Ductile Iron for worm gears? 2

[BrianE22]

We currently use aluminum bronze but the per price piece of the castings has nearly tripled from the price about 4 months ago. Many years ago the company made the blanks out of cast (gray) iron but found that if they came out of the foundry above a certain hardness level, the teeth would break easily. I was around when that was happening - very frustrating for the owner of the company. He decided the "hell with this I'm just switching over to aluminum bronze".

Now I'm thinking of switching over to ductile iron (I'm still leery of cast iron). Have you people seen many ductile iron worm gears? It's a 4" pitch diameter and 10 DP teeth. From what I've read it's still fairly new for gears (10-20 years) and it does look like it is a bit sensitive to cooling rates (strength, toughness, etc.). Both the aluminum bronze (954) and the old cast iron gears work (and worked) fine (at least when the CI ones didn't break off teeth).

 

[mfgenggear]

Not recommended, the worm gear is easier to fab. There fore why aluminum bronze is used. It is sacrificed for wear.

 

[hydtools]

Buy a piece of ductile iron bar in the grade you want and machine the gear.

Ted

 

[Tmoose]

How are the gears lubricated?

Are the working loads and stresses of the gears known?

What is the approximate diameter of the worm gear?

 

[mfgenggear]

so tmoose has posted great questions. and what is the fit form and function has not been posted.
but lets discuss what a worm gear and worm is normally used for. and I will assume it is lubricated by oil bath.
the name of the game is gear reduction, high torque, low rpm. the stresses and wear is high on the worm.
so the worm and worm gear are to be machined as a set. to obtain central gear pattern and the correct center distance.
the worm gear is sacrificed for wear. depending on the function the worm is normally is a more difficult and more expensive component.
the center distance controls the correct back lash. the worm gear can not be measured measurement over wires correctly. thus has to be measured with a master worm.
and takes special fixturing and cutting tools. since the worm gear is the stronger component it is made from bronze.

 

[TugboatEng]

I disagree with the statement that any gear is manufactured to be sacrificial. Worm gear sets require differential materials because of the high amount of sliding at contact, unlike other gear forms. The wheel gear is usually the lower strength material because it sees less bending stress due to larger diameter or distributes wear over a larger number of teeth.

Some alternative options would be to face an iron or steel wheel gear with bronze to reduce material costs. This may not be viable without mass production.

Another option could be to make the larger wheel gear out of an inexpensive iron/steel and the smaller worm of an exotic material such as a Nitronic alloy.

 

[mfgenggear]

here is comment from the American Machinery handbook

 

[mfgenggear]

web check

https://moviecultists.com/why-worm-wheel-is-made-of-bronze

 

[BrianE22]

Thanks for all the inputs. I've been searching online for "ductile iron for worm gears" and have found a few things. AGMA has a paper on ductile iron for gears but it does not mention worm gears. Seems like the tried and trued are:

1) Cast iron and soft steel worm (what we used to use).
2) Aluminum bronze and hardened steel worm (what we currently use).
3) Phosphor bronze and hardened steel worm.

I'm going to re-examine:

https://www.accuratespecialties.com/spuncast-gear-blanks

Our quantities haven't been justified to use this method but with the large increase in price for aluminum bronze it might be more attractive.

To address some of the issues you mentioned:

We have not noticed the current aluminum bronze gear wearing so we don't consider it as sacrificial although I see there is a lot on stuff online that calls it sacrificial. It wears in nicely and stabilizes. It's probably not stressed as highly for our application.

I may very well buy a piece of ductile iron and experiment.

We use a worm gear lubricant, Mobilgear 600 XP 320. Forces are known. We also have a long history with cast iron and aluminum bronze. The strength and ductility of the cast iron we used (Class 45) and aluminum bronze (C95400) are easy to compare to the current ductile iron products. It's a 4" PD and 10 DP tooth gear.

Worm of Nitronic Alloy - hmm, I'll have to look at that.