Do railroads limit the number of powered axles on a given line? And what are the reasons behind the limitations?
Yes, the railroads do have a limit on number of powered axels. Don’t know why.
Yes, many railroads have a limit on powered axles. The reason is that on lines with heavier grades in is possible for locomotive consists to break knuckles. Standard freight cars have couplers capable of withstanding either 300,000 lbs or 360,000 lbs of force (tractive effort). Most cars used in coal train service can withstand 500,000 lbs of force. If you exceed this force the knuckle breaks or worse. An AC motored GE can produce 180,000 lbs TE. a SD70ACe about the same, older SD70MACs only slightly less. So two of these can produce the maximum allowable force. Now if the locomotives slip a little and slack is put into the train it is possible to get much higher amounts. You can rip whole drawbars out or even tear a freight car apart. Now this is why GE developed the CTE software for its AC locomotives, when they are hauling ordinary freight trains the Engineer can select CTE mode and the Locomotive will automatically limit maximum TE to the same amount as a Dash-9, about 140,000 lbs. The reason that axles are limited is that it is a simplier calculation for Engineers and Power Dispatchers, an AC4400CW is not rated at 6 axles (which is what it has) but rather as 11 or 12 axles, based upon a SD40-2 as having 6 axles and being the standard.
…Good explaination…I’ve wondered how power was controlled to the point of tearing the train apart, etc…as you have very well explained in your post.
Beaulieu, I am going to guess that the forces that you stated are for tension (or pulling). Wouldn’t the forces be much higher for compression (or pushing)?
It causes me to wonder why railroads were not developed as a “pushing” technology instead of a pulling technology.
Why doesn’t the horse go behind the buggy ?
Because it can pull more than it can push ?[?]
There’s alot of factors involved as mentioned.Most r.r’s count a new EMD or GE six axle as eight powered axles because of their " tractive effort ".And some four axles as six.Meaning the amount of pounds that locomotive can pull or stop before it is suppose,not too.
All the locomotives are rounded up in groups or classes ,such as group 1,group 2 ,and so on.It is based on the TE mentioned,not according to one paticular brand.
Dynamic braking abilities also have a scale as to how many units can have the dynamic brake cut in.
I have seen cases where a derailment was caused by too many dynamic brakes being cut in .It causes " high-jacking" of the train.[2c][:)]
I suspect that with the proper harness, a horse could push or pull with an equal amount of force. The big factor is being able to steer. With a train, steering is not an issue.
Are you saying that locomotive has greater tractive effort if it is pulling than when it is pushing?
If you want to get technical, both the locomotive and the horse are pushing their load.
The horse is pushing against its harness, to which is attached its load.
The coupler is a little harder to explain, so let’s use a drawer handle instead. You place your hand behind the handle, and push it towards you. The drawer follows. A coupler works the same way.
Seems like I read somewhere that only a magnet or a planet can pull…
That doesn’t change the fact that in common usage, bringing something toward you is called pulling…
I’m glad this finally came up, as I was wanting to ask about this but didn’t want to start a post about it.
I know the railroads do limit the number of units (or powered axles) online, but I was wondering exactly when this rule was in effect. It cannot be all the time. Just take the BNSF over Crawford Hill in Nebraska. When the heavy unit coal trains get to the hill, 2-3 pushers are added to the train that already generally has 3 big units online. This definitely blows the limit on the number of units allowed online. I am assuming that the limit is only in effect during standard flat running, and when hills are encountered those limits are not in effect. Am I correct?
There are standards for front and rear locomotive consists. We are allowed 52 Equivalent Powered Axles (EPA) and 28 Equivalent Dynamic Brake Axles on the head end for most trains and 23 EPA on rear consists.
A few examples from the chart:
SD-40-2 7.1 EPA 5.9 EDBA
C44AC 12.1 EPA 9.8 EDBA
SD70ACe 12 EPA 10.5 EDBA
SD80MAC 12 EPA 10 EDBA
C44AC-CTE & C45AC-CTE 12.1 EPA 9.8 EDBA When in a lead consist or in a remote consist operating in Full Tractive Effort (FTE) mode. When in a remote consist operating in Controlled Tractive Effort (CTE) mode, 11 EPA and 9.8 EDBA.
Jeff
It does not matter if the load is being pushed or pulled. It takes the same amount of force to achieve the same result either way.
I’m sorry, I guess that I wasn’t making myself clear. I wasn’t talking about the forces to move the train. I was talking about the forces that the knuckle and the drawbar are capable of withstanding.
Others may come up with a more exact answer, but my reading says:
There is a limit on the amoung of tractive effort that can be applied to a train because of the limit on coupler (DRAWBAR) streingth. This is the first limitation on the number of powered axles, and although tractive effort is the criterion, the usual ways of expressing it is in horsepower or just number of powered axles if there isn’t much variation in TA/axle among the locomotives rostered.
There are limitations on total weight on bridge that can limit the number of locomotives in multiple.l
another even bigger concern with limiting the amount of powered axles has to do with track train dynamics, specificaly “stringlining” and “kicking out”. Stringlining is when the units on the head of the train are pulling around a curve. If there is a car behind the units with less downward force on the railhead and the flange and inner railhead the car will lift off the rail towards the inside of the curve and derail. An easy way to figure this is to try and pull a small wire around a curve…it will try and go straight rather than follow the curve unless you have something (in this case gravity acting on the wheel/rail interface) holding it to the form of the curve. It is very similar with kicking out, which is where the train is being pushed, either with DPU units, or in a similar form, when comming downhill while in dynamics or stopping using only the engine brakes. The lightest and/or light and long will be shoved over the top of the rail and shoved towards the outside of the curve or switch. Again think of a wire, if you try and push it around a curve it will bow out towards the outside of the curve. In most cases different subdivisions have different requirements on the number of axles of power or dynamics allowed as well as limits on DPU axles. These limits are associated with both the divisions curves and hills and takes into account tonnage limits as well. A line with relatively straight hard pulls will have higher allowable axles than one with similar grade but tighter curves. This is where DPU’s really shine as they allow a much higher total number of axles provided strict guidlines are followed for placement in train and tonnage as well as train makeup. The AC4400CTE’s are specifically built for this purpose, particularly when opperating as a pusher with light cars within a certain number of cars ahead of them.
The problem with pushing as opposed to pulling, is that when pulling the couplers will stay straight on the centerline of the track, when pushing the coupler can deflect to one side or the other. Road Locomotives have limiter blocks to restrict how much deflection can occur, freight cars and Switchers do not have this so that they can navigate the tight curves found in industrial trackage. The limitation on powered axles applies to headend power only, when you have pushers or DPUs you can put more power on the headend, but the calculations are more involved, get them wrong and you will have a derailment.
For Dave Klepper horsepower is not the reason for the limit nearly as much as TE. For example a GP60 and a SD60 both have 3800hp., but the SD60 is rated as having more powered axles, since at low speeds it will produce more TE than the GP60. When speed rises enough that the SD60 cannot produce more TE than that produced by the GP60, neither locomotive will be producing enough TE to cause problems. TE falls as speed rises, because a DC electric motor has maximum torque at or very near stalling.
For those with engineering interest: http://www.newton.dep.anl.gov/askasci/eng99/eng99164.htm.
Each carrier has it’s own rules concerning the allowed number of powered axles and their own rules for how to calculate the number of powered axles. Those that have been represented by the NS and UP employees in this thread are not the same and they are not the one’s used by CSX. I have no idea how BNSF calculates their restrictions, but I feel certain they are different from CSX, NS & UP - then throw KCS and the Canadians and you will find there is no standard calculation for limiting the number of powered axles.
Locomotives have alignment control couplers (usually) that do more than just limit the travel, they have V shaped blocks that actually will center the coupler on tangent track.
The axle limits for pulling the train are based almost entirely on knuckle strength. There are other factors as have been discussed already, but draft tensile strength is the 900 # gorilla.
First: [#welcome]
Second: Just to ‘activate’ this link (that would be me, too !):
Once again, I refer anyone with an interest in this to former BNSF locomotive engineer Al Krug’s essays and explanations, in this instance “How Much Force can a Coupler Withstand?”, at: http://www.alkrug.vcn.com/rrfacts/drawbar.htm
- Paul North.