How Does Hovercraft Technology Aid in Heavy Lifting?

[Bob Wood]

I work in a hospital, and recently we acquired a device by which air flow from a small low-pressure pump is fed into an inflatable mattress with many tiny holes on the bottom side. As this mattress inflates, it creates a cushion of air beneath it, capable of lifting a 1,000 lb person so that moving this person from a bed to an operating table is almost effortless.

I've seen these types of low pressure lift systems used in industry, but it struck me that maybe I can use the lift design in my shop to move around extremely heavy objects, if I build a platform (rather than that inflatable mattress) sealed air tight except for the tiny holes in the bottom. Trouble is, I can't find any information that can help me find the equation for lifting in such scenarios (pressure vs. flow, how big a contact area, and how many of what size holes must be placed on this lift contact area) for a given weight.

Does anyone have any experience with this type of lift, or can point me in the right direction for figuring this out?

Thanks all.

 

[Fish4Fun]

Hey Bob,

Welcome to PF!

I know nothing specifically about the air bags you referenced, but a precursory investigation suggests to me two different things are happening.

First, the "lifting" part is achieved by Pressure * Area. If you have a bag with 10psi of pressure and a surface area of 100sq inches, it can support 1000lbs if the weight is evenly distributed. In a similar example, a car with four tires inflated to 30psi each and a total weight of 3000 lbs will have 100 square inches of tire touching the road (in theory, 25 sq. in. per tire).

Now, the next part, the part about it acting like a hovercraft. This is fundamentally different than a bag supporting or lifting some amount of weight. I do not doubt your real life experience with such a device "working", but I cannot imagine how it could "slide effortlessly" across dissimilar surfaces. In general hovercraft are inefficient machines requiring a fairly high power to weight ratio. Assuming the bag you mention is being moved across hard, non-porous surfaces, I suppose "pockets of air" could reduce the sliding friction considerably by reducing the amount of the bag in contact with the surface.

So, from what VERY LITTLE I understand about your bags, I think it is very reasonable to assume you might "lift" a heavy piece of equipment with one, but "moving it" would depend a lot on the surface beneath the bag. I could certainly be wrong.

Fish

 

[Cleonis]

[...] maybe I can use the lift design in my shop to move around extremely heavy objects, if I build a platform (rather than that inflatable mattress) sealed air tight except for the tiny holes in the bottom.

For such a platform to work the limiting factor is the evenness and smoothness of the floor of the shop.

Let's say the floor is very even, and very smooth. Let's say bumps and dimples of the surface are all less than half a milimeter in height/depth. As lifting pads underneath the platform I'm thinking of something like the suction cups that are used to lift window glass (but instead of pullng a vacuum you push air out.)
Lifting the pads one milimeter above the floor would do it. I think the one milimeter gap will present a lot of friction to the flowing air. Keeping the air flow up is doable, I think. And the smaller the gap, the more friction.

Several years ago I assisted in a theater. The lowest level floor had been constructed for the purpose of enabling air-flow lifting. The stage could be made larger or smaller by adding or removing large blocks. A supply of pressurized air was connected, and a section of the stage weighing several tons was lifted from the floor (just a fraction; you couldn't see it was lifted), so that we could move it to another location, simply pushing it around.


The common factor with a hovercraft is that there is an air cushion, but the big difference is that a hovercraft is useful only if it's able to travel over uneven terrain. A hovercraft needs a huge amount of ground clearance, and the skirt provides only a poor seal, so to maintain lift a huge flow of air is necessary.

Inside, in a shop: the more even and smooth a floor is, the less air flow you need to keep a platform (with pads underneath it) lifted.

 

[Bob Wood]

Thanks for the response, guys. Here's a video of the system that gave me the idea:



Today I took one of the mats and counted the holes. On the underside of the mat, there were 10 rows of about 220 holes, each about a half of a millimeter in diameter. Very little air flow, for a mat that claims a 1,200 lb max weight. Do the math, and if you were to have all the holes on a contact surface at one time, you have about a millimeter of air flow per pound. I can tell you from using the darned thing, that it works well, almost easier than it looks on the video.

I told one of the clinical departments to give me one of the mats when they're done with it and I'll test it. Soon, I hope. Then I'll post with the result.