Best Material for gear? 1

[nittany77]

We need to make a timing gear for an automobile engine. What tool steel would be the best to use for this application?

 

[DAVIDSTECKER]

Since the timing chain is already a hardened steel, I'd reomend you look to ductile iron for your gear.
Dis-simular metals and hardness' give a longer wear life and the iron has graphite which is a lubricant.

 

[diamondjim]

You might want to consider 4340 or 8660
steels which have nickel in them and give
you good ductility as well as strength and
good cold temperature properties.

 

[nittany77]

Actually, this is a very old engine and the gear will be running against a phenolic gear, not a timing chain.

If we use the 4340, how hard should it be heat treated to?

 

[mariusdoc]

The optimum hardness for any gear submited to normal stresses is between 28 - 32 HRC, because in this domain, you will have troostite and bainite as the main structural components. However, the best durability for a gear can be obtained by gazeous or plasma nitriding (but it is not cheap!).

Vlad Marius Dorin, UNIMET-CUG Co., Cluj-Napoca, Romania

 

[Metalguy]

Nittany77,

I am curious about why you need a new steel crankshaft gear. I've seen old Chevy inline 6 and 4 cyl. engines with failed phenolic cam gears, but the steel crank gear always looked fine.

In any event, a case-hardened 8620 gear should work fine.

 

[diamondjim]

If the mating gear is phenolic,
the loads cannot be very high.
Almost any tool steel should
be sufficient. I too assumed
initially you were using a
metal chain. Even a normalized
steel should work in this
application.

 

[rnd2]

The wear resisting mechanism for regular re-enforced phenolic is the inter-polishing of sliding surfaces either dry or with oil or grease lubrication. It is better not to use hardened steel.

 

[nittany77]

The gear is not worn, but pitted. The tooth shape is fine for the most part, the original gear appears to be made from a casting, and there are many pits showing in about half of the teeth.

This is a very early Rolls Royce engine, the original gear is some type of hardened steel, but I don't know what the alloy is.

Would 8620 be better than A2 or D2? Is low 30's HRC enough?

 

[rnd2]

Is low 30's HRC enough?
Yes.

 

[TVP]

I agree, 30's HRC is sufficient for this type of low load application. Tool steels like A2 and D2 should not be used.

 

[nittany77]

So would 8620 be the best choice then?

 

[rnd2]

Ahh, best choice would be a stainless steel, preferably with Cr greater >12%. Any 3XX stainless works really well because a high polish is developed that is less subject to moisture driven corrosion than regular mild steel. However, if the sliding surfaces are known to operate where corrosion need not be taken into consideration, regular common or garden mild steel works just fine. We have phenolic/mild steel bearing/shaft combos in service now for more than 20 years and all they get is a shot of grease when someone remembers to do it.

 

[metman]

I would not expect corrosion to be a problem in this application. Therefore I suggest 41L40 quenched and tempered to 32-38 HRC. It will easily respond to this heat treatment and since it is a low stress applicaion, the negatives of leaded steel can be ignored. So then you have a free machining grade which is really nice for a machining intensive part like a gear.