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rhpope (Mechanical)
12 Apr 06 14:32
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???
GregLocock (Automotive)
13 Apr 06 0:26
"Has anyone everyone heard of static balancing a crankshaft for a "V" type eingine?"

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.

Tmoose (Mechanical)
13 Apr 06 12:54
A reasonable job can be done statically balancing a V twin, like HD, but a bob-weight must be attached to the crankpin to simulate the rods and pistons.  Multi-cylinder Vs must be bob-weighted based on the actual rod, piston, etc weights.

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.
GregLocock (Automotive)
13 Apr 06 18:51
Just thinking about it, maybe other companies no longer balance the assembled engines - a lot of engines are not run before installation.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

ivymike (Mechanical)
13 Apr 06 22:34
The company I work for doesn't do any balancing during or after assembly.  Nonetheless, all engines are run for a couple of minutes before shipping.
franzh (Automotive)
14 Apr 06 8:15
Agreeing with Greg AND Ivymike, modern production tolerances are guite good, often better than slightly older vehicles (25 years ago) with manual balancing pre-assembly).  This tightly maintained production tolerance during casting, machining, and assembly can produce a final product that may never see its own power until it is started from the end of the assembly line and frequently directly loaded on the transporter.  All fluids and refrigerants are loaded under pressure in predetermined amounts.

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

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evelrod (Automotive)
14 Apr 06 13:05
One of my part time sponsors is a balance shop...I've had a tiny bit of practical, do it yourself, experience there.  My 4 cyl cranks are normally not static balanced with the possible exception of one special case where we were changing the size of the counterweights.  I do balance EVERY crank that goes into any of my race engines to closer than OEM tolerance.  As to the 90 degree V8's---I was told that no static balancing was done but it is common practice with race cranks to "overbalance"...ie, to actually make the bob weights slightly heavier than the formula would indicate (the rotating wt plus half the reciprocating wt).  I am not sure where this 'fudge factor' originated (I don't build V8's), but it appears to be commonly practiced at other balance shops. The 'overbalance' is to 'smooth' out vibrations at extreme rpm and is part of the last of the 'black arts' of engine building. No one wants to tell me 'exactly' how the overbalance is calculated.

Rod  
GregLocock (Automotive)
14 Apr 06 20:15
You can run any amount of crank web balance, from zero through 100%  to as much as you like. The more you add, the smaller the peak loads the main bearings will see (roughly).

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

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waross (Electrical)
14 Apr 06 20:47
Hello GregLocock;
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
GregLocock (Automotive)
14 Apr 06 21:01
Bearing in mind that I've never done this by hand, I'm just reporting the results of other people's work on the computer:

"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.

waross (Electrical)
14 Apr 06 21:18
My mistake, sorry;
"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.
Northstarlc3 (Automotive)
4 May 06 0:33
Do I need to balance connecting rods in a straight four cylinder engine end to end, or can I just make the total weight of each connecting rod the same?
Tmoose (Mechanical)
4 May 06 8:15
Magnaflux rods and crankshaft before considering them for re-use. Always balance end for end.  When removing material from connecting rods keep all grinding and machining marks running lengthwise, not crosswise.  Don't remove material that reduces the "footprint" at the parting line, lest extra bolt bending and big end distortion will result.  Pre-balancing rods before machining or reconditioning will result in a better job.

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