As I said at the start of this debate a 911 crank can be simply balanced as a rigid rotor and no 'bob weights' are needed.

I also agree that individual cranks need to be balanced due to manufacturing tolerances.

The discussion with regard to 'software' used on balancing machines is interesting.

Back in 1965 I would think that most balancing machines used soft pedestals and inductive transducers and simple analogue instrumentation.

The skill of the operator is of paramount importance when using this type of machine.

The introduction of hard pedestal machines (Hines Industries?) has helped with automation andthe current generation of production line balancing machines are almost fully automated and even carry out correction within the balancing cell.

The standard of balance of a modern production engine is way beyond the measurnment capability of some of the older soft pedestal machine that I have seen used in some engine shops and I am sure that if the balance was changed using this tyoe of machine it would only get worse.

I am not sure that I agree that counterweights are not needed on a 911 crank.

I could agree that the inertia torques balance at all of the even orders but what about the gas torques which can be either odd order of half order?


To look at the torsional behaviour of a crank you would need to analyse both torques and superimpose.

Counterweights would counteract the higher and odd order torsional vibrations generated by the combustion process.

These torsional vibrations would not be measured using accelerometers but would eat into crank fatigue life.

To measure this effect you would need to equip a dyno with a key Phasor or similar instrument to measure speed change with an rev.

Sorry to drift off topic but I do believe counterwights are important in prolonging the life of 911 cranks at high rpm.

I am still certain that the crank can be balanced without bob weights and that its inherent balance is good.

I also agree that manufacturing variation means that cranks must be individually balanced and that if a crank has an unknown history it would be good practice to have it checked.

The manner in which is is balanced in after market shop is interesting and in my experience quite unsettling.

My local 'shop' has an old Avery Soft Pedestal machine with analogue electronics and could never be calibrated to better than G6.3.

They don't carry out any checks when they vary crankshaft weight andd use all of the same settings.

it is common practice for this shop to belance cranks from modern Ford engine produced at Bridgend and balanced on a fully automated system (Schenck - I believe) that cost many millions of dollars and is very advanced.

Using soft machines and analogue electronics does need operator skill and I am sure the early cranks were balanced using this type of machine.

The subject of counterweights is interesting as is the firing angles is interesting but maybe a little off topic.

There are two types of vibration that are intersting

Lateral vibration and torsional vibration.

Lateral vibrations are bad from a 'shaking' and main bearing point of view and are significantly affected by dynamic out of balance.

Torsional vibration is affected by the interia and gas generated torques and will be influenced bt the mass-elastic behaviour of the crank.

Torsional vibrations tend to eat up fatigue life and casue cranks to crack and fail.

In general balancing will not help with the torsional vibration of a crank.

Whist I agree that a 911 crank is well balanced from an inertia perspective and that counterweights have little influence in this behaviour the superimposed gas torques will make a significant contribution to torsional vibration.

There will be affects at odd and half-orders due to combustion.

To model this behaviour it would be possible to consider the engine as 3 hosizontally opposed unit connected by shafts of a known stiffness.

The gas torque could be superimposed onto the interia torque and the torque examined.

I have thought for some time that the counter weights are designed to deal with gas torque related torsional vibration and removing them could be an issue for high rpm race engines.

Yes, and we can correct first order issues with proper balancing. Second order issues are addressed in four cylinder engine with shafts rotating at twice the engine speed. Third and higher harmonics are deemed "not practical" to design out.

As someone above said, even a cylinder, which signifies very symmetrical rotor, would need to get balanced for high rpm operation. A good approximation of a cylindrical rotor is a gear rotor. Yes we balance those, and consider that the machines to make the gear teeth need to keep a higher class tolerance range than turbine rotors. Granted those live at high speeds 24/7.

Perhaps the imbalance in combustion forces from cylinder to cylinder far exceeds any mechanical crankshaft imbalance. Just something to think about.

Yes, you need to balance for manufacturing tolerances, but balance shafts in inline fours make up for the inherently poor balance. It is quite practical to design out all the inertial harmonics- just make it a boxer motor.

It would seem to me that you would want less counterweight if the torsional stiffness of the crank was a concern. That would raise the natural frequency, right?

It's very small, less than 5.0% increase. It's more sensitive to journal diameter, overlap and crankshaft length.

I agree that this sounds reasonable but it all depends on the details.

The high order vibrations will be quite small in terms of amplitude at least until they reach the natural frequency, which I agree could be reduced by the increase in inertia.

If the vibrations are absorbed by the counterweights then there will be no forcing function to excite the crank and it may run quite well.

If you take away the influence of the counterweight then although the natural frquenecy may be a little higher the amplitude of the vibration could be greater and hence more damaging.

Lycoming and Continental Flat 6 Aero engines have tunable conterweights to deal with high odd order harmonics (5th typically).

The counterweights on a 911 crank are relatively small and we are just drawing a 906 style crank now and will try to estimate the inertia of the counterweight on this crank to see what influence it has on behaviour.

Aero engines need to keep the prop excitation away from the crank Fn. Different deal.

Porsche discovered a recent need for add'l crankshaft balancing. This via an external vibration dampener on late model 911s:

Vibration Dampener: M97.01 (item 29)
http://forums.pelicanparts.com/uploa...1369424027.jpg

Sherwood

Since we're talking cranks and we mentioned John @ Costa Mesa, I thought this video subscription I got today would interest some considering we're talking cranks. I found it pretty wild that John took a 3 main crank and made it a 5 main.

5-Main Crankshaft for the Fiat 850 - YouTube

Would be interesting to hear how he has to balance it after making that modification. I understand that he's not adding any "throws" by creating two more main support journals. But it still has to be balanced after a modification such as that!

Interesting idea...but it would depend on the crank and case being able to accomodate more bearing surfaces.
It would help in the RPM of the engine (holding the crank to a more precise plane)....but unless our engines have a problem with RPM...I don't see the need for the mod.
Also...the FIAT engine is a straight four...which is inherently out of balance.
Bob

So could I think of it as making several flywheels - one for each rod, that help to smooth out the torque that reaches the shafts connecting the "flywheels" (throws+counterweights)?

I think you need to eliminate the torsional vibration from the engine to prevent exciting the propeller but I would agree that propeller induced vibrations would be a significant issue and would also need to be avoided.

I only used the Lycoming example to illustrate the potential for problems due to gas torques which would tend to be high order.

Propeller Vibration: Issues, Examples, Consequences, by EPI Inc.

I think the several 'flywheels' analogy is reasonable and we need to think of them as absorbers and not dampers.

Jack Kane of EPI is quite a resource Chris you indeed use excellent resources.

Regards

They discover this periodically, since they are constantly changing displacement. When the main bearing and the rod bearing move further apart, the crank gets more twisty (torsional stiffness is lower). The same thing happened when the 3.6L 964 engine came out. The stroke went from 74.4 to 76.4 and the damper went on that engine too.

When the 993 cam out they skinnied up the bearing and beefed up the thickness of the crank arms, and the damper went away again.

It is a common solution. I would not view it as a weakness.