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 Posted - May 14 2024 : 11:26:33 AM
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My layout has a 5 percent grade on the mainline, so trains bucking downhill are a big issue for me to overcome. Lately, I was reading the June 1968 Model Railroader and it had a discussion of this issue. Boiling down what it said, here’s what happens:
The engine begins to roll downhill faster than the gears are turning. If there is end play in the worm shaft, the worm rides forward easily, then it comes up against the forward thrust bearing. Now all the force of the train presses against this bearing, and the friction of the bearing and gear teeth acts like a brake. The train's speed slackens, only to have this process repeat itself.
Usually reducing the play reduces the bucking. The train still balks, but so often and so lightly each time that you no longer notice it.
A rheostat throttle aggravates the problem. While the gears are turning freely, the motor draws minimum current so little voltage is lost in the rheostat and the motor keeps up speed with the coasting train. When the train finally overtakes the motor, gear friction loads the motor and the motor draws more current. The voltage wasted by the rheostat increases. The motor is slowed, not only by direct braking action but also by loss of voltage. At very slow speeds, when the most rheostat resistance is in the circuit, this can be very severe. Using a transistor throttle or variable transformer helps.
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I have two Mantua Prairies that share the job of yard switching and way freight duty.
Engine 52 rolls smoothly down the 5 percent grade with little or no bucking.
Engine 50 was bucking mildly taking a heavy train downhill.
Ironically engine 52 has about twice the end play in the motor/worm shaft. Why was 50 the engine with the problem? Bucking can be overcome by brute force of low-end torque. The motors in both locomotives have had their magnets replaced by more intense rare earth magnets, but they were not the same. Engine 52 has the older style motor with a screw by the magnet, so the replacement magnets were closer to the armature and therefore more powerful, resulting in more torque, lower top speed and a lower current draw. Engine 50 had the newer style motor with a rivet, which is placed farther forward, forcing the magnets back farther away from the armature thus slightly weakening the field. It was enough to make the locomotive buck downhill a bit. Fortunately, I had another motor around with the screw arrangement and put the powerful magnet in it. With that motor, 50 is as smooth a runner as 52.
On more thing:
I only use rheostat throttles, and I have learned that an automotive taillight bulb wired in parallel to the track makes for better control of low-draw motors. It also smooths out the downhill bucking somewhat by eliminating the problem described in Model Railroader Magazine. It’s no longer just a balance between the motor and the rheostat. The light bulb shares the voltage with the rheostat, and the motor inherits the bulb's voltage.
Most taillight bulbs I’ve tried worked, but I found that the ones used in my last car (a Jeep Patriot) were too high wattage and overloaded the transformer. One kind that does work is this:
I also have a switch wired to each light bulb so I can choose not to use it. The light bulb is not needed with motors with a high enough current draw nor when the train has more than one locomotive.
Note also, that my power packs deliver a full ampere of power. It's possible that a small trainset pack might not handle the train and the taillight bulb, both.
Carpe Manana!
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Posted - May 15 2024 : 12:12:41 AM
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| This was a fascinating read! Neat modification with the motor magnets too.
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 Posted - May 15 2024 : 12:39:23 AM
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Nice photos Don. You certainly have the ingenuity to have your collection handle those steep grades with ease.
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Posted - May 15 2024 : 5:49:26 PM
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| Most ingenious!
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Posted - May 15 2024 : 6:16:18 PM
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| I have found that on DCC I can overcome this problem by adjusting the deceleration rate to give a momentum effect. Also, decoders with the back EMF adjusted will overcome this cogging effect.
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Posted - May 15 2024 : 10:26:54 PM
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quote:I have found that on DCC I can overcome this problem by adjusting the deceleration rate to give a momentum effect. Also, decoders with the back EMF adjusted will overcome this cogging effect.
Originally posted by jward - May 15 2024 : 6:16:18 PM
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Interesting to know how to cope with the problem in that world, too.
DCC question for my own curiosity:
I have the impression that DCC throttles the speed of the motor by pulse width modulation rather than changing voltage. Is that true?
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Posted - May 16 2024 : 09:34:28 AM
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| Yes, motor speed is controlled by PWM. Full voltage is present on the rails at all times. Speed is controlled by varying the width of the pulses rather than the track voltage.
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