Boing! has gone on a diet – both in terms of size and in terms of hitting power.

Let’s display that ever-so-loved spec sheet.

  • 2 x NTM 4258 Propdrive Motors (500KV)
  • 7.2:1 Reduction, 80mm driven diameter – 14.5mph top speed.
  • 2x 6S Batteries, 2650mAH NanoTech’s.
  • ESCs are TZ85A’s, Simon K’d
  • Pneumatic Axe weapon (details below)
  • Tank tracks! (Because, in the words of the infamous Ollie Lang – Why F***in’ Not)

Seems – if anything – pretty standard. What’s not standard is the weapon system. None of this off-the-shelf pneumatics that only run to 10 bar at maximum. No, this is a custom made pneumatic double acting piston, running at 68 bar (986.257 PSI). It’s got a 63mm bore, with a 160mm stroke. If we plug those numbers into a calculator by our good friends at Trelleborg, we get a cylinder with a volume of 0.498 litres, delivering just over 21000 newtons of force at the full 68 bar.

I’m planning to run a 1 meter hammer arm on this bot, so that should (with any luck, taking into account pressure cushioning, ram speed, etc) be in the region of 20Kj delivered to the top of another robot. It’s not quite what I wanted (the goal was 50Kj), but it should be powerful enough to dent the top of a few hardox-clad robots.

But Rob! You’re Crazy! (At least, that’s what I believe Chris’s inner monologue says)

Well, yes, to a degree. The hard bit of this is making sure that the piston & cylinder arrangement in the middle of Boing! runs a large enough safety factor for me to be OK with. This, for me, means that the cylinder should be able to operate safely at twice the pressure that it’s ever going to see, and will fail completely at 6 times the pressure. We need to find out what pressure the cylinder will work at – this is done with Barlows Formula.

P = Working Pressure in PSI
S =  Yield Strength of the Material
T = Wall Thickness in inches (america!)
OD = Outside Diameter of the cylinder

SF = Safety Factor (in our case, 3, resulting in a complete failure at 6 times the operating pressure)

Our material for this is 7075 Aluminum, which has a yield (note: not complete failure) strength of 500 MPa (73,000 psi).

The thickness of the wall is 2.5mm (or, 0.137 inches), with an O/D of 2.760 inches (or 70mm)

Our safety factor is 3.

All of this spits out the following:

Working Pressure (P) = 2433.333 PSI
Bursting Pressure = 7300 PSI.

That, to me, says that the cylinder will work fine at 2433 PSI, and work near perfectly at the 985PSI I’ll be putting through it.

The above image shows what I’ve got at the moment: The piston, cylinder (the grooves are easy weight to loose and look cool), and the gear mount, from which I’ll drive the hammer arm. These mounts, the cylinder & piston come in at just under 800 grams.

In my next post about Boing!, I’ll be talking about how I’m going to go about feeding the cylinder with all of the CO2 it needs to work. That’ll be coming once I’ve put some thought into designing a high flow, 5/3 solenoid.



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