7th Street Shops: The Imbalancing Act - Post 27

Something happened in the late 1970s and 1980 that had a direct bearing on the topic of this post. Motor miniaturization advanced because high potency magnets that used rare earth materials became available. This allow significant reduction to all parts of the motors yet increased their efficiency. Model manufacturers realized the advantages of the new technology and began to use them.

The engine and beam are at balance with the small amount of weight added to the cab roof

Before Can motors, heavy open pole motors would have had a significant impact on model balance and of course, the motors of steam models are typically over the back axles. The worst condition of imbalance is for the rear drivers to carry more of the weight. I'm nearly convinced that the entire purpose of adding boiler weight to a brass model was to counter-balance the weight of the motor that was used.

By the time Can motors caught on there were already millions of brass models with open pole motors in the world. Many models already had worn out motors. But of course, brass models do not get throw away. Rather, they usually wind up in a closet tucked safely in their box. Sometimes they were fixed but the arrival of the new motors logically inspired many modelers to fix non-operating models. Then many more old motors were simply replaced because the "modern" types were demonstrably better.

When commercial DCC came along more old motors were replaced because they could easily spike several amps more than the decoders could handle. Conversely, the better Can motors typically operate at less than a few hundred milli-amps.

It's always a good idea to replace that old motor. They are inefficient to begin with but over time magnets tend to loose potency and of course, this only degrades motor performance further.

However, there is a caveat to replacing an original open pole motor with a smaller Can type. 

I have no information that would describe the in-house approach various manufacturers used to address balance of the models they produced but clearly most of them did. The best builders were very good at it. You can determine this for yourself if you have the tools and a model that hasn't already been been altered. But herein lies the problems. Set aside that the number of untouched models is dwindling; how do you actually determine engine balance? What tools - and where do you get them - do you need to do that? You might be able to diminish the issue with a lot of trial and error but without the tools and know how, your success will likely be a matter of luck

The manufacturer probably engineered the balance of a given model based on known data that included the weight of the motor. After the fact, when we replace that old motor we don't particularly have all of the data. But we can weight the two motors and find their differences. We will likely find that the old motor is 2 or 3 ounces heavier. What ever the difference, it will be displaced to the front axle when the new motor is installed. This isn't as serious as overburden of the rear axle since train pull-down will relieve some of that but it still robs the model of performance. 

What compounds the problem is that it isn't detected when the deed is done; unless the installer knows what to expect. There isn't an overt change in how the model runs. Rather, the issue distills into observation of the model over time and it is slowly realization; this model just doesn't pull quite as good as it use to... what happened? Then, as is often the case, the issue is further buried under the arbitrary addition of more weight. Folk, the laws of physics are immutable and they do not forgive.

Motor to left is typically replaced with the  can motor at top.

In the photo to the left the large open pole motor is readily replaced by the can motor (top). Another NWSL 2032D-9 is disassemble at right. That armature measures just 13mm in diameter and the total weighs is less than 1.5oz. The open pole motor weighs 3.6oz and has an armature nearly 18mm in diameter. The Can motor is more powerful but if it replaces the open pole motor more than two ounces would be improperly displaced. A motor's power becomes irrelevant if the drivers do not engage the rail tops.

The condition that is manifest in an unbalanced model is that one or more of the driver sets float. It is true that springs can also cause drivers to float; where they pass above the rails ineffective. What most often causes this condition however, is a model that is out of balance.

Model balance is not well understood by the hobby community. I took a model and the balance to a "new" local hobby shop to show the younger proprietor what the beam could do. It was quickly apparent none of what I shared with him was of any value. Nor was it certain he even understood the point. The "math" of my situation became very clear as I looked around at all the modern diesels in the store. Is the age of steam over in the hobby? Tractive balance of steam engines is of no real relevance to diesel traction and the coming generations of modelers are progressively less connected to that old railroad charm. The response to diesels traction issues (as well as shays and other shaft driven models)  is that blunt-instrument paradigm; add more weight wherever it will fit.

Unfortunately, this topic has always been obscure and digging into old magazines to find related discussions will bear that out. When discussions of poor pullers come up the general and relatively easy solution is "add more weight" - wherever it will fit. Adding weight indiscriminately can exacerbate a balance error. It can also create risk to motors, bearings, driver tires and perhaps other power train parts.

Yet, again, even if the modeler suspects a locomotive is out of balance how does he effectively identify the causes and arrive at the proper solutions? Does he even know what the factors are and where does he get the tools needed to accomplish all of this?

Beam is gauged to (lt. to rt.) O, S, On3 (not shown) HO and Sn3

As I described in the last post the rewards are clearly worth the effort to find practical solutions to this issue. Over the years I have gathered materials and thought about how they should be used. I quickly recognized that a rocking beam would be the best means to gauge balance.

I recently built the beam that you see here since I know of nothing commercially available. This tool is substantial in that it is heavy, rigid and smooth in operation. The beam itself is milled and calibrated and it is grooved for the gauges we service. The only thing that moves the beam (and only in a rocking motion) is any slight imbalance of weight.

Using the beam is relatively simple if all the principles are understood. The.entire primus is based on centering the driver coupling over the proper location of the beam's cross axis. A "driver coupling" is the joining of 2 or more axles by means of linking bars, or more commonly, side rods. What matters most to achieving balance of a model steamer are the two outboard axles (front and rear) and the exact mid point between them. Inboard axles are incidental to these 3 elements. It is the mid point between the two end axles that we must place exactly at the rocking axis of the beam.

Before the actual test is conducted we may remove lead and trailing trucks. They have little effect on the driver coupling itself because most of their weight is born by their own axles. Therefore, unless they are massive multi-axle trucks that would bear down on the beam itself, they probably won't significantly alter the balance on the drivers.

At this point loose pieces of weight are used to bring the engine into balance. In the title photo a few small weights can be seen on the cab roof. Weights should rest somewhere on the model itself. Placing them on the beam within the dimensions of the model is permissible but I prefer to place the weights directly onto the model as near to their mounting locations as I can determine. Certainly, weights should not be placed on the ends of the beam.

Buckshot is added to the cup a little at a time

The two places weight usually winds up are in the smoke box and in the cab roof.  Placing weight evenly over the driver coupling can achieve greater traction without too much impact on balance but that is not always easy to achieve. If the model is already balanced this is the only place that weight can be added "where ever it will fit", but the weight must be distributed evenly within the center-to- center limits of the coupling. Keep an eye on the balance.

I sometimes find a small measuring cup useful when seeking the point of balance. It is easy to add just a few bee-bees at a time since exact balance can be achieved in this way. Be careful however; buckshot and bee-bees are notorious escape artists and a bunch of tiny balls on the floor can be hazardous.

Once balance has been achieved I adjust the weight equation to lean 1/4 to 1/2 ounce to the front of the engine. Again, this goes to what happens when an engine pulls a train. There is a slight pull downward on the rear axle as the train weight takes affect on the engine; therefore tilting the weight slightly forward helps to counter that pull down. At this point I install the weight and test the improvements to the model's "Locomotive Tractive Effort" (LTE). 

If you suspect this is the reason some of your prized brass steamers are disappointing pullers and you don't want to do this yourself contact 7th Street Shops. We can help.

When we continue this discussion I will share a "live" example to demonstrate how adjusting balance improves performance.

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