Home Chapter 16 Motors and Muscles
Motors and Muscles

"The function of Art is to disturb. Science reassures." ~ George Braque

The choice of using a motor or an artificial muscle to make something move in a certain way can be a complicated task and requires testing and research. Though, motorized aspects can also be important elements that give your robotic interactive work of art or invention motion and a life-like quality. The motor and muscle aspects of your project need to come together in parallel with the mechanical and electronic control system.


Festo Rubber Muscle Robot image complements of Festo Corporation


The kind of motor or muscles you choose will determine much about the electronics you choose, as well as how you integrate these into a functioning project both mechanically and aesthetically. One of the more challenging aspects of using electromechanical and motorized elements is finding just the right part.

A great resource for seeing what parts are out there is Thomas Register at http://www.thomasnet.com. This site lists many industrial companies worldwide and has an excellent index, which will help you search for the products you need by part type, brand name, and company name.



DC gear head motors are very common, but when selecting a motor try to find one that is close to the maximum speed you will need.

For example, if you want a mobile platform to move at five revolutions per minute, try to find a motor that is close to this speed, as the gear ratios will allow for enough torque to drive your platform. If you were to select a 100 or 300-rpm motor, you would likely not have enough torque to drive the motor, as the power is reduced using PWM or variable voltages.

Think also about the voltage and amperage necessary to drive the motor. Will it be on a platform that is battery powered or will you be driving the motor with a power supply and an AC to DC converter?

If you are going to use batteries, look for a low-current motor and go for the highest quality motor at the lowest cost.

3-12 volt DC motors are recommended simply because it is easier to find power supplies for these than, for instance, a 90 VDC motor. There are many sources of surplus motors though remember that if you purchase a surplus motor for your work of art or invention you may not be able to get this motor again if the motor fails.

Good surplus houses are: http://www.herbach.com/ and http://www.73.com/ and http://www.allelectronics.com/ and http://www.goldmine-elec-products.com/

Good sources for new motors are: https://www.solarbotics.com/ and http://www.robotstore.com and http://www.grainger.com.

Nominal voltage is the motor’s input voltage that is applied to move the motor under normal use. The published ratings of the motor from specification sheets define how the nominal voltage is related to the torque, speed, and amperage consumption during use.

You can use less voltage on DC motors, which will allow them to spin slower, as you learned in Chapter 12. PWM is a way to provide less voltage over time, but do not exceed the maximum voltage ratings. Torque is also related to the voltage, and the motor will have the rated torque at its maximum voltage. However, it will not have the same torque at a lower voltage, so consider this in your design.

Torque refers to the power of the motor in creating force. The motor’s torque will be listed in a specification sheet that you should request or find online for each motor you use. The torque will be rated at the nominal voltage.

Operating current is the amount of current used by the motor. Stall current is the current the motor draws when you stop (stall) the output shaft (if you can). Stalling a motor is very hard on the motor and can burn open the motor windings and ruin the motor. If you want to test for stall current, grab the output shaft with your hand or a pair of pliers (only if the motors is not too powerful) while measuring the current drawn. Remember some motors are turn so fast and are so powerful they can hurt your hand and arm so do not test the current with this method if this is the case.

As the motor approaches stall, the current usage will climb. Refer to Chapters 12 and 13 for control of DC motors. A good source of used DC motors is C & H, and for new ones try Grainger and Jameco. Micromo makes some of the top-of-the-line motors in the business, though they tend to be expensive and more industry-oriented.

When thinking about using the motor to transfer the rotation of the shaft to a cam, mechanism, gear, or lever, the closer an object is to the shaft center the more drive torque is available and levers farther from the motor will have less torque available. Motors are rated according to ounces per inch. A lever one inch out from the center of the shaft of a motor that is rated at one ounce can lift one ounce or 28 grams. If a motor is rated at 16-ounce-inches, a lever one inch from the shaft will lift one pound.

Speed refers to the motor's rotational speed in revolutions per minute (RPM). The speed will vary with varying voltages and the load you place on the motor.

The gear ratio of the motor is the amount or degree of gearing of the motor up or down. This only counts for gear motors. If the motor has a gear ratio of 30:1, every 30 revolutions of the main motor will turn the output shaft one time. The higher the revolutions of the main motor in relation to the output shaft, the more torque you will have and the more powerful the motor will be. Incidentally, this also translates into more noise for the motor.

In order to connect a lever, piece of metal or a piece of wood to the motor shaft you will need a shaft arbor (Below) with an insert to clamp onto the shaft. The hex nuts (Below) that are placed into the motor arbor must be screwed down tight to the flat surface of the motor shaft, so they engage and hold the motor shaft tightly to the arbor. A good source for these is http://www.grainger.com item # 6L105.

Another good source is McMaster Carr and search for shaft coupling and you will find many options. Another key search phrase on McMaster Carr is arbor attachment for electric motors and generally the motor shafts are 1/4 inch on many DC gearhead motors however they also have sizes in 3/8, 1/2 inch etc for larger shafts on AC and DC motors. 


Hex bolt Shaft Arbor


Hex wrenches are commonly found at most hardware stores and are an important tool when working with motors, as generally you will need one of these to work with the arbors to motor shafts. A very high-torque hex bolt has no slot and needs to be tightened with a hex wrench. (McComb, 168)


Hex wrenches