break - it's not such a good deal!

 

[plasgears]

Monkeydog,
The switch in mfg process by your source to PM is very suspect. It's a breach of good quality ops. However I am an adherent of PM. We used the process: press, sinter, air quench, and drop into oil. You end up with HT properties and prelube, too. Consult with a PM house. They have seen many applications.