« on: August 05, 2013, 11:25:50 PM »
link http://juicemotoparts.com/crankbalancing
Crank Balancing for Single Cylinder 2 Stroke Engines One way of increasing both the reliability and lifetime of your single cylinder 2 stroke engine is to properly balance your crankshaft. Balancing a crankshaft can yield higher RPM’s because it equalizes the forces involved during operation. An unbalanced crankshaft can lose something like 2000 rpms off the top. Generally speaking, multi-cylinder engines have multiple crankshafts and/or flywheels that allow them to balance themselves out from precise computer calculation (dynamic crank balancing). Since our motorized bicycle engines is only single cylinder we can balance the engine to within a certain RPM range. There are two difference kinds of balancing: static and or dynamic crank balancing. Here we will be focusing on static crank balancing because dynamic requires the use of complicated moving machinery and computers which most of us dont have access to. In our experiments, we have managed to get a 66/80cc motorized bicycle engine up to about 8800-9000 RPMs under load, and it balanced itself between 6800-8500. This does not mean that my engine will not vibrate at all; it simply means that at around 6800-8500 rpm’s the engines internal crankshaft forces will balance themselves out and there will be little to no vibrations within that range. Now that’s impressive seeing that with a rear sprocket of 41t gives about 37-40 mph without the vibrations to tear itself apart!
Here we will be showing you how to statically balance a crankshaft on a single cylinder engine. Statically balancing your crankshaft involves removing or adding material to the flywheel of the crankshaft while it is stationary. This will be particularly useful if you know your kind of riding style (eg. long distance rider vs city rider vs racing). Typically, a stock 66cc comes balanced within 3500-5000 rpms and any much after that it turns into a cheap massage chair like most of you know. It’s those internal vibrations that cause the engine to tear itself apart and bearings to fail prematurely. If you are one to ride your motorized bicycle for longer distances at higher cruising speeds outside its balanced RPM ranges then you might want to consider crank balancing to ensure the maximum life of your engine.
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The Concept
Balancing a crankshaft assembly for any reciprocating piston engine presents a variety of challenges and compromises. Unfortunately, there are virtually no designs that completely cancel all the competing primary and secondary forces that produce shaft vibration. What this means is that in reference to our small 2 stroke engines we can only balance the crankshaft to within a certain RPM range.
Understand that a single cylinder engine produces vibration primarily from two sources:
1. Rotating imbalance
2. Vibration induced by the reversion of the reciprocating mass at TDC and BDC
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Preliminaries
READ through Some science of balance by Tony Foale Designs to get some idea of what balancing is and how it works.
DOWNLOAD this spreadsheet Crank Balancing Calculator. It could be useful later on for some of the calculations.
Access to the Crankshaft
Step 1- Refer to Changing Bearings to get access the crankshaft
Step 2- Remove the piston and keep all the parts in a safe place.
Step 3- Removing the crankshaft from the bearings could be a little tricky if it is seated in there tightly. You can use a little heat and a rubber mallet to remove it from the case halfs. What you have to do here is fully remove the crankshaft (with connect rod) from the engine motor. See picture below.You can choose to leave the main bearings on if you like as this can help with the balancing process.
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Instructions
Step 1- Get a precision electronic digital scale. The more accurate the better your balancing will be. Usually the lower the maximum weight a scale can measure is the more precise it is.
Step 2- You will now have to weigh the top end of your engine. This includes the piston & rings, wrist pin, wrist pin bearing, e clips, and small end of the crankshaft. Weighing the small end of crankshaft could be a little tricky so placing the crankshaft on an elevated platform and having the top end rest on the scale is probably your best bet. See picture below. To save you some time, generally the small end of the crankshaft is around 31g +/- 5g.
(adapted from motorbicycling.com)
Step 3- Calculate the sum of weight of the top end and multiply this number by the balancing factor. (eg 140g x 59%). Choosing a balancing factor can be a little tricky to decide because it is based on the RPM range in which you plan to run the engine in. According to Tony Foale, “In practice, balance factors are usually between 50% and 85%.” I personally have tried a balancing factor of 58-59% with engine RPMs equalizing between 6800-8200 RPMS. You will have to play with balancing factors to find out what’s best for you 60% is a good place to start.
Step 4- Measure the weight of the bottom end of the crank shaft and bottom end wrist pin bearing. To do this you’ll have to break open the halves of the crankshaft to have access to the lower wrist pin bearing and bigger end of the crankshaft. If you do not have the tools to this this, the value is usually around 47.5g +- 1 gram.
Step 5- Calculate the sum of step 3 and step 4. This is your “bob weight” or the weight that you will attach to your crankshaft to judge whether you need to add or subtract weight from your crankshaft.
Step 6- Carefully measure out your “bob weight” using nuts/bolts/weights and attach it to the crankshaft pin. You can use a threaded rod and bolt the weights to the side. See example picture below
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