Powering Models - Part 1: Gears
|Some Common OS Gear Types|
Over the years, we have received many emails and message board postings dealing with powering models. Issues have included how to prevent belts from slipping, gears vs belts, using chain drive, and many more. Those of us who have been building for many years have had to answer these questions for ourselves on more than one occasion. So, to help those of you who may have similar questions, I'm going to present a series of features on powering models here on Girders & Gears. This first installment is something of a primer on gears, as well as on indirect drive mechanisms like belts and chain.
Most builders, even inexperienced ones, have at least some familiarity with gears. However, given the number of gear types and combinations that can be used in model building, there is some confusion about what gears to use in different applications. There is also confusion about gear terminology. According to Wikepedia, a gear is defined as "a toothed wheel designed to transmit torque to another gear or toothed component". Combining two (or more) gears of equal diameters transfers equal torque at equal axle speed, whereas combining two (or more) gears of different diamaters varies both the amount of torque transferred as well as the speed of each shaft. The two-gear pair shown in the animation at right illustrates what is probably the most common use of gears in model mechanisms. Note that the smaller of the two gears is turning faster than the larger one. In any such arrangement, the larger gear is called the wheel and the smaller gear is called the pinion. Flat, straight-cut gears such as these are known as spur gears. Regardless of their size, thickness, or number of teeth, they all have the same basic form and are designed to mesh directly with another gear. When used in combination, as in the animation above, axle rotation is in the same plane, i.e., the shafts on which the gears are mounted are parallel to one another.
A specialized type of spur gear is called a sprocket. A sprocket is designed to mesh with a chain or track rather than with another gear. This yields an advantage over a pair of standard spur gears because it allows the sprockets to be separated from one another and located in different portions of the model while still being connected together (although gear shafts are still parallel). Sprockets designed for use with chain have teeth that are typically coarser, rounder, and more widely spaced than standard gear teeth. The gears themselves are also relatively thin in cross section, while standard spur gears may be quite wide. The images here show some examples of common sprockets. The photo above shows a variety of Temsi (basically Meccano) sprockets that were included in the special Temsi Gear Set. The photo below shows three Gilbert Erector sprockets from the mid 1920s (note: the two photos are NOT shown at the same scale). Notice that the teeth on the Temsi gears are shorter and more pointed than those on the Erector gears. In both cases, however, the teeth are coarser and more widely spaced (as described above) than those on regular spur gears.
In 1927, Gilbert redesigned the sprockets shown at left by adding additional teeth to each gear and making all of the teeth much finer, which effectively converted them to standard spur gears. The P13 pinion went from 8 teeth to 12, the P49 from 12 teeth to 18, and the CJ gear from 24 to 36 teeth. (The P12 crown gear, or contrate, also changed, going from 24 to 36 teeth). Gilbert eventually phased out the old sprockets, and with them the ladder chain that they were designed to be used with.
In the early part of the last century, sprockets and ladder chain were common components in many systems, including Gilbert Erector, Meccano, Mignon, Stabil, TEMSI, and others. Chain sizes varied slightly between systems, as did the materials they were made from. The image below shows a diagram of a typical ladder chain link, along with a photo of original chain from two different systems. If you've ever used a belt and pulleys to power a model and had trouble with the belt slipping, consider using sprockets and chain. You can occasionally find original OS chain, but new chain can readily be purchased in a variety of sizes and materials (see my Links page for sources). If you need to match new chain to original gears, you can use the diagram below as an aid. You can also buy new gears from the same source(s) as the chain, and match them together easily. In this case, the chain/sprocket combination does not need to have the same specifications as the original parts (unless you are restoring a set, of course). The only thing you'll need to do is to be sure that the bosses on the new gears have the same inner bore ("B" on the gear diagram below) as that of original gears so that they will fit properly on your original axles.
In addition to pinions and gear wheels of various sizes, arguably the most frequently used gear types in model building, other gear types commonly found in construction systems include crown gears (also known as contrates), worm gears, and bevel gears. In addition to transferring torque and controlling axle speed, these gear types provide an added function over spur gears (pinions/gear wheels/sprockets) by changing the plane of rotation. Typically, this is 90º, but certain pairings of bevel gears can vary this angle (flexible shafts can also be used to change the plane of rotation). A crown gear, or contrate, resembles a gear wheel with the teeth oriented 90º to the plane of the gear disk. Crown gears are typically used in conjunction with a wide pinion. Worm gears are essentially just a threaded cylinder, like a fat screw, the thread forming a single winding gear tooth. Worm gears are useful when you need a significant gear reduction for your model. This can be 20:1 or greater for most gears. They are typically paired with gear wheels of varying sizes. Most Gilbert Erector fans will be familiar with the worm gear that was permanently fixed to the main drive shaft of the famous A49 electric engine. An interesting property of many worm gears is that while they can easily turn the gear wheel they are mated with, the gear wheel cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.
Bevel gears, less common than the other gear types, were specifically designed to mesh with other bevel gears. In form, those used in construction systems usually have straight teeth angled at 45º to the plane of the gear disk (see photo along right side of the page). When a pair of such gears are mated together, their shafts form a 90º angle. As with spur gears, two paired bevel gears with equal diameters rotate at equal speed, while two gears of unequal diameter will rotate at different speeds. In your model building, if you need to change the plane of rotation but wish to keep the rotational speed constant, a matching pair of bevel gears will do the job. In real world use, bevel gears are most commonly found in car and truck differentials, where two opposing pairs of equal size bevel gears are mounted in a housing (see diagram at right). To see this mechanism in action, click here. You can create similar mechanisms for your models, as well - see my feature on Powering Merkur Models - Part 1 for a photo.
There is one other gear type that you might find in an OS set, although they were rarely used: helical gears. The teeth on helical gears are cut at an angle to the face of the gear (see photo above right). When two teeth on a pair of helical gears engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in fully engaged. This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions. However, there is a drawback: because of the angle of the teeth on helical gears, a thrust load is created in the gears when they mesh. Real world mechanisms that incorporate helical gears utilize special bearings to support this load. Fortunately, bearings are not necessary when using helical gears in model building. If you'd like to try them in one of your models, but you don't have any in your parts inventory, high quality reproductions of Meccano helical gears are available from several sources.