# I Force consideration on a gripped object

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1. Aug 9, 2016

### umair20

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?

2. Aug 9, 2016

### Krylov

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.

Last edited: Aug 9, 2016
3. Aug 9, 2016

### CWatters

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 dependant 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.

4. Aug 9, 2016

### umair20

What about the centripetal force? Would we have to consider it?

5. Aug 9, 2016

### Nidum

Draw the free body diagram for the object . Identify what forces have to act on the body to make it follow a circular path .

6. Aug 9, 2016

### CWatters

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!

7. Aug 10, 2016

### umair20

Well, the system is a lathe machine in which a chuck is holding a shaft and it is being turned at high rpm.

8. Aug 11, 2016

### Nidum

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 .

9. Aug 11, 2016

### umair20

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. Aug 11, 2016

### Nidum

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 .

Last edited: Aug 11, 2016