Designing a Low Speed Crank Mechanism: Counterweight Calculation

[kevjcarvalho]

heyy...i'm designing a crank for a mechanism...and i need to counterweight it...is there any formula which tells u how much counterweight to add...its a low speed mechanism and very light...material mostly plastic or wood

 

[xxChrisxx]

There is no set formula to plug numbers into that I am aware of, but you can use an approximation which is treating the masses as point masses rotating about an axis and balance them that way. Basically you want all the little CoG to balance to the CoG ove the overall crankshaft lies on the axis.

This can easily sort the crank's static balance, sorting the dynamic balance and not making the counterweights overly large will affect dynamic balance.

However it probably doesn't need a counter weights if its very light, even if it's not totally balanced. Many counterweights are actually not used to balance the crankshaft (as it could be manufactured with no counterweights and still be balanced), they are put on to reduce peak bearing loads.

I'm assuming this is probably a single/twin cylinder thing if its made of plastic? What sort of RPM are you looking at?

 

[DaveC426913]

Well, the counterweight "formula" is pretty straightforward:

(crank-mass x crank-radius) = (counterweight-mass x counterweight radius)

So, if your crank masses 1kg and and is offset from the axle by 1 foot, then your counterweight should be:
1kg at 1 foot
or
2kg at .5 foot
or
4 kg at .25foot
etc.

 

[kevjcarvalho]

The thing is i am designing a mechanical calculator...for which i am using a geneva mechanism for units to tens carry...the crank must therefore move by a precise amounts...owing to its size something like 8 cm radius it posesses a lot of inertia...even if it moves a little more the wrong number is outputted...the material is most likely deodar(wood)...it is a light weight application which is rotated by hand...so kinda high torque but low speed...thanks