Passive suction cup design rules 1

[RonRivera]

Hello,

I have not been able to find good design documentation for non-vacuum pumped suction cups. I'm working on a passive suction cup application where the cup needs to provide resistance to being pulled up, but otherwise lay flat. Sliding is not as much of a concern.

My first attempt's results are less than spectacular (sketch attached). The cup does not seem to be conformal enough on real-world surface imperfections. The material is silicone with a hardness of about 55 shore A. I think thinning the cup to about 0.7mm and flattening it will provide better results. I can also hopefully package a bit of a diameter increase as well. Does anyone know of design guidelines for suction cup stiffness and angles of attack?

Thank you!

 

[Pud]

Any reason you just can't buy them? All sorts available for end-of-arm robotic tooling as used in plastics manufacture for one.


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[IRstuff]

seems to me that one issue is that as you compress your cup, the contact surface gets smaller, and when it's flat, there's almost no contact surface to keep air out.

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[RonRivera]

I appreciate the feedback

Off the shelf: I'll look into more off the shelf options to benchmark. We have a one-piece molding, but I could incorporate some of the design.

Contact area: It looks like a bit less angle of attack should help in this regard.

 

[3DDave]

Consider how a passive suction cup functions. To cling it needs to produce a lower internal pressure than the surroundings which means it has to force its way away from the contact surface with both enough force and signficant displacement to decrease the pressure of the trapped air.

The design you have encompasses a lot of volume, even when compressed, and little axial spring load. More contact area can make the labyrinth seal back to the atmosphere a bit longer, but a smaller contact area will increase the sealing surface pressure to better conform.

This is why any passive cup I've seen is nearly flat, with only a slight depression. This allows expulsion of nearly all the air. As the load on the cup increases that slight amount expands, lowering the pressure and increasing the working area.

 

[RonRivera]

Thank you all for the feedback. I've updated the design as in the attached. It is flatter with an extremely small nipple on the top. I am frankly not sure how important the nipple is, but I've seen them on some example cups and I believe it will help avoid contact from the center of the cup that defeats the axial load from the cup sealing surfaces. I reduced the wall thickness as well, it gets as slender as 0.6 mm at the leading edge.

Unfortunately I'm constrained by previous geometry, so I can't make too dramatic of changes.

Thanks again!

 

[RonRivera]

Here's the drawing.

 

[Compositepro]

Where on the cup, exactly, do you expect the cup to form a seal to the surface? The bevel at the circumference of the cup is on the wrong side. When you press the cup against the surface, the bevel tip will lift off of the surface. You will still get some sealing at the I.D. of the beveled area but the rubber at greater diameters is useless.

The correct design depends entirely on the surface you are attaching to. If the surface has any dirt, texture or scratches, a good seal is almost impossible and there will be air leakage. The only practical way for a suction cup to work on these surfaces is to use a vacuum pump.

Seals, like o-rings, normally use a very small contact area with the surface so that a small contact force creates a large contact pressure. The contact pressure and the hardness of the rubber determines how well the rubber conforms to the surface.

Keep in mind that the seal is at the O.D. of the cup, and contact force has to be maintained there under all operating conditions. Other areas of contact are good for transferring loads by friction, but are not part of the seal.

 

[RonRivera]

Those are great points.

For at least every commercial suction cup I've benchmarked, the ring of material in contact with the surface when the cup is in use ends up being about half the diameter of the outside of the cup. The bevel does indeed fold up on all of them. I might just be looking at the wrong examples, though some of them work pretty well.

I haven't tested an example with the profile of that in the link below, maybe this would work measurably better.

https://etol-rubber.com/Products/custom/suction-cup/products228.html

 

[Compositepro]

That looks like an effective design. If you are considering manufacturing your own design, another factor you need to consider is the manufacturing process. Molding sharp sealing edges without imperfections, such as microscopic voids due to air bubbles, or resin flash at mold part-lines, can be challenging. These edges are often formed by slicing the rubber after molding with a razor sharp knife blade. This is often done to make windshield wipers or squeegees.