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post #1 of 17 (permalink) Old 03-01-2002, 09:32 AM Thread Starter
 
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Function of counterbalancer

Some engines have a counterbalancer to help reduce vibrations. What produces the vibration that a counterbalancer is trying to reduce?

Last edited by Andy; 03-01-2002 at 09:49 AM.
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post #2 of 17 (permalink) Old 03-01-2002, 01:39 PM
 
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The 4 cylinders are usually 180 degrees out of phase, 1 and 4, then 2 and 3. Because they are not firing in phase, or 360 degrees out of phase, there is some inherit vibration. The counterbalancer is wheighted opposite of the firing sequence to reduce vibration.


that it?
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post #3 of 17 (permalink) Old 03-01-2002, 04:08 PM Thread Starter
 
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Sorry Kevlar, I think you got it wrong.

The added weight (Fig. A) on the crankshaft cancels the vibration caused by the piston moving up and down.
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post #4 of 17 (permalink) Old 03-01-2002, 04:35 PM
 
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A counterbalancer has nothing to do with the pistons. A crank with a big weight on one side will create its own imbalance.The piston's up and down motion is offset by the crank. But the crank has an imbalance that is front to back, so a counterbalancer is used to offset the crank's weight. I think I am right, but its hard to describe.....
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post #5 of 17 (permalink) Old 03-01-2002, 04:37 PM
 
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What I mean is that a counterbalancer is used to offset crankshaft vibrations
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post #6 of 17 (permalink) Old 03-01-2002, 04:40 PM
 
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Quote:
Balance shaft A shaft designed so that, as it rotates, it vibrates in a way that reduces or cancels some of the vibration produced by an engine. Not essential to an engine's operation, balance shafts are nonetheless becoming increasingly common as a means of engine refinement. Balance-shafted four-cylinder engines use two shafts turning in opposite directions on either side of the engine's crankshaft. A single balance shaft is used when fitted to three-cylinder and V-6 engines.


The vibration comes from the combustion pulse of the engine. On a 4 cyl its every half crankshaft revolution. On a V8 its every quarter rev.

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post #7 of 17 (permalink) Old 03-01-2002, 05:18 PM Thread Starter
 
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You got right ninjabeater!

I always thought is was to reduce the piston vibrations also. But a counterbalancer reduces the crankshaft vibration in the X axis.

Here is the link:

http://www.offroadhaven.com/counter_balance_design.htm

Andy
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post #8 of 17 (permalink) Old 03-03-2002, 10:11 PM
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The added weight to the crankshaft at the opposite side of the piston is there to offset the weight of the piston mainly.
The counterbalancer shaft is there to reduce the Harmonic Vibrations mainly.
Don't confuse the two, every crankshaft has counterbalance weights, but not all engines have a counterbalance shaft!
If you ve ridden a single without a counterbalance shaft, you will understand better
The article makes a very simplistic assumption that the only source of vibration are the reciprocating parts.
Truth is the most important source of vibration is the extremely high pressure at the top of the piston at TDC! This is the cause of Harmonic Vibrations (of many orders!) that are a nightmare to calculate and reduce in order to manufacture an engine that can work! Configuration and firing order are crucial for the design and the vibrations handling.

The inline 6 has an advantage, and the V2-90deg (like Ducati) has another advantage. What are the advantages of these engines?

(tell me if I overdid it here , I couldn't help it because my engineering spirit woke up!)

Aris
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post #9 of 17 (permalink) Old 03-04-2002, 09:47 AM
 
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A 90 degree twin has perfect balance of the pistons because they are moving opposite of eachother. They do not need a counterweight on the crankshaft.
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post #10 of 17 (permalink) Old 03-04-2002, 05:09 PM Thread Starter
 
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No Aris... you did not over do it here... I'm glad you make us think about physics and engine design!

Here is some information as to why the inline 6 is such a good design regarding balance.

"As I understand it, in simplified but essentially correct terms, due to the mass of the piston moving up and down in the cylinder, the engine block wants to shake up and down (i.e. along the cylinder bore axis); this is the primary or first order force shaking the engine. Due to the mass of the connecting rod swinging from side to side, the engine block wants to shake from side to side; this is the secondary shaking force.

From what I know at this point, it is these two forces which cancel out in a spinning - BUT NOT FIRING - inline-six, flat six, or V-12. By "cancel" we mean that the interrelated motion of the various pistons and connecting rods act in such a way that no net force acts on the engine block to shake it. As one piston moves down, another moves up; as one piston rod swings left, another swings right. Net result: no shaking force in either the axial direction (along cylinder bore) or at right angles to this."

Link source: http://www.zhome.com/ZCMnL/tech/harmonics.htm

Here is a picture of a inline 6 crankshaft...



And here is a front view of the same crankshaft...



From what I gather, if you look at 2 & 5 the rods big end would be coming towards you, while 3 & 4 the rods big end are moving away from you.

Andy
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