Air pressure tech question

Harry Bloomfield

Harry Bloomfield

Joined
30 Dec 2018
Messages
22,080
Reaction score
3,949
Location
Up North
Country
United Kingdom
Imagine a flat until inflated air bladder, around 330mm x 740mm, onto which a caravan wheel is driven, then you inflate the bladder to raise that wheel to get the caravan side to side level. Assume that caravan weighs 1400Kg and so the bladder has to raise half of that 700Kg and is a single axle, just two wheels on the road. Question is - how much pressure is needed in the bladder to raise that wheel off the ground. The bladder is rated for a maximum pressure of 30 psi / 2bar.
 
Sponsored Links
The problem I would think is the lack of weight distribution , the bladder would simple inflate around the wheel . The wheel would need to be driven onto a flat plate larger than the bladder and would still be very unstable when inflated .
 
foxhole said:
The problem I would think is the lack of weight distribution , the bladder would simple inflate around the wheel . The wheel would need to be driven onto a flat plate larger than the bladder and would still be very unstable when inflated .
Click to expand...

No, it works well enough - the bladder inflates around the edges of the tyre, then the tyre begins to lift once there is enough pressure. Imagine that reinforced flat hose, used as the discharge pipe for a portable pump - a section of that. Each open end is clamped by bolted together heavy steel bars. near middle at one side is a Schrader valve to inflate it.
 
Sponsored Links
It will depend on the surface area of the wheel that makes contact with the bladder.

To raise 700kg/ 1540lbs up would require a surface area 51 square inches at 30psi
So approximately a footprint of 10" x 5"
As the Caravan is raised, it will need slightly less and less force, as the centre of gravity moves over.
 
Basically, an air jack will lift a lot.
but you’d need screw-down legs or similar to stabilise the caravan.
 
Tigercubrider said:
Basically, an air jack will lift a lot.
but you’d need screw-down legs or similar to stabilise the caravan.
Click to expand...

Yes, which it has - this is just the initial task of making it side to side level. I'm curious as to what sort of pressure is involved.
 
I had air ride on my motorhome. Minimum was recommended as 1.5 bar and maximum was about 3.5 bar. 2 bar raised the rear about 4 inches on the ride height.
 
www.ps-stage.com

PNEUMATIC LIFT ROLLER II

PNEUMATIC LIFT ROLLER II WITH WHEEL OF 50 MM (HEBEWUNDER)
www.ps-stage.com www.ps-stage.com

These lift 400Kg per unit, and operate on 2Bar.
If for a caravan, although the above too expensive for your needs (?) the jack may do if it works on similar pressure?

The stage ones work with a pre-charged reservoir . You could have a small cylinder in the caravan topped up at a garage.
 
Tigercubrider said:
www.ps-stage.com

PNEUMATIC LIFT ROLLER II

PNEUMATIC LIFT ROLLER II WITH WHEEL OF 50 MM (HEBEWUNDER)
www.ps-stage.com www.ps-stage.com

These lift 400Kg per unit, and operate on 2Bar.
If for a caravan, although the above too expensive for your needs (?) the jack may do if it works on similar pressure?

The stage ones work with a pre-charged reservoir . You could have a small cylinder in the caravan topped up at a garage.
Click to expand...

Those are the same idea, but obviously just the bare bladder. My concern is the sheer weight of steel used at each end (similar idea to the black strips in that link) - 2/3rds of the weight of the unit is in the steel. Were it not for that weight of steel it would be much lighter. I'm a bit tight on payload, so looking for alternatives to replace the steel.

I have tried inflating it with the weight of the caravan on it, using an high pressure, bike pump of the full height vertical two handed variety. That works reasonably OK, but they normally use a 12v car tyre inflator.
 
13" X 30" gives 540 square inches of surface area. 1540lbs over that area would be 3psi. That assumes a perfectly square surface which is obviously not correct but it gives a starting point.

Or another way to think of it is that at 30psi one tenth of the wheel would need to be in contact with the bladder. Or a plate one tenth the size of the bladder would be needed on top for the wheel to rest on.

A larger bearing surface would mean the caravan would lift at a lower pressure.
 
Last edited:
My mate had some purpose designed bags that you inflated via your exhaust pipe

They would lift a car up
 
transam said:
My mate had some purpose designed bags that you inflated via your exhaust pipe

They would lift a car up
Click to expand...

The exhaust would need to be proof against the pressure, a petrol engine would be able to produce >130psi.

I used to have a device which screwed in, in place of a spark plug, which included a valve, that then used a long flexible pipe to blow up a tyre. You would run the engine on the other three/ five cylinders. I don't know what effect the petrol air mix had on the rubber of the tyre, but it worked extremely well.
 
You must log in or register to reply here.
Sponsored Links
Back
Top