Prototyping a spinning cup oil burner

The prototype rig with all it's shortcomings has reached it's limits after being pushed further than I imagined, that U turn tube worked well enough to prove the concept, it always was a an awful compromise to the problem. The copper was holding up well to the heat but had black deposits inside and out and had black copper oxide which forms at 350 deg C (662 F) on the last few centimetres. That's more than enough heat to boil oil into vapour (desirable for some furnaces) and cause the surging burn seen in the video. This time the unit was withdrawn from the furnace face on shutdown so what you see occurred during normal operation.




The next unit will be a hollow drive shaft with an oil feed tube running up it's length, all that remains is to find some straight round stainless tube in the right sizes to allow ball bearings to be run on the outside. The shaft tube and feed tube may be larger in diameter than needed to find bearings that fit the tube, I'll have to sort through a pile of offcuts to get suitable pieces for shaft, tube and cone. As far as an oil diffuser at the end of the fuel tube opening: what about a wadded up piece of stainless steel normally used as dish washing scourer with a press fit retaining ring into the fuel tube? (1/2" dia. tube). It'd clog with large particles of crud but allow fine dirt through ok.

 

I'd sooner use a dime or penny with some deep flats filed into it, and then press fit it into the cup. I think your scouring pad will clog up fairly quick with anything other than well filtered fuel.

Whatever is easier I guess...

 

I think the “coin”is the way to go. I would just find or turn a piece of 16ga steel to the correct diameter and cut notches in the periphery. Press it into a groove or tack weld it.

The scouring pad sounds too subject to imbalance—-at high revs it will want to centrifuge off to one side, will tend to plug with combustion products, might easily be ejected, etc.

 

I'd mill four key ways into a piece of round stainless stock, turn it to fit with slight interference and part it off thin.

Figured everyone could get their hands on a penny or dime, and might not have the means to machine something out.

 

I was imagining a stationary fuel tube that extends through the shaft all the way into the base of the cup with the scourer material, stainless swarf really, fitted to the mouth of the fuel tube and be removable if it does clog with some tweezers for replacement. It'd be ideal to have a unit that can be made without too much difficult machining or complex parts if it's going to be used by others who don't have machine shop access.

After my last run with waste oil, I was purging the siphon hose and fuel line into a container and the oil was flowing at least three inches out of the tube in a downward arc, more than enough to miss the cup completely without some kind of deflection sideways. Whatever I end up using, it will need to be removable for cleaning/unblocking when in use.

Edit: if I end the stationary tube short by a few inches inside the rotating shaft with a metal shielded ball bearing on the end, there'd be extra distance in the shaft to spin the fuel and even out flow before it hits the cup taper.

 

...or you could make the "deflector" (if you will) have large enough openings that you need not be concerned with blockages. That's why I suggested the 'dime' approach over the steel wool.

Wondering how long a stationary tube within the drive shaft would last...? You'd be begging for a jam up due to the spalling (sp?) of the inner tube.

 

I'd hope two things would happen: the metal shielded spacer bearing would block off the spinning shaft tube to a certain extent, and if any did get past the shielded bearing, the fluid would have more flow friction travelling down the longer length of the shaft and tube and preferentially go out the cup end. Currently I'm eyeing off a 18" length of mystery stainless tube with an outside diameter of 18.02mm and bore of 16mm as the spinning shaft. Ball bearings seem to jump from a 17mm bore to a 20mm bore so machined sleeves would be necessary unless I can locate 18mm bore bearings. Ideally I'd find a thin wall tube with and O.D. and bore that fit readily available ball bearings. R.P.M. is currently 28 000.

 

...man. I keep forgetting about the scale to which you're building this to...lol. 18mm od seems massive to me. Then again I personally wouldn't be inserting another tube within that one.

 

I don't believe an oil seal was ever needed. We're not talking about a high pressure application here.

I'm more concerned about the life span of the stationary tube within the spinning one.

 

This is what I came up with after much thought about simplifying the design and having a hollow drive shaft design. It has only two ball bearings but relies on the rigidity of the inner fuel tube: I have some high pressure seamless stainless tube intended for high pressure hydraulics that may be the ticket.

 

There ya go.

 

Well, I am unconvinced of the benefit of the fixed-feed shaft design. My reservation is it increases the necessary drive shaft diameter and twice the number of bearing are needed and bearings and shifts of greater diameter are needed. All this to void a small o-ring oil seal.

Before committing to the double-tube design, I would encourage “torture testing” an o ring like you might use for a seal on the ccxombined drive/feed tube. Just hook up a simple joint mock-up lubed with diesel or other oil and spin it for a while to see how it holds up. A pretty small ring (1/4” OD) would be adequate and I think would last a very long time.

I like the creativity and out-of-the-box designs you are coming up wIth and then putting in the energy to actually test them out. Very interesting. I am feeling a bit inferior and like I cheated a bit by using such an off-the-shelf Hago burner.

 

Unless you have a insanely rigid feed tube, I think you're begging for a failure by using it to carry the load of the drive shaft and the loading that the belt drive will place on it. The tiniest bit of deflection created by the drive belt will be increased over the length to the cup (heaviest component) and will may create one hell of a speed wobble. Proof is in the pudding as they say. No way to know for sure without trying it.

I would maintain the drive belt concept of spinning the drive shaft. This would allow access to the back end of the hollow tube. Next would be to fab a cap or create a taper on the inlet end of the hollow shaft with an ID slightly oversized to the fuel line. The fuel line only needs to be inserted beyond this taper. The point is, not only does the fuel need to travel upstream (path of most resistance) to leak out the back end of the drive shaft, but also climb up the taper while overcoming the centrifugal force of the drive shaft.

blue: cap
red: fuel
brown: belt
black outline: hopefully doesn't need explaining at this point...lol

If we're running in bearings and machining parts, it won't be very difficult to get within even wide tolerences. .005" difference between the fuel tube OD and the drive shaft cap ID would allow the fuel to seal itself, especially if we're talking about waste oil.

At that point of me making that comment, I had thought the fuel tube was simply going to be laying full length in the spinning drive shaft. All the cool air in the world wouldn't prevent the softer of the two tubes from wearing out due to friction.

 

Balance issues would be serious issue, just like that You Tube video of a lathe whipping a length of 3" steel bar stock around. I measured the fuel tube and the bore of the shaft and a 1210 needle roller bearing would be a perfect fit but has a 17 000 RPM limit with grease lube. About all I can do with my design is shorten the fuel tube and shaft

That tapered rear cap to pump fuel flow in a given direction is a great idea, even simpler than my concept, it solves the oil flow problem neatly and I'll use it if I can scale it down. Bearing speed is now an issue: only bearings with a 12mm bore can run much over 25 000 RPM. I lucked out with my first design by using such tiny bearings I had lying around.

I have to scale things things up and slow RPM (makes a tapered end cap easier too) or keep it fast and shorten the length as much as possible.