Rather than starting with some more exotic designs that use a flexible shaft or even an articulated shaft fitted with swivel joints, let’s instead jump right to the method that is used the most—a chain drive. This type of interconnection between the wheels and motors offers a lot of pluses. If the proper chain is used, it has the capacity to transfer a lot of power to the wheels. It also has the ability to take up
“slop” in the system without requiring precise spacing between the motor and wheel/axle sprocket.
Buying the Chain
What is the proper chain for your robot? You might be tempted to use a bicycle chain. Hey, you can pedal hard, even stand on the pedals when going uphill, and still not break the chain. The quality of mass-marketed bicycle chains is not up to industrial standards, however. Invest a few bucks in some good roller chain. It will be money well spent and can save you from a few headaches in the long run.
The proper term for this type of chain issingle strand roller chain. Generally, the pitch on these types of chains ranges from 1/4 inch to 3/4 inch. A 1/2 inch pitch means that the spacing of the sprocket’s teeth are 1/2 inch apart (or the chain’s rollers are 1/2 inch apart). The industrial roller chain is specified with an ANSI number, generally 25 to 80. See Table 6-1 for a list of some of the common chains.
A typical ANSI #40 industrial roller chain, for example, will have a 1/2-inch pitch and a 5/16-inch roller width; it will have a maximum allowable load of 810 pounds;
and the chain will break when the load gets up to 4,300 pounds. The maximum al-lowable load is based on continuous operation. Exceeding the maximum alal-lowable load will shorten the life of the chain. If you exceed the average tensile strength, the chain will break.
Some builders have ganged up two sprockets on each end to double the strength. In actuality, the strength is not quite doubled due to slight differences in
ANSI No. Pitch,
60 3/4 1/2 0.98 1,950 10,000
80 1 5/8 1.28 3,300 17,700
TABLE 6-1 Standard Chain Size and Load Specifications ■
chain-link spacing and subsequent uneven loading on one of the two chains, but we won’t cover the dynamics and physics of this scenario. This is still an accept-able method of applying redundancy for safety. When one of the chains fails, you still have another to carry most of the load. Double-strand roller chain is the best way to increase load capacity, and the cost of this type of chain is only about twice that of single-strand chain.
Most supply houses will supply the chain as a random-length loop or as long pieces of various lengths. Cutting the chain may require that you punch or drill out the rivet on one part of a link. You can buy a set of chain maintenance tools for in-the-field chain repairs; these would include a roller chain breaking tool, which is far easier to use than a hammer and a punch. Also available are chain pin ex-tracting tools and a unique roller chain puller that allows you to tighten the chain be-fore inserting a master link connector. For maximum chain strength, a chain can be custom ordered from the manufacturer in the exact length you need. If you choose to go this route, you will not need a master link.
Themaster linkis a separately purchased connector link that allows you to cre-ate a continuous loop of chain. You should also buy several extra master link con-nectors to fasten the chain together at the length you’ll want. This fastener consists of a side piece of a link with two pins that fit in the roller parts of the two ends of the chain, and a figure-8 side piece to fit over the pins on the other side. A clip snaps over the slotted ends of the pins, locking the master link in place. Figure 6-7 shows a typical chain.
Chain Sprockets
The sprockets used with roller chains look a little bit like gears, but they have more rounded teeth and are not meant to mesh with each other like a “standard” gear.
For combat robots, you should buy only steel sprockets for their strength. These sprockets are specified by an ANSI number (sprockets and chains must have the same ANSI number, or they will not mesh together because the pitch lengths will not be the same), the number of teeth on the sprocket, and the shaft bore size.
Most sprockets you will find include a keyway to lock them to a shaft with a similar
FIGURE 6-7 A typical ANSI #40 chain.
keyway. Some of the smaller diameter sprockets may have one or two set screws in the place of a keyway. These will work adequately with a flattened area on the shaft for lower torque applications, such as for small hobby robots. For combat robots, use keyways on all sprockets, gears, and pulleys. Doing so is a battle-proven method to secure components to shafts.
You might also want to apply one or more idler sprockets to take up slack in the chain. Quite often you place your motor(s) and wheel(s) in set locations and then apply the chain. More than likely, you’ll find that the chain is too loose (or maybe too tight). Having a bit of slack in the chain and using a sprocket idler on a small spring-loaded lever arm will keep the chain at a specified tightness and will pre-vent the chain from flying outward with centrifugal force under high speeds.
When implementing a sprocket and chain system, all of the sprockets must have the same pitch as the chain to which they are connected. When calculating the speed and torque ratios, you should use the number of teeth instead of using the actual diameter. If you use the sprocket diameter, use the specified pitch diam-eter, not the outside diameter of the sprocket. The pitch diameter is the actual di-ameter in which the chain will wrap around the sprocket.
To locate the sprockets on the robot, you can determine the distance between the sprockets in two ways. The proper method would be to calculate the center distances and then design the robot to accommodate the dimensions. Appendix C shows the calculations for determining the center distances. The other method, which is used by many beginners, is to place the two sprockets wherever you want them and then take a long length of chain and wrap it around both sprockets, holding the two ends in your hand. Then you cut the chain at the appropriate place, apply the master link, and possibly use an idler sprocket to take up the slack. Figure 6-8 shows a sprocket.
FIGURE 6-8 A typical 12-tooth ANSI #40 sprocket.