Force consideration on a gripped object

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Discussion Overview

The discussion revolves around the forces that need to be considered when gripping a cylindrical object in a rotating frame, specifically in the context of a lathe machine. Participants explore the implications of static friction, centripetal force, and other forces affecting grip stability during rotation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that static friction is a critical factor in determining the necessary grip force to prevent slipping.
  • There is a proposal that the maximum friction force must exceed the torque transmitted between the frame and the object, which depends on the clamping force and the coefficient of friction.
  • One participant raises the need to consider centripetal force in the analysis, questioning its relevance in different rotational scenarios.
  • Another participant emphasizes the importance of drawing a free body diagram to identify the forces acting on the object to maintain a circular path.
  • Discussion includes the specific context of a lathe machine, where the workpiece is constrained radially by chuck jaws and the axial movement is prevented by friction.
  • Concerns are raised about the impact of tangential forces during turning on the grip between the workpiece and chuck jaws, as well as the effect of centripetal force at high RPMs.
  • One participant notes that at very high speeds, the radial jaw closing force may reduce, potentially affecting the gripping force, although this is not common with modern equipment.
  • There is mention of safety concerns regarding large chucks at high speeds, including the risk of bursting, particularly with hydraulic and pneumatic chucks.
  • The complexity of the situation is acknowledged when dealing with unbalanced setups and irregularly shaped workpieces.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the relevance of centripetal force and the conditions under which grip stability may be compromised. The discussion remains unresolved as various factors and scenarios are considered.

Contextual Notes

Limitations include assumptions about the coefficient of friction being constant, the specific conditions of the lathe machine setup, and the varying impacts of different types of chucks on grip stability.

Who May Find This Useful

This discussion may be useful for engineers, machinists, and students interested in mechanics, particularly in applications involving rotating machinery and grip stability analysis.

umair20
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If an object say a cylindrical body is gripped in a frame and that frame is being rotated by some mechanism, what forces should be considered on the gripped object in order to know the necessary force required for a proper grip?
 
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I'm not sure what the situation would look like exactly, but I think that at least you need to consider static friction. If you would include a sketch of the model, one could be more precise.
 
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In order for the grip not to slip you would want the max friction force between the frame and cylinder to be greater than the torque transmitted between the same.

The max friction force is usually dependent on the force clamping the two surfaces together (eg the frame to the cylinder) and the coefficient of friction (which depends on the materials). However that's a simple model which assumes the coefficient of friction is a constant and it isn't always like that.
 
CWatters said:
In order for the grip not to slip you would want the max friction force between the frame and cylinder to be greater than the torque transmitted between the same.

The max friction force is usually dependent on the force clamping the two surfaces together (eg the frame to the cylinder) and the coefficient of friction (which depends on the materials). However that's a simple model which assumes the coefficient of friction is a constant and it isn't always like that.
What about the centripetal force? Would we have to consider it?
 
Draw the free body diagram for the object . Identify what forces have to act on the body to make it follow a circular path .
 
umair20 said:
What about the centripetal force? Would we have to consider it?

You will have to tell us more about the system. I've been imagining something like a jar opener used to unscrew the lids of jam jars. They don't tend to rotate the lid fast enough for centripetal force to be an issue!
 
CWatters said:
You will have to tell us more about the system. I've been imagining something like a jar opener used to unscrew the lids of jam jars. They don't tend to rotate the lid fast enough for centripetal force to be an issue!
Well, the system is a lathe machine in which a chuck is holding a shaft and it is being turned at high rpm.
 
Workpiece held in a lathe chuck is positively constrained in the radial direction by the chuck jaws .

Axial movement of the workpiece is prevented by friction acting between workpiece and chuck jaws .
 
Nidum said:
Workpiece held in a lathe chuck is positively constrained in the radial direction by the chuck jaws .

Axial movement of the workpiece is prevented by friction acting between workpiece and chuck jaws .
Do tangential forces acting on the chuck during turning have any impact on the grip between workpiece and chuck jaws? Also, will we consider the effect of centripetal force on workpiece?
 
  • #10
The workpiece is driven round against the cutting load by the tangential friction forces acting between chuck jaws and workpiece . For any given chuck and workpiece combination the maximum available friction force is set by the chuck jaw radial closing force .

It can happen that at very high turning speeds the radial jaw closing force becomes reduced and hence the gripping force becomes reduced . This is not a common problem though with modern equipment and normal working practices .

Tends to be more of a problem with hydraulic and pneumatic operated chucks than with simple mechanically operated chucks .

Main problem with large chucks running at very high speeds is that they can burst . Again not a common problem though it has happened . For this reason all new chucks have a maximum safe operating speed specified by manufacturers .

Discussion so far has been about balanced chucks and round bar workpieces . The situation with unbalanced set ups and odd shaped workpieces is much more complex .
 
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