Worm & Worm Gear Material 1

[bryansonnier]

We are designing device that will utilize a worm and worm gear internally to actuate a fluid orifice. I was hoping to make the worm gear out of a 17-4 ph h1150 as the through bore of the gear will be in the fluid stream, the worm and the gears of the worm gear will not. Is there a material of comparable strength that would work as the worm? The application will be high torque low rpm intermittent actuation. The worm will make 330 degrees of rotation to the worm gear’s 100 degrees roughly.

If there is no suitable combination for the 17-4, what would be the nearest material combination for the worm and worm gear that will be corrosion and h2s resistant (at least for the worm gear?

 

[TheTick]

If parts are to be molded, I strongly recommend you use a molder that specializes in gears. There are lots of not-good things that non-specialists do. Also, they can help with material chooices.

 

[BrianE22]

Standard materials for worm gearing are aluminum bronze/hardened steel (gear/worm) or cast iron/soft steel (gear/worm). I'm not sure I'm reading your post correctly, are you saying the gear teeth are not exposed? If not you may want to consider a composite gear - stainless hub fastened to an aluminum bronze or cast iron ring with the teeth.

 

[3DDave]

Does that ratio seem low? 330 degrees of input to 100 degrees of output? That's more in line with crossed helical or bevel ratios.

I agree with boring out an existing gear (the worm (wheel) gears are often aluminum bronze to avoid wearing out the worms) and fitting a hub made of a suitable material for the fluid passage.

 

[bryansonnier]

Its a 6 thread worm gear. Looking for some torque reduction but really the material is the biggest issue. The main purpose of putting the rotation in the comments was to say that the gears will see very little use compared to a drive system or something.

 

[DAVIDSTECKER]

Assuming you chose 17-4PH H1150 as the best choice for dealing with the corrosion and H2S, what are the other materials that were on your short list?
I'm not sure about the galling properties of 17-4 but with 316 it is a huge problem for us even for nuts and bolts.
They will lock up just running them together by hand.
Both bronze and iron are good bearing materials but are also dissimilar materials which is what you want to look for in you gear teeth match up.
I would find the best pairing based on that, then scale up the gear teeth as required to carry the loads you need.
BrianE22's suggestion of a composite gear is a very cost effective means of getting the properties you need too.
If it's a one off you can combine the materials using interference fits to create any extremely strong interface between the materials.
I have gear to shaft assemblies with 0.005-0.010 inch interference that are impossible to separate what out properly heating the gear.
Best regards, David

 

[EngineerTex]

Have you considered making the parts out of a normal low-alloy material (4140, 4340), then either TIG or laser welding an Inconel overlay into the wetted ID of the part?

1 - Hob the gears
2 - N,Q&T
3 - Carburize teeth
3 - Weld overlay Inconel 625 on ID
4 - PWHT to reduce hardness in the HAZ

You may talk with your HT guy about exact steps for the carburizing and N,Q&T. Carburizing is not my area of expertise.


Engineering is not the science behind building. It is the science behind not building.

 

[bryansonnier]

Its looking more and more that i will have to go with some sort of composite, i'm thinking purchasing some standard gear or gear set and modifying it to fit onto the main piece and the drive. This would give the best of all the worlds, ideal gear material for the gearing and proper design material for the application.

@EngineerTex, i think if this were a mass produced item where we could have an assembly line of processes or a complete one off where extreme costs were justified this would be the perfect solution but in our world of low run items we are really ham strung. The costs must be kept low but we don't have volume to justify the requisite number of processes. Thank you for your input though.

 

[shu60]

17-4PH has poor unlubricated galling and wear properties mated to itself and other "poor" alloys not designed for this type service. It had done well over the years as chrome plated or nitrided. I have tested Al Bronze (Ampco 18) and found it exceptional for galling (ASTM G98) and also metal-to-metal (ASTM G83) wear resistance. Not sure if it's available in cold worked higher strength conditions to meet your strength requirements.
Another possibility is Nitronic 60 stainless steel used widely for anti-galling components especially threaded assemblies like helical inserts, anchor bolts and ball screw shafts. It has run well against many stainless grades including 17-4PH. It is available in strengths comparable to H1150 17-4PH using cold working practices...not sure of bar size limit, but it's at least up to 2" dia.

shu60

 

[enginesrus]

I worked at an out fit, we machined mostly Nitronic 50 and 60, I think the 60 round stock bars we received were about 14 inches diameter may have been closer to 12 inches I do not remember.

 

[shu60]

enginesrus
The Nitronic alloys are available up to 12-14" dia in the annealed condition with a minimum YS=50 ksi. To match the 17-4PH alloy, the Nitronics have to be warm or cold worked which limits their size availability depending on the producer's capability. I do want to clarify one thing I've seen re the 50 & 60 alloys-they are not interchangeable...the 50 was designed as a higher strength austenitic stainless steel with superior corrosion resistance to 316. It's galling resistance is marginally better than 316 depending what it's sliding against. The Nitronic 60 stainless was designed as an anti-galling stainless and is superior to the standard austenitic and duplex alloys. It has been widely used for threaded components, valve trim, pump parts and chain belt pins. It also performs well at elevated temps up to 1500F.

shu60

 

[enginesrus]

And 60 was easier to machine too.