Can someone give a hand with this?
I heard the saying "...any diesel can start a train it can't pull and any steam loco can pull a train it can't start"

Just out of curiosity, which one of the two has a harder time doing the following:

Does a diesel or steam loco have a harder time, pushing (helper) from the rear, mid-consist help or pulling AND is it tougher for either to do it forward or reverse?
Just wondering but please help???

That is an old saying. There is/was some truth to it, but there are many factors that can come into play. Basically, a rod (not geared) steam locomotive's peak horsepower is at speed.

This is an example I cut and pasted from the SteamLocomotive.com. (Check that site out!)

"N&W A 2-6-6-4 5,300@43MPH (DBHP) DBHP is drawbar horsepower."

To keep things simple; the diameter of the wheels has a lot to do with it.

Diesels with generally smaller wheels or "drivers" if you will, gain better traction. Traction is also better because all of the locomotive's weight was used for pulling power as opposed to many steamers that may have had leading and/or trailing trucks carrying some of the weight.

This is why you see freight steam locomotives with generally smaller drive wheels than passenger locomotives. The taller drivers of a passenger engine were more "slippery", but could roll a train faster.

Until recent history most diesels had a "short time rating". An engineer could only operate at maxium output to the traction motors for X number of minutes. (There is more to it, but this is enough for now)

Here are a couple of more reasons why that quote may not hold water any longer:

Modern diesels with advanced truck systems have a higher factor of adhesion then diesels had back when that quote originated. Also, newer power with A/C traction motors virtually have no "short time rating" as opposed to earlier D/C locomotives that could damage themselves if the engineer wasn't paying attention and left the traction motors under load too long.

As far as your question about pushing, pulling etc, I will leave that to someone else.

So why wouldn't that also be true of a geared locomotive?

Way to go, Bob. You've got everyone going "umm-m-m" before their first cup of coffee. I kinda think that a geared locomotive such as a Shay would conform more to the rules of a conventional automotive engine. The cylinder rods are applying force to a crankshaft which is transferring power to the wheels via ring and pinion gears.

Probably (I'm no engineer) the Shay will also reach peak horsepower at "speed", but that speed is inherently going to be considerably less than the more direct type of power transmission of a rod engine. Maximum drawbar h.p. would be available at very low speeds, which would be desirable for starting heavy loads.

Fred
SRI

Wouldn't the opposite be true here? A larger wheel diameter would result in a somewhat larger contact patch with the railhead. While it might be a minute difference on a steel wheel, this is the reason for larger tires on farm tractors, as well as the basic theory behind tracked machines such as bull dozers. A smaller driving wheel on a steam locomotive would result in a mechanical advantage as far as the moment arm for torque production, but I'm not sure about the actual effects on traction issues. Geared locomotives and switchers would have a traction advantages due to all the engine weight being on the driving wheels, but then there's those small wheels again........ Hmmmmmm

I think there are many questions and factors related to the top post, but I want to focus on the part above. I believe that a diesel locomotive would produce the same tractive effort whether running forward or reverse. However, in a situation where a diesel is starting a train into motion, I believe it would produce more potential tracive effort pushing as opposed to pulling.

When pulling, the two trucks are pulling a load that is on a line above the trucks, so the rear truck lifts the locomotive, tilting upward from the rear truck. This transfers weight off of the front truck and onto the rear truck, so the front truck loses traction. However, the rear truck gains traction from the added weight, so I am not sure how this shakes out. It would seem like there would be no such weight transfer when pushing.

RTK

Hey Warren!

I qualified my statement! I said "generally". : ) Actually, that is a good point about farm tractors and bull dozers.....

I have spent the last hour going through old books & magazines. I have book somewhere that explains relationship between wheel diameter, speed and tractive effort. It also gets into factors of adhesion and where on the wheel & axle the force is applied. I will find it, but not today!

Anyway... Just to add to the confusion. Remember, often railroads put smaller drivers on passenger engines to make them less slippery for use as dual service or freight power. Take PRR (4-4-0 D 16 sb) 1223 as an example.

The issue is not related to driver diameter between steam and diesel. The basis for the statement quoted is the difference between the torque and power curves of an electric motor as opposed to a reciprocating engine.

An electric motor develops its maximum torque when it is stalled. A reciprocating engine develops zero torque when it is stalled.

The diesel is a fixed horsepower, variable torque machine. Because the horsepower is fixed, as it goes faster, it's tractive effort drops. It develops maximum tractive effort at a very low speed. In other words, it could start a train that it could not get over 8 to 10 mph.

A steam locomotive, like every reciprocating engine, is a variable horsepower machine. The faster it turns over, the more horsepower it makes (up to a point.) As the speed increases, more power is required to move a train. As the steam locomotive goes faster it develops more horsepower. Therefore, any train a steam locomotive could start, it could pull at speed. Maximum speed would be reached when the horsepower required to move the train at speed equaled the power being developed by the steam locomotive.

To add what little I know to Rich's points above, the characteristics of the Diesel-electric drive are involved in the ability of a Diesel to start a train it can't pull. An electric motor can be overloaded to some extent before its insulation starts melting, the duration depending on the amount of overload. The traction motors of a locomotive can be overloaded for some period of time when starting a train, relative to the amount of work the motor is rated for continuously. A steam locomotive just doesn't have this ability to find extra power temporarily.

There have been a few magazine articles by demonstrators of the first Diesels, and the awe they inspired in railroaders by their ability to start trains. I recall reading that one road posted signs in Diesel cabs saying something like, "This locomotive will move anything it's coupled to." Meaning that at least the knuckle or drawbar will move, even if the rest of the train doesn't.

I've never heard of weight transfer being considered in the tractive-effort calculation, but I presume that it exists to some extent. I know that Diesel-locomotive truck designers consider the friction between the wheel and rail very thoroughly. When SRI volunteers were given a tour of the former EMD London plant, I learned a phrase that has stuck with me: "friction budget." We were shown the new steered-axle 6-wheel trucks, which were designed to fully exploit the friction available at each wheel. The axles on these locomotives are steered around curves by linkages, so as always to be tangent to the rail, to avoid wasting any of the friction budget on wheel scrub. That's another thing you can't do with a rigid-frame steam locomotive. The EMD designers said they were shooting for 1,000,000 miles between wheel jobs, compared with about 40,000 for a 2-8-4.

AC versus DC traction motors enter into this equation as well, in ways I don't understand.

Correct me if I am wrong, but I would take issue with some of your explanation.

A steam locomotive does indeed produce torque at a stall. It could not start a train if it did not produce torque at a stall. In fact it produces its highest torque at a stall.

Both steam and diesel locomotives are variable horsepower, with the horsepower of each rising with the engine RPM. I think the difference in pulling ability is more a matter of the transmission of each type of motive power. A diesel has a true transmission that can change the gearing in electrical terms of amps and voltage.

A steam locomotive has cutoff, but this is not a true transmission that can very the torque through leverage principles. It is simply increases efficiency by reducing cylinder backpressure at higher speeds where full admission is not needed. So a steam locomotive develops its maximum horsepower at its highest speed, and that horsepower is mostly expressed as speed. At a lower speed it develops more torque, but it has less horsepower to express in terms of torque at that lower speed.

A diesel can open up to full RPM and deliver its full horsepower to start a train. At the lower speed, the diesel expresses its horsepower mostly in amperage, which translates into torque. And since it can produce full horsepower with the engine running at full RPM, when starting a train, it can express its full horsepower as torque. As its speed increases, it expresses its horsepower more in voltage and less in amperage, which translates into more speed and less torque. So the diesel has a true transmission that, in effect, shifts gears from lower to higher; whereas a steam locomotive is always in high gear.

RTK

There is another component of this topic of discussion known as 'factor of adhesion'. Having run both steam and diesels, the primary problem with a steam engine starting a train is wheel slippage. I have encountered wheel slip with a diesel but, rarely on trying to start a train. Usually only after the train is underway.

Subject to being further enlightened by people who are more enlighened about this than me, it seems that the explanation for a diesel's ability to start a train it can't pull is in the diesel's higher factor of adhesion, coupled with the horsepower/torque factors discussed by some of the other posters.

It is my understanding that the older (DC) traction motors could not run a low speeds for very long before the windings would overheat and melt down. Modern (AC) traction motors can be ran down at low speeds for as long as you care; I read in TRAINS magazine about one of the early AC locomotives being hooked up to a large train in an emergency situation (I think the other locomotive was down), and pulled the train at less than one MPH for an extended period of time.

They also have anti-skid controls built in to maximize the amount of power delivered to the wheels before they break loose. That is my understanding, anyway.

There is some weight transfer that occurs to a diesel loco truck when pulling hard, be it moving or static. The load (the train) is transfered to the loco frame through the coupler, but the center of the pulling force is the same as the centerline of the axles. The axles are lower than the coupler, or more significantly, the bolster pivot, where the trucks transfer the force to the frame, thus the coupler. Because of this the trucks will try to rotate in the direction of movement about the bolster pivot point, thus reducing the weight on the lead axle of each truck. That is why the wheelslip circuits are wired to sense a slip on 1 and 3, or 2 and 4. Either your picking the front axles up in a forward movment, or the rear axles up in a rear movement. I believe the high adhesion trucks do something to counter this effect, although I have never had a chance to really look at them closely.

I'm not sure there is really much of a like effect from car to loco (pushing vs pulling), unless the grade is severe.

Ryan

Ron:

Let's look at it this way and I'll use two relatively similiar locomotives for the comparision: the SD40-2 and the NKP 765. If I were called to run either one on,let's say,a 5,000 ton train,level ground,normal weather,I could easliy start the train with either locomotive. When it comes to getting over the railroad,the 765 would leave the SD40-2 in her smoky wake. The reason is a diesel is peaked out at 3,000 horsepower...that is all you will get out of her,end of story. The 765 will surpass 3,000 horsepower and will top out (help me Rich) at around 4500 horsepower at speed. The higher a steam locomotive's speed,the higher her horsepower until she reaches the speed at which horsepower and tonnage equal out.
When it comes to actually starting a load,the SD40-2,at around 80,000 pounds of tractive effort,will outpull the 765 from a standing start to whatever speed the diesel can manage,which ain't gonna be much if she is loaded to capacity. From what I can remember,those Berks could pull 8,000 tons on the level with a tractive effort of 64,500 pounds (right Rich?)
Now if you could shove that big train which could be started by the SD40-2,but put the Berk on her,shove her up to speed,the Berk could pull that train as long as she did not have to stop for water or fuel because at speed the Berk is more powerful than the SD40-2. Remember horsepower is work at speed...something Super Power locomotives had plenty of.

Not really how it works......

To simplify it on a typical EMD you start out the S contactor closed and the wheel slip relay connected in a bridge circuit so if one motor turns faster than the others the voltage drop accross the armature causes the coil of the wheel slip relay (WSR) to pick up. When WSR picks up it drops out the battery field contactor and the shunt field contactor which pics up the governor overiding solenoid driving the load regulator toward minimum field reducing power.

After transition S opens and the parallel contactors P1 & P2 close and the wheel slip is sensed by passing a cable through the magnet frame of the wheel slip relay. It is also posible to introduce a Wheel Creep Relay (WCR) in the power circuit that is more sensitive than the WSR and will result in arresting a wheel creep by reducing power at a lesser rate than when WSR picks up.

I never put a lot of stock in weight transfer but I will say that a locomotive with worn trucks will hold the rail better than a locomotive with fresh trucks.

Simple circuit like I described, IDAC, WS-10, or Super Series all work great......The only thing that is the great equalizer is clean traps and full sandboxes.

E.B. Levin