Well, that is what Sir Issac Newton said…to get it moving, you have to apply an acceleration. That is the overcoming inertia part…after it is moving, you only have to overcome friction…untill you want to move faster! I understand the modern Roller Bearings have much less friction from a standing start than the older friction bearings…as another poster mentioned above somewhere, which makes them easier to get moving to start with. So that probably means it takes about hte same amount of force to accelerate from 0 to 1MPH as to accelerate from 10 to 11 mph…any differernce in those amounts of force would be due to the force required to get the roller bearings moving…
I differ slightly - While your observation about steam engines is probably accurate, I suspect the move to roller bearings from friction bearings played a significant part. The incident I mentioned in my first post occured with Diesels, but most likely with friction bearings.
I agree that roller bearings would certainly reduce the starting inertia. I only mention dieselization as eliminating the need to take slack for starting because I have heard that assertion dozens of times. While roller bearings would play a part in the issue, the widespread adoption of roller bearings seems to lag a decade or so beyond the period of dieselization, so I conclude that they alone are not the deciding factor if it is true that slack-taking ended with steam.
There is a relationship to the premise that dieselization ended the need to take slack by the fact that diesels can apply more of their horsepower to the rails at lower speeds compared to steam. However, I am not sure if that is the full explanation because once the wheels slip, the further horsepower is useless. Perhaps one would need to compare the actual maximum tractive effort on dry rail of a given horsepower between steam and diesels to get a better picture. It might have something to do with having more drivers per horsepower with diesels, as compared to steam.
And also, it may be a mistake to assume that taking slack to start was a universal practice during the steam era, or that it was so during the entire duration of the steam era. It may have been more common in the earlier period than it was in the super power era, for example.
There’s a commonly overlooked difference between most diesels and steam locomotives. The steam locomotive doesn’t have problems with traction motor heating and low speed continuous tractive effort should be close to starting tractive effort. Most diesel locomotives have a starting (short term) tractive effort that is considerably higher than the continuous tractive effort. In other words, the diesels inherently had a lot more starting tractive effort for a given train size than a steam locomotive, where the train size was based on the continuous tractive effort of the locomotive assignned to the train.
Equate torque as the ability to pull…
Diesel electric locomotives are constant horsepower, variable torque machines.
You get the same available horse power in notch 1 as notch 8, but more torque at the lower throttle range.
This means you can start out with a high amount of pulling power…but the trade off is that somewhere along the power curve you top out in both speed and torque…the horsepower limit.
Steam locomotives are constant torque, variable horse power machines…in theory, the more steam you can cram into the piston, the faster it will go…but the torque on starting is limited to the tractive effort created by the locomotives weight…to much throttle and the horsepower overcomes the tractive effort, or torque, and the wheels slip.
Applying the physics already mentioned, once you get it all moving, it takes less pulling effort to accelerate and keep it moving.
So once a steam locomotive gets the train moving, the speed it can travel at is limited to the max pressure the pistons can handle.
With a diesel electric, once you get it all moving, the top speed is limited to the max horsepower and the gearing.
A diesel electric locomotive uses an electric motor to turn a bull gear…with an electric motor, the torque it can create is limited only to the amount of electricity the motor can handle, more juice, more torque.
With a steam engine, the driving force, or torque is limited to the rate the steam can expand and the length the piston can travel, once it reaches the end of its travel, the pull, or torque it produces ends…it is a direct drive machine, no gearing beyond the crank pin on the driver and the size of the driver.
Part of the power the expanding steam creates is used on the exhaust side to push the piston back, and part of the power created by the other piston on the opposite side during its expansion cycle is also used to drag the opposing driver and side rods back too…the torque is c
Ed,
I understand what you are saying and generally agree, however, I would not have thought that diesel locomotives develop the same horsepower regardless of where the throttle is set. I thought their horsepower increases as the engine RPM increases along with the rate of fuel being burned. My understanding is that both steam and diesel locomotives produce more horsepower at higher throttle settings than they do at lower throttle settings. But a diesel is able to vary the current sent to the traction motors by trading amperage for voltage. This used to be called transition, but I am not sure if that term is still used. The effect is the same as a mechanical transmission in a vehicle. Amperage produces torque and voltage produces speed, so in starting a train, the current is sent to the traction motors with relatively high amperage, and as the speed increases, the amperage drops and the voltage rises. This has the effect of putting the locomotive in “low gear” to start, just like a truck, except with a locomotive; the “gears” are not mechanical.
Steam locomotives have no transmission, mechanical or otherwise. So they are like a vehicle with a manual transmission being in high gear all the time. They do have variable cutoff, but I speculate that it would be inaccurate to call that a transmission. Although it does provide a long steam admission at low speed, which produces high torque and then shortens that admission as the speed increases and the need for torque correspondingly lessens. Not only does the shortening cutoff reduce torque that is not needed for higher speed, but it also eliminates backpressure, which would otherwise increase with the speed if the