Balancing a crankshaft

[Sebastien]

I would like to ask a question to the technically inclined members of this forum:

We are currently rebuilding a Lambda engine. The company balancing the crankshaft and flywheel is asking if there is a known balancing factor to be used specifically for a Lambda engine to help compute the weights to be  screwed on the crankpins during the balancing operation.

The balancing factor, from what I was told, is expressed as a percentage of the oscillating weight (piston with rings and wristpin + rod reciprocating weight).

If no Lambda specific value emerges from this request, the company will use a factor from similar engines.

[JohnMillham]

Ask Bill Smith in Australia. He's the expert on these matters.
 I have had my engines balanced at "Vibration Free" near Bicester in Oxfordshire, but complete with rods, flywheel and pistons (without their rings -but with washers amounting to the same weight superglued onto the piston crowns.) Their technique is to run the engine on their machine with oil in it (as there's oil in the crank when it's running, which weighs something)  at a few hundred rpm and the machine tells them how much weight should be added to or removed from the flywheel or front pulley. In all three cases (2 Lambdas and 1 Augusta) they have had to drill a few holes in the flywheel and add a bit of weight to the pulley. I'm having a new Lambda crankshaft made by Phoenix at the moment, which will be ready by Christmas, so will have to go through the balancing process yet again.
 Regards, John

[davidwheeler]

I have also used Vibration Free with excellent results.   I've also read Bill Smith's dissertation on the subject (it may possibly be on the forum somewhere?) and the maths is incredibly complicated and depends critically upon the angle of the V.  Using balancing factors derived from straight 4 engines simply will not work!   I would not be too keen on screwing weights on to the crankpins (?) either, the crankshaft is quite slender and is machined from the solid and screwholes may well act as stress points to initiate cracks.   There is very little room to spare either.  I would very strongly suggest going to Vibration Free, they transformed Old Boot's engine.

[Sebastien]

Thank you John and David for your comments, and recommendation of "Vibration Free".

I am in touch with Bill Smith, we had a good discussion at Castlemaine, and he has sent me his article on balancing.
His recommendation is 32.1 % as the factor to use on the reciprocating mass (piston + wristpin + rings + reciprocating weight of the conrod). That factor is only applicable to the 7th series engine, because Lancia changed the geometry (V-angle) almost on each series of Lambda engines.

So I am happy to use that factor, and will report later, hopefully in the next months, about the vibrations, or lack of them!!

Regarding screwing the balancing weights to the crankpins, it is not done that way I believe. The machinist doing the balancing will manufacture half weights, just like conrod big ends, of the required mass, that are then mounted on the crank throws. Thus you simulate the weight of the moving parts that act on the crankshaft, and you can then spin the crankshaft in a balancing machine.

Bill Jamieson in Capolavoro clearly mentions that the crankshaft was balanced before the engine was assembled, and there is a photo on page 105 of a Lambda crankshaft in a Norton balancing machine.

I like also the "Vibration free" process as it uses the real engine parts, including the flywheel - however it seems that you have to assemble and disassemble the engine twice, if you have to spin it without rings fitted to the pistons, as described. I wonder also what then happens to the oil which is in the crankcase - you would have to cover the head face of the block, to avoid being covered in oil!

[Sebastien]

And just for reference, additional to the Capolavoro description, there is that illustrated article, over 4 pages:

An Unusual Crankshaft and how it is made
by J.A. Lucas
that was published in American Machinist, March 15th 1928,
and was reproduced by Geoffrey Goldberg in the booklet "Lancia machining"

I think the decription is from the period Lancia manufactured the 7th series Lambda.

One year later the same J.A. Lucas also wrote an article on the bus and lorry Lancia crankshaft (must have been for the Omicron 6 cylinder): they too were balanced on a Norton machine.

All quite interesting!

[JohnMillham]

I wonder also what then happens to the oil which is in the crankcase - you would have to cover the head face of the block, to avoid being covered in oil!

Using a bearing puller, I pressed a piece of metal over the oil hole to block it. Simples!

[Sebastien]

Thank John.

As you were there, could you explain what happens in the cylinders, when you let the pistons run without piston rings, "... at a few hundred rpm" with oil in the crankcase?

Did you look into the cylinders then?

Must be quite a sight!

[JohnMillham]

Thank John.

As you were there, could you explain what happens in the cylinders, when you let the pistons run without piston rings, "... at a few hundred rpm" with oil in the crankcase?

Did you look into the cylinders then?

Must be quite a sight!

Nothing unusual.  As you might expect, the pistons go up and down, lubricated from below by oil splashing around and from above with an aerosol of some sort of thin oil. There was some piston slap, but that was reduced each time the oil aerosol was applied. It didn't need to run for long, as the balancing machine was very quick to decide what needed doing. We didn't look down into the cylinders for long, as one of the superglued weights came unstuck and even at only 250 rpm flew off with some force. You wouldn't want to get in the way of it!
Regards, John

[Sebastien]

Best to wear glasses when looking into the cylinders!
John: I see that your engine was equipped also with the flywheel when at Vibration Free.
Once the machine has done its work, I assume it is a bit like a wheel balancing machine: you turn the flywheel in the right position and add a balancing weight to the flywheel - or did they take material off?

However, I shall stick with the bob weights on the crankshaft, using the Bill Smith balancing factor, and dynamic balancing of the crankshaft alone.
I hope to be able to report good results with the engine back in the car in early spring. Shall check the rear view mirror, if I get a clear view back at all speeds in 4th up to 75 mph, then the balancing will have been successfull.

One eye on the rear view mirror, the other alternating from oil pressure to water temperature, I shall be busy...

[JohnMillham]

Best to wear glasses when looking into the cylinders!
John: I see that your engine was equipped also with the flywheel when at Vibration Free.
Once the machine has done its work, I assume it is a bit like a wheel balancing machine: you turn the flywheel in the right position and add a balancing weight to the flywheel - or did they take material off?

However, I shall stick with the bob weights on the crankshaft, using the Bill Smith balancing factor, and dynamic balancing of the crankshaft alone.
I hope to be able to report good results with the engine back in the car in early spring. Shall check the rear view mirror, if I get a clear view back at all speeds in 4th up to 75 mph, then the balancing will have been successfull.
One eye on the rear view mirror, the other alternating from oil pressure to water temperature, I shall be busy...

I must admit I've never driven a Lambda as fast as that. I doubt that I've ever exceeded 65 mph, as I usually cruise at 60 in it and only go faster if I'm late for a ferry, etc. Tony Stevens used to race his (ex Maltby) Lambda at 80 or so. I look forward to hearing how you get on.
 Vibration Free added weight to the crankshaft pulley and removed it from the flywheel, but i imagine it could just as easily have been the other way round.
 Regards, John

[davidwheeler]

Well, Sheila is quite happy at 75.  She has the benefit of Colonial large wheels and a notably smooth engine.  Boot is lower geared so unable to attain that speed but certainly 70 down the motorway was often achieved when in a hurry and probably a bit more when overtaking downhill one day.  The vibration (before Vibration Free and proper balancing of the prop shaft) was terrific!

[davidwheeler]

Hope your balancing works or you will have to take the engine out and strip it down again!     Perhaps you could spin it up out of the car or even run it up before installing?

[casarokardo]

There was the famous demonstration to motoring journalists of the first Hudson super six, when they ran it up to 3000 rpm and then took off the balancing weights. The crankshaft broke at about 2,200 rpm. Now I am not an engineer, but it is said that the reason that V12s were invented was to even out the SECONDARY rotational forces that caused in line six crankshafts to break (a six is inherently balanced for primary forces). That makes me wonder whether something similar could happen to V4 crankshafts when the balancing weights (or balancing weight removal) is/are located at a distance along the crankshaft from the crankshaft webs. It seems to me inevitable that if you take weight off the flywheel for example and that weight removal is not symmetrical about the centre of its rotation, you will set up reciprocating torsional forces in the crankshaft that would not be present if you balanced the crankshaft itself by attaching weight or removing weight from the webs. Could we have a comment please by an engineer who understands the infinitesimal calculus and how to use it ( like the 1916 Hudson design team)?