connecting rods weighing question

[digger]

To me this does not sound right w/o making some far reaching assumptions about the distribution of mass in the rod. I think I can see a scenario where two rods have the same total wt and center of gravity yet have drastically different end weights.

If the total mass is the same and the end weights are the same the c.of.g is the same. It’s a statically determinate problem

The GKN rods (first large scale production engine use of sintered rods) could have two rods with coincident small end borings relative to rod morphology but with one boring pair rotated around the centre of the small end. This would shift the positioning of the big end bore through an arc. This would yield two rods with the same big and small end weights but varying centre of gravity (due to the centre of mass following the same radian sweep but along a shorter arc.


Anyways, small things can make a difference.

Kevin,

I'm not sure what you are describing but its a 3 point force system for static equilibrium. So if:
- the total weight of two rods are the same;
- both are same length; and
- the big end and little end masses are the same

then the c.of.g is in the same point along its axis where the axis is defined center to center.

it doesn't matter if ones a straight rod and the other is a banana shaped one.

[Kevin Johnson]

If the total mass is the same and the end weights are the same the c.of.g is the same. It’s a statically determinate problem

The GKN rods (first large scale production engine use of sintered rods) could have two rods with coincident small end borings relative to rod morphology but with one boring pair rotated around the centre of the small end. This would shift the positioning of the big end bore through an arc. This would yield two rods with the same big and small end weights but varying centre of gravity (due to the centre of mass following the same radian sweep but along a shorter arc.


Anyways, small things can make a difference.

Kevin,

I'm not sure what you are describing but its a 3 point force system for static equilibrium. So if:
- the total weight of two rods are the same;
- both are same length; and
- the big end and little end masses are the same ...

The structure of your argument "attempts" (granting it agency) to import an assumption that equivalent topological masses have an equivalent centre of mass.

... then the c.of.g is in the same point along its axis where the axis is defined center to center.

it doesn't matter if ones a straight rod and the other is a banana shaped one.

... In many equilibrium situations, one of the forces acting on the body is its weight. In free-body diagrams, the weight vector is attached to the center of gravity of the body. For all practical purposes, the center of gravity is identical to the center of mass, as you learned in Linear Momentum and Collisions on linear momentum and collisions. Only in situations where a body has a large spatial extension so that the gravitational field is nonuniform throughout its volume, are the center of gravity and the center of mass located at different points. In practical situations, however, even objects as large as buildings or cruise ships are located in a uniform gravitational field on Earth’s surface, where the acceleration due to gravity has a constant magnitude of g = 9.8 m/s2. In these situations, the center of gravity is identical to the center of mass. Therefore, throughout this chapter, we use the center of mass (CM) as the point where the weight vector is attached. Recall that the CM has a special physical meaning: When an external force is applied to a body at exactly its CM, the body as a whole undergoes translational motion and such a force does not cause rotation.

When the CM is located off the axis of rotation, a net gravitational torque occurs on an object. Gravitational torque is the torque caused by weight. This gravitational torque may rotate the object if there is no support present to balance it. The magnitude of the gravitational torque depends on how far away from the pivot the CM is located. ...

Moving from a uniform gravitational field to the non-uniform accelerational field of rod movement, torque values will be introduced about the centre of mass and when this set of values differs from rod to rod, they are dynamically imbalanced.

[digger]

The GKN rods (first large scale production engine use of sintered rods) could have two rods with coincident small end borings relative to rod morphology but with one boring pair rotated around the centre of the small end. This would shift the positioning of the big end bore through an arc. This would yield two rods with the same big and small end weights but varying centre of gravity (due to the centre of mass following the same radian sweep but along a shorter arc.


Anyways, small things can make a difference.

Kevin,

I'm not sure what you are describing but its a 3 point force system for static equilibrium. So if:
- the total weight of two rods are the same;
- both are same length; and
- the big end and little end masses are the same ...

The structure of your argument "attempts" (granting it agency) to import an assumption that equivalent topological masses have an equivalent centre of mass.

... then the c.of.g is in the same point along its axis where the axis is defined center to center.

it doesn't matter if ones a straight rod and the other is a banana shaped one.

... In many equilibrium situations, one of the forces acting on the body is its weight. In free-body diagrams, the weight vector is attached to the center of gravity of the body. For all practical purposes, the center of gravity is identical to the center of mass, as you learned in Linear Momentum and Collisions on linear momentum and collisions. Only in situations where a body has a large spatial extension so that the gravitational field is nonuniform throughout its volume, are the center of gravity and the center of mass located at different points. In practical situations, however, even objects as large as buildings or cruise ships are located in a uniform gravitational field on Earth’s surface, where the acceleration due to gravity has a constant magnitude of g = 9.8 m/s2. In these situations, the center of gravity is identical to the center of mass. Therefore, throughout this chapter, we use the center of mass (CM) as the point where the weight vector is attached. Recall that the CM has a special physical meaning: When an external force is applied to a body at exactly its CM, the body as a whole undergoes translational motion and such a force does not cause rotation.

When the CM is located off the axis of rotation, a net gravitational torque occurs on an object. Gravitational torque is the torque caused by weight. This gravitational torque may rotate the object if there is no support present to balance it. The magnitude of the gravitational torque depends on how far away from the pivot the CM is located. ...

Moving from a uniform gravitational field to the non-uniform accelerational field of rod movement, torque values will be introduced about the centre of mass and when this set of values differs from rod to rod, they are dynamically imbalanced.

Kevin,

don't complicate simple concepts unless it is needed.
We are talking about weighing rods on a static fixture, not engine operation and how they behave. As noted above C.og.G and mass moment of inertia are different. Two rods can weigh the same have the same C.og.G but behave differently in motion. This is not what is being discussed.

[modok]

A rod that's fat in the middle VS another that's fat at the ends can be equivalently balanced.

But, what if one has a performance advantage over the other?

Then it's interesting

[Kevin Johnson]

...
Kevin,

don't complicate simple concepts unless it is needed.
We are talking about weighing rods on a static fixture, not engine operation and how they behave. As noted above C.og.G and mass moment of inertia are different. Two rods can weigh the same have the same C.og.G but behave differently in motion. This is not what is being discussed.

The static fixture in the ad uses multi-dimensional torques.

In general, I think it is important to notice that "points" in a 2-d analysis are in fact provided by lines (edges) in 3-d or multiple contact points within erstwhile concentric cylinders. It is easy to play fast and loose with simplifying assumptions. The danger comes when the assumptions made are forgotten.

[digger]

...
Kevin,

don't complicate simple concepts unless it is needed.
We are talking about weighing rods on a static fixture, not engine operation and how they behave. As noted above C.og.G and mass moment of inertia are different. Two rods can weigh the same have the same C.og.G but behave differently in motion. This is not what is being discussed.

The static fixture in the ad uses multi-dimensional torques.

In general, I think it is important to notice that "points" in a 2-d analysis are in fact provided by lines (edges) in 3-d or multiple contact points within erstwhile concentric cylinders. It is easy to play fast and loose with simplifying assumptions. The danger comes when the assumptions made are forgotten.

i haven't even seen the fixture you refer to was responding to Nut124's comment.

it's not simplified assumptions it doesn't matter if 2D point or 3D line its actual fundamental force equilibrium i.e. laws of physics. if you're measurements don't align then this is because of artifact with the test apparatus e.g no such thing as frictionless or a knife edge, equipment accuracy/ tolerances, physical setup e.g. how level it is etc etc. Those effects can be all but eliminated with good equipment

[Kevin Johnson]

Mr. Daminato from Rossi & Kramer wrote this morning concerning the device gunt asked about:

any thoughts

https://theultimatetooling.com/en/produ ... -balancer/

https://translate.google.com/

~~~~~~~~~~~~~~~~~~~~~

Buongiorno Sig. Kevin,

Il sistema operativo prevede:

Pesare separatamente tutte le bielle
Scegliere la più leggera come biella di riferimento
Eseguire l’operazione di controllo della distribuzione dei pesi con il nostro attrezzo

A sua disposizione per eventuali ulteriori informazioni.

Cordiali saluti.

Antonio Daminato

Translation:
...

The operating system provides:

Weigh all connecting rods separately
Choose the lightest one as the reference connecting rod
Perform the weight distribution control operation with our tool

At your disposal for any further information.

Best regards.

Antonio Daminato

~~~~~~~~~~~~~~~~~~~~~~~

Rossi & Kramer obviously has excellent customer service. They created and uploaded a video specifically because of questions asked in this thread.