Balancing Happy Time style 70cc Crankshaft

[Fabian]

Ok,

Just want to get peoples ideas and if possible, photos of what their balanced crankshaft looked like; the balance factor they used for the calculated rpm speed and if they used stuffers to fill the balancing holes.

Photos are of a standard 70cc crankshaft that has travelled 700 kilometers.
If you look closely, you can see where the connecting rod has cut into the crankshaft halves because the factory does not use thrust washers.

Fabian

 

[clay]

53%

 

[Fabian]

Hi Clay

You have used a crankshaft balance factor of 53%

Can you please explain the mathematics of deriving a balance factor for a specific rpm.
From what i understand, the balance factor changes for the rpm range an engine will be "mostly" operated.

The commonly used rpm range best suiting my application is 3000 - 4500 rpm - could you advise the most effective balance factor.

For my own curiousity, what were the weight measurements of your big end bearing, the big end of the conrod, the small end of the conrod and the piston, piston pin, bearing and spring clips.

Cheers Fabian

 

[210061741]

Math Behind Crankshaft Balance

This is a very good book (and FREE ) with all the mathematics needed to figure out anything to do with ENGINES.
A chapter in the end of the book called Balancing describes everything you need to know and at the very end even has an example of balancing a single cylinder engine.
Simple certanly not.
But i'm sure much more effective than guessing or following someone elses guess.
Most likly you would need some type of engineering experience to understand the Force diagrams and have the ability to do the math.
When i get time i will go through all the calculations and see what i come up with.

Fabian
If you go through the math please share and we will compare the results to what i calculate.
This is gonna take some time to understand and do correctly.
But for what you are putting into your motor i would go the extra mile.
If this math is used to get your answers i'm positive the very best result will be achieved.

Have fun I will.

Link to the free book. "The theory of heat engines By William Inchley"

http://books.google.com/books?id=SN...resnum=3&ved=0CAwQ6AEwAg#v=onepage&q=&f=false


A SIMPLER APPROACH BUT WOULD NEED TO BE TRIAL AND ERROR.

By placing the crank in a stand that will allow it to turn freely.
V-Blocks with 2 bearings on each side which the crank spins on.
We could follow the method of balancing a grinding wheel.

You would need some round magnets to use for weights place them on the crank to find the correct position of the weights.

The round magnets 2 for each flywheel (4 total) should weigh more than the total weight of the unbalance. " piston conn rod pins bearings ect.

Once you find the proper placement to achieve balance you will need to drill out the crank flywheels and place weights in there = to the weight of the magnets + the weight of the removed material.

This would be the simpler non mathematical less accurate method of balancing.

Let me know which way you go.
If you get stuck in the math somewhere let me know and i'll try to help.
I have quite an engineering background myself and know some old timer engineers that would help with the math just for fun.
Also if you are going to school the math professor would love this one.

 

[Fabian]

I've had a look at the book "the theory of heat engines".
Lovely mathematics on balancing but it doesn't give an explaination for balance factor and how it relates to rpm.

I have gained an understanding as how to set up the bob weights for a given balance factor but i haven't found any information as to how balance factor changes due to a selected rpm range.

I can find my bob weight for 50% balance factor or 53 or 55 or 60% balance factor but what number is correct for say, 3,000 rpm or 3,500 rpm or 4,500 rpm.

This is where i'm completely lost.

 

[210061741]

From my studies today it seems there is no such thing as balance in a single cylinder engine.
As the piston speed is constantly changing.
Balancing the crank will only produce balance at 90 degrees to the cylinder.

So unfortunatly it seems to be a trial and error implementation.
I don't think i will bother trying to balance my shafts.

Rather it seems time would be best spent on designing a crank weight "the screw on plates as light as possable " trial and error" and increasing the size to take up more volume in the crankcase.

Higher Psi in the Crank = Higher HP

AKA (BMEP)

But anyway let me know how it all goes when your done.

 

[tacoshell4]

check out jim the manic mechanic creative engineering he has the whole scoop he uses software to figure out the sweetspots

 

[Fabian]

Hi tacoshell4

Can you please post a link for that information on balancing

 

[tacoshell4]

any of his parts are available from justin at pirate cycles he is his his distributer I recomend all his billet stuff trust me
http://home.roadrunner.com/~bikeparts/

 

[Fabian]

Ok,

I,ve been on a quest for solution to crankshaft balance.

From what i understand and relating the the weights of my engine components:

1) Take the complete weight of the connecting rod big end and the big end bearing - 40.3 grams + 7.1 grams = 47.4 grams
If your engine has thrust washers, add the thrust washer weight to "step 1"

2) Take the complete weight of the connecting rod small end and the total weight of the piston and rings, piston pin, small end bearing and clips - totals 105.5 grams

3) Multiply the figure 105.5, by your desired balance factor. For arguements sake, i'll use a common balance factor figure of 55% > 105.5 x 0.55 = 58.025

4) Add the total weight of "step 1" to the balance factor figure of "step 3" - 47.4 + 58.025 = 105.425

The required bob weight to balance the crankshaft to a balance factor of 55% will be 105.4 grams

5) Assemble the crankshaft by lightly pressing the two halves and crankshaft pin in place, only so it will hold together and you can true the crankshaft on a set of V-blocks for the balancing proceedure.

6) Place the trued crankshaft ends or shafts on parallel knife edged surfaces or use two reasonably sized drill bits that do not have any scratches or deep grooves - you want a perfectly smooth and horizontal surface for the crankshaft to be able to rotate on.

7) Make some sort of hook like device to hang off the crankshaft pin that weighs exactly the same as the figure you've calculated in "step 4"

8) You want to be able to rotate the crankshaft in any position without the crankpin falling downwards.
Generally it seems to be accepted that a slightly heavy counterweight is desirable (the side opposite the crankpin).
If the crankpin falls downwards, you can either remove metal from the crankpin end, or add heavy metal (like brass or malory metal) to the counterweight end, till you get the crankshaft to balance.

9) Once the crankshaft has been balanced, take it apart and install the big end bearing and connecting rod (thrust washers also if the crankshaft used them) and press the crankshaft together and true the crankshaft.

10) Take your crankshaft and put it in a plastic bag and place it it the freezer. After about an hour, take your crankshaft bearings (for the left and right side shafts) and remove the dust seals if they are installed.
Now drop the bearings into a pan filled with hot oil.
The bearings will expand in the oil and the crankshaft will shrink ever so slightly.
Take the crankshaft out of the freezer and prop it up so the shafts are vertical.
With a pair of smooth faced plyers, take a heated bearing out of the oil and carefully drop it on the shaft, don't delay, do the other side as well.
The bearing should just drop straight on, no pressing or force should be needed.
Make sure you have pressed the bearings into contact with the face of the crankshaft before the bearings cool down and the crankshaft heats up.
If they are not seated squarely on the shaft after everything has equalised in temperature, you will have no hope of bashing them flat and straight - you'll only damage the shaft and the bearings.

11) Before assembling the crankshaft in the case halves, along with the clutch shaft, place the assembled crankshaft with it's crankshaft bearings in the freezer and use heat to expand the cases allowing for an easy fit of the bearings into their respective recievers.
Assembly should take place without too much force being applied on the crankshaft.
If you need heaps of pressure to get the case halves to close up, you'll press the crankshaft out of "true" and the engine will vibrate, regardless that it has been balanced.

12) It's best to install the crankshaft seals after the cases are clamped together.

Although my tutorial may not be perfectly worded, it may be of use to someone wanting to balance and reinstall their crankshaft.

Fabian