Author Topic: The crank is the foundation to your engine.  (Read 7686 times)

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Offline BAKeng

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The crank is the foundation to your engine.
« on: June 05, 2016, 03:55:52 pm »
Every engine just like any structure needs a good foundation. It all starts with a good rotating assembly. At the heart of that is your crank. All cranks regardless of the engine needs dynamically balanced with the exact rotating assembly compensated for. The crank does not create horse power but it can steel it if not balanced properly.
Keep this in mind, a crank that is out of balance by 28 grams (about 1 ounce) spinning at 6500 rpms is creating over 160 pounds of lateral force. 28 grams or 1 ounce is the approximately the weight of a slice of bread. That's not much. It is common for us to see Briggs singles out of balance over a 100 grams. Those cranks are generating more vibration than the whole mower and rider combined.
Out of balance cranks are the most over looked thing we see when someone builds an engine. An unbalanced crank will cause the following;
Rider fatigue (from engine vibration)
Vehicle control issues (hard to drive something thats vibrating severely)
Mechanical break down (broken parts, severely worn bearings, pistons, rings,etc.)
Loss in horse power (it takes power to keep something spinning that's not balanced)
Loss in acceleration

You can not purchase a balanced crank. Every crank must be dynamically balanced to its specific piston, rings, wrist pin, bearing, and rod.
You also can not balance a crank statically. Static balancing (setting the crank on two rollers and moving it until it does not move on its own at any position) at its best might get you to within 10-15 grams. That's if you do all the calculations and get the bob weight correct. Static balancing also does not correct for the 2 planes a crank needs to be balanced in.

Whether you send use your crank for proper balancing or use someone that can do it properly, this is the "foundation" to your engine. Get it balanced and build a good, reliable, engine from there.

Brent Kasper
BAK Performance
Zanesville, Ohio 43701

Offline trbinrat

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Re: The crank is the foundation to your engine.
« Reply #1 on: June 14, 2016, 09:24:58 pm »
I agree with 90% of what you have posted. Balancing a single cylinder motor is a misnomer unless you are using an active balancing system. Balancing dynamically or statically you are balancing for a specific RPM. We as mower racers run our engines from 2000 to 7000 and sometimes 10,000 Rpm's every lap thus passing the unbalanced zone twice  a lap.

I have a kawi flathead that I balanced staticlly and at idle it is purs like a kitten at 7000 it's perfect at 1500 to 2000 it shakes like it going to come a part.

I welcome your comments as I'm very positive I don't know everything about balancing but have been a student of it for a long time
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