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Regular poster 331 posts A. C. Hubbard Regular poster 331 posts
A. C. Hubbard

I created a calculator for estimating tonnage ratings for my locomotives. This is spreadsheet based (open office).  Using the estimator is fairly simple. If you know the actual  draw bar pull of your loco , enter that number. There is also an estimator at the bottom if you only know engine weight. 

Next is the Average car weight on ounces. I suppose it could be grams, as long as units are kept the same throughout.

The next two items are grade percent and curve radius. These will effect how much can be pulled. I used a drag factor of 2 for the grade and .04% per degree of radius for the curve drag factor. A lot goes into these factors in real life.. you can research if you really want.

The result is the amount of cars that can be pulled for the given locomotive based on info you entered. The results are rounded down, as this is an estimate.

The draw bar pull estimator section is based on locomotive weight and an adhesion factor of 20%. 

Yellow color boxes are user input areas. 

This is still pretty much a Beta test type thing .. but so far results are good. Let's see how this goes...

Download from Dropbox here : https://www.dropbox.com/s/z7vzkmqrw094a0g/Drawbar%20Pull.ods?dl=0

Tony

 
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rhammill Regular poster 100 posts
rhammill

So this is to determine the

So this is to determine the tonnage rating of the model locomotive - that is, the number of ounces of train a given model locomotive can pull? Interesting.

I just load up a train and just remove cars one at a time until it can pull the train up the helix. But my cars aren't all weighted the same. I'll have to play around with this.

Not knowing the physics behind this, would the number of powered drivers make a difference? How do you determine actual drawbar pull?

Randy

 
Randy Hammill
Prototype Junction
Modeling the New Haven Railroad 1946-1954
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A. C. Hubbard Regular poster 331 posts
A. C. Hubbard

@ Randy

At the bottom of the spreadsheet, you can estimate drawbar pull by entering the locomotive weight. There are a few physics factors that I used, and so far it has not been far off from what I measured at my bench ( I used a spring scale to measure draw bar pull)  for example, the draw bar pull is about 20% of engine weight..(to a certain point). I believe full size trains is around the 20-25 % range. 

As to car weight, most of mine are between 4.5 and 5 oz. So I use 5 oz as my avg car weight. 

There are a few reasons I did this. I had seen many posts elsewhere, that questioned how to get such info for operating purposes. As we know, curves and grade effect how much can be pulled. This brought me to thinking of how can this be estimated, especially when considering operations and new locomotives. 

Example If a person knows the max grade is 2% and max radius is 22 deg. at one location on the layout. Let's say the avg car weight is 5 oz. knowing that I have a Proto 2k GP30 that has 3.5 drawbar pull (loco is 17.5 oz) and the train is 20 cars long. By running the estimator ( to run all the math) I find the loco will only pull about 13 cars at that spot, so a second locomotive would be needed. 

If you do play around with it, I would be interested in what you find. If it is in the ball park on how much can be pulled, some may find this useful..or at least fun to mess with. 

Thanks!

 

Tony

 
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