counterbalanced crank welds

[Phil Brown]

Originally Posted by oldspert

This pic is from your comment on the oil pressure to the center main that you state is the way you do the tube. The picture shows a lot more weld than tack to keep the weight from moving. I still believe the bolt should be strong enough to keep the weight from slinging off into the pan, but this crank has a lot more weld than just a tack.
Ed

Probably no bolts holding the counterweights on that crank, just 100% weld around the weight. Two different ways to get the same result, both work fine

[Pete]

If you knew the minimum weld that would hold the weight WITHOUT any bolts, you would be surprised.

[oldspert]

Originally Posted by Pete

If you knew the minimum weld that would hold the weight WITHOUT any bolts, you would be surprised.

I would think that would depend on the alloy of the rod being used to weld the weights. 7018 would be stronger than say 6011 because of the tensile of the alloy, and greatly depend of the penetration depth of the weld too.
Ed

[BRENT in 10-uh-C]

Originally Posted by Pete

If you knew the minimum weld that would hold the weight WITHOUT any bolts, you would be surprised.

Yes, this is very true.

Pete, I am not sure I know the answer to this, but I am curious. Does the weight have greater forces that would cause it to want to separate from the crankshaft at 2,500 RPM vs 7,500 RPM? My instinct would say yes. If this is indeed true, I think this affects the method of how the mounting of the weights should be.

Originally Posted by oldspert

I would think that would depend on the alloy of the rod being used to weld the weights. 7018 would be stronger than say 6011 because of the tensile of the alloy, and greatly depend of the penetration depth of the weld too.
Ed

You are really overthinking this. As a teenager, I used to build car trailers and small utility trailers using a welder. I sucked at welding (-honestly still do ) and so my mindset was weld on all four sides, -and add more weld if I could find a place. Later I learned from multiple people that my additional welds did not make it any stronger. In my picture above, that was one of the first 20 or so that I did, but as I progressed and thought about it in detail, I came to realize I really was not gaining anything.

[katy]

Take it to a place that does welder proficiency exams and have them inspect the welds.

[nkaminar]

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Centrifugal force is the force trying to separate the weight from the crankshaft. The formula to calculate centrifugal force is F=mw^2r, where F is the force, w is the angular velocity in radians per second, and r is the radius of the center of mass of the mass. The angular velocity is squared in the formula so twice the rpm will give 4 times the force. The center of mass for a two dimensional part, like the counter weight, is the center of area as viewed from the side. It can be estimated or measured by hanging the weight and a string with a weight on it at the same point. A mark is drawn on the weight were the string lies. If you do this at two points, where the two lines cross is the center of mass. If the weight is symmetrical (they are), then the center of mass lies on the axis of symmetry.

In Brent's example above, the force at 7,500 rpm is 9 times what it is at 2,500 rpm. 3 squared = 9.

The angular velocity can be calculated by multiplying the rpm by 2 pi radians per revolution and divided by 60 seconds/minute. The radius has to be in feet. The mass has to be in slugs, which is the weight in pounds divided by 32.2 feet/seconds squared (gravity).

If you do the calculation, check the units. The result should be pounds force.

The centrifugal force is only part of the story. There is vibration and acceleration to consider. A big factor of safety has to be added, say 3 times or more what is calculated.

There will be a pop quiz on Monday.

[oldspert]

Originally Posted by nkaminar

Centrifugal force is the force trying to separate the weight from the crankshaft. The formula to calculate centrifugal force is F=mw^2r, where F is the force, w is the angular velocity in radians per second, and r is the radius of the center of mass of the mass. The angular velocity is squared in the formula so twice the rpm will give 4 times the force. The center of mass for a two dimensional part, like the counter weight, is the center of area as viewed from the side. It can be estimated or measured by hanging the weight and a string with a weight on it at the same point. A mark is drawn on the weight were the string lies. If you do this at two points, where the two lines cross is the center of mass. If the weight is symmetrical (they are), then the center of mass lies on the axis of symmetry.

In Brent's example above, the force at 7,500 rpm is 9 times what it is at 2,500 rpm. 3 squared = 9.

The angular velocity can be calculated by multiplying the rpm by 2 pi radians per revolution and divided by 60 seconds/minute. The radius has to be in feet. The mass has to be in slugs, which is the weight in pounds divided by 32.2 feet/seconds squared (gravity).

If you do the calculation, check the units. The result should be pounds force.

The centrifugal force is only part of the story. There is vibration and acceleration to consider. A big factor of safety has to be added, say 3 times or more what is calculated.

There will be a pop quiz on Monday.

You sound like a mechanical engineering, or physics professor. But all you have pointed out is true.
Ed

[Pete]

The answer to the test question is:
At 48 million rpm, the .200 inch diameter steel ball bearing vaporized.
It was levitated in a vacuum in a magnetic field and rotated by an RF signal applied to a fixed stator.

In the case of the standard 3 main model B crankshaft, it had Dan Price full circle press on weights with 3, 1 inch diameter Mallory slugs pressed into each weight and the overall diameter of the crank reduced by 1 inch. After the weights were pressed on they were tig welded all around with ER70 rod.
As much weight as possible was machined off the throw sides of the crank. It was final straightened and the journals were nitrided.
This flathead 4 intake port engine turned 6500 twice a lap for one season of 6 vintage race events.
It was not as smooth vibrationwise as a V8 but was far better than any stock crank model I ever saw.