F1technical.net

Another challenge faced when chasing such high revs were the connecting rod and big end bearings, which displayed their weaknesses very publicly when Nico Rosberg's failed spectacularly in Malaysia. The rods are titanium and, unlike some other manufacturers, still have separate shell bearings. Initially, the rev limit was reduced to ensure the life of the engines while the problem was tackled at the factory. The problem is ensuring the big end adopts the correct shape under load to achieve an even surface pressure across the bearing face with an uninterrupted film of oil. Extensive work on the rod design, bearing specification and choice of engine oil allowed the full 20,000rpm to be built up to again as the season progressed and confidence grew. This is indeed an impressive feat, when you consider that the piston has an equivalent mass of about 2.5 tonnes at 20,000rpm. With peak gas loads greater than 65kN, and peak inertia loads verging on 60kN, there is little wonder the connecting rods can stretch as much as 0.6mm during a single cycle.

malbeare wrote:very excellent pics.
I wonder why the end cap has that groove cut in it across the bolt hole?
malbeare

Brian.G wrote:
Titanium has a very high modulus of elasticity in comparison to lets say 4340, a common rod material, so this eccentric, or ovality machining cannot be ignored on a Ti rod. Im sure this is what they have to have done in the case of this engine, but perhaps something the naked eye would not notice, or perhaps be even thought of by just looking at my pictures.

Tim.Wright wrote:

Brian.G wrote:
Titanium has a very high modulus of elasticity in comparison to lets say 4340, a common rod material, so this eccentric, or ovality machining cannot be ignored on a Ti rod. Im sure this is what they have to have done in the case of this engine, but perhaps something the naked eye would not notice, or perhaps be even thought of by just looking at my pictures.

The elastic modulus of titanium is actually about half that of steels (like 4340). To my mind, a more compliant material like Ti would require greater ovality because it will deform more than a steel part.

Is that consistent with what you see in the Machinery Handbook?

Tim

Tim.Wright wrote:No you said Ti has a higher modulus of elasticity than steel, whereas is actually about half that of steel.

Steel is the stiffer material, not Ti.

Tim

Brian.G wrote:

malbeare wrote:very excellent pics.
I wonder why the end cap has that groove cut in it across the bolt hole?
malbeare

Im thinking to maximise contact force in those particular areas.

mep wrote:hmm ok I just keep on thinking little bit on this and write down what I get into my mind....
The cut is on the lower half. It increases the pressure in contact zone and should therefore increase deformation of the lower part when you put in the screws. There are two load cases for the connection rod.
1. The piston pushes down, con rod is in compression.
2. The piston gets pulled down, con rod is in tension

The second case is important for the lower semi shell because it gets stretched in the area where the cut is. Now this area with reduced cross-section is already compressed by the screw initial load. The two loads act against each other reducing the pressure in the smaller are and causing the hole to get into its perfect round shape during that load case.
That's my guess about it.

malbeare wrote:does the bearing shell have the same ovality of the conrod in the big end when installed? If so then would this mean that the bearing clearence is 0 at the top and bottom when the engine is not running? would it be neccessary to have a very high idle (say 5000) when running to keep the dynamic forces in play to keep the big end round when running ?