MATERIAL AND HEAT TREAT FOR DOG CLUTCH

[2500CT]

I have a new product that involves a shaft passing through a housing. The shaft has external dogs on it. The housing has internal dogs. The design requires that the shaft slides forward and reward selectivley engaging and disengaging the housing. The shaft has the ability to rotate at 300rpm. The rotational torque is 3500 ft/lbs. I have made the dogs out of 4140 with an ht of 45-50 rc. The housing is filled with a 40 wt oil with additives.
The system requires that the dogs are engaged by feeling there way. This will most likely cause a hammering effect. The Problem that I am having is that the dogs are gallding and shaving each other, leaving the shavings in the oil that also lubricates the bearings.
Is there anything that I can do to correct this problem.

Thanks

 

[kclim]

I'm assuming the damage is mechanically related, rather than chemically/corrosion related.

If you were to redesign the shaft/housing, perhaps you might want to go with a case hardening steel, which will allow hardnesses of up to 60 Rc (which should slow down the wear), while still allowing you to retain core toughness.

To prevent galling, you should use different materials for the contacting parts, or failing that, ensure there is a sufficient different in hardness between the two parts (10 difference Rc is a rule of thumb).

If you are stuck with the 4140 parts, I'm not too sure what you could do. I don't think 4140 is suitable for case hardening. It might be possible to heat treat to a higher (through thickness) hardness, but you would lose shaft toughness.

As far as the shavings in the oil go, is it possible to filter the oil to minimise any possible contamination?

 

[TVP]

It would be helpful to know the stresses involved. Options for improving the wear resistance include the following:

1. Nitriding of the 4140 after quench & temper heat treating. The core hardness of the 4140 won't be 45+ HRC anymore (nitriding is performed around 580 C, so ~ 38 HRC) but the surface hardness will be considerably higher due to the nitride layer. Typical surface hardness is > 750 HV or > 60 HRC.

2. Change from 4140 to a carburizing grade like 8620 or 4320. Carburize then quench and temper. Surface hardness will be > 60 HRC, core hardness likely will be in the 30's HRC. Case depth requiremetn depends on stresses, both contact and bending.

 

[2500CT]

kclime and tvp thanks for your help. The damage is mechanically induced. I am not married to 4140. What do you think of using 17-4 ph for one of the pieces and and using the nitrided 4140 for the other. It is not possible to filter the oil, but I have already introduce magnets to capture what they can.

 

[arunmrao]

Sealed bearings can be considered to avoid risk of contamination. Also how often does the dog engage and disengage to create such large amounts of wear.

Is it possible to have carbide insets on the mating surfaces?

 

[TVP]

2500CT,

The description that you provided leads me to believe that the contact stresses are high and that sliding is involved. This means that a PH stainless steel like 17-4 (Type 630) will likely be a poor choice. What is the material for the housing/internal dogs? This will have a significant effect on the material choice.

 

[unclesyd]

That's a lot of power to use this type clutch to make and break.

I've replaced a lot of clutch parts with with either S7 or D2 tool steel.
Where a large impact was involved I went with S7. Where wear was a problem I went with D2.

 

[2500CT]

arunmrao, I do not have the room to use sealed bearings. and the dogs engage quite often. I think carbide is to brittle.
TVP, currently, I am using 4140 to a rc of 52. the 17-4 was recommended to me by a machinist.
unclesyd, the S-7 might be the answer.

1 additional item I stated that the rpm is 300. The operator has been instructed to use as low an rpm as possible and to use a high lateral displacment as possible. This is believed to decrease the possibility of the faces of the dogs coming into contact. The faces are the areas that are giving me the problem.

Do any of you thick hardox or manganal could work?

 

[Fabrico]

Why are you using thin oil for such a low speed application?

Despite recent trends toward mileage, etc, there are plenty of automotive and motorcycle gearboxes that still must use 85wt to 140wt lube.

 

[arunmrao]

If carbide is brittle you can try hard facing with Co based alloys to get a good wear resistant surface.

 

[2500CT]

Fribico, the thin oil is used so that it does not resist the shifting that takes place between the shaft and the housing.

 

[NickE]

you might wnat to try a lube with a solid suspension for shock....

http://www.redlineoil.com/products_gearlubricants.asp?subcatID=20&prodID=61

There are three different grades. Super Light; Light; and Heavy.


Nick
I love materials science!

 

[Fabrico]

Something seems off if you have too much pressure for thin lube and too much drag for thicker lube.

Is the material coming from the face or the corners of the dogs? Is there full engagement each shift? Is something starting at the edges and wiping across the faces? Engagement is critical with dog teeth.

Can they be greased?

4140 should be able to handle the situation. I would review the layout and usage, and see what's really wrong.

 

[2500CT]

NickiE thanks for the link this looks real interesting. I will investigate.

Fabrico the damage is mainly on the face of the teeth. this mainly occurs when the dogs barely engage and the operator starts to rotate the shaft. This disengages the dogs violently. There is no way for the operator to know for certain that the dogs are completly engaged because the machine is used underground with just the shaft showing.

Thank ya'll for all the help.

 

[NickE]

Anybody-- What about Hadfield's steel? do you think that might help him?

Nick
I love materials science!

 

[CoryPad]

Nick,

2500CT already asked that question when he mentioned manganal.

I would think something hard would be best, so a carburized layer similar to gears should do well.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

Nick,Corypad,

If manganese steel weld deposit is acceptable then hard facing with Co alloys should be better.

Let us see how 2500CT resolves the issue. Any redesign actively considered?