Measuring "Pulling Power"

Manny,

Having been in the force measurement field earlier in my career I generally agree with you, but ...

It would be important to make sure that the gauge doesn't reach the end of its travel during the test.  The way to do this is to pull a rolling test train acting as the load so that the gauge is always within the limits of its range.  The important thing is that the anchor point for the gauge that you're adding must move with the locomotive, and a moving train accomplishes this.

If you pull against a non-moving point, the gauge will at some point "bottom out", and it won't read properly in that condition.

This is a case of "dynamic" measurement vs. "static".

Mike

Pull meter:

Item#84518 MicroLux tools

I use one of these...

https://www.amazon.com/Ajax-Sc...627056941&sr=8-6

@Mellow Hudson Mike posted:

Manny,

Having been in the force measurement field earlier in my career I generally agree with you, but ...

It would be important to make sure that the gauge doesn't reach the end of its travel during the test.  The way to do this is to pull a rolling test train acting as the load so that the gauge is always within the limits of its range.  The important thing is that the anchor point for the gauge that you're adding must move with the locomotive, and a moving train accomplishes this.

If you pull against a non-moving point, the gauge will at some point "bottom out", and it won't read properly in that condition.

This is a case of "dynamic" measurement vs. "static".

Mike

Having a moving train connected doesn't guarantee the gauge won't bottom out.  If the train has more drag than what the locomotive can pull, you'll still bottom out a gauge that can't measure that force.

If you do measure with a moving train, the drag of the train will have to be at or near the the maximum pulling power of the locomotive, to get some semblance of the actual pulling power of it.

Putting a gauge between a locomotive and a moving train only tells you the drag the train creates.

@rplst8 posted:

Having a moving train connected doesn't guarantee the gauge won't bottom out.  If the train has more drag than what the locomotive can pull, you'll still bottom out a gauge that can't measure that force.

Look carefully.  I didn't say that it would guarantee anything.  I said that it's a way, one way, to adjust the load in order to do prevent over-range problems.

And, you're absolutely correct.  If you hang too many cars on the load train it will still exceed the capability of the gauge.

Mike

Ok,

So how bout we weigh a gondola car, hook it to the engine, and slowly add #9 birdshot or similar fill to the car, until the locomotive wheels spin, and the loco can't pull it anymore, and then weigh the gondola with its load?

I'm not sure whether that just measures drag, or whatever, but it would give a pretty good idea of how much weight the loco can pull.

Mannyrock

@Mannyrock posted:

Ok,

So how bout we weigh a gondola car, hook it to the engine, and slowly add #9 birdshot or similar fill to the car, until the locomotive wheels spin, and the loco can't pull it anymore, and then weigh the gondola with its load?

I'm not sure whether that just measures drag, or whatever, but it would give a pretty good idea of how much weight the loco can pull.

Mannyrock

Measuring the weight of the gondola doesn't tell you anything unless you know the coefficient of friction.  If doing so from a dead stop, there is also starting friction at play (and possibly inertia).  If you did know all this, you could use those along with the weight of the gondola to calculate the "drag" of the load on the locomotive and then derive it's pulling power.

I use a little digital scale hooked to the locomotive.  While it might not represent true pulling power accurately, it does allow me to do comparisons of pulling power between various locomotives.

When the engine stops pulling I eliminate a car or 2.

Simple solution. You are overthinking it Manny.

@RSJB18 posted:

When the engine stops pulling I eliminate a car or 2.

I add a locomotive, the freight must go through!

Years ago we used to tie the rear coupler of the loco to one of those little hand scales fishermen have in their tackle boxes to weigh fish. As others have pointed out, not accurate but good for comparison between loco a and loco b.

Frank, that's basically what my little digital scale is, a fisherman's scale.

@gunrunnerjohn posted:

I add a locomotive, the freight must go through!

Lets not forget adding lead weights to the chassis.                j

Seems to me that what the OP is asking is how to make measurements of the pulling power of locomotives for comparison purposes.  AFAIK, there is no 'standard method', but from a physics point of view, I don't see an important difference between the three methods suggested here:  a fisherman's or luggage scale, the string, pulley and water container and the loaded gondola car(s).  True, there will be some small amount of friction in using the loaded gondola, but I find it difficult to believe that that frictional component of the drag is significant wrt the total 'stall' load.  Even then, if the measurements are all made in the same way, it certainly ought to answer the question of which loco is the 'strongest', etc., even if the units aren't quite right.

As someone else noted, there is the question of getting the right range for the scale.  Off hand, I have no idea how 'strong' any of my Flyer locos are.  I have a digital luggage scale (last used to establish bragging rights about whose load was heaviest before the last backpacking trip with my kids*).  If the static load is a few pounds on that scale, I'm not sure how accurate and repeatable the digital scale is at the low end of its range.

*the 'winner' was the person whose pack was the lightest...

The gondola method will not give you the right answer.  If a locomotive can pull a 5 lbs car, that does not mean it has 5 lbs of pulling force.  These two things are very different.

A rail car of a given mass presents some fraction of its weight as drag when acted upon by a force.  Think about a disabled car in the roadway.  A human can usually push it to the shoulder, but that does not mean the human has 1-2 tons of pulling power.

In addition, using weighted cars while adding weight to them as a comparison between locomotives is suspect, because the amount of “resistance” or drag produced by the axles may not scale linearly with the amount of added weight.

@rplst8 posted:

The gondola method will not give you the right answer.  If a locomotive can pull a 5 lbs car, that does not mean it has 5 lbs of pulling force.  These two things are very different.

A rail car of a given mass presents some fraction of its weight as drag when acted upon by a force.  Think about a disabled car in the roadway.  A human can usually push it to the shoulder, but that does not mean the human has 1-2 tons of pulling power.

In addition, using weighted cars while adding weight to them as a comparison between locomotives is suspect, because the amount of “resistance” or drag produced by the axles may not scale linearly with the amount of added weight.

The weight of the train is what it is, it doesn’t matter if it’s one gondola loaded with 5lbs of weight, or a string of cars that weighs the same. If the locomotive can’t pull the loaded gondola, it’s not going to pull the string of cars at the same weight. The resistance will show up as a maximum number on a scale required to get the train rolling. And yes, the gondola method does give you a pretty good idea of where the stall load would be ( in no. of lbs.) that’s how I do it,……but so many factors come into play, grades, curves, etc. There’s two numbers that one can use to gauge the pulling power. The weight needed to make the train roll, and the weight needed to maintain the pull. This is the grey area that’s hard to fine tune, as an example, I have a 40 car mixed bag freight on O99 curves, generous straights, etc…that 40 car train when pulled by hand with a digital fish scale shows a quick blip of 9lbs, but quickly settles down to 5-6 lbs. as I pull it along. I can duplicate the same scenario with a few gondolas and a lot of weights. Although I don’t go as far as stall, ( there’s no real need)  the object of my tests is for specific user needs and the measurements of amperage needed to start a heavy train so I’m not exceeding that. Actually, I’ve never even come close, but the tests are fun to do…….the bottom line is, there is no one set method that could be adopted, because everyone’s home layout is different, …..rolling resistance, curves, inertia, everything could easily skew a number. It’d be up to the individual to devise a system that works for them, make it consistent, and stick with it,…

Pat

Those of you who are digital subscribers can see how Jim Barrett measured tractive effort IN THIS VIDEO.

Thanks for pointing to that video, Rich. Tractive effort is really what the OP wants to measure and Jim did a great job in the video of showing how this can be measured with the engine pulling against a 4 lbs spring scale.

Interestingly, the video also demonstrates the advantage of using DC motors in modern engines with respect to their pulling abilities vs. post-war AC motors.

I always thought the digital scale method was reasonable.  As for the accuracy, I actually did check the accuracy of my digital scale, and it was almost spot-on to the actual weight applied.

There's a guy that's from Ohio that came to our modular club meeting with a home-brew pulling power measuring car.  He was using a car with an embedded strain gauge and Bluetooth to transmit the results to a phone.  We were having fun measuring pulling power for all our locomotives.  I was happy to see that my Vision Line Big Boy topped the list for a single locomotive right at 7 pounds of pulling power, I guess a lot of weight and a big Pittman motor are a good combination.

You guys are mixing up force and power engineering units of measure. It’s a little upsetting to me as a professional calibrator.

You will also need to measure distance and time to complete the power calculation.  P=(FD)/T

Be sure to take a few runs at the measurement, discard the data, reset the load measuring device to zero force, then record last run. This will remove hysteresis from the load train and the sensor/scale.

@Norm Charbonneau posted:

You guys are mixing up force and power engineering units of measure. It’s a little upsetting to me as a professional calibrator.

You will also need to measure distance and time to complete the power calculation.  P=(FD)/T

I think we are using "power" here in the layman's sense, meaning force, in this context considered as "pulling effort." But you are of course absolutely correct in the technical sense.

Yes, clearly we are talking about pulling force and not pulling power as indeed that would be a far more complicated thing to measure with the primitive tools we're using.

I had it beaten into my skull that power = ‘work done’.

Well,  it is my fault, because I should have been clearer and stated that I was not looking for a mathematically correct measurement arising from the application of all engineering and physical factors.  Just a handy procedure for approximating how "much" a locomotive can pull, or how many average weight cars it can pull, when compared to other locomotives.

In the same vein, when contractors do estimates of roofing jobs and siding, they roughly count and approximately how many "squares" of surface they will have to cover, a square being 10 feet by 10 feet.  They will not go forth to use a tape measure and measure everything to the nearest foot, because that type of measurement would be extremely time consuming and would have no practical application for what they are trying to achieve.

For the same reason, many yardsticks and even carpenter squares, measure only down to the nearest 8th of an inch, not to 16ths and 32nds.

I guess that even if an engineer calculated all of the drag coefficients on every test car, and all of the other physical factors, and applied the correct mathematical formulae, an argument could break out that unless he carried the computation to four decimal places, instead of rounding to the nearest whole number, the measurement wasn't correct.  :-)

Thanks,

Mannyrock

I could talk about resolution all day. Especially with someone trying to rip me off on a roofing job!

Although as noted it’d be a difficult task to measure pulling power, you can get an idea of amperage consumed ( in our world) to start a specific test, which does come in handy when figuring out whether the system being used ( if applicable) can handle the load being demanded ,…..case in point would be the recent topic on driving a bunch of motors with a single M, ….knowing how much force is needed to start a train moving will allow the tester to know the limits that can’t be exceeded ……but yeah, I agree with Norm, we shouldn’t confuse force with power,…that’s apples and oranges,,..😉

Pat

I use a digital scale too. Luggage scales and fishing scales are essentially the same. Cheap and effective. I use it for relative measurements, before and after a modification, motor swap, one engine vs another. Not convinced the numbers it gives can be assumed to be actual tractive effort though.

Pete

Agree with the comments above that a cheap digital scale provides excellent relative readings.  Here's mine:

@harmonyards posted:

The weight of the train is what it is, it doesn’t matter if it’s one gondola loaded with 5lbs of weight, or a string of cars that weighs the same. If the locomotive can’t pull the loaded gondola, it’s not going to pull the string of cars at the same weight. The resistance will show up as a maximum number on a scale required to get the train rolling. And yes, the gondola method does give you a pretty good idea of where the stall load would be ( in no. of lbs.) that’s how I do it,……but so many factors come into play, grades, curves, etc. There’s two numbers that one can use to gauge the pulling power. The weight needed to make the train roll, and the weight needed to maintain the pull. This is the grey area that’s hard to fine tune, as an example, I have a 40 car mixed bag freight on O99 curves, generous straights, etc…that 40 car train when pulled by hand with a digital fish scale shows a quick blip of 9lbs, but quickly settles down to 5-6 lbs. as I pull it along. I can duplicate the same scenario with a few gondolas and a lot of weights. Although I don’t go as far as stall, ( there’s no real need)  the object of my tests is for specific user needs and the measurements of amperage needed to start a heavy train so I’m not exceeding that. Actually, I’ve never even come close, but the tests are fun to do…….the bottom line is, there is no one set method that could be adopted, because everyone’s home layout is different, …..rolling resistance, curves, inertia, everything could easily skew a number. It’d be up to the individual to devise a system that works for them, make it consistent, and stick with it,…

Pat

I stand by the fact that the weight of the cars does not equal the pulling force required to start that train.

A modern train might have 100 70 ton cars. No locomotive ever made has 14,000,000 lbs of tractive effort.  Yet trains of that weight are regularly started by 2 or 3 SD70Ace locomotives, each having around 150,000 lbs of tractive effort.

Friction coefficient of bearings times static (assuming flat plane) mass. Friction coefficients for roller bearings are very low.

Agreed about roofing jobs Norm.  That's why whenever I get a new roof, I get up on the roof and measure it myself, "after" the contractor has estimated it.  Then they will quickly cave.

I guess that stuff like this is why manufacturers of locomotives never want to quote a pulling power factor for the products, although they could easily state something like "Easily pulls X number of Y inch cars, weighing Z ounces each, on a flat run.

Mannyrock

@gunrunnerjohn posted:

...snip...There's a guy that's from Ohio that came to our modular club meeting with a home-brew pulling power measuring car.  He was using a car with an embedded strain gauge and Bluetooth to transmit the results to a phone.  We were having fun measuring pulling power for all our locomotives.  I was happy to see that my Vision Line Big Boy topped the list for a single locomotive right at 7 pounds of pulling power, I guess a lot of weight and a big Pittman motor are a good combination.

Was that perchance a highly modified Atlas/Roco extended-vision cabin car painted a dark red?

@Bruce Brown posted:

Agree with the comments above that a cheap digital scale provides excellent relative readings.

Relative and repeatable readings, that's all we need.

All the physics aside, my fish scale has served my purpose.
Alan

I've seen locomotive reviews that list info like "Drawbar pull = 1lb-15ozs.  Will easily pull 15 standard boxcars on level track".  I think the purpose is to give a relative comparison between different model train locomotives rather than an absolute value.

John

The following OGR link will show the effect of engine weight, traction tires, Magne-Traction and rail type.

https://ogrforum.com/...pes-of-track-article

A simple way to judge pulling power of an engine is to see how many heavy cars it can pull verses other engines for your track.

Charlie

@Choo Choo Charlie posted:

A simple way to judge pulling power of an engine is to see how many heavy cars it can pull verses other engines for your track.

While that no doubt would work, it would also probably be pretty time consuming to actually come up with definitive results.  I think the digital scale seems to be a much quicker way to come up with quantifiable results that can be compared across more engines and layouts.

Then there's the Gomez Addams method...

Ram the locomotive at top speed into an increasingly larger pile of bricks, until the bricks (or the loco) explode.  Perhaps Arttista can create 1/48 crash test dummies.

Sorry, I just needed to inject the absurd into a very interesting but futile discussion. Figuring out the RELATIVE pulling power would seem to give useful information. "My old Lionel switcher can outpull your MTH Hudson" etc. The absolute power in kilowatts x 10^-n delivered to the drawbar seems to be an elusive answer, given the many variables: Voltage, track and wheel cleanliness, humidity, temperature, phase of the moon, mother's blood type, etc.

Some guy here often says "they're just toys."

@Arthur P. Bloom posted:Some guy here often says "they're just toys."

This is all in pusuit of science.