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Crankshaft Balancing:
An imbalance in the crankshaft in relation to the reciprocating weight of the upper end causes vibration and a loss of power. Making sure your engine is balanced correctly is essential, especially if you are modifying the engine to work in a different rpm range than what it was designed for. Using a lighter wrist pin lightens the top end by 4.5 grams which is enough to make a significant difference but not enough to completely balance the crankshaft.
50% to 85% of the top end weight (piston, rings, wrist pin, bearing, upper half of connecting rod) is recommended as the weight to be "missing" in the balance holes on the crankshaft halves close to the connecting rod pin. (Making weight disappear on one side of the halves is like making it appear additionally at the opposite side.) The conrod pin adds 3.3 grams to the balance area (over what would be if no holes were made for it). In the Tony Foale information (www.tonyfoale.com/Articles/EngineBalance/EngineBalance.pdf) he said that the balance factor corresponds to the square root of the rpm. So here I list the square roots from 5,500 to 10,000 rpm:
10,000 100
9,500 97.5
9,000 94.9
8,500 92.0
8,000 89.5
7,500 86.6
7,000 83.7
6,500 80.6
6,000 77.5
5,500 74.2
So this gives us the relation of the balance factors to each other for the different max rpm. If we assume 5,500 as needing the minimal 50% balance factor then we can derive these other balance factors with the percentages changed to fractions:
10,000 .674 (67.4%)
9,500 .657
9,000 .640
8,500 .620
8,000 .603
7,500 .583
7,000 .564
6,500 .543
6,000 .522
5,500 .500
As an example of how to calculate the needed "missing" balance weight I will use the 48cc Grubee engine. What I first notice is that the existing balance holes are not the same distance from the center of the crankshaft as the connecting rod pin is. That is important because the farther a weight is from the centerpoint the more centrifugal force it has for the same rotational velocity. Using a test weight of 1kg at http://www.calctool.org/CALC/phys/newtonian/centrifugal I see that 1kg at the 36mm distance of the balance holes gives 1.9 times the centrifugal force as 1kg at the 19mm distance of the conrod pin. So the final formula will take the upper end weight, multiply it by the balance factor, add the additional weight (3.3gm) of the conrod pin, and then divide that result by 1.9. The upper half of the conrod weighs 31 grams and the piston assembly weighs 79 grams for a total of 110 grams. 110x.50=55 +3.3=58.3 /1.9=30.7gm. The two factory-placed holes of 11mm diameter equate to 23 grams of missing weight. 30.7 minus 23 equals 7.7 grams that still needs to be removed. A single extra hole of 6.3mm diameter (1/4") drilled at the same 36mm distance will remove 7.5 grams of weight. I would also buy a 1/8" drill bit to use as a starter hole. It is hard slow going drilling a hole in that metal. (Grainger has good prices on carbide bits. On their site search "jobber drill aircraft" and the size needed.
The missing weight of any hole can be calculated at http://www.ralingroup.co.uk/weights.html but you have to multiply the resultant weight of kg by 1000 to get grams. Use half of the diameter as the wall thickness.
If the lighter weight wrist pin is used then that shaves 4.5 grams off of the piston assembly to reduce the total weight from 110 to 105.5 grams. Recalculating gives 29.5 grams needing to be counterbalanced. Subtracting 23 grams leaves 6.5 needing to be removed. A 6mm (15/64") diameter hole will remove that.
Below is a picture of my crank assembly with an additional balancing hole just above the conrod pin. The 6 blue holes are lightening holes for better acceleration. The blue is foam filling half the hole. The ends of each hole were later filled with JBWeld. I used foam just to reduce the amount of expensive JBWeld used. The conrod hole and two factory balance holes are already filled with JBWeld for increased crankcase compression.
An imbalance in the crankshaft in relation to the reciprocating weight of the upper end causes vibration and a loss of power. Making sure your engine is balanced correctly is essential, especially if you are modifying the engine to work in a different rpm range than what it was designed for. Using a lighter wrist pin lightens the top end by 4.5 grams which is enough to make a significant difference but not enough to completely balance the crankshaft.
50% to 85% of the top end weight (piston, rings, wrist pin, bearing, upper half of connecting rod) is recommended as the weight to be "missing" in the balance holes on the crankshaft halves close to the connecting rod pin. (Making weight disappear on one side of the halves is like making it appear additionally at the opposite side.) The conrod pin adds 3.3 grams to the balance area (over what would be if no holes were made for it). In the Tony Foale information (www.tonyfoale.com/Articles/EngineBalance/EngineBalance.pdf) he said that the balance factor corresponds to the square root of the rpm. So here I list the square roots from 5,500 to 10,000 rpm:
10,000 100
9,500 97.5
9,000 94.9
8,500 92.0
8,000 89.5
7,500 86.6
7,000 83.7
6,500 80.6
6,000 77.5
5,500 74.2
So this gives us the relation of the balance factors to each other for the different max rpm. If we assume 5,500 as needing the minimal 50% balance factor then we can derive these other balance factors with the percentages changed to fractions:
10,000 .674 (67.4%)
9,500 .657
9,000 .640
8,500 .620
8,000 .603
7,500 .583
7,000 .564
6,500 .543
6,000 .522
5,500 .500
As an example of how to calculate the needed "missing" balance weight I will use the 48cc Grubee engine. What I first notice is that the existing balance holes are not the same distance from the center of the crankshaft as the connecting rod pin is. That is important because the farther a weight is from the centerpoint the more centrifugal force it has for the same rotational velocity. Using a test weight of 1kg at http://www.calctool.org/CALC/phys/newtonian/centrifugal I see that 1kg at the 36mm distance of the balance holes gives 1.9 times the centrifugal force as 1kg at the 19mm distance of the conrod pin. So the final formula will take the upper end weight, multiply it by the balance factor, add the additional weight (3.3gm) of the conrod pin, and then divide that result by 1.9. The upper half of the conrod weighs 31 grams and the piston assembly weighs 79 grams for a total of 110 grams. 110x.50=55 +3.3=58.3 /1.9=30.7gm. The two factory-placed holes of 11mm diameter equate to 23 grams of missing weight. 30.7 minus 23 equals 7.7 grams that still needs to be removed. A single extra hole of 6.3mm diameter (1/4") drilled at the same 36mm distance will remove 7.5 grams of weight. I would also buy a 1/8" drill bit to use as a starter hole. It is hard slow going drilling a hole in that metal. (Grainger has good prices on carbide bits. On their site search "jobber drill aircraft" and the size needed.
The missing weight of any hole can be calculated at http://www.ralingroup.co.uk/weights.html but you have to multiply the resultant weight of kg by 1000 to get grams. Use half of the diameter as the wall thickness.
If the lighter weight wrist pin is used then that shaves 4.5 grams off of the piston assembly to reduce the total weight from 110 to 105.5 grams. Recalculating gives 29.5 grams needing to be counterbalanced. Subtracting 23 grams leaves 6.5 needing to be removed. A 6mm (15/64") diameter hole will remove that.
Below is a picture of my crank assembly with an additional balancing hole just above the conrod pin. The 6 blue holes are lightening holes for better acceleration. The blue is foam filling half the hole. The ends of each hole were later filled with JBWeld. I used foam just to reduce the amount of expensive JBWeld used. The conrod hole and two factory balance holes are already filled with JBWeld for increased crankcase compression.
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