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Build a Reciprocating Electric Motor


This page features a couple of simple electric motors, made mostly of pine! Most electric motors contain only rotating parts, but this design has certain advantages in that it is unusual, more fun to watch, and possibly a little easier for the student to understand. It also offers some insight on how other types of engines (steam, and internal combustion) work.

reciprocating motor

Click the image to download a 250K MPEG--see and hear the motor in operation!

This is a simple engine. Although this engine contains all steel shafts and bronze bushings it could easily be made completely from wood. The motor is about 10 inches tall. The solenoid (coil) is made from approx. 150 turns of 22 gauge copper wire. This could be found at Radio Shack, or any shop that rebuilds electric motors. We also sell magnet wire in many sizes on our Products Page. This motor barely runs on 6 volts, and runs real strong on 12 volts.

reciprocating motor

] The back of the motor shows clearly the connecting rod and crankshaft. Although I used steel shaft and bronze bushings here, it would work just fine with all wooden parts. The coil is wound around a brass tube which fits tightly over a 1/2 inch steel shaft. Although there is very little play between the brass tubing and the shaft, the tubing slides easily over the shaft, making for an excellent cross-head. (A cross-head is the part of a reciprocating engine in between the 'piston' and the connecting rod). At the top of the engine are two large 1.5 inch diameter ring magnets which have a 1/2 inch inner diameter to accept the end of the steel shaft (See item 45 on our products page!). The steel shaft serves not only as part of the 'cross-head' but also helps direct the magnetic field through the coil. The shaft must end near the bottom of the stroke in order for the engine to run efficiently. This engine has a 1 inch stroke, and the coil is 1 inch long, so 2 inches of steel shaft hang below the magnets.

reciprocating motor

In the above picture you can see detail of the connecting rod, the crankshaft and the 'brush/commutator'. The brushes and commutator are the parts of an electric motor which allows electricity to flow into the moving parts, in this case, it serves to turn the coil on, and off. The brush is simply a carbon graphite block glued to the back side of the connecting rod. Carbon graphite is easily cut and machines with simple hand tools. It can be found in motor/generator brushes, or...we sell nice big blocks of it, see item #36 on our products page! Near the bottom of the stroke it contacts a brass plate and allows current to flow into the coil, which pulls the coil up towards the magnets. Once the coil hits the top, the brush moves away from the brass contact, thus turning off the coil, and the flywheel carries the motor around for another cycle. This would be considered a 'single acting' motor, meaning that there is only power when the coil is being pulled up - the rest of the time the flywheel is required to carry the motor around. With slightly more work the motor could be made 'double acting', so that the coil would both attract the magnet on the upstroke and repel the magnet on the downstroke. Most steam engines are 'double acting', almost all internal combustion engines are 'single acting'.

reciprocating motor

Pictured above is another motor similar in design. This motor has all wooden parts, except for the magnets and the coil. The same sort of brush/commutator assembly is used here, although in this one we just used pop can aluminum instead of the brass contact. The carbon in the brush is a small piece taken from the inside of a common D battery. This motor has all wooden shafts and bearings. The flywheel is a section cut off from a log, it's about 5 inches diameter. In this motor there is a separate cross-head and the coil is wrapped around the same wooden dowel that the crankshaft is hooked to - there is no steel core. It's important to keep in mind, neither of these motors are efficient. Electric motors can be very simple, with only one moving part (the armature) which rotates. Reciprocating parts waste energy, but...they sure are fun to watch and have some educational value!

Both of these motors were made from wood, with simple hand tools and a small drill press. They are completely glued together with medium viscosity super glue. It's excellent for this type of project because it dries very quickly, especially with the optional accelerator, and pieces can be tacked down with only a small drop of glue, and easily broken loose with a chisel should adjustment need to be made. It's critical in a motor of this type that all the parts be fairly square, and adjusted properly. The motor should be assembled one piece at a time so that it turns as freely as possible by hand. So...tack down all the parts, one at a time until everything seems well adjusted, then glue it together tightly! This type of glue is available at some hobby stores, mail order catalogs, and we sometimes offer it on the products page of our website.


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