Gear material selection- investment cast
Gear material selection- investment cast
(OP)
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.





RE: Gear material selection- investment cast
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RE: Gear material selection- investment cast
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.
RE: Gear material selection- investment cast
Ted
RE: Gear material selection- investment cast
RE: Gear material selection- investment cast
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?
RE: Gear material selection- investment cast
Ted
RE: Gear material selection- investment cast
RE: Gear material selection- investment cast
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