Stuka Stunt Main Forum · 11 of 11 known posts recovered
These are very good questions. I have seen model airplane engines die cast. They can be cast at a machine cycle rate of about 12 seconds in a die cast machine. If the mold will cast 4 parts at a time that works out to a case every 3 seconds assuming all parts are good. They still have to be machined. Scrap rate and down time my amount to 2% to 10% depending on the part, maintenance, the mold, set up operator, etc. Parts are placed in fixtured and machined. These are usually automatic operations that take only a few seconds per part to bore out each cylinder, bore for bushings or bearing, drill screw holes, tap screw holes, etc. Bushings are made in an automatic screw machine. Automatic screw machine is a very fast and accurate operator. Bushings can be made at a rate of about 1 ever 3 seconds. Bearings bought in quantity are cheap. I bought a single bearing for an engine it was about $2.00 while a quantity price will make them probably less than a $1.00 each. Each engine part has its own operation, die cast, screw machine, etc. The assembly area has all the pistons sized to within .0001" diameter. When the parts come off the machines it is very hard to hold the tolorance 100% exact to .0001" on every part. 70% of the parts may be perfect while the other 25% will be + or -. To avoid scrap the other 25% are used too.
It is much easier to have 10 boxes of different size parts. If an engine has a .750" bore the parts may range in size + or - .0005". The parts are sized in boxes at the assembly table. Box in the middle is bore size .7500". Boxes 1, 2, 3, 4, 5, to the right will be sized .7501", .7502", .7503", .7504", .7505". Boxes 1, 2, 3, 4, 5, to the left will be size .7495", .7496", .7497", .7498", .7499". Pistons are sized the same way. At the assemble table the works puts all the .7495" pistons in the .7495" cycliders and all the .7505" pistons in the .7505" cylinders and so on. I saw this done like this in 1973 that was 32 years go. Technology has gotten better and I am guessing it is done a little different than it was 32 years ago.
Gary Weaver
Not much differently, Gary. The same operations still have to be performed in order to make a useable part,and to make the parts fit together. The main change over the years has been in the improvements in CAD/CAM,making for much better accuracy and less scrap.Automatic gauging has made selective fitting much simpler to do on a volume basis,and better materials have improved engine life.
As to the plain vs ball bearing,most plain bearings in model engines are cast in place. That is,they're fixtured in the mold before the molten metal is poured for the casting. In low end engines they are at finished size in the mold and clearance is controlled by selecting the shaft. More expensive engines are finished sized during the machineing operations. For ball or roller bearings the case absolutely must be machined to very close size, roundness, and straightness limits,or the bearings won't live.
In another thread the question was raised about the advantage of ball bearings. It's really a simple matter of the fact that (most) model engine cranks are cantilevered and have no support behind the crankpin.This puts very high loading on the rear bearing (or the back end of a plain bearing). Ball or roller bearings not only are better able to carry this load,they're readily replaceable. They're also much more tolerant of marginal oiling,than plain bushings.
Ron B.
Tom B
Instead, the circular crankshaft disk had the effect of limiting the amount of oil seen by the bearing, and caused a faster failure. This was on the O.S. 50 H, I believe.
Tom B
Ron B.
F4Fguy
I avoid RCM like the plague,and Mr.C.L. almost as diligently, so I don't know what O.S. was referenced,but, the problem is almost never lube related.The basic problem is that the crank is cantilevered.The rear main takes a disproportionate share of the load.If there is significant imbalance(and there always is),the situation is worse yet. The real solution (short of a fully supported crank) is BIGGER (or better or both) rear bearings. Anti friction bearings such as ball or roller bearings need very little lube,in fact they do best with a light mist. If in fact this urban myth is accurate re O.S., it's a classic case of misinterpreting test results.
If the application in question is plain bearing (FP/LA)the only real cure is to increase shaft/bearing diameter.This is a diminishing return situation ,as increased diameter increases surface speed and worsens the problem ina journal bearing.
Ron B.
The article did say that the full disk crankshaft resulted in a rougher running engine. It was an O.S. 50 SX-H, essentially an over-stroked 46 FX.
Mr. Lee went on to say that the actual mechanism of failure was probably the lack of after-run oil able to circulate into a bearing shielded by a full disk crankshaft. While the engine was running, of course, adequate lubrication is easy to maintain. With the synthetics run in today's fuels, though, rust will set up quickly if fuel residue is not displaced with after-run oil. A little rust in the races or balls, and premature failure would result.
Tom B