Drawbar pull & tractive effort

Tractive effort, pulling force, for locomotives is equal to the coefficient of friction times the weight on drivers of the locomotive.   This works for all scales, from 1:1 down to whatever.  Coefficient of friction is in the order of 0.25 for metal on metal, and a good deal higher for rubber on metal (traction tires).  Merely weigh the locomotive and you know how hard it can pull.

   For HO, you add as much weight as can be fit into the locomotive.   The extra traction far outweighs the extra omph needed to heave the locomotive up a grade. 

 There have been methods of attaching a string to the coupler thru a pulley system and putting weghts in a carrier with known OZ weight amounts. You could find an ounce scale placing it sideways and let the engine push the scale.

Theoretically 1 ounce pull is about 10 cars on the level I believe if I remember right thats for HO.

I tend to like the 3% grade measure having a string of cars and just put cars behind the engine and test how many cars it can take up the grade, since so many layouts have heavy grades, but kinda difficult like me if you own the PCM Y6b or Bowser Big Boy that can pull the tar off the wall, its not easy to get a good measure, I tend to like the sideways spring scale for the bench test.

Put a pulley at the end of the track with weights on a string/fishing line hanging down.  Add weights until the engine can pull no more, then add up the weights.  Adding weight to the locomotive can help to a point.   Putting rubber tires (or a coating of rubber) on one or more wheels can help. Both could also lead to overloading and damaging your locomotive.

Enjoy

Paul

Besides weight another consideration is how many wheels are on the track. 6 wheel trucks will have better traction than 4 wheel trucks with the same weight. It also depends on the cars being pulled.  Some roll better than others and there are differing weights.  Also turns on a grade add resistance and add to the load.   I find it's typically more fun to try adding rolling stock to each engine until it starts slipping, then back off one or 2.  If you consist you'll usually not have to worry about it at all.

Adding weight is the way to go, but the weight must be added to the right locations. Traction tires probably work great, but they require changing and detract from electric pickup. Adding weight to the engine will not alter the amount of cars you can pull the way you think. You might be able to add a lb of lead and pull 3x the cars, it's all about the current and wheel slippage. Most engines will spin wheels before the max amps are reached on the motor. To do this right, you're going to need an ammeter and the specifications of the motor in the engine. You'll also need some sort of way to gauge pull, a spring of some sort will work fine, as long as you can see if "this way stretched the spring more than that way", your good. What you are going to be doing is increasing traction until the max amperage rating is reached, and this should be at the point where slippage happens. At no point should the motor exceed the amp rating to prevent damage, usually by heat (don't turn your engine into a blob). For a brass engine, if handrails and piping starts falling off, it's tooooooo hot.

1-rig the engine on a section of track with the spring holding it back. Make sure it's a straight pull, no down/up force on the engine.

2-hook the ammeter in series with the track power on the positive lead

3- turn track power all the way up and take note of the current draw. If it is at the max rating for the motor then make sure the motor/driveline is in good shape and lubricated.

4- add weight, usually over the drivers, until the max current draw is reached before slippage happens. Move the weight around to get the best draw on the spring. You might be able to add a suprising amount of weight before you reach the limits of the motor. It depends on the motor and how good the system of getting it to the wheels is.

IRONROOSTER

Put a pulley at the end of the track with weights on a string/fishing line hanging down.  Add weights until the engine can pull no more, then add up the weights.  Adding weight to the locomotive can help to a point.   Putting rubber tires (or a coating of rubber) on one or more wheels can help. Both could also lead to overloading and damaging your locomotive.

Enjoy

Paul

As ol' Gary Cooper used to say "Yep!"

And it doesn't matter whether you have 6-wheel trucks or 66-wheel trucks, the amount of weight a locomotive will lift vertically is it's drawbar pull or tractive effort!

This subject recently was discussed on the Atlas Forum.  I started a thread over there to talk about how our club does it.  In the interests in saving time, here's the direct link to my thread over there with 16 pics:

http://forum.atlasrr.com/forum/topic.asp?TOPIC_ID=55024

This is basically what we do:

Paul A. Cutler III
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Weather Or No Go New Haven
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A small digital scale is a good idea for these projects.  Digital scales tend to be cheaper and more available than spring scales, and more accurate, too.  Look for mail scales in Staples.  You can set one up with the table in a vertical plane, and make a stiff wire harness so the horizontal pull of the loco on a test track will register as "weight" on the scale.

I routinely use a digital scale for stall-torque readings on the electric motors I test. With the  maximum torque known, and the no-load RPM measured with a non-contact digital tach,  I can compute the power.  Motor power is only of passing interest in model railroading, but tractive effort is of high importance (since model railroading operations seem forever in the drag-freight era, and never get to the point where power is required to keep a schedule).  Tractive effort is a function of motor torque and tire traction, the former modified by gear ratio and driver diameter, and the latter being a combination of coefficient of friction and weight on drivers (as mentioned earlier in this thread).

Electric motors can withstand large overloads for short periods of time, so I wouldn't worry too much about burning the motors out unless you are pulling heavy trains continually, like an all-day run at a show.  Heat dissipation can be increased with heatsinks near the brushes and with ventilating holes in the case, provided the motor isn't inside a sealed plastic body.       

You can also measure drawbar pull through use of an incline plane.  Simply multiply the engine weight, in ounces, by the maximum grade which it will climb, expressed as a decimal.  For example, if a 16 oz. locomotive will climb a 25% grade, multiply 16oz.x.25=4oz. drawbar pull on level track.

dstarr

Tractive effort, pulling force, for locomotives is equal to the coefficient of friction times the weight on drivers of the locomotive.   This works for all scales, from 1:1 down to whatever.  Coefficient of friction is in the order of 0.25 for metal on metal, and a good deal higher for rubber on metal (traction tires).  Merely weigh the locomotive and you know how hard it can pull.

   For HO, you add as much weight as can be fit into the locomotive.   The extra traction far outweighs the extra omph needed to heave the locomotive up a grade. 

David is correct on the 25% figure:  I had the loco shown below tested at a local club, using an incline plane, and drawbar pull was calculated at 8.3oz, almost exactly 25% of its 33+oz. weight.

Two of these locos easily pulled a 22lb. train of loaded hoppers up a curving 2.5% grade - I ran out of hoppers and "live" coal loads before reaching their limit, and there's no telling what would have been the capacity of all three locos.  All of the locos used the older Athearn sintered iron wheels, btw.

With steam locos, it's important to balance the weight around the centre of the driver wheelbase - weight too far forward or too far towards the rear decreases the amount of weight applied at the opposite end of the driver wheelbase, actually lessening tractive effort.

The loco shown below has room in the cab for another couple of pounds of lead, but, as shown, is perfectly balanced around the third driver's axle, with no room ahead of that driver for any additional weight.  

At 32oz. (locomotive only), its drawbar pull is probably less than 8oz., though, due to friction losses through the drive train.  In addition, its single motor is just able to slip the drivers when the load exceeds its pulling capacity.  Adding additional weight, even if balanced, would risk burning out the motor. 

Wayne

 KD

 I have a Spectrum K4 4-6-2 that could barely pull 3 HW cars up the clubs long grade. I took the boiler off and put just a few ounces of sheet lead in front of and behind the drivers. Now it pulls 9 HW cars up the grade with out spinning its drivers. If you run steam check the front pilot and rear trailer truck for springs that tend to lift the loco off its drivers. I usually take a few coils out of the springs or remove them and put just a tiny amount of weight on the truck to keep the wheels on the rails. Traction tires can be a pain and they also pick up dirt real easy. Adding just a small amount of weight in the right places can make a big difference that will amaze you. Balance front to back. Don't be afraid to add weight but do it smartly. Don't take a 10 oz loco and add five pounds of lead and expect it to last. I basically do like the prototype and add more locos on front and back of the trains. It is a real treat to see and hear 2 steamers in front of 60 cars  with pushers on the rear. When the pushers and lead engine gets cut it also gives time for the brakeman to set retainers.

  Diesels I treat like the real rail roads do. If I need more traction then I consist  more locos or put pushers on the hills.

   Pete