Crankshaft Balancing
Crankshaft Balancing
(OP)
Has anyone everyone heard of static balancing a crankshaft for a "V" type eingine?
Is it correct that on a 4 cylinder, you only have to match the weights of the pistons and rods to balance the engine? Nothing has to be done to the crankshaft.
What about an inline 6 cylinder where the rod journals are 120 degrees apart? Same as the 4 cylinder???
Is it correct that on a 4 cylinder, you only have to match the weights of the pistons and rods to balance the engine? Nothing has to be done to the crankshaft.
What about an inline 6 cylinder where the rod journals are 120 degrees apart? Same as the 4 cylinder???





RE: Crankshaft Balancing
No
"Is it correct that on a 4 cylinder, you only have to match the weights of the pistons and rods to balance the engine? Nothing has to be done to the crankshaft."
No
"What about an inline 6 cylinder where the rod journals are 120 degrees apart? Same as the 4 cylinder???"
Same as the 4 cylinder, you have to statically balance the rods and pistons, and dynamically balance the crank, and usually dynamically balance the assembled engine.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Crankshaft Balancing
I wonder if On your 4 and 6 examples what was intended was no bob-weights are attached to the crank when dynamically balancing it. Or when changing to lighter or heavier components the crank can be left alone.
RE: Crankshaft Balancing
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Crankshaft Balancing
RE: Crankshaft Balancing
A tour through any modern ISO engine and auto assembly plant is a real treat. I took my first in Detroit in 1970, and again in 1993, WHAT A DIFFERENCE!
Franz
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Crankshaft Balancing
Rod
RE: Crankshaft Balancing
A logical way to do this would be to start with a crank that is strong enough, and then work out the main bearing pressures throughout the speed range, and then optimise the counterweight sizes to minimise the peaks.
Then work out the torsional and bending frequencies of the resulting crank and check that they are OK. They won't be, so then you reduce the counterweight sizes a bit, to reach a compromise.
I don't know how backyard shops do it.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Crankshaft Balancing
Have I got this right?
Please note, this is a question, not a statement of fact.
On a 4 cylinder with 0,180,180,0 crank positions;
1> Overall the front and back throws will counteract the center throws.
Balancing the throws will reduce the tendency of the crank to deflect, and reduce bearing pressures.
2> The front and back pistons going down counteract the two pistons going up. If an attempt is made to balance the pistons with counter weights it will reduce bearing forces vertically but add to the forces horizontally.
3> However, added counter weights will be effective in counteracting the mass of the lower end of the connecting rods. The counterweight may be further increased to counteract the mass of the big end of the rod. Past that amount, a small amount of additional counter weight may be added to counteract the effective lateral inertia of the connecting rod shank.
Basically, with a symetrical crankshaft, and matched components, the engine will be self balancing overall, but the crank shaft will tend to deflect in the direction of the offset mass at each throw. This will add to bearing pressure. Balancing the throws and the big ends will reduce the tendancy of the crank to deflect and reduce balance induced bearing pressures.
Any comments are welcome Greg.
respectfully
RE: Crankshaft Balancing
"1> Overall the front and back throws will counteract the center throws.
Balancing the throws will reduce the tendency of the crank to deflect, and reduce bearing pressures. "
Yes
"2> The front and back pistons going down counteract the two pistons going up. If an attempt is made to balance the pistons with counter weights it will reduce bearing forces vertically but add to the forces horizontally."
Yes, vertically, I think so horizontally.
"3> However, added counter weights will be effective in counteracting the mass of the lower end of the connecting rods. "
Yes
"The counterweight may be further increased to counteract the mass of the big end of the rod. "
The big end is the lower end?
"Past that amount, a small amount of additional counter weight may be added to counteract the effective lateral inertia of the connecting rod shank."
...and some of the piston mass as well.
"Basically, with a symetrical crankshaft, and matched components, the engine will be self balancing overall, but the crank shaft will tend to deflect in the direction of the offset mass at each throw. This will add to bearing pressure. Balancing the throws and the big ends will reduce the tendancy of the crank to deflect and reduce balance induced bearing pressures."
Crank deflections are more complex than that, that is why it is important to do a speed sweep. Most 4 cylinders I have worked on have crank resonances in the running range of the engine when excited by second order (L/r) forces.
An inefficient alternative is to split the engine up into 4 coupled single cylinder engines, work out the counterbalancing for each, and then glue it all back together.
That tends to be a heavy solution as you aren't taking advantage of some complex interactions, such as the #1 cylinder being able to 'see' some of the #3 counterweight's inertia.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Crankshaft Balancing
"The counterweight may be further increased to counteract the mass of the big end of the rod."
I meant to delete that.
Thank you very much Greg for your time to respond.
respectfully.
RE: Crankshaft Balancing
RE: Crankshaft Balancing