Balancing of a roller bearing crank

[Reinhoud]

Guys,

 

For my GS1000 turbo project I need to build a stronger crankshaft, I build a press, I've got a dial indicator and a copper hammer and enough time on my hands  

so I'm going to do it myself, I already had trial run fixing my old crank, it wasn't too difficult.

 

I already figured out how to do everything, so the next thing came to mind, is it worth the effort to press the crank together without the con-rods and have it balanced?

Does anyone know if the crankshafts are properly balanced from factory? But another thing, say I press the crank together without the con-rods and have it balanced, would I fuck up the balance when I take material from the throws and weld it?

 

This is something what gets me in doubt.

 

Hopefully some has experience with this..

 

Something else; my GS750 I had in the past, and my GS1000 always had some kind of vibration when the engine is running, my mates CBR1000rr (2012) hasn't, his GSXR750 (2006) vibrates too, is this caused by an unbalance in the crank?

 

Thanks for your input

Edited April 15, 2017 by Reinhoud

[Gixer1460]

Balancing should be the absolute last operation done on ANY crank. Pointless balancing before welding and pulling it apart will lose any corrected indexing. There are always very slight variances between static and dynamic balance and ultimately you want ALL the rotating components to be together and balanced as an assembly - easier with plain bearing crank IMO as the pin index will never vary. How do you plan to measure where the variance is without the machinery?

[Reinhoud]

I know, but apparently you can't balance a crank with con-rods on it.. That's why this topic..

 

What do you mean with measuring variance without machinery?

Did you measure the indexing from a stock crank?

[arnout]

Ha.. I've tried my hand at doing some crank work myself too a while ago, and looked hard for any info wherever I could possible get it.

So I'm no crankshaft expert, but from what I understand you are right with regards to the difficulty to balance cranks with roller bearing big end conrods. I believe I once saw a video of crank being dynamically balanced with the rods fixed into place by some sort of clamps, but the method is questionable as the situation doesn't reflect the normal operation setup.

Taking the crank apart to remove the rods, pressing it back together for balancing, taking it part again to install the rods, and finally press the whole thing back together again, would in theory be the right way to do it , but it would wear the press fittings down a lot. On each press operation the fit becomes looser, and the risk of damage (like galling) also plays a part in ending up with a sloppy fitting crank. This will also require welding to keep it from spinning, but the added welding material would of course throw off the fresh balance again. Generally you want to do as little press operations as possible.

So.. As I understand it, in practice you do not dynamically balance such a crank, and just accept any imbalance there may be. You can only balance the small ends (compare weight as per manual). I've never tried a static balancing job (letting the rods hang down), but I suspect that method would be far too crude to find the small weight differences that could cause the imbalance. But just by inspecting the crank for trueness (straightness and indexing) you should be able to find any major causes for  imbalance.

These air cooled engines are slow-revving anyway compared to newer water cooled ones that are designed to make power at a zillion revs. So there is less need for a perfectly balanced crankshaft on older engines than there is on modern ones.

[Reinhoud]

It's very hard to find information about this.

What you say about pressing and getting sloppy I already knew..

 

So, best thing is just to leave it like it is?

 

 

[nlovien]

the section on dynamic balance alludes to the reasons for a machine  - my very basic understanding = static is fine but is limited to seeing the assembly as a whole  - like your presuming all the mass is on one and the same vertical axis, but it isn't, its distrobuted along the horizontal axis of the crank  - and the distance between say two mass's that statically balance induce's what is called the rocking couple and the further apart, the more this has an effect ( hence why static balance works better on a single versus a 4 cylinder ) - anyway, let a man of knowledge better explain it - can't attach the pdf but the link to it is https://www.google.co.uk/search?q=4+cylinder+crankshaft+balancing+principle&oq=4+cylinder+crankshaft+balancing+principle&aqs=chrome..69i57.12468j0j7&sourceid=chrome&ie=UTF-8

Dynamic balance. Figure 2a illustrates the concept of dynamic imbalance. The object shown is clearly in static balance, because the moments of the two masses balance each other about the spin axis as in figure 1a. The offset of the two masses along the spin axis gives rise to what is often known as a rocking couple. As such an object rotates, the orientation of this couple also rotates, attempting to move the axis in a conical manner. To achieve dynamic balance, we must rearrange the masses such that the rocking couple disappears. In practice, this is done by either removing material from the object, or sometimes by adding material. Figure 2b shows how the addition of two extra masses can achieve dynamic balance, which also guarantees that the object is statically balanced. The idea of the possible generation of a rocking couple is very important to the subject of engine balance. 2a. 2b. Fig. 2 a. Each of these two masses create centrifugal forces, and because of their separation along the axis or rotation these forces produce a rocking couple. b. The addition of two opposite but otherwise identical masses is one way to remove the couple and restore dynamic balance.

Edited April 18, 2017 by nlovien
added info

[Gixer1460]

Yeah - what he said LOL!

Arttu - some of these AC engines rev 'slow' - but check out the ProStock motors 14k+ but they tend to only last 6 passes - how about 1.5 miles and maybe a minute total run time and $5-8,000 cost!