Jabiru crankshaft balance

[VFR Pilot]

Has anyone had any issues or experiences with crankshaft balance in their 3300 engine

 

I am thinking about getting my new cranks balance checked

 

 

[AVOCET]

What's the history with the engine?

 

 

[VFR Pilot]

The engine is brand new, but I have been told other people have found that their cranks have been out of balance all along and when they get them balanced they can't believe how smooth their engines run ?

 

I wondered if this could be one of the reasons for through bolt failure. I just want everything to be right when I install it into the airframe.

 

 

[facthunter]

The level of forces caused by a bit of imbalance are nothing compared with what is going on down there. Having said that I understand that there could be a significant improvement in "feel" of the engine and all vibration isn't helpful to things ( engine mounts etc.). The "six" is a smooth engine and will be smoother if balance work is done. Make sure they remove metal in the places that don't concentrate stresses in the shaft. An engine feeling smooth doesn't indicate that it won't blow up, but if vibration develops it should be investigated immediately ( if not sooner). Nev

 

 

[jetjr]

I have had very nice result with dynamicaly balancing prop

 

Could be related to inherant crank balance tolerances as prop is well made carbon type which shouldnt have much balance problem. Also cheap and easy process.

 

In RSA they have a workshop offering some sort of Kiele treatment which sounds like balancing is a key part of it

 

 

[facthunter]

If I had a motor sitting on the bench in pieces I think I would be inclined to have it done. Even though they are CNC'd I have heard there is some left over out of balance in the crank. It should also be trued at the same time.( obvious really).. Nev

 

 

[AVOCET]

Apart from crank balance , i found that a poor fuel distribution can feel like something's out of balance . Changing to multipoint fuel injection made a huge improvement in smooth running ,

 

Also balancing the prop on the engine ( dynamic balance) also takes pistons , rods , and every thing else that moves into account,

 

Pulling down a new engine and only doing the crank only does half the job ,

 

Cheers .

 

 

[facthunter]

That's a bit funny Avocet . If you correct everything with the prop why would that be OK?

 

You hopefully do ALL moving parts. The crank dynamically as it's long. Each conrod end and other end and total, each piston assy total. Flywheel if it is thin enough, static, and prop if it tracks well,and accurately crafted, static. Should be good enough for what we use. I know people do what you are talking of but it can't be the ultimate job Nev

 

 

[AVOCET]

That's a bit funny Avocet . If you correct everything with the prop why would that be OK?You hopefully do ALL moving parts. The crank dynamically as it's long. Each conrod end and other end and total, each piston assy total. Flywheel if it is thin enough, static, and prop if it tracks well,and accurately crafted, static. Should be good enough for what we use. I know people do what you are talking of but it can't be the ultimate job Nev

 

That's true , I agree , I would balance all the bits for a total job , but balance is balance , if you carnt get the prop in balance , dynamically , that will show somthings amiss , when the props balanced with the engine runing doesn't that also bring every thing else into an overall balance ?

 

I could be totally wrong here , it wouldn't be the first time .

 

Cheers Mike .

 

 

[bexrbetter]

when the props balanced with the engine runing doesn't that also bring every thing else into an overall balance ?

Yes but not for the right reasons but it is better than nothing.

 

I always balance components individually then final assembled balance of crank and anything that hangs off the ends again all together (flywheel, front pully etc).

 

The engine is brand new.

You wouldn't be the first to pull a brand new engine down and have everything balanced, if you got the time and inclination then it's worth doing.

 

 

[VFR Pilot]

I think I might as well do it at least I will know everything is right then.

 

Can someone pm me and let me know of a good company in Oz who can do the balancing

 

Cheers

 

 

[Bruce Tuncks]

What is the theory of balancing just a crankshaft? I would have thought that balancing the whole thing, complete with prop, made more sense. For example, I can imagine the big ends being of different weights and these weights cancelling or exacerbating crankshaft effects.

 

And how much does it really matter anyway? One day I was balancing my prop when a mate pointed out that in most flight modes, one prop blade has several degrees more angle of attack than the other. And he was right, it is only when the oncoming airflow is exactly perpendicular to the prop disc that both prop blades pull the same. Most of the time this is not so. This effect gives kilograms of "unbalance".

 

But I still reckon that a dynamic balance is a good idea. In fact I tried to get up a syndicate at Gawler to buy a dynamic balancer, but nobody else wanted to be in it. I have to admit these other guys are not falling out of the sky, so maybe I'm being fussy about balancing.

 

Bruce

 

 

[jetboy]

I have had Continentals dynamic balanced and that was a worthwhile thing, recommended if a static balanced prop has any vibration - such as rudder pedal, instrument shaking etc. I've only flown in one Jab aircraft that in my opinion was out of whack, the owner took it to a dynamic balance outfit but they said it was in limits. I dumped my Jab prop of 200 hrs because one blade had a degree more pitch than the other.

 

You say the engine is brand new. Exactly how old is it? I would be reluctant to open up an engine and remove the crank just on suspicion that your one might have missed being balanced in the first place, passed thru the dyno run and sat in the crate.

 

If its an older one (without the roller camfollowers), then there are many and various compulsory SB which need doing for the privilege of putting a spanner to it. In many cases such work becomes uneconomic, if not seriously questionable.

 

Then there is the other matter, which way do you refit the pistons? the Jabiru way, or the Holden way? According to some, who have reversed the piston offset on more than 1 engine, its noticeably smoother.

 

Your choice, but go for the prop first, the more times you strip a Jab, the more likely it has used up all its 'cycles'

 

Ralph

 

 

[Garry Morgan]

You will need to balance the pistons ,flywheel conrods etc. but will most likely do more good with a well balance prop

 

 

[facthunter]

The principle of correcting engine balance only at the prop is flawed. You are putting an out of balance force in one place to correct one in another. Whilst it might work in the field and feel better "feel' is not all you are after. The original balance on a jab crank may rely on the accuracy of the cnc machining. IF it was done extremely accurately it would be close to perfect balance. Most crankshafts are forged with less dimensional accuracy so require balancing. You can't static balance a crankshaft because of it's length. Nev