Discussion in 'Burners and their construction' started by Mark's castings, Feb 4, 2018.
Most impressive! That outer tube is nice and straight. What is the wall thickness?
It's a piece of mystery stainless steel alloy with very close to 16mm bore and 18mm outside diameter which would give it a 1mm wall thickness. The 12mm tube came from a hydraulic shop and both are seamless tube.
The 19" long test failed in part as the tube had a slight bend at one end which I cut off for the 12" test piece.
The new cup has been machined and tested, again it's mystery stainless probably intended for marine prop shafts and it machined surprisingly well. It's 44mm versus the original cup unit's 27mm which translates to a 63% larger diameter and should have the same rim surface speeds as the original at 17K RPM. I've tested it to 24K RPM and the balance is not perfect, as it has several low vibration sweet spot speeds including 18K RPM which is again ideal for this unit. The vibration is not visible but can be felt by touching the inner fixed tube.
It has opposed grub screws drilled in one operation in an attempt to aid balance. I had to remove it from the four jaw chuck to machine both ends as it was a short stub of bar: a longer length would have allowed complete machining in one session on the lathe which would be a far more accurate and balanced object as you could bore inside and machine outside before parting it off in the lathe. The only flaw in design so far is there's nothing to stop the shaft moving along the tube as needle roller bearings don't retain the shaft in position, possibly a drive pulley and tensioned belt will be enough to maintain position. Cone angle is 20 degrees included angle which had to be truncated due to the length of the piece. Grubscrews are not very tight to avoid crushing the tube over the needle roller bearings
I realize this is a prototype. For a more balanced cup could you braze the cup stock to the tube and do the finish machining with the tube held in the chuck? That way it would be dead on concentric and balanced. I love this device already.
One simple way to locate the bearings on the fuel line would be to simply drill a small,say, 1/16” cross hole in the fuel line and then peen a pin in the hole. Make the pin a little longer than the outside diameter of the fuel tube. Place pins on either side of desired bearing locations. Even if the locations were spaced at somewhat greater distances than bearing width and the bearing could travel a fraction of an inch, it would still be acceptable.
Or, most likely the tube is cooled enough by combustion air and fuel flow so that Loctite could be used. But, if on shutdown and the burner assembly was not removed, the tube would heat up, the Loctite would, obviously, melt. I would probably use pins.
I took the easy route and Loctited them into the cup shaft, I'll have to retract the unit on shutdown. The whole cup shaft can move up and down the fuel tube if the belt and pulley don't keep it in position, I'll have to fit collars to the fuel tube: one could also be the end cap fuel deflector inside the cup. Retracting will also stop the cup end bearings from burning the lube and coking up....hmmm the design is getting complex again.
The bearings are not much more than an outer cup and needles, they are a snug fit for the cup shaft although tube tolerances wont guarantee that. I haven't even checked them to see if they have a metal cage or a phenolic one....probably metal.
If stainless tube is anything like stainless sheet, it will distort badly when hot enough to silver solder, if my machining skills were better I'd go for a shrink fit.....actually that's what I was aiming for: it was very snug but not tight . I think being able to machine the entire cup and thin the cup in one fixturing in the lathe would make it light and balanced enough to work well.
"if my machining skills were better I'd go for a shrink fit.....actually that's what I was aiming for: it was very snug but not tight "
Isn't that what they make knurling tools for? Not that I've ever used them for fixing an undersize journal...
Keep up the good work!
Are you going to adding a central defuser of some sort to this new cup prior to testing...?
Yes: I'll either fit wadded up stainless swarf or a interference fit end cap with four holes and rely on a pulley and belt to keep the shaft from riding up the fuel tube. Any end cap made from bearing alloys could get too hot in theory. By the way your earlier drawing of a fuel tube free design in post 157 is shaping up to be a simpler, more rugged, stronger design that keeps the ball bearing well away from the furnace heat.
Got a bit of work done on the Mk3 unit: The plan is to have a base for the fuel tube and shaft that can support the fuel tube and shaft when spinning and slide up and down the air pipe to move the cup away from the furnace heat when the air is shut off. So I have an aluminium block that clamps the fuel tube with some grub screws and has two bolts on the bottom to attach to the 3mm thick stainless base. the base is welded to two rings that have reduced diameter to fit inside the air tube. I now have to mount a drive motor, pulleys and belt with some cover to prevent major leaks yet allow sliding up and down the tube 6 inches or so. The little motor tool is 135 Watts, I pulled it apart to see if I could fit the tiny motor inside the pipe with a belt drive but it has integral brush holders and the bearings are also mounted in the plastic molding.
I don't know that the duty cycle of that tool will permit you to get as much use out of this as you'll need or not. I am watching with much curiosity.
I have a few units, each with at least 100 hours use time spread over 15 years of time with no failures. that red one has a minimum of 30 hours use, after 4 days routing MDF with 3mm carbide cutters and grinding rust out of a car body. Yesterday's look at the internals showed no serious issues with it (and no way to remove the motor from the plastic case) so I closed it up again.
They seem to be pretty cheap and fairly common, I've yet to even try it with this shaft assembly to see if 1/6th Hp is enough to drive it without overloading. If you cut the first inch or so off the plastic case at the collet end with a bandsaw, you expose a steel shaft about 5/16" diameter that's closer to the ball bearing which would be ideal for the drive pulley.
If it does fail I have spares, so I can get it running quick enough to finish a casting session before fitting a unit with more grunt like a plunge router.
I've been struggling with the "Fire Brand" multi tool driving the shaft up to speed with it running too hot. I chalked it up to a lack of horsepower/duty cycle (Like Jason suggested) and went out and bought a cheapo $20 angle grinder today and carefully cut off the right angle gearbox. The unit runs at 32 000 RPM no load rated at 500W, it has a tiny stub of soft steel and I grafted a pulley on the end and gave it a run. Things seemed pretty good, bit more vibration than before with the router motor but it was still getting hotter than expected. Right about then, with the bore of the pulley getting flogged out by the motor shaft, I realised I had the 2:1 pulley ratio the wrong way round and was trying to drive things at 60 000 RPM ....I knocked off for the day at this point and will leave it alone until tomorrow
I machined the pulleys today, the small one on the grinder stub shaft is mild steel with a single grub screw: if it fails I'll have to graft a shaft on the end of the armature and fit a larger end bearing. The shaft pulley is lightweight aluminium and I was able to shrink fit it to the shaft nicely. The final reduction ratio is about 2:1. After some thought on mounting the grinder so belt tension is adjustable and thinking of elaborate hinges and springs, I noticed the threaded holes for the left/right side handles on the grinder and was able to weld up some mounts that bolt on the holes. The weight of the grinder provides the tension....simple, elegant and it runs so smoothly now I had to video it running on the bench and put it on Youtube. I seem to have hit the vibration cancellation jackpot, I once built a belt drive disc sander with a 2:1 pulley ratio that also ran smooth and had no critical speed where the belt flaps. I'll start work on cutting a section of stainless tube for it to slide in and make a removable stainless sheet cowling....then it's time for a test!!!.
Nice stuff... I am a little concerned again with the duty cycle of your motor. Speaking strictly for myself, my "burner time" for a melt is in the 30-45min range. Do you think an angle grinder can be ran for that long...?
I'm concerned about the build quality of the bearings and the carbon brushes in a $20 grinder. In it's favour, it's not heavily loaded with the tiny contact area of the pulleys unable to load it much. In operation the tacho shows close to the no-load running speed. Finally it's got a cooling fan....and situated in a forced air chamber. It's possible the airflow is too fast for adequate cooling due to poor transfer of heat: maybe boundary layer effects.
Worst case scenario is that I derate the universal motor with a speed control and size the pulley accordingly. Halving the power dissipation has got to be good for the motor lifetime.
The target speed for this furnace is 20 minutes for an A20 of bronze when cold and 15 minutes per pot after that (hopefully). This is in line with an existing propane gas fired furnace performance.
Watching "how its made" the other day, i saw them using a spinning cup for painting new cars on the assembly line. Certainly a testament to the evenness and ability to atomize materials.
I had a look around online, there's a lot of study going on for industrial painting use. I vaguely remember seeing a video of a cabinet factory using one. It seems they solve the flat droplet pattern with electrostatics to steer them towards the object. One German guy did his masters degree on the topic and has a nice photo of "strings" of paint coming off the cup a fair distance before breaking up into tiny droplets.
So it's slowly edging towards completion, I spent today "chassis bashing" (sheet metal working) and have the framework for the cover in place and a 0.6mm sheet stainless cover made. The air tube has been cut away to allow the spinning cup "sled" to be fitted and allows about 10" of cup retraction. The sled can be moved by holding the fuel valve to slide in into position and I'll fit a plastic elbow with a hole in it to allow the fuel tube to exit the air tube.
Can't wait to see the fire up.
You could turn the belt inside out so the smooth side is on the pulleys for better contact.
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