Hum, Buzz, Chatter, and Scrape
In Bachmann Spectrum and Other Engines

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This page was engendered by questions about the noises sometimes generated by Bachmann Spectrum engines when running downhill. The opinions expressed are those of the author. You the reader are certainly entitled to disagree. Informed discussion of alternative theories of the causes of these noises and possible ways to overcome them are welcome. Jim Banner 17 March 1999 After two days in the laboratory without food or water, the mad scientist emerges. No, no. Make that after a couple of pleasant days spent poking around in the innards of a dozen or so locomotives, the happy model railroader (me) thinks he has the answer to the question of why Spectrum (and other) engines hum and make a variety of other noises. I believe I have identified four separate and distinct sources of noises in Spectrum and other locos. For want of better terms, lets call these noises Hum, Buzz, Chatter and Scrape. I didn't realize I had all these noises - mostly they are masked by wheel noises from the cars when running trains. Also the noises are reduced by running the engines in pairs, which is normal practice on the Lorraine Valley and James River Railway. Let's look at these noises and what I think is causing them. Hum - a low frequency throb not unlike the rumble of a full size diesel , it can be felt in the roadbed as well as heard. It is caused by unbalanced flywheels. I think the flywheels themselves would run true if their plastic inserts ran true, but they do not. You can check your own flywheels by running your engine with the shell removed and lightly running a pencil back and forth the length of the flywheel. If the flywheel is perfectly centred, this will leave pencil lead all around the flywheel. If it is eccentric, the pencil lead with only be on the high spots. Along with three Spectrum F40ph's, I checked two Spectrum H16-44's, two Bachmann Plus GP-35's, one each Athearns F-7 and GP-7, a Kato GP-9 and a Kato powered Steward Hobbies RS-12. The Kato had no run out and no hum. The Kato powered Stewart had very little run out but was the noisiest engine of the bunch, probably due to a thin, hard shell with a high resonance frequency. The Spectrum, Bachmann-Plus and Athearns units all had some run out and the amount of hum seemed about proportional to the amount of run out. I personally do not find this noise objectionable, but I did try to reduce it in the worst offender, one of the F40ph's, by truing up the flywheels using a Dremel tool to gently grind the outsides of the flywheels while spinning them at high speed using the engine's motor. This attempt was only partially successful. After grinding, the outsides of the flywheels ran true but the insides of course did not, so while the hum was reduced, it was not completely eliminated. The hum problem is made somewhat worse by running the engines on rectified 60 Hz power such as from the MRC Tech II 1440 power pack used in some of the testing. This undoubtedly occurs because the motor of the F40ph is geared for 60 revolutions per second at 32 scale miles per hour, which means that for speeds of about 30 scale miles per hour used in these tests, the hum caused by the power pack and the hum caused by the off balance flywheels will add and produce low frequency beat notes. This effect was worse going down hill, presumably because the power pack was turned down, giving a higher percentage of pulse power in its output, when the train was going down hill. Buzz - a higher frequency than the hum, buzzing seems to depend on load. It can be heard when the engine is working hard pulling a train up a grade or braking a train going down grade. For testing, ten free rolling, four ounce box cars were pulled up and down a long, straight 3% grade. All the Bachmann engines plus the Kato and Kato powered engines buzzed when pulling hard up hill. The Athearns engines did not. Braking down hill, the Bachmann engines were more prone to buzz than the others. I think this buzzing is associated with the drive shafts, specifically with the horns on the balls at the ends clicking in their sockets. At high speed (3600 rpm at 32 scale miles per hour) the clicks merge into a buzz. The use of a relatively thin drive shaft by both Bachmann and Kato helps broadcast the clicks/buzz. A heavier drive shaft might suppress the noise, but the larger rotating and probably off balance mass would likely create problems and noises of it own. Finding that both the Bachmann and the Kato engines buzzed going up hill but only the Bachmann ones did down hill was something of a puzzle. I suspect there is something akin to the old steam engine surging problem going on here. If you have been in model railroading for a long time, you probably remember the early worm gear steam engines that would run smoothly up hill but jerk like crazy going down hill. The problem was that when the wheels tried to drive the motor, the pinion gear would push against the worm gear and cause it to bind. This binding would slow the motor, causing even more binding. This would slow the train until the binding was reduced, then the motor would pick up speed again and the train would accelerate down hill until once again the train was trying to drive the motor, and the cycle would repeat. This would happen typically 1-2 times per second, all the way down the hill. I suspect in the Spectrum engines something similar is happening, but at a much faster rate. I think it is applying torque to the drive shafts and is causing the ball-and-horn universal joints to click at high speed, causing a buzz similar to that heard when pulling hard up hill. Sometimes there seems to be another noise at the same time, and it may be that there is some wheel chatter going on at the same time. See 'Chatter' below. Chatter - This is the noise of wheels dragging along the rails. With the power off, push your locomotive along the track. That noise is chatter. Add cars until the engine can no longer pull them but instead sits there slowly turning the wheels. That noise is chatter. Back the throttle off too quickly going down hill so that the wheels cannot hold back the momentum of the train. That noise too is chatter. Chatter, specifically wheel chatter, is caused by the slip-grip-slip-grip of the wheels on the rails. Chatter can occur by itself or in combination with other noises. An obvious example is drive shaft buzz and wheel chatter when the train is just to heavy to pull. Or going down hill the binding of a worm gear may momentarily cause wheel slip, and the wheel slip in turn may momentarily release the binding of the worm gear, which may contribute to the cause of buzzing in the drive shaft attached to the worm gear. The alternate binding and releasing of the worm gear by the pinion may be a source of chatter in itself. And what causes the worm gear to bind in the first place? Probably tool chatter when the worm gear was made in the first place. This may explain why the Spectrum engines buzzed going down hill but the Kato ones did not - the Spectrum worms appear to be lathe turned but the Kato worms appear to have been either hobbed or lathe turned then ground and polished. This may also explain why my oldest F40ph, or rather, the one with the most miles on it, runs the smoothest and quietest, at least down hill - the worm appears to have worn smooth. It would be interesting to know if anyone has tried the Pearl Drops treatment on one of these engines. Scrape - This is the noise that makes my teeth stand on end and my pocket book cringe. This is the sound of dry surfaces running against dry surfaces, of shafts walking around inside of bushings, of bearing races dragging balls against bearing races, of money flowing rapidly out of my pocket and into repairs. In short, this is the sound of a mechanism crying out for lubrication. This noise showed up only once during this test - in a brand new engine from a factory sealed box. It was probably significant that the engine had been in the box for close to five years. It was well lubricated, but rather than let it chew itself to pieces while it redistributed its oil and grease into more useful configurations, I stripped it and lubricated it prior to testing. I had hoped to use this engine as a control to compare engines with various amounts of wear to, but in the end I felt that comparing them to a freshly lubricated new engine was probably not a fair test. In any event, thorough lubrication, while it stopped the scraping sounds, did not noticeably reduce the other three noises.