Crankshaft balance

[James Baxter]

Has anyone ever dynamically balanced their 2 cylinder Jowett engine?
I am finding that the counterweights are way too heavy.

My thinking is that you dont need to bolt on weights to simulate the con rods. But i am open to alternative views.
The crankshaft is a 1925 rivetted on counterbalanced type.

All thoughts appreciated.

[george garside]

Those counterweights are an essential part of the sweet running of the Jowett twin which is why they were used throughout . I think removing them would produce a serious lack of balance and put a huge strain on the crankshaft, bearings etc as well as making the car jump up and down on its springs!.

On the other hand Jowetts did lighten the flywheel for some purposes eg the 24 hr class g record and I think on the engines they prepared for Victoria Worsleys competition car.

Perhaps somebody of a more scientific bent can add their three penneth!

george

[Keith Clements]

I am definitely bent scientifically.

Static balance is the first thing to try to get correct. Pistons should be the same weight and the crankshaft should have its weight/mass evenly balanced around its rotational centre. You could go farther and make sure the centre of gravity is on the centre line of each component. For instance a piston should not have more mass on one side of its centre than the other. The con rods should all balance in the same place so that the mass of the small ends and the big ends are the same on each. Static balancing can be done with scales, weights and knife edges.

Dynamical balancing is considerably more difficult, particularly if the rotating masses are not in the same plane. Balancing needs a machine to rotate the components and the assembly of them and have sensors to check strains and forces.

Think about having two buckets filled with sand on a length of rope. If the buckets and rope are statically balanced, that is they weigh the same and have the same dimensions, then if you start rotating them, one on each arm, and you pirouette, then you should not fall over. If however you lengthen one of the ropes then you will fall over because there will be more centrifugal force on the longer rope.

Even more of an issue is when the rotating masses are not in the same plane. That is when you spin the buckets around your outstretched rotating arm. You will experience a twisting motion around your spine if the buckets are out of phase (that is one is behind you whilst the other is in front).
If the buckets are in phase then you will experience a disastrous twisting motion about your feet and you will fall flat on your face. But if you stopped that falling over by putting a bearing around your neck you would find an alternating force both side to side and up and down as the buckets rotated in phase. On a horizontally opposed engine some of that force is balanced, but not all, as the two sets of piston(s) in each bank do not rotate in the same plane.

Anyhow having now visualised, or perhaps even experienced dynamic balance, you can now read up on the maths which is what I had to do at University a very long time ago.

Wikepedia helps to understand static and dynamic balance.

[James Baxter]

A clever chap on the vscc
Forum gave me this answer
" As long as the rod-piston assemblies have equal mass, the crank can be balanced without them. [If not, the piston masses should match, the small ends should, and so should the big ends - and be careful what you grind away ....]
Such an engine will produce a 'rocking couple' because the two sides are not exactly in line with each other. This couple can be reduced by adding counter-balance masses to the crank, but the couple cannot be removed completely in this way. What happens is that a side-to-side rock is replaced by an up-and-down rock as more extra mass is added each side, and a mid-way compromise is arrived at.
The couple can be removed completely by adding an extra shaft with its own out-of-balance, timed so that the two effects cancel each other out - but this involves serious engineering and would undoubtedly fail to impress the scrutineers. "

I did balance the rods and pistons, and pins , all to within 1 gram. They were almost perfect before!
The flywheel was almost perfect too.
What proportion of rotating weight, and reciprocal weight do you add temporarily to the big end bearing, while on the balancing machine?

[BobCulver]

A flat twin operates in a state of inherent dynamic unbalance. The extent is chosen to counter the reciprocating unbalnce to some nominal degree, so there is a bit of both and not a lot of just one sort.So called dyanamic balancing is acheived with added weights. If wrong, often end up reducing both crank counterweights, a not uncommon, negative step, very expensively acheived.The factory counterweights will not be too large unless perhaps using some ultra light pistons. Often the firms doing have a limited understanding.

Balance the crank statically, preferably by adding not removing weight. Then redo with the flywheel, then the cover plate. I roll on ange iron milled to narrow edges and bolted across my lathe bed. Use precison level.

The topic was extensively covered in Flat Four some years ago . I will hunt out sometime.

Bob Culver

[BobCulver]

To elaborate somewhat. A flat twin can be in perfect balance if the pistons are exactly opposite. Such engines have been made using a 3 throw crank and two conrods on one piston). Otherwise the offest pistons rock the engine. By deliberately making the crank out of dynamic balance with over large counterweights the reciprocating unbalance can be more or less cancelled, but only in one plane. So it is about half matched, a significant dynamic unbalance remaining in other planes.
So some unintended dynamic unbalance is not of great consequence, and generally preferable to much reduced counterweights, often the outcome of commercial attempts at dynamic balancing.
If correcting static balance best to remedy near middle of crank as less chance of worsening dynamic balance.
Good balance of the flywheel and clutch is worthwhile.Static balance requires patience. If to be done commercially, and assuming the flywheel runs true on the crank, it is quicker and cheaper to balance the flywheel and cover by rotating it on a dummy shaft.

Bob Culver

[BobCulver]

To rabbit on a little further and to attempt to answer James question.

To dynamic balance firms fit supposedly identical shaped weights which match the b.e. plus about 1/2 the reciprocating weight (top of rod, piston assembly). If chosen too large cannot acheive a dynamic balance on the machine, if too small end up reducing both crank counterweights...a common wrong outcome. The exact weight reqd and involving the least metal removal and from just one palce varies with the individual crank casting. If examine the cranks carefully can see that the late CC Bradfords are more effectively counterweighted than early CC.
Some firms use adjustable weights, but the errors likely with these seem to match any dynamic irregularity in the first place!

Bob Culver