Question about connecting rods in V-engine

[Adaware]

Question about bearings, do i need some kind of bearing between the two rods or is the rods themselves sufficient when oil gets in between?

[dynosor]

Question about bearings, do i need some kind of bearing between the two rods or is the rods themselves sufficient when oil gets in between?

You need to start with a nominal clearance between bigend bearings, even when allowing for all the tolerances between parts in the assembly.

If the rods should float into each other, the contact pressure should be very low, so just oil should work. Now the question is what material are we talking about. If aluminum, there is still some risk of galling. This risk is significant if the part relationship is designed as "line to line". Steel should be more forgiving, but should still be designed with a nominal clearance.

[BobWarfield]

That fork and blade style can be seen in this radial aircraft engine:





Cheers,

BW

PS Adaware, do you own AdAware, the anti-spyware software co?

[Adaware]

What do you mean by nominal clearance?

Don't have anything to do with the spyware company, just a random name i came up with when i registered.

Nice engine!

[dynosor]

What do you mean by nominal clearance?

Start by considering the maximum material condition: conrod bosses at maximum thickness.

Then look at the how the cylinder centerline positions would be controlled - there is always some uncertainty for placement - add this value to the "space" requirement.

Design a deliberate minimum clearance between conrods of perhaps 0.2mm; add the tolerance on conrod boss thickness (actually half x 2); then add clearance based on how accurately you can place the cylinders sharing the crankpin; then add clearance for the tolerances and clearances between piston and conrod; then add tolerances and clearances between piston and bore (cold).

You will probably end up with a maximum clearance of 0.5mm between conrods in this example because of the tolerances and clearances between all the parts involved, but that way the conrods wll never come closer than 0.2 mm.

If you deem 0.2 mm minimum too generous make it 0.1 mm minumum - do your sums from there.

Remember that the tolerances over the length of the engine between cylinder 1 and 4, and 5 and 8 need to be approached in the same way when considering the crankshaft journals and their clearances - you don't want anything in the engine to be loaded axially along its bearing because it is being forced out of its nominal position. Causing the conrods to bend or run at an angle should be avoided.

That said, the crankshaft does need some sort of axial control or thrust bearing.

[greg b]

Check out the harley davidson design con rod,although its a press apart roller bearing set up you might be able to get some ideas,these things when amped up crank out some good horse power and seem to still hold together.regards greg

[Adaware]

Tried to find pictures of the rods you were talking about but no luck, help me?

[stevehuckss396]

Tried to find pictures of the rods you were talking about but no luck, help me?

Here

[greg b]

This is the con rod i was talking about,if you google up harley davison conrod design you should find some good imformation,regards greg.

[youngjim]

That fork and blade style can be seen in this radial aircraft engine:


r
Cheers,

BW

A master rod arrangement like this is necessary in a radial, although it is not a fork and blade as only the master rod is connected to the crank journal, as in a v-type engine. See here about 1'03" for a good shot of such an arrangement. "http://www.youtube.com/watch?v=CVu6vjPvxkg"]YouTube - Rotec Radial Engine R3600 150hp

This is because there is only one crank arm for however many cylinders (9 in each row on this engine, a Pratt & Whitney R-2800 I believe) so one cylinder's rod gets attached normally, and the others are attached to bosses on that master rod.

You could do a master rod in a v-type engine, but the geometry for the opposing cylinder would be very unusual, and that would make tuning difficult. It might make an interesting sound running though!

I know what a pair of R-2800s at full throttle/full fine make going overhead about 200', that is the best kind of scary!

Jim