Weight on the drivers determines the max tractive effort, when the wheels start spinning, and you obviously need a motor that can provide sufficient torque to generate that force
replacing the motor won’t increase the “pulling power” if the weight doesn’t change. But if the wheels are spinning, adding weight will increase the “pulling power”.
if there’s a point where the motor can’t spin because because there are too many cars, then a motor with more torque is needed.
a common DC motor for models has ~9 mNm (milliNewton-meter) of torque. it can produce ~5.8 oz-in of force at the rails requiring ~5x as much weight on the drivers.
9 mNm
1.27 oz-in
24 gear-ratio
30.6 oz-in
0.190 rad driver
5.8 oz force at rail
29.0 oz weight on drivers
Exacty correct. It usually pretty hard to get 2lbs of weight in our locomotives. Nearly all the motors are strong enough, and the gear ratios are balanced enough for torque and speed. More weight and/or traction tires are the key to pulling more.
So if cars can track well at under 3 oz rather than 4 oz, you can pull 20% to 25% with any given locomotive.
So if you want pull more, read my post on how trucks work, put the weight down low where it does the most good, don’t over weight cars, and use free rolling equalized trucks to minimize all the sources of friction.
But if you are only pulling 12 cars, then don’t worry about it…
Sheldon
how do you measure “free rolling” and what “equalized” mean"
which post (link?)
Right. So if the goal of model railroaders is to pull more cars, companies will want to design more powerful motors.
You don’t have to think about weight for that statement to be true.
Driver diameter matters too. Switchers tend to have smaller drivers for a reason. A tall drivered loco might not even get their wheels to turn under the same circumstances. But you don’t have to think about that either.
There is a link to my original post “How trucks work” in the NMRA thread. I can post the link here later, but my phone skills are limited so I have wait until I am home on my desk top.
Equalized means the truck can freely flex so that all 4 wheels are equally loaded on to the rails all the time. Rigid plastic frame trucks are not equalized.
Sprung trucks are equalized, Kadee HGC trucks use a two piece frame that makes them equalized, old three piece trucks with shouldered screws that let the side frames move on the bolster are equalized.
More later when I am home.
Sheldon
Douglas, most of our motors already have plenty of power to handle much more weight. Usually more weight than we can squeeze in our locos.
I have added lots of weight to lots of locos, never burnt up a motor of made a motor stall. The drivers always spin first.
Sheldon
I agree. Whenever one of my locomotives is inadequate to pull a load, it doesn’t stall out, it spins out. A beefier motor can’t fix that. More weight can.
no
if the wheels are slipping, the motors are not the problem.
the loco needs more weight so the wheels won’t slip with more cars
Not talking about the wheels slipping as the problem. Look I fully understand that weight is more important. Not mentioning it doesn’t mean I don’t think that
I get why an 0-8-0 configuration works for a switcher. Also tiny drivers. Pulling power isn’t simply about weight either.
The idea that you need a powerful motor to move the wheels under a heavy weight or load stands as a factually correct statement. And if pulling power is the goal, motor power matters.
Why did railroads go from buying 1500 hp diesels to 4000 hp diesels?
Why did emd increase The hp from a gp7 to a gp9. And the sd7 to the sd9.
you seemed to suggest is was either or
this statement ignores weight
As far as models. What about can motors. How about moving away from rubber and drives. And what about gearing
Is your perspective on this totally about Steam?
Yes the statement ignores weight. Because the issue is whether or not they want more powerful motors or not.
it’s about physics
Also. For steam locomotive models. You could compare driver size and worm gearing for limiting wheel spin too.
Big deal. Compare worm gearing and driver size. Or is weight the only issue
I can answer the question without math. And I’d be correct. .
Douglas, it is about matching the whole package, motor toque, total gear ratio including drive wheel size, and weight for tractive effort. Fact remains, most of the motors in our models have plenty of torque. And gearing needs to be in line with both slow speed operation and prototypical top speeds. That just leaves weight.
In the last 20 to 30 years most models are geared for a good balance of speed and power.
I need my trains to run at prototypical mainline scale speeds as well as have good pulling power and good slow speed performance.
Sheldon
Some more math facts you may not want to hear. Max motor voltage is a consideration. I operate with DC at a max of 14 volts.
The old NMRA DC Standard was that locos should reach a typical top scale speed at a load voltage of 12 volts.
That Srandard has been mucked around with because of DCC and dual mode decoders.
Another technical fact. Electric motors produce their max torque at zero RPM’s when you energize them. As they speed up torque goes down and horsepower goes up.
Once again, our motors are strong enough we have to get that power to the ground.
Sheldon
How powerful are the can motors of today compared to the motors in 1961.
And because I ignored smoothness doesn’t mean I don’t think it’s a factor
I have mentioned all of the factors you have mentionend in my several posts. I just didn’t bother mentioning them in the OP because that statement was in the context of another discussion.
But again Doug, weight is the one factor that is typically inadequate with our models, not gearing or power.
I have done extensive pulling power tests with many of my locos, I have never been able to stall a motor. But it easy to make the wheels spin for lack of tractive effort.
Sheldon