17-11-2017, 05:23 AM
Out of balance forces increase as the square of rpm so of much more concern to 7000 rpm racers than the ordinary Seven for which about 4,000 rpm is typical max used. (49 cf 16)
Ideally parts should be balanced as an assembly, otherwise presence/absence of keys, non concentric parts etc causes confusion., although parts then more interchangeable. The force associated a few grams unbalance is impressive, but barely significant cf normal forces in an engine. Significant unbalance does make for less smooth running, although the money spent on felt will often achieve more.
Re the claim that balancing significantly reduces loads on bearings etc, at 4,000 rpm the two pistons and little end assemblies at top of stroke exert an upward force some 855 times weight, and the two at bottom of stroke 515 times. The difference 340 times weight acts every half stroke to lift the crank and engine. An unbalance in flywheel at bolt radius exerts 2000 times weight. I will leave you to insert figures. Firing pressure is about 100 x c.r. in psi and piston area exceeds 3 square inches. Even a totally missing bolt will not generate bearing loads comparable normal running.
As with many other cars of the era the makers did not see it as worthwhile to dynamically balance the crank assembly
The figures do demonstrate the importance of reasonable flywheel/cover plate balance. Being more or less in one plane static balancing is quite effective. (Commercially it is easier to dynamically balance)
I have balanced cranks for other cars rolling along milled angle iron clamped across my lathe bed and checked with an engineers level. Can detect a thin washer, about the normal production tolerance.
I don’t know how sensitive a Seven crank in bearings (not necessarily in crankcase) with very thin oil is.
It is worthwhile matching pistons and little ends and whole conrods within reason.
From a myriad workshop manuals typical limits are 3 to 10 gms for each. The VW Beetle gave stringent 8 gm.cm for flywheel unbalance ie less than 2 gm at bolt radius.
E and OE!
Ideally parts should be balanced as an assembly, otherwise presence/absence of keys, non concentric parts etc causes confusion., although parts then more interchangeable. The force associated a few grams unbalance is impressive, but barely significant cf normal forces in an engine. Significant unbalance does make for less smooth running, although the money spent on felt will often achieve more.
Re the claim that balancing significantly reduces loads on bearings etc, at 4,000 rpm the two pistons and little end assemblies at top of stroke exert an upward force some 855 times weight, and the two at bottom of stroke 515 times. The difference 340 times weight acts every half stroke to lift the crank and engine. An unbalance in flywheel at bolt radius exerts 2000 times weight. I will leave you to insert figures. Firing pressure is about 100 x c.r. in psi and piston area exceeds 3 square inches. Even a totally missing bolt will not generate bearing loads comparable normal running.
As with many other cars of the era the makers did not see it as worthwhile to dynamically balance the crank assembly
The figures do demonstrate the importance of reasonable flywheel/cover plate balance. Being more or less in one plane static balancing is quite effective. (Commercially it is easier to dynamically balance)
I have balanced cranks for other cars rolling along milled angle iron clamped across my lathe bed and checked with an engineers level. Can detect a thin washer, about the normal production tolerance.
I don’t know how sensitive a Seven crank in bearings (not necessarily in crankcase) with very thin oil is.
It is worthwhile matching pistons and little ends and whole conrods within reason.
From a myriad workshop manuals typical limits are 3 to 10 gms for each. The VW Beetle gave stringent 8 gm.cm for flywheel unbalance ie less than 2 gm at bolt radius.
E and OE!