Gear material selection- investment cast

[Bradbb]

I'm working to reproduce an old bevel gear set that was originally sand cast and secondary machined. They were originally an iron gear and were quite soft. I had someone hardness test them and they came out to about a HRC 2. They are slow speed gears that are actually turned by a crank. I intend to investment cast them, but am not sure which steel material to look at and whether I should heat treat or not. Most of the investment cast gear materials are probably going to be stronger and tougher than the original, which is what I want. For purposes of quoting I need to pick one though. We're talking about a 3" dia bevel gear 22 teeth. Some of the original gears occasionally broke off a tooth or developed a lot of tooth wear all caused by worn bushing/bearing surfaces and resulting misalignment. I've got that solved. But Any suggestions for material selection and heat treat? Thanks.

 

[Bradbb]

Here is a chart of material choices from one of the investment cast vendors.

Here is a link to a larger picture
Link

 

[tbuelna]

Investment casting will produce a more accurate cast part than sand casting will, but the cost for tooling and for each casting will be much higher.

Before you decide on a particular alloy for your castings, it would be helpful to do some basic gear analysis to determine just what level of strength/hardness your gear really requires. Then you should also do some manufacturing cost trade studies to determine which combination of raw material cost, heat treat cost, and machining cost provides the best value.

You might actually find that using a more carefully controlled sand cast, cast iron gear blank, combined with improved heat treatment and machining, will give the best combination of performance and cost.

 

[hydtools]

You might consider ductile cast iron like 65-45-12.

Ted

 

[Bradbb]

tbuelna, I have tried to reproduce these gears by taking my best set and filling pits and such, and spraying with shellac. Then I had the caster use them directly in the sand mold. The parts came out with finish too coarse, too much flash and voids. I don't know of anyone using a finer grade of sand to try. If I step up to a more professional process, I'm afraid the cost vs result will go down. So I know that investment will produce the finish and accuracy I need, so I got the gears correctly modeled on the computer(which cost me). Now I need to find out how the costs for investment cast are to see if this is cost feasible. To start quoting I need to choose a material and heat treat if needed). I would like a little better wear resistance than the originals. There probably isn't a lot of cost difference between material and heat treats, but I need to nail it down for quoting.

 

[Bradbb]

hydtools,
Looking up Ductile iron for investment cast in machinery handbook it shows Ferritic and Pearlitic. It does not give specific numbers like you mention. What do each of those numbers represent?
The Ferritic is list as 143-200 Bhn, which is roughly Rc 0-13 ish. So perhaps that was close to the original material the gears were made of?

The Pearlitic is listed as a much higher yeild strength, hardness(Bhn 243-300), and lower elongation. I wonder if then I am more susceptible to fracture?, or if that would work out well?

In order to get hardness numbers in a steel alloy you'd have to move up to a hardened 1030 (Rc 20-50), a 1045 or higher with hardening.

So I'm not sure if I should be looking at the Pearlitic ductile iron or a hardened steel alloy. I wonder if one flows better, is easier to achieve the properties needed? Not sure of advantages and disadvantages one way or the other. Maybe it's time to speak with an investment casting shop?

 

[hydtools]

Ductile cast iron pours very nicely. The numbers indicate ultimate strength, yield strength in thousands psi and elongation in percent. Find a 3-d printer to make meltable positives of the gears that an investment caster can use to cast your parts.

Ted

 

[Bradbb]

Ted and tbuelna, thank you so much for your replies so far. I have 8 different gears, all in the same general size range, and I plan to have 100 pieces of each made if I can make the costs work. I'm not sure if they'd do 3D printer for that quantity, or if they would force me into aluminum molds for wax.

 

[tbuelna]

Bradbb-

For a qty of 100 pcs of each of your bevel gear designs, your lowest cost option will probably be to machine them from ductile iron bar stock. You can purchase continuous cast ductile iron bar stock at reasonable cost, it machines very nicely, and the finished gear will be of much higher quality than you will even get from investment casting.

Best regards.
Terry