TT scale trains and models

OK, let's see if I can get the math right!
If you figure a 40 inch wheel (a factor), that's .333 inches diameter so 1.05 inches circumference.
Top speed is say 70 miles per hour, figure 44 feet per TT scale mile so that's 3080 feet per hour or 51.3 feet per minute or 616 inches per minute.
So if you divide 616 by 1.05 that's 586 RPM to do 70 MPH in TT scale

So if the typical max speed of the 1024 from the chart is 12400 rpm at 12 volts, 12400/586 is roughly 21:1!
Now that assuming you'd want to run full throttle for top speed, I'd rather keep everything a little less stressed. And that's not including any of the engineering variables like friction losses, etc.- no idea how much it would affect it?

If that's right, higher than 14 or 15 to one would be better? And the only other thing might be when you start to get into gear or pulley diameters, I seem to remember Geoff from Hollywood saying if there's a bigger gear ratio that the gear sizes increase? I think that was an issue in the GP38, the gear version would work but the 15:1 belt drive might have had clearance issues when swiveling on tight radius curves.

Reply #2!
After looking at the chart again, it says the max power produced is 1 watt at 8300 RPM, so that would give you 14:1 gearing. But I'm not sure which parameter matters more, if it would be the max power (I'm assuming the peak of the torque curve?), or gear more towards a max RPM of 10 or 11K? and how would added weight affect it on either scale?
The prototypes must have had the same occasional headaches- the Erie Lackawanna Railroad regeared their E8 passenger locomotives down for freight service after passenger operations ended, for better low speed "pull" on 100 car freight trains vs. higher speed operation pulling 8-10 passenger cars.

"higher than 14 or 15 to one would be better? "

It's a balancing act between what works best and what is simpler, easier to source, and more reliable. Experience has shown that 10:1 ratio is a bit too high for model diesel locos and 15:1 very suitable. 20: 1 might be theoretically better but in practice there's no need for more than 15:1 so most scale diesels are built with a ratio somewhere around that number. If there is surplus speed with 15:1 it's a simple matter to limit it by decoder or throttle setting.More complicated gearing or belt or chain drives creates more problems that usually offset any theoretical performance gain.Ernst once made high ratio gear sets for Athearn HO locos that doubled the gear ratio but they also doubled the friction so a loco didn't run any better at slow speeds ,it just made a lot more noise as it overcame the friction :>) . Steamers have their own optimal gear ratios based on their individual driver diameters BTW.. .......DaveB

There is one more parameter. If you plan to run your locos with DCC than you can forget the motor power chart (power versus RPM) as this is valid only for smooth DC power. In DCC the decoder delivers PWM power, where the motor torque characteristic is almost linear and very often the motor gets more voltage as in analog version. How often do you ride with full throttle on analog pike? Almost never. In DCC almost always! The motor voltage is usually track voltage minus 1.5-2.5V!
With the higher gear ratio you reduce the load on the motor, ie it will run on higher speed (if all other conditions remain the same).
This is always tricky, that is why I tried to use in all my models the same motors, however today it is almost not possible.

Basing a road engines speed to be 60mph, a 40" wheel needs to do 504rpm to travel one mile in a minute. Somebody check my math. I did bad in math back in high school.

So if my math is correct you would need to get the motor speed down from 16,000rpm at the motor to 504rpm at the wheel. Does that come out to 32:1 reduction?

Bernd

Your math is fine Bernd! I got 503 for 60 mph.
After talking to my resident mechanical engineer (my son) he said it would be better in the long run to gear to the max power RPM, in this case 8300 RPM. The free winding speed is 16500, and the max "load" speed says 12400 but if you ran it constantly at that speed it would get hot and burn itself out. So it looks like 14 or 15 would be a good ratio? It would have a higher top speed than we want but you'd have to set that by the throttle or DCC settings?

Al,

Thanks for checking my math. Didn't take into consideration this statement.

After talking to my resident mechanical engineer (my son) he said it would be better in the long run to gear to the max power RPM, in this case 8300 RPM

Makes sense. Glad you have a resident mechanical engineer in the family.

Bernd

areibel wrote:Your math is fine Bernd! I got 503 for 60 mph.
After talking to my resident mechanical engineer (my son) he said it would be better in the long run to gear to the max power RPM, in this case 8300 RPM. The free winding speed is 16500, and the max "load" speed says 12400 but if you ran it constantly at that speed it would get hot and burn itself out. So it looks like 14 or 15 would be a good ratio? It would have a higher top speed than we want but you'd have to set that by the throttle or DCC settings?

The problem is that this value is calculated, you can control this only by mechanical load. This is a bit misleading parameter beacause it says how mach mechanical load can be added to the motor before it burns.
Usually it is measured in lab or workbench when the motor gets the nominal voltage ( 12V) and one increases the mechanical load, while in the same time the mechanical power is calculated. It is something like on the layout you add cars and cars until the loco can pull them. That means the 8300 RPM is practically the max power what you can expect from the motor before it burns. Do you really want to do this?
From electrical point of view you have to run your model in less than half throttle (max RPM is around 18000), and increase the throttle when you add any car ( to stay on 8300 RPM).
In DCC it will be even worse, because there you have to set the speed CV ( PWM) to 50% and the torque compensation CV to the highest value to keep tge motor on 8300RPM. This is unfortunatelly the worst case for the motor ( highest torque=highest temperature).
If you do not do it and the mechanical friction of the gear is low your loco without a load will run extremely fast ( twice of the max soeed - the motor RPM will settle somewhere between 12000-18000 depends on the motor and gears).

For my U30B project, the latest parts I used were a Mashima 1024 mated directly to the MTB trucks/towers/wheels from the SW1200. I measured full speed (analog) @ ~65mph. From all the work I've done on this project (years worth now), I believe the above is the best solution for price/performance/availability of parts. My 3 cents. An extra cent for the tons of R&D I've done on this.

ConducTTor wrote:For my U30B project, the latest parts I used were a Mashima 1024 mated directly to the MTB trucks/towers/wheels from the SW1200. I measured full speed (analog) @ ~65mph. From all the work I've done on this project (years worth now), I believe the above is the best solution for price/performance/availability of parts. My 3 cents. An extra cent for the tons of R&D I've done on this.

That is correct. The Mashima and the original MTB motors are from the speed point of view the same category, this means there is no head ache for the gears.