The Quarter Scale documentation, initially released in 2003, showed a 32DP 50 tooth gear on the rear of the countershaft for driving the supercharger. In 2007 a drawing for an optional slip clutch was added to the documentation, and in 2013 its design was revised. Gunnar's engine was built according to the original documentation, and a photo of his wheel case clearly shows a simple driving gear. John's documentation is later than Gunnar's, and he included a slip clutch; but it's one of his own design. The Quarter Scale notes explain the purpose of the clutch but also mention that it's optional, untested, and may not be required. I don't know when that particular note may have been added to the documentation, but it's possible that testing eventually showed the clutch was required after all, since a design revision showed up so very late in the project's development.
When the engine speed is changed, the 10x geared supercharger follows, but its angular momentum creates stress on the gear train. The magnitude of this stress is a result of the applied torque which is a product of the rate of change of the speed and the moment of inertia of the supercharger's impeller assembly. Without a clutch in a full-size Merlin, enthusiastic use of the throttle during combat could raise the drivetrain stress to destructively high levels.
One of the highly stressed components in the Quarter Scale version is the 12 tooth gear on the countershaft. In order to achieve the rpm multiplication for the supercharger, the number of teeth on this particular gear was reduced to as few as practicable. Rather than using a separate gear, it was integrally machined into the 1144 alloy countershaft for maximum strength, but its small diameter and few number of teeth still resulted in a tooth profile that was at the lower edge of good design practice.
A slip clutch can absorb the transient energy dumped into the gear train by either the throttle or the supercharger's angular momentum and dissipate it as heat rather than dangerous gear strain. Although adding a clutch to the gear train may seem like a no-brainer, it doesn't come without cost. The clutch needs to be inserted into the gear train behind the starter which means it will add its own angular momentum to the high-revving rear end of the system. Also, its high speed rotating parts are potential sources of imbalance.
The first photos show exploded and sectional views from my SolidWorks interpretation of the clutch design. The provided documentation was an assembly drawing that isn't really clear about some of the hub and spacer details. But, basically the clutch is a steel ring gear sandwiched between a brass backing plate and a flexible brass friction disk. A spacer partially fills a gap between the two and establishes the gripping force on the ring gear. The Quarter Scale drawing lacks the dimensions to determine this gap, and so it must be empirically determined during assembly. Its value, in any event, is dependent upon the particular brass alloy used as well as the surface finish of its various parts.
There are several potential sources of imbalance in the clutch including the TIR of every component, and so the challenge became to work out a machining strategy to minimize them. The narrow widths of the parts greatly complicated their work-holding, but each was indicated-in before every machining operation. Other than zero, I wasn't sure what the final TIR goal should be, but I figured I'd know it when I saw it.
The steel hub was the first component to be machined since I planned to use it as a mandrel to machine the brass frictional parts. The ring gear was the most tedious part to machine because I had not been concerned with its final size and shape when I initially machined its huge starting blank. The numerous holes in both the backing plate and friction disk have several functions. They reduce the momentum of the rotating parts, create spring in the friction plate, and they help air cool the frictional components. A copper alloy might have been a better choice for the frictional surfaces, but soft brass was the closest material I had on hand in the diameter needed.
The spacer was the real PITA part. It had to be very flat and very thin. I wasn't sure exactly how thin, though, because I had no idea about how the breakaway torque might vary with its thickness. The pre-assembly gap that I could measure was evidently 4-5 thousandths different from its assembled value which I couldn't measure. It turned out that only 1-2 thousandths separated a usable spacer from an unusable one. I had considered machining the spacers from thin shim stock and then stacking parts to build up the spacer, but I couldn't figure out how to align their screw holes during assembly. Individual spacers were turned on the lathe and parted off from a pre-drilled Stressproof blank before being surface ground to their final thickness. It took several spacers to zero in on the correct thickness.
I selected a breakaway torque of 1-2 ft-lbs for the clutch as this felt like a safe maximum for the gear train. I cobbled up an adapter for my torque wrench so I could measure the torque for each spacer that I tested. A bit of freshman physics allowed me to estimate the supercharger's maximum angular acceleration while protected by the final clutch. This calculation showed the clutch would limit the supercharger's acceleration from 20k rpm to 30k rpm to about 1 second which probably wouldn't be noticeable on a model.
After machining all the parts, I trial-assembled the clutch on the countershaft so I could measure the TIR. The full ceramic bearings that I ordered for the supercharger had arrived, and so I used them to support the countershaft in a vee-block for the measurement. The TIR was disappointing with an unacceptable .002" measured at the gear teeth. Most of it appeared to come from a .005" lateral wobble created by one or both of the brass parts. Even though they had been turned on the hub used as a mandrel, neither had come out uniform around its center hole. Evidently I had not fully cleaned out the sharp corner adjacent to the hub's flange, and the close-fitting parts distorted when the mounting screws drew them against the flange for machining. I was able to correct the backing plate, but I had to make a new friction disk.
The second assembly looked better. Both the wobble and TIR were less than a thousandth. I was eventually able to find a relative angular position between the backing plate and the friction disk that reduced the errors to less than .0005". Witness marks were scratched into the two disks so they could be reassembled with the same relative orientation.
When all the measurements were completed, I performed a final calculation just to see if the clutch had really been worth the effort required to add it. The computed moment of inertia of the clutch was less than half of what I had estimated for the supercharger, and by definition its angular velocity will be one-half as well. Therefore the angular momentum of the clutch is less than a quarter of that of the supercharger which is a relatively small price to pay for the protection it provides. As will be seen later, the torsional load of the supercharger adds unbelievable complications to the design and machining of the main shaft.
I'm anxious to play more with the ceramic bearings, and so my next step may be to work on the supercharger's bearing assembly. - Terry
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