Engine Balancing

[M3Evo 0]

Keep hearing things about knife edging a crank to reduce pumping losses and have a reasonable idea of what that's about but:

 

It occurs to me that the material that's removed in the process of knife edging the crank must've been there for a reason. Presumably part of the reason the material was there, other than to balance the crank istelf, is to balance out some component of the out of balance (OOB) force created by the reciprocating and partially reciprocating components.

 

Logically, if that is the case, knife edging a crank will cause the engine to become unbalanced, or in the case of most engines, more unbalanced.

 

Am I missing something here, or is that right?

 

Cheers!

 

Alex

[Jonmurgie 2]

I thought having a crank knife edged was done first and THEN the whole bottom end balanced... makes sense, no?

[Mandic 0]

I agree with You, running stock components (pistons, rods) knife edging a crank is not a good idea at all, imo.

 

But when using aftermarket rods and pistons (which presumably are lighter) one can take away some of the material.

 

Cheers

 

Ziga

[M3Evo 0]

Yeah, there's gotta be a limit to how light the reciprocating assemblies can be made?

[Mandic 0]

Sure, having 50g lighter pistons doesn't mean one can take 50g off the crank.

 

Cheers

 

Ziga

[Beastie 1]

It would be possible to remove the counterweights altogether and a crankshaft could still run perfectly in balace *if* it were considered as a whole. A good example of this is the billet crankshaft in the 3.5 and 4.3 litre Alvis engines produced between 1936 and 1940. These were very smooth engines and the cranks were drilled and tapped for the fitment of bolt on counterweights - but the manufacturer never used them on the production models.

 

If you consider a conventional 4 cylinder engine the throws are disposed evenly around the shaft so it can be balanced without conterweights. Similarly the weight of the reciprocating parts cancel each other out by virtue of their even disposition around the crank.

 

If you start to consider the out of balance forces across different parts of the crank then a different picture starts to unfold: A four cylinder crank for a vertical engine has two throws together in the middle which are balanced out by one throw either side. Therefore a cross section considered isolated in the middle of the crank will be out of balance unless it has a counterweight. These imbalances across the shaft cancel each other out but there is a force applied as this happens which cause some degree of twist and which can cause vibration. These forces are known as "rocking couples" and it is the job of the counterweight to reduce or as an ideal to eliminate rocking couples.

 

Removing mass from counterweights can be done without affecting overall shaft balance but it does increase force generated by rocking couples.

[M3Evo 0]

Do do the counter weights cancel out none of the OOB forces of the reciprocating masses?

 

Thinking of the crank in isolation it's easy to see how it can be balanced regardless of how much mass is removed, but it's the bit about the reciprocating components that throws me.

 

Out of interest, how many cylinders and what configuration were the Alvis engines?

[Beastie 1]

Do do the counter weights cancel out none of the OOB forces of the reciprocating masses?

 

Thinking of the crank in isolation it's easy to see how it can be balanced regardless of how much mass is removed, but it's the bit about the reciprocating components that throws me.

 

Out of interest, how many cylinders and what configuration were the Alvis engines?

 

 

I have to confess that being a vintage and classic specialist I am likely to be a bit out of date on those specifics Traditionally counterbalancing has varied according individual design. Usually four cylinder crankshafts have been weighted to cancel out rocking couples generated by the shaft only. Six cylinder engines have often been designed with small counterweights. Engines with odd numbers of cylinders have been counterweighted to balance out the forces of the reciprocating masses. The perfect example of the latter being single cylinder engines which always have huge counterweights to balance out the mass of con rod and piston assembly for obvious reasons. A great historical example of the variation which occurs in designs is the 2 litre AC engine which at one time held the record for the longest production run of any engine (until the VW flat 4 surpassed it) The AC was a straight 6 which was launched in 1919 and finally bowed out in 1956 having been developed from 30bhp to 135bhp. The original had a bad crankshaft vibration period. So did the final version The manufacture tried umpteen different crankshaft designs with different counterweights over the years, some of which were over weighted and some underweighted.

 

Engines with even numbers of cylinders have opposing forces generated by the reciprocating components which cancel each other out - although they do of course contribute to the effect of rocking couples in the crankshaft.

 

The Alvis engines were straight sixes. They had an aluminium crankcase with plain bearings and the timing chain was at the flywheel end. As with nearly all sixes they had torsional vibration dampers. RPM was modest by current standards at 4500 maximum sustained revs for standard models and 5000 for special models.

[Dom9 2]

I think we had quite a long chat about this some time ago when someone was buliding their race engine, was it Mark? He spent a lot of money getting the counterweights chopped about and I was dead against it... Dave Baker threw in his usual £8.94 worth and came up with some interesting points... If you can find it, it's worth a read!!

[Mandic 0]

Yes, that topic goes about 4 cylinder, Beastie is talking about I-6 which is far better in this aspect, but anyway...

 

http://forum.205gtidrivers.com/index.php?s...opic=36351&st=0

 

Cheers

 

Ziga

Edited August 23, 2006 by Mandic

[nick 3]

I like the link Mandic, but I can't see which bits are out of balance.....it all looks fine to me.....

 

Nick

[Mandic 0]

 

Doh, this is what happens when You don't use Your own computer

 

Keyborad didn't accept Ctrl+C so I pasted what was in the memory, but after checking the link I must say You're right, everything seems well balanced

 

Cheers & sorry,

 

Ziga

[nick 3]

Mmmm, the old "not my computer" excuse eh!

 

No apologies needed...........

 

Nick

[M3Evo 0]

Hmmmm, this is still playing on my mind!

 

With an Inline 4 cylinder engine (I4) you can balance the crank as an entity so that assuming it doesn't deform for any reason, it'll be balanced at any speed.

 

Also with out I4 engine, the deceleration at TDC of say pistons 1&4 doesn't make the engine jump because pistons 2&3 are doing the opposite, barring the geometric differences in the rods giving rise to a slight jiggle as the pistons decelerate at a different rate when comparing TDC and BDC.

 

So based on that, with the crank nicely balanced and the pistons and rods balanced as well as they can be, we should have an acceptably smooth I4 engine where the counter weights on the crank just balance the crank and not the masses of the pistons?

 

However, take something like an I6 engine:

 

Again we can balance the crank as an entity, but when it comes to the pistons, we've got say pistons 1&6 coming to an abrupt halt at TDC, whilst the other 4 are 120degrees away on either side, so what cancels out the jiggle?

 

So in this instance, do the crank counterweights play a part in balancing out the OOB forces of the pistons and rods?

 

We know that you can perfectly balance an I6, and that you can't perfectly balance an I4 so there's obviously much more going on than meets my untrained eye, but I guess what I'm asking now is do different engine configurations rely different amounts on the mass of the counterweights on the crank?

[welshpug 1,626]

erm, try go and talk to an engine builder/balancer, bear in mind that this is a pug forum and I dont remember any recent I6 engines, V6 yes but not I6

 

surely your 325 is smooth enough already!

[arry 0]

http://www.sxoc.co.uk/vbb/showthread.php?t...ht=knife+edging

 

Not sure if you can read that but there's some fascinating debate from Ruariadh in the later pages - well worth a read.

 

With regards having aftermarket rods/pistons and them being lighter - in my experience forged pistons have actually been heavier than the stock items, which obviously would lead to an increase in the reciprocating mass of the engine.

[jackherer 543]

I found this site very useful: http://www.autozine.org/technical_school/engine/smooth1.htm

 

(its continued on a second page, thats where he talks about I4s)

[M3Evo 0]

erm, try go and talk to an engine builder/balancer, bear in mind that this is a pug forum and I dont remember any recent I6 engines, V6 yes but not I6

 

surely your 325 is smooth enough already!

 

More just wondering about the theory than the particulars of who uses what engine configuration, and people on here seem to have a pretty broad knowledge base

 

I found this site very useful: http://www.autozine.org/technical_school/engine/smooth1.htm

 

(its continued on a second page, thats where he talks about I4s)

 

Crikey! That's gonna keep me in reading material for many a lunch time to come

[Mandic 0]

I found this site very useful: http://www.autozine.org/technical_school/engine/smooth1.htm

 

(its continued on a second page, thats where he talks about I4s)

 

 

aaaa, finaly get it!

 

Thanks

 

Cheers

 

Ziga

[Rom 0]

I dropped my bottom end off last week to Julien Godfreys in Heathfield. They do all of Dermot Carnegies (sp) Work, aswell as a lot of other high spec race engines. They did/do hold the record for the worlds fastest YB Cosworth engine. 7.7 seconds on a 1/4 mile drag race @ 170.5 miles an hours

 

Anyway, i was talking to some of the guys there. They said even on thier 800bhp+ engines, they dont knife edge or polish the crank. They were explaing,knife edging is not so much to cut down on weight. But to reduce drag of the crank turning in the oil. The idea being it would slice through, rather than smash into it.

At least that is what they were saying. Apparently they just lighten and balance in the usual way, and also lap the crank. Which is what im having done.