So, as we've covered, motors have predictable turning speeds which can be controlled by voltage. But suppose you want the motor to spin faster but at a certain voltage. Or what if you want more torque at a given voltage? Or think about the forces a motor shaft might experience. Is there some way to keep the motor from being directly impacted by impacts? And what if you want to have 4 driven wheels, but only want to use one motor per side? (And those teachers tell you never to start your sentences with "and", "but" or "or"..) Well, one other method can solve any of the above problems: power transmission.

Power transmission does what it says: it is a way of transmitting power. Specifically, though, it's a way of transferring spinning energy from your motor to something else in the bot. In other words, your motor turns something which then in turn spins what you wanted to spin in the first place. You can have many stages in between, but the goal is to get spinning energy from your motor shaft to the wheels or a spinning weapon, in most cases. There are many, many ways to do this (and in some the rotation gets skewed: some for example make a reciprocating action from your motor: in, then out), but some of the simpler ways are the ones I'll talk about below: direct drive, gearing, belts and pulleys, and chain and sprockets. All of these ways provide a simple transmission of the motor's spin to whatever you wish to transmit the energy to.

Take for example direct drive. This is, as far as I know, the simplest way of spinning something: just stick it on the motor shaft! Okay, this kind of defies my own definition of "power transmission" as it goes straight from the energy source to the spinning object, but a) it's frequently used, and b) I don't really care. While this method is simple, cheap and light, it obviously has its own drawbacks. First of all, you must find some way to attach the shaft of the motor to the wheel or spinning object. With larger motors this isn't a problem: you can tap the spinning object, creating threads in it which you can srew the shaft onto (if the shaft has no threads, you can cut them on it with a die. Erm, we haven't covered all of this tap and die stuff, I'll mention it in the tools section). Or you can run a screw through the wheel's lip (if it has one around the hole) and screw it in until it touches the shaft. But with smaller motors it's harder to mess with the shaft on them. Sometimes you just epoxy the wheels on! But I don't recommend it... Also, the motor will be subjected to any trauma the wheel takes. If the wheel is hit by a spinning weapon, the motor will receive some of the shock. Typically motors are harder to replace than wheels, so this is probably something you don't want!

Now we move on to gears. This is probably what most people think of when they think of power transmission. Gears are basically disks with interlocking notches ("teeth"). When one gear's teeth mesh with another's, the unpowered gear rotates (really its teeth are being pushed by the other's teeth, causing this), but in the opposite direction of the powered gear. So if two gears are used, and the powered gear rotates counterclockwise, the unpowered gear rotates clockwise. But wait, it gets better!

Suppose you want to do more than just change directions and use a different shaft for your wheel than your motor. Suppose you want to increase your speed, or increase your torque. With gears (and any form of power transmission other than direct drive), you can do that! Let's say Gear 1 is powered and has 12 teeth (and is 1" diameter), and Gear 2 is unpowered and has 24 teeth (and is 2" in diameter). Gear 1 is the gear where the motor shaft is exerting torque, Gear 2 is receiving it and turning the wheel. (Gear 1: input. Gear 2: output.) And then let's say Gear 1 is rotating at 120 RPM with 3 oz-in of torque. Gear 2 is twice the size of Gear 1, so a tooth on the rim of Gear 2 has more distance to travel to turn once (C=pi*D, see the wheels section). But because the teeth are moving at the same speed on both gears, and the teeth on Gear 2 have twice as far to go in one revolution than those on Gear 1, Gear 2 will spin at half the speed of Gear 1 (or at 60 RPM). Yet because Gear 1 transmits its torque at the edges where the teeth meet, and the distance from Gear 1's teeth to its center is much less than Gear 2, the torque is doubled (to 6 oz-in)! This is because T=Fd, and while force is the same at the juncture of the teeth, the distance from the center on Gear 2 is twice as great, making T=F(2d), and T=2Fd. Woo! Physics! This is called gear reduction, since the output gear has a lesser RPM than the input gear, but also has greater torque. In this particular case, the gear ratio would be described as 2:1. Gear 1 has twice the RPM as Gear 2. In any x:y gear ratio, multiply the input RPM by y/x and multiply the input RPM by x/y to get your output stats. If x>y, it's called gear reduction. If y>x, it's called.. er.. well, it's gearing for a higher RPM and a lower torque. 1:1 gearing just gives you the same RPM and torque as before, only slightly lessened as nothing is perfect: some energy is lost due to friction and other small things.

With that behind us, gears spin in opposite directions, a peculiarity that may be good or bad, depending on your situation. Some advantages to gears is that they are light and take up little space (unless you greatly overuse them). However, they are prone to "stripping" (having their teeth broken off so they stop working right) in high torque or high stress situations, so make sure you use good ones (and ones that are appropriate for your situation)! They are also hard to set up: if you can't find a gearbox that's perfect, you may have to make one, which requires some precision and machining skills which may be frustrating. However, they are available in many different forms: spur gears are your typical gears, looking like discs with teeth cut out (and sometimes part of the middle cut out as well). Miter gears are gears in 3-dimensions (hard to describe; I'll get a picture up soon) which provide a 1:1 ratio, and allow you to change the rotation at a right angle, nice for compact designs with little internal space. Bevel gears are exactly like miter gears, only they come in ratios other than 1:1 (2:3 or 1:2 or the like). And there are many other gearing forms as well (ie rack and pinion, where rotation by the pinion, which is always the gear on the motor shaft, causes the rack to move linearly, but this is besides what I wan tto talk about here), more than I could mention in a short paragraph. So gears offer unique options as well. And there's something awesome about hearing a 10,000 RPM spinning weapon spin up if it's powered with gears: a screaming which makes others feel a little nervous!

Next type: chain and sprockets. What do you think of when you hear "sprocket"? A bicycle? The Jetsons? (Isn't it strange how, in a future where spaceships are rampant and you can press a button to materialize any food, they still sold sprockets in huge quantities?) Well, anyway, a chain and sprocket system is, in short, like gears which do not touch each other but instead transmit their energy through a chain which the gear teeth lock into. Check a nearby bicycle for a clear picture. The gears are the sprockets, the chain.. a chain. The main difference between gears and a chain and sprocket system is that, although they function on the same principles, the teeth don't touch, they just share a chain. The sprockets must have the same force and outer (linear, tangential, whatever you want to call it: D*pi*RPM) speed, and this creates the same relationship of torque and speed I mentioned above. However, chain and sprockets rotate in the same direction as the teeth aren't actually touching each other. One nice aspect of this approach is that, due to the interlocking chain, the sprockets transfer torque fairly well. This is also a popular way of starting to make tank treads. But also you have to worry about alignment again, as well as the chain's tension, and keeping the chain in: getting a chain knocked off the sprockets is a common failure point of fighting bots. They also aren't light. However, it still is useful, and not too hard to pull off, so don't discount it immediately for its disadvantages! And you can change the chain's length merely by popping in a link or two! Worth thought, at least.

And then we come to belts and pulleys. Belts and pulleys are chains and sprockets without the teeth. The pulleys are turned simply by the friction and tension created by the rubber belts. This may make you think that this is a weak form of power transmission, but they are fairly commonly used. V-belts, often used in cars, tend to be the standard in the sport for belts and pulleys, as they are a great way to transmit torque between pulleys. However, timing belts do this job well also: timing belts and timing pulleys are similar to chain and sprockets in that they have notches to transfer energy with. One advantage to any belt-and-pulley setup is that it is a nice way to control spinning weapons. Not counting timing belts, all belts provide some measure of slippage, meaning if a weapon hit something and stops, the belt may still turn some, reducing the strain on the motor, and making it harder to stall the motor. Timing belts are also very efficient at transmitting torque, and most pulleys can come with "flanges" which are extensions of the sides to make it harder for the belt to slide off! This is one point where belts have chains and sprockets beaten. However, overall the transmission may not be as effective as other types, it is hard to adjust the length of a belt, and belts are probably more vulnerable than chains or gears in that they are usually made of rubber, or some material that may melt due to heat or may be easier to break than a chain or gear teeth. Still, this is a nice way to go. Even the transmission sounds quieter: my drill press uses belts to run its chuck, and I can barely hear the motor running it's so quiet. But this may be the opposite of what you want, as you may want to frighten the opponent when your weapon spins up!

In any case, all of these are ways to transmit power. When two gears, two pulleys, or two sprockets share a shaft, the shaft spins at the same speed and torque, making basically a 1:1 ratio between the two transmission devices. So intermediate shafts can provide more reduction or can help you achieve higher RPMs. Remember, though, any power transmission at all will cost you weight. After all, the shaft must be supported, usually by bearings, or small devices which have a half which stays stationary for mounting and a half with low friction materials (in some, ball bearing separate the halves). These can add up quickly, greatly increasing weight, so add shafts (or transmission at all) sparingly.

And what if you want to know how much you can get out of a motor? Taking straight from the advanced motor math section: horsepower. At a given voltage, a motor has constant horsepower, which takes into account the motor's torque and RPM (HP=(no load RPM)*(stall torque (in ft-lbs))/5252). So, if you know a motor's HP, and know your target RPM, you can determine the stall torque, then gear accordingly. Note: the no-load RPM times the stall torque is constant at a given voltage, so that explains the gearing: if you have 2:1 reduction, your RPM is divided by 2 but your stall torque is doubled! So, that's one reason why no load RPM always does the opposite of what stall torque does and vice-versa.

Had enough? That's all right, I'm done for today! But here are some web-sites to visit to practice what you've learned!

McMaster Carr (http://www.mcmaster.com/)
Heck, you could probably build an entire robot from McMaster Carr's products.. But, yes, they sell many power transmission components, probably more than any builder has ever seen. Take a look around and explore any types that interest you!

W.M. Berg (http://www.wmberg.com/)
Well-known for being both high-quality and high price. Again, though, another perspective on the power transmission ideas. Take a look around.

SDP-SI (http://www.sdp-si.com/)
Usually the favorite company of bot builders for power transmission needs. They seem to have about everything. I have several of their catalogs put aside to look through now and then! Definitely worth a look.

Boston Gear (http://www.bostongear.com)
Another well-known transmission specialist that has been around for a long time.. You ought to check their products as well, but the thing I'm going to stress that you check out is their reference materials. They have a course for budding Boston Gear representatives that teaches "Gearology" via PDFs to the self-professed "non-mechanical engineer". This covers most, if not all, of what I have covered here in different words, and then some details I didn't get to. It keeps things understandable for the most part as well and has plenty of pictures! You probably won't need to know everything in there, but it's worth reading towards the beginning, and at least skimming through the rest. You will pick up oodles of info, if you try.

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