Back in August we had a thread "What should be the relationship between a worm and a worm gear ? " As is common the original post, about gear lash and depth of tooth engagement, started to wander and ended up about ratios. It prompted me to dig out four Williams Samhongsa built locos a PRR L1s Mikado, PRR B6 0-6-0, N&W "J" and a Jersey Central 4-6-0 Camelback. Also built by Sam. a Weaver PRR M1a Mountain. I found some things which some may find interesting. All the Locos have basically the same gearbox but the final drive ratios were either 21:1 or 42:1. The difference between the two was that the 21:1 box uses a double helix worm and the 42:1 uses a single helix. The worm as well as the worm wheel with skewed teeth were the only differences between the two gearboxes. One thing I found ironic was that the two locos which one would consider mainline high speed passenger or fast freight locos, N&W J and PRR M1a had the 42:1 gearbox and the locos which would be considered plodders for freight and commuter passenger service had the 21:1 gearbox which would give them a much higher top speed. It is one of those things which make you scratch your head. One can imagine the guys in MD. and PA. on the phone requesting a "HIGH SPEED " gearbox in the J and the M1a and the guys in Korea listening through a translator hear " HIGH RATIO" I think the same problem exist when oval track racers talk to dragster guys. Well thankfully the two gearboxes are easily swapped. So I pulled the 42:1 boxes out of the J and the M1a and swapped them for the 21:1 boxes on the PRR B6 and L1s Mikado. So while off the locos I decided to open the clam-shells and take a took inside and found one more surprise. I had bought all five of these locos from different sellers over several years. However the worm wheels in the J and the L1s were the wrong worm wheel. The worm wheel from the 42:1 gearbox out of the N&W J should have been in the 21:1 out of the PRR L1s and the worm wheel from the L1s should have been in the gearbox from the N&W J. I had been very careful when disassembling to keep parts from the two locos on two separate tables while removing them from the loco pulling apart and cleaning. I used a red and green marker to mark all the parts before I made the photos. It was not till I was setting up the first photo that I noticed the problem with the worm wheels. The Skew angle on the teeth of the worm wheel was wrong for the worm in their respective gearbox. However it did not seem to affect performance in any meaningful way. They may have run louder than the locos which had the correct worm wheels but with the loud Mabuchi 550 motors who could tell. Never mind that the Samhongsa gearbox is no where near a precision device. When I got to swapping the boxes in the Weaver M1a Mountain and the Williams PRR B6 both locos had all the correct parts. Well, they did have that 550 Mabuchi. After test running all four locos involved in the swap I was happy with all but the L1s Mikado. Remember it was one of the locos which ended up with the 42:1 gearing and the Mabuchi 545 which I installed in place of the 550 just did not turn fast enough to achieve the 40smph I was hoping for. I had two likely candidates for motors a Maxon coreless and a Mabuchi. I was hoping to use the Maxon but could not install a flywheel on it but may at some point try again. I want to use a tach reader for cruise and I did not have a drill bit that matched the shaft diameter to drill a brass rod for a step-up spacer to mount the flywheel. I was able to make a spacer for the 385 Mabuchi shaft to allow mounting a MTH flywheel off a Pittman. It took four tries to make a spacer that is concentric but the flywheel is on and running true. it is much heavier than a typical flywheel for a 385 motor and will coast over three feet when cutting power at 12v The 385 Mabuchi I used is good for 12,500 rpm about 55smph at 12v and is faster than I actually wanted it has a static resistance of 2 ohms so stall current at 12v will be 6A ,just fine for most TMCC motor driver boards. If you look at the photos you will see that I mounted the 385 in a magnetic shield from a Mabuchi 550 motor I lined the shield with 1/8" thick foam tape so that the motor floats in the foam not touching any metal and I lined the firebox with more of this foam tape. When the body is on you cannot hear the motor but you can hear the gearbox. I had been planning on selling this loco but was so happy after installing the 385 that I decided to keep it and set about dressing it up with a few details. These Williams brass locos left the factory rather naked and I always hated the big gap between the rear truck and the frame and all the detail they left out. I almost forgot in my attempt to isolate the frame and boiler from any motor vibration I used thick wall silicone tubing between the flywheel and the gearbox which worked well at low to medium speeds but when running flat out the tubing would stretch in the middle and cause a low frequency vibration that the foam suspension could not filter out. I inserted a piece of 1/8" brass tube inside the tubing and the problem went away. So far I have about a third of the plumbing that I plan to install and nearing start to the right side of the loco. I just added the photos and I'm sorry if I got some out of order. I got tired of writing and did not give an explanation of everything though I think that most are self explanatory but I don't mind answering questions. j
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