connecting rods materials

[_mark]

hi all.

I need to know on which base a projecter of a connecting rod can choose steel or alluminium for material in which is made.
Taking account the fact that the major stress are those due to inertia loads, the 2 materials have same performances:
σ / ρ = between 100 and 200 for both alluminium and steel alloys (berillum 300)

So connecting rods in alluminuim and steel have the same shape and volume for the same performance under inertia loads.

Secondary effects are those related to axial stresses due to gas pressure (here steel takes an advantage because of his major yield stress) and those due to vibration trasmission on the ground and bearings (alluminum takes an advantage due to his lower ρ)

thanks to who will help me

[Apex]

Not sure what you mean by a 'base projector', please explain.

Ali is great when in bending, and thats about it... A conrod sees a lot more axial force than bending forces.

What i can tell you is that you must consider a lot more than just the stress : density ratio.

Here are four other things to consider:

1 - In this case I feel Youngs Modulus is probably the most important material property, this is the ratio of stress / strain. Steel has a modulus of + 200 GPa, where ali has a modulus of + 70 GPa. So for a given stress the steel conrod would enongate / compress only a third as much as ali. When you consider the tolerances (valves and all baby) inside the combustion chamber you must keep the dimensional integrity of all components.

2 - Steel can be forged which increases its strength significantly.

3 - You must consider fatigue. It is possible to achieve infinite life with steel and not ali.

4 - What temperature will this component encounter?

Hope this helps

[Ian P.]

Just one choice for now in F1.... Titanium.
Same modulus as aluminum, similar strength as steel and no fatigue limitiations.
Desnity is 1/3rd of steel (same as aluminum).
Berilium and carbon fiber, both legitimate and attractive choices, have been effectively banned as far as I understand.
Old Triumph 650s had aluminum con rods. Production Honda NSX has Ti con rods. Virtually all other road cars use high strength steel.

Ian P.

[m3_lover]

I think the Infiniti Q45 the flagship luxury sedan has connecting rods and valvles made out of titanium

[Ciro Pabón]

I've read that the problem with titanium con rods is that it has only half the elastic modulus of steel, so deflection of crank end has to be considered. Unless you make your crank of titanium, too.

[rizo]

main problem with titanium is that even if it doesn´t oxidise at low temperatures it does at high temperatures.

[Carlos]

Here is an article commenting on the merits of aluminum, steel and titanium connecting rods.

http://www.hotrod.com/techarticles/choo ... ting_rods/

Edit - IMO aluminum is the most elegant material for connecting rods.

[riff_raff]

Where to begin?

"What i can tell you is that you must consider a lot more than just the stress : density ratio"

I believe the correct term is specific stiffness, the ratio of E to density. From that standpoint aluminum, titanium and steel are all about equal. Beryllium is the best metal for specific stiffness.

"Steel can be forged which increases its strength significantly"

Aluminum and titanium also forge quite nicely. Forging does not increase strength, it improves the metal's grain structure. Which allows a higher fatigue limit.

"Just one choice for now in F1.... Titanium.
Same modulus as aluminum, similar strength as steel and no fatigue limitiations."

Titanium has a M of E 55% that of steel. It's density is also about 55% that of steel. So they're fairly equal from that standpoint. But titanium (like alloy steel) definitely has an endurance (fatigue) limit.

The fully reversing, high cycle loads encountered in an F1 engine preclude the use of aluminum for con rods. Aluminum con rods are only used where loads are very light (lawn mower engines) or life is very limited (drag race engines). Titanium is very expensive and difficult to machine, so it is only used where cost is no object (racing engines).

[Dantheman]

Would bearings need to be used in the big and small ends on aluminium conrods?

[Carlos]

Yes they would - bearings, different types for different purposes - something I haven't thought about in a while - this is an interesting article:

http://science.howstuffworks.com/bearing.htm

I remember a 2-stroke with needle bearings for the connecting rod ends and a compound crankshaft - interesting technology.

[Dantheman]

But I thought aluminium was a good bearing material. If a conrod is made of aluminium then I presume seperate plain bearings wouldn't be needed.

[Apex]

Aluminium is too soft to use as a bearing material.

When you start the engine there is no oil pressure and you have metal on metal contact, the ali will just get eaten up!

[riff_raff]

"Aluminium is too soft to use as a bearing material."

Not true. Aluminum makes a very good bearing for some applications. It's inexpensive, but it has limitations with regards to fatigue life.

However, for high performance engine journal bearings, the typical material is a steel shell with a tri-metal overlay. The tri-metal overlay is usually a copper-lead alloy, a nickel intermediate barrier layer, with a soft lead-tin overplate to give the bearing surface some embedability.

The modern, mass produced tri-metal bearing shell is truly a masterpiece of engineering. Especially when you consider the loads and operating environment it survives in!

[adamo]

[quote="Ian P."]
Berilium and carbon fiber, both legitimate and attractive choices, have been effectively banned as far as I understand.
quote]

Do you know where i may find more information on the use or possiblility of carbon fibre conrods?

thanks

[ss_collins]

Theres a full article on F1 con rod design going into RE soon