Tuesday, February 19, 2019

Tennis ball pump

It may be remembered that I have an abstract interest in pumping up tennis balls.

A plastic tube, all this past fortnight.

This interest resurrected itself when a tubeless tyre refused to hold pressure and I had recourse to the bicycle shop which has a compressor.  There I found a New Device called a flash pump into which we will not go since Google will take us there if we're interested and anyway nobody is. Reading this. Unless I'm sore mistaken.

Persons unknown have made their own flash pumps all over Youtube and you may youtuble them, I expect, if the subject enthralls.

I have confined myself to gluing ends on a bit of PVC pipe, one of which screws off and didn't quite seal on an 80mm O-ring cos I couldn't get enough torsion twixt lid and pipe.  Accordingly I machined a large tight hole in a bit of oak and chopped it in half and got a better purchase on the pipe in the bench vice and that worked: the tyre pressure gauge revealed that 60psi still remained within from Waitangi day until this afternoon, viz., for two weeks.

JE Gordon (p119-123, Structures) told me that the pipe is twice as likely to burst like a sausage than the ends are likely to shoot off, but they - the ends - are a bit bulging and I housed this thing out in the shed in case I got that wrong and it transformed itself into a pipe bomb.  Which is itself stupid: I should have put it in the grapefruit tree and then only grapefruits rather than sheds will be damaged.

The balls were sealed inside and the test to see if some of the 60psi leaked into them was done by dropping them on a concrete floor and seeing if they bounce as high as a new tennis ball. They didn't. Well, one did, but four others were mediocre and one bounced with little enthusiasm. So they're back in the pipe and back up to 60psi for another week and we'll see how long it takes before they all go bouncy.

The pipe, if this is of interest, has an OD of 82mm and a wall thickness of 3.4mm and somehow or other I calculated this would give a safe working pressure of 115 psi but I can't recall what possessed me to come to that conclusion. Was it a website somewhere? Anyway it seems safe at 60 psi. The inlet valve is a Presta valve cut from an old inner tube, sealed on the inside merely with the remnant of the rubber inner tube and the locking ring tightened with pliers on the outside of the end cap. 

What all the Youtublers do is fit a second Presta valve, connect it to the putative tubeless wheel valve with a bit of soft plastic pipe, fold the plastic pipe over and nip it closed with a Mole grip.  The device is pumped to something approaching pressure vs volume, the Mole grips flicked open, and the outrush of air is claimed to seat the tubeless tyre on its rim.  Some Youtublers claim that a plastic fizzy drinks bottle will hold 150 psi and so it might if it's a small one, but I have bitter experience of this and can assure you a three litre fizzy drinks bottle will swell and burst at 130psi and woe betide your hand if you chance to be holding it while this bursting occurs.  Compressed air is a lot more dangerous than folks imagine.

There are so many videos online that I can't be bothered to locate one for viewing pleasure, but the general clues used by Mr and Mrs Google are cheap ghetto tubeless inflator.

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