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first its important to know that all coasting drives are not designed the same. some use a mechanism that disengages the gears and others rely on back driving the worm and gear. I have a car and locomotive shop steam engine that had a coasting drive.  the coasting feature worked by the axle gear back driving the worm. this design is accomplished by have a worm that has a double thread. the helix angle of the worm is such that the axle gear can easily back drive (rotate) the worm. the performance of the engine was compromised by a very high gear ratio which made for jack rabbit starts and poor slow speed operation. as far a down hill run-aways, the engine by itself would not run away down hill on a 3% grade but if you added cars behind the engine it would. the only way to control it was to put the throttle in reverse and use it like a brake. this was with DC control not DCC. I did not like this arrangement and defeated it by replacing the motor with a gear head motor that could not be back driven. this solved the run away issue and give me a much lower over all gear ration resulting in much improved slow speed control. i think that coasting drives was mainly a gimmick whos time has come and gone. 

I agree with David completely, for small scales from Z, N, HO to O. For large scales such as Gauge 1 and G, the Coasting drive is usually built in and you can push the engine by hand, while drivers are rotating. But then the masses of the locomotives and cars are completely different and the coasting drives are necessary if you do not want to break the gearboxes during loss of power or brutal stops.

Yves

@Ace posted:

Around 70 years ago there was interest in fluid drive couplings for O and HO, as evidenced by discussion and ads in Model Railroader magazine. Motors would rev up before trains moved, plus coasting effects. How did those pan out? Leakage problems?

Here's an ad from an old magazine on the Schulz fluid drive, never saw one but I bet they made things messy:

schulz fluid drive250

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... the coasting feature worked by the axle gear back driving the worm. this design is accomplished by have a worm that has a double thread. the helix angle of the worm is such that the axle gear can easily back drive (rotate) the worm. the performance of the engine was compromised by a very high gear ratio which made for jack rabbit starts and poor slow speed operation ...

Interesting. I recall reading that an associated factor for the coasting feature was the use of non-cogging motors if DC powered.

In "Pop Off" (letters to the editor) on page 64 of OGR Run 110 (April 1990), Donald K. McClure describes the drive mechanism in the Lionel 1937 scale Hudson: " . . . the Lionel worm gear has been so expertly constructed that it is not self-locking.  Just think of the realism that adds!  When you shut off the power, the momentum of the moving locomotive will continue to carry it onward – it will coast!"

As Mr. McClure remarked in his next paragraph, it was amazing that Lionel had a non-locking worm gear drive in 1937. 

I believe this drive may have been a factor in the naming of "Dammit Hill" on Frank Ellison's Delta Lines as the hill had a steep downgrade in a tunnel that came out at a crossing.  The Delta Lines has at least two of the Lionel Hudsons, and it is said the name of the hill came from locomotives on the downgrade coasting out onto the crossing even after their engineers had cut the power.  This would have been a very unpleasant surprise for newbies or engineers inexperienced with the handling characteristics of the Hudson, especially if they caused a collision which apparently did happen with some regularity at this location.  "Dammit" is probably among the more gentle terms that were used.

One other note.  This is just one of the many instances where I have found the OGR digital archive to be a great resource.  Thank you, OGR, for making this store house of valuable information so readily available.

Last edited by PGentieu

I had an OMI BL2 with a Kleinschmidt drive. It was awful and totally unusable with a DCC decoder.  Basically it had a centrifugal clutch mounted between the motor and the drive shaft. It was supposed to slowly engage smoothly slipping until lock up when it became a solid connection. In theory it was supposed to let the motor turn faster than drivetrain for a smooth slow start and then have the drive train catch up as the speed increased and then lock up.  The same thing in reverse as you slowed the motor the clutch was supposed slowly smoothly disengage allowing the drive train to slowly coast down until it reached the same speed as the motor.  In practice it didn’t make smooth starts and sort of stuttered until it locked up very quickly.  When slowing down it would disengage and then engage making for a stuttering slow down. I was fascinated by the concept and fooled  with it trying to get it to work smoothly. I tried different oils, grease and even tried that sticky stuff the R/C cars use in their differentials.  Nothing would smooth it out except a long heavy train made the stuttering of engagement and disengagement less noticeable. The only way to use it with a decoder was to lock it up totally and that defeated the purpose of it so I returned it to the seller.  It seems to me it was old technology that would work only on DC and with a long heavy train.

Peter

as I mentioned I replaced the original motor in a car and locomotive shop engine that had a coasting drive. by added a gear head motor it defeated the coasting feature. But another reason for doing this was I was unsure how a coasting drive would function when a DCC sound decoder was added. decoders with sound function by reading the back EMF from the motor. with that said, I was thinking that the motor being back driven by a coasting drive producing back EMF going downhill would drive a DCC sound decoder crazy. with coasting drive the motor is acting as a generator. anyway I never found out if the decoder would function properly because when I replaced the original motor with the gear head motor I added a Soundtrax DCC decoder at the same time.

I still believe that a coasting drive is a gimmick. what is is purpose? the physics of O scale is such that an engine will not coast to a long stop like a real steam when power is cut to the motor. the mass and momentum is just not there. It may be that collectors just like to push the engine back and forth and watch the drive wheels go around and they think that is a cool feature. but on a model railroad it worthless and a dangerous downhill runaway.

bob2, I do believe that DCC encoders with all of their performance settings have made things like flywheels and coasting drives, clutches and fluid drives unnecessary. A DCC encoder will improve a models performance greatly compared to a straight DC model. DCC uses PWM (pulse width modulation) frequency to control a motors speed not voltage as in DC operation. so its similar to pulse power which improves the motors overall performance.

Last edited by David Eisinger

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