Checking a plate grab for lifting glass

[steelnz2003]

Hi all,
Plate grab is used in a manufacturing facility for lifting. The supplier document allows lifting a single pane or double pane for the same weight.

I am trying to check if we can lift 2 panes of glass at the same time or not.

My judgment would say lifting 2 panes of glass will require more clamping force to keep them in place due to the lower friction between glass and glass but the calculation does not reflect this because we got more contact faces.


I did a spreadsheet attached would appreciate any feedback.

 

[phamENG]

In a single pane of glass, the load to each arm of your plate grab is going to be W/2. With 2 panes of glass, the load to each arm is still W/2. There's no load being transferred between the plates so it's irrelevant. That's why you can't lift 3 panes with the same weight - you'd be relying on a pair of slip planes with a friction coefficient of 0.4, and the center pane would slip. But with only two, each one is in contact with one of your rubber coated arms and they can't slip.

 

[steelnz2003]

Thanks, @phamENG I am more aligned with what you said but when looking at one pane try to slip does it will still have friction between it and the remaining pane. If we assume 3 panes also and you try to pull the middle one it will still get a friction surface.

 

[3DDave]

The friction between the two thinner plates doesn't matter because there isn't a load transferred between them.

If you make a shear free-body diagram there would be a shear load up at each face where the cups are and then the load would decrease to zero through the thickness of the glass at the place where the two pieces meet and then the shear would increase due to the weight through the thickness of the other glass plate until it met the other cup.

The same diagram would represent a continuous thickness of glass.

The same is true for any simply supported continuous beam. Of course in a beam there needs to be moment continuity but in the case of the two pieces of glass the plates are large enough that the moment can be resisted by contacting loads on the faces, though if the pair of glass is gripped too closely to the upper edge that might not work out well.

If there is a third piece of glass in between two others the zero shear is still half-way between the cups, but it will not be zero where the interfaces with the outer glass plates are. It is there that the friction of glass-on-glass will be a problem and since there is no compression except by the moment-carrying loads, it is likely the middle one will slip.

Edit: I'm assuming the use of suction cup grabbers; if they are clamps then there will be more friction for the third plate, but it doesn't change the free-body shear diagram as the clamp loads are normal to effects of gravity.