Dissimilar metals and lubrication. 1

[mgb23]

Hello, I'm new to the forum. I want to design a gear set. One gear is 4140 steel, one will be a grade 2 cast iron. I do not want to lubricate the gears. Lubricating the gearset becomes a hygeine issue with the lubricant attracting dust particles. The bending stress is 1100 psi. The contact stress is approx 41000 psi. Needless to say, these gears are severely overdesigned from a power transmission standpoint. How do I determine if these gears require lubrication? Is there a way to calculate it? Thanks,
Mark

 

[israelkk]

To my best knowedge All gear calculations according to the respecteive specs (AGMA, ISO ,DIN etc.) assumes appropriate lubrication they are not valid for dry gears at least for contact stress, pitting scoring etc. One of the design methods even uses the oil flash point which checks that the oil temperature is not beyond the oil flash point because than the oil looses its lubrication ability.

 

[diamondjim]

Your stresses are low. If your teeth
are ground and of a very high quality
and recessive type gearing you might
get by without lube. Erichello is one
of his seminars spoke of one customer
who ran gears without lubrication and
had no problem. They were very high
quality gears. You do not mention rpm
or life expectancy. What quality are
you specifying for these gears.
Will you run them in before installing
at final site?

 

[Philrock]

I assume it's the pinion thats 4140.

Your gears are not severely over-designed in terms of contact stresses. 41,000 psi is in the realm of typical contact stresses for cast iron. I'd say your chances of success without lube are not great.

If you really have to run metal gears without lubrication, consider a special coating or impregnation with something like molybdenum disulfide on one or both gears.

If possible, it's always a good idea to enclose gears to protect them from contamination. Enclosing and lubrication will probably be a lot easier than developing gearing that will run dry in a contaminated environment.

 

[diamondjim]

Philrock,

Many ball bearings are run at 375000psi contact
stresses in life test situations but with lube.
I would assume that line contact stresses
of 41000psi would be low. Of course the races
and balls are in the 63Rc hardness range. If the
pinion 4140 is surface hardened to 54Rc or higher,
I would think that it would be ok. If the pinion
were long addendum designed, I think it would
help reduce the contact stresses.

 

[mgb23]

Diamondjim, both gears have the same number of teeth,27. The 4140 material would be hardened to 54 Rc. The cast iron I mentioned would be made by Durabar. The cast iron has contact stress limit of 75,000 and is impregnated with graphite. These gears run 858 rpm at top speed. We are expecting these gears to last 70,000 hours minimum at 24 hours a day. Gear quality would ideally be Q10 or higher but delivery could be an issue. Would like to stay with Q8 for simplicity sake. An enclosure would present design problems since the rolls in which these gears attached allow complete disengagement of the gear train. Any more opinions? You guys are great!

 

[diamondjim]

I think I would still go with the recess action
type gearing with the pinion addendum being
50 percent long and the gear 50 percent short.
I do not know if you need the direct 1 to 1
ratio. I would spec the greater accuracy in the
pinion if possible because I assume it will wear
in the gear. Regardless of what you use, please
keep us posted. I assume you will adjust the gears
or set them up taking into account their runout
parameters or runout signature, setting the highs
with the lows basically.

 

[diamondjim]

After some considerable reflection, you might
want to only go with 25 percent long and short
addendums. 50 percent is too much for the
small number of teeth. I have not run these
on a gear program but should take a look at
these profiles to look at their respective
sliding velocities. I seldom work with 1 to
1 ratios like your design.

 

[KnifeKnut]

Militech-1 might solve your lubrication problem needs. It is not a lubricant, but it reduces surface friction between metal surfaces. Unoffically, Infantrymen love it for their guns as an alternative to no lubrication at all on rifles as it does not attract dust in the desert.

 

[tbuelna]

mgb23,

lubrication in a gear train serves two functions:

-reduction of friction at the contact point (a dry film lubricant can fullfill this requirement).

-transfer of heat away from the gears, due to to mechanical losses. Even with a very well designed spur gear set, you should assume that you will need to be able to dissipate at least 2% of the power being transmitted thru the gear mesh as heat losses. If you have something like a worm gear set, the heat rejection can be as high as 40%.

Thus, you should review your design from a heat transfer standpoint.

Good Luck.