Die-cast spur gear material

Question

Die-cast spur gear material

BerthoBoman (Electrical)

(OP)

13 Jun 19 02:34

I need to improve the life of a small gearbox used in coin dispensers. The current material is similar to Zamak 2.
There are 5 die-cast gears, M0.8 12 through 42 teeth. The shafts are 1/8" dowel pins.
They are driven by a 12 Teeth brass gear on a small electric motor.
Unfortunately dirt from coins being dispensed is also entering the partially open gearbox (not my design).
I see two failure mechanisms: The hole is worn with some related galling on the shaft and the teeth are getting worn out.
Of course, an enlarged hole will quickly damage the small teeth. Although in some cases the holes are OK and the teeth still fail.
I cannot make any significant changes to the basic setup. The gearbox is part of an injection molded assembly.
What suggestions do you have for a better die-cast material?
Am I using the wrong shaft material?
I am alternatively looking at using metal injection molding and steel gears. Tooling is expensive but I need to improve the gears.
What are your comments about that process and materials?
Current field life is typically 2-4 years.
Life testing 24 hours/ day, typically 3-6 months.
I have attached several pictures:

Bertho
www.Vinland.com

3DDave (Aerospace) · Answer 1 · 2019-06-13 04:48:06

My experience with copper in gears is the copper is sticky and will transfer to the mating gear, causing the action to become increasingly poor and increasing the contact loads which increases the rate of transfer. The transfer bond is not particularly strong. As copper builds up it eventually gets to be a chunk that can be leveraged loose, which is where the big flakes come from.

The cure would be to make the pinion out of steel and ensure lubrication. Likewise, I would press fit a steel sleeve into the diecast gear. I would use felt and oil rather than grease.

One failure that "life" testing can have is that the oil film formation from grease is not fast. It takes time for the oil to creep out and if the item is tested at too high a rate then the lubrication might as well not be there at all.

I saw one poorly done test that concluded that bushings would overheat. The joint in question was used in application for 15 second max at 1 hour intervals - they put the max load on the test and upped it to actuating every 10 seconds or less. Oil isn't that fast.

BerthoBoman (Electrical) · Answer 2 · 2019-06-13 12:51:47

Thanks 3DDave,
I have tested pressing in a "pre-oiled" bronze bushing. See:

Although helpful, the labor cost of adding that to the 5 gears is not practical.
Good point about life testing. In this case it appears to be OK since the failures seen are duplicating field failures.

Bertho
www.Vinland.com

3DDave (Aerospace) · Answer 3 · 2019-06-13 13:09:48

The bushing could be inserted into the mold and the gear injected around it.

hydtools (Mechanical) · Answer 4 · 2019-06-16 18:32:45

Is investment casting grey iron an option? Or ductile iron?

Ted

BerthoBoman (Electrical) · Answer 5 · 2019-06-16 21:02:46

Thanks Ted,
I have been looking at "metal injection molding" as an option but worry about tolerances and how well the sintered metal will perform.
I am familiar with investment casting of jewelry but not for steel.
Is investment casting cost-effective for low-cost consumer items?
The gear teeth are small, Module 0.8 so the tolerances are critical.
I need to learn more.

Bertho
www.Vinland.com

RolMec (Mechanical) · Answer 6 · 2019-06-19 06:20:36

two observations:
a) seemingly no lubricant
When i sometimes open up small toys where gears are used, the gearboxes are also made from plastics and are not closed tight. The gears are often completely covered in a white viscuous and very sticky paste. Dirt particles adhere to the surface of it, but rather outside of the reach of the gears as it is generously applied within the gear.
--> get a lube counsellor.
b) teeth (esp. first pic) seem also bent, not only worn
How much load is transmitted, does the load increase over time?
--> increase in strength of material / e.g. sintered iron / as per others above
--> press fit of bronze or steel gears on dovel pins.

Improve performance and keep all of the setup and be as cheap or cheaper than /ZAMAK 2 die cast/ might not be achievable.

Roland Heilmann

BerthoBoman (Electrical) · Answer 7 · 2019-06-19 13:06:15

Thank you Roland,
I realize that improved performance is likely to add cost for better gears. I am just trying to find the right compromise.
There is good lubrication to start with. The mess seen is the particles from the worn out gears mixed with the original grease and some external dust/dirt.
The gears are not breaking. Once the center holes get worn, the gear to gear spacing increases which then increases the wear on the gear tips.
The gearbox cannot be sealed because of the original design concept that I do not have control over.
I modified about 500 gears: The center hole was drilled, then reamed, and then a pre-oiled bronze bushing was pressed in.
The process was very labor intensive but it is one possible option.

Bertho
www.Vinland.com

Pud (Mechanical) · Answer 8 · 2019-06-19 13:43:09

Have you considered sintered powder metallurgy? Cheaper tooling than metal injection, and I would guess the material is cheaper too.

H

www.tynevalleyplastics.co.uk

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dgallup (Automotive) · Answer 9 · 2019-06-19 13:51:43

I agree a PM gear would be better than a MIM gear. There are of course geometry limitations inherent in the PM process but it should be OK for these gears. Also lots of molded plastic gears used for all kinds of things that run for years and years.

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BerthoBoman (Electrical) · Answer 10 · 2019-06-19 21:19:17

Thanks Pud and Dgallup.
I had not thought about PM but I will find out and get some quotes.
Thanks again,

Bertho
www.Vinland.com

tbuelna (Aerospace) · Answer 11 · 2019-06-20 02:32:27

I can see some profile differences between the pinion teeth shown in your OP photos. For example, the brass pinion teeth in the 1st photo appear to have far less undercut than the molded pinion teeth shown in the 3rd photo. Are you sure your gear meshes all have the correct pressure angle, profile mod, mounting distance, etc?

3DDave (Aerospace) · Answer 12 · 2019-06-20 02:51:44

tbuelna - the brass pinion teeth probably did have undercut until they were turned into that pile of brass fragments.

mighoser (Aerospace) · Answer 13 · 2019-07-08 20:12:37

Not sure the gear loading but perhaps cast nylon with MoS2 filler (Nylatron GSM) could replace the larger metal gear. Might need a metal hub still depending on the shaft loading.

________________________________________
Mitsubishi Chemical Advanced Materials
www.mcam.com

romke (Automotive) · Answer 14 · 2019-07-09 09:53:16

I understand that the gears are grease lubricated. What type of grease is being used?

A soft grease with some antiwear additive might be sufficient (NLGI 000 - 0), but may not be sufficient during the running in period.

A alternative could be to dip half of the mating surfaces in a coating containing a dry solid lubricant. After hardening the plastic base of the coating will initially wear somewhat, freeing up the solid lubricant in the process. Subsequently some of the solid lubricant will then be transferred to the mating surface forming a protective layer on both surfaces that may be capable of reducing the wear experienced.

A coating will not be effective immediately, to free up the lubricant contained within needs some time under load. Thus this solution might call for a short running in process under sufficient load (a 10 minute may be sufficient)

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Die-cast spur gear material