Designing a Low Speed Crank Mechanism: Counterweight Calculation

In summary, you need to balance the mass of the counterweight with the mass of the crank to keep the mechanism balanced. You can approximate this by treating the masses as point masses rotating about an axis.
  • #1
kevjcarvalho
42
0
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
 
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  • #2
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?
 
  • #3
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.
 
  • #4
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
 

1. How do you determine the required counterweight for a low speed crank mechanism?

The required counterweight for a low speed crank mechanism can be determined by first calculating the unbalanced force of the rotating mass and then using this value to calculate the required mass of the counterweight. This can be done using the formula: Counterweight mass = (rotating mass x radius of rotation squared) / (distance from center of rotation to counterweight x acceleration due to gravity).

2. What factors should be considered when designing a low speed crank mechanism?

When designing a low speed crank mechanism, the following factors should be considered: the desired speed of rotation, the size and weight of the rotating mass, the size and weight of the counterweight, the distance between the rotating mass and the counterweight, and the strength and efficiency of the materials used for the crank mechanism.

3. How does the counterweight affect the performance of a low speed crank mechanism?

The counterweight in a low speed crank mechanism helps to balance the unbalanced forces of the rotating mass, which in turn reduces vibration and increases the efficiency of the mechanism. Without a counterweight, the crank mechanism would experience excessive wear and tear and may not function properly.

4. Can the counterweight be adjusted to improve the performance of a low speed crank mechanism?

Yes, the counterweight in a low speed crank mechanism can be adjusted to fine-tune the performance of the mechanism. By adjusting the mass or position of the counterweight, the unbalanced force of the rotating mass can be better balanced, resulting in improved performance and efficiency.

5. Are there any limitations to using a counterweight in a low speed crank mechanism?

One limitation of using a counterweight in a low speed crank mechanism is that it adds additional weight and complexity to the overall design. This can be a disadvantage in certain applications where weight and size are critical factors. Additionally, the counterweight must be carefully designed and positioned to ensure proper balance and performance of the mechanism.

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