[ARC5] Locomotives

[Richard Knoppow]

----- Original Message ----- 
From: "Roy Morgan" <k1lky at earthlink.net>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Wednesday, December 19, 2012 10:11 PM
Subject: Re: [ARC5] Locomotives


>
> On Dec 19, 2012, at 10:46 PM, D C _Mac_ Macdonald wrote:
>
>> Added weight on the rails for any locomotive means
>> that heavier train weights can be moved.
>
> Right on.
>
>>  Friction
>> coefficient of steel wheels on steel rails..  .25.
>>  This means that the maximum weight
>> that can be moved .. is 25% of the weight of the loco.
>
> Actually, the maximum PULLING force would be a quarter of 
> the weight  of the locomotive.  I'd expect the force 
> needed to get a freight car  going would be a small 
> fraction of the weight of the thing.
>
> Rail yard workers used to use a long handled prying device 
> to get a  freight car started in motion. It was stuck 
> under a wheel and the  various lever actions provided 
> mechanical advantage enough to roll a  loaded car a short 
> distance.
>
> Roy
>
> Roy Morgan
> k1lky at earthlink.net
> K1LKY Since 1958 - Keep 'em Glowing!
>
     For long trains the locomotive could compress the 
coupler springs in the draft gear by backing slowly, then 
move forward slowly stretching the train out. That way the 
train was started progressively.
     I am not sure of the physics of traction but I think 
the available tractive force is greater than this suggests. 
The maximum weight of a train that can be started depends on 
the maximum tractive effort of the locomotive. This is where 
electric motors are superior to a steam engine. An electric 
motor can be made to have maximum torque at standstill, that 
is the electric locomotive will have maximum tractive effort 
from a standing start where a reciprocating steam locomotive 
has relatively low starting tractive force and needs to get 
some speed up to gain it.
     If you want to calculate the frictional forces at the 
wheels keep in mind that both wheel and track are deformed 
by the weight of the weight of the locomotive so that the 
effective area in contact is larger than one might expect. 
The adhesion between wheels and track depend on the weight 
on the drivers. Some large steam locomotives had only about 
half their weight on the drivers, the rest being carried by 
non-powered trucks.
     Until pretty late horsepower was not considered 
important, only tractive effort or force.  It was Lima 
Locomotive that first realized that horsepower was very 
important and began making high horsepower locomotives. 
These rapidly proved themselves in service so Baldwin and 
Alco followed suit. This was about the mid-1920s.
     Even though diesel-electric or electric locomotives can 
be coupled together to yield almost unlimited tractive 
effort the amount of force that can be applied is limited by 
the ability of the couplers to transmit it without breaking. 
So, even with the greater ability to start a heavy train the 
use of pusher-helpers continued to lessen the stress on the 
cars.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com