Bushings have a disadvantage. During turning they eat a lot of power.
On the other hand, when used well, those sinterbronze bushings are durable, simple and cheap.
If 40 000 tons battleships could depend on them for the main shafts, I don't think it's a bad option.
A fixed shaft makes for a stronger bot but there will be problems with bronze bushings at higher speeds. Bushings have a maximum velocity value they can run at before they start to overheat and breakdown. The velocity value (in imperial units) is V= .262 x RPM x shaft diameter in inches. For a disk rotating at 15K rpm on a 20mm shaft, V= .262 x 15,000 x .787 = 3,092.91
Unfortunately, when I looked up the maximum V for various types of bushings, the highest value is 1,200 so your bearing will be operating at more than double its maximum rating. You will be losing extra disk speed to friction and the bearings will need replacing frequently. The highest rated bearings are made of oil infused SAE 841 bronze, which are probably called Super Oilite bearings in the UK. Other types had much lower velocity ratings, so if you do go ahead with bronze bearings check that velocity rating carefully!
The ratings of those bushings are measured in 100's or 1000's of hours of use.
How many real working hours do our machines have?
And replacing a £3 bushing after each fight.... Almost the same as refilling a CO2 bottle.
And yes, I wrote that bushings ain't the most efficient bearings. But I like'm.
also… the bushings in battleships will have a constant pressurised oil feed id guess…
After a string of failures, Aussie builders gave up on using bushings for weapon bearings. Back in the day, those failures were probably in bearings only doing ~ 5,000 RPM so at 10K or 15K, the failure rate will be magnified and the friction losses could mean that a larger motor is needed to get the target speed. I like bronze bushings too, I'm just quoting what my engineering books say about the RPM limitations.
Most 20mm ID ball bearings have an OD between 42 and 47mm, which makes fitting them in the hub of a small diameter disk a PITA. Perhaps a heavy duty needle roller bearing (or two) is the best alternative? They can take high RPM have a small diameter and are relatively cheap. The only drawback is that they have relatively low load performance, so they may need replacing after major impacts.
Here is an idea: find a bushing with the same OD as a needle roller bearing; if the bushing doesn't work out, you can just graduate to the roller bearing.
In my limited experience I'd say go for needle bearings over bushings, I used a bushing on Lynx's bar and even at half speed (4500ish rpm) there was enough friction to wear it down to the point it now wobbles on the shaft it was once pretty much a perfect fit for, it also deformed pretty badly. Bearings whilst probably not as strong will be much more efficient.
Yep, in battleships the bushing got pressurized oil, and a lot lower RPM.
On the other hand, the running time was measured in years and 100 000's of miles.
We'll see, Valkiri ran on bushings up to now, and the rebuild will use the same bushings.
That sounds a good shout there Nick! If I can adapt a design, then it's solid really. Would rather get it first time but the way I see it is that bearings have more moving parts, therefore there's more to go wrong. That said, pushing the bushings that far over their intended speed could induce failures still...
I think I might up the disc width to 20mm instead, which may leave more room... Needle rollers sound a good option though, if they're cheap enough I can definitely stock up on spares if that's the case. Will get on the CAD programs, get designing it and see what I can fit in, probably the best bet here!
Right, almost got everything sorted ready for the build parts-list wise, going to start cardboarding the lot up to see what fits and what doesn't, bearing in mind I need 3 ESCs, bulkheads, a disc, two batteries (unless I can get everything to run on a single 4S) and two right angle drives in the machine it'll be a tight fit. Already got it to fit just about in CaD but I'd rather fit it together in the actual machine to make sure something hasn't gone wrong. Not buying anything quite yet though until I know whether I'm in or not, because the World Champs will be the only event I can make this year, if I don't get in I don't see much of a point spending this much on a machine I won't be able to use for a long while yet!
I am going to downsize the drive a little however, if it were a pure drive machine it'd be alright but sticking a pair of 2.4kw inrunners in severely limits the internal space (who'd have thought?) so going to go for something maybe around the 500w-1kw mark, like this one on 4S http://www.hobbyking.co.uk/hobbyking...kv_15_5T_.html on 36:1, 100mm wheels. Still plenty enough power and speed, but not overly huge either. Probably could get away with a smaller battery/esc as well which helps.
That motor with 36:1 reduction should do just fine, its quite a bit more power than I am using. What gearbox are you using to get the big reduction?
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