Thunderbolt Siren Restoration
Sirens Main Page
The chopper is the part of the siren that actually makes the noise. The chopper consists of a motor with a slotted wheel attached to the motor shaft. The slots in the wheel cause the air to be "chopped" into pulses as it passes through the openings when the chopper is spinning. A Federal Signal manual describes it as a "rotating air valve." All mechanical sirens work this way. The Thunderbolt siren pumps air through it's chopper with the blower to increase the sound volume coming out the horn. The Thunderbolt chopper unit is the same siren unit as the Federal Model 2 siren. The single tone chopper has 5 openings for air to pass through. The dual-tone Thunderbolt chopper has 2 rows of slots. One row has 5 slots and one row has 6 slots. See a dual tone chopper on this page. Fire Signal Sirens.Here is the chopper unit. You can plainly see the horn attach point with the slot in the bronze stator housing. The model 1000 uses a single tone chopper. The brushes (seen in the photo at the right on the bottom of the motor assy.) ride on the copper collector rings in the bottom of the chopper housing as the horn rotates.
Here is a close-up of the chopper in the stator housing. You can see the bearing retainer through the 2 spokes of the chopper wheel. I had to repair the threads in this retainer with Heli-coils because they were stripped out. The stator reminds me of a brake drum from a car. The inside of the stator is turned on a lathe to allow minimal gap between the chopper and stator. You can see in the picture how close the fit is between the chopper and the inside of the stator. The chopper is pressed onto motor armature shaft.
Here is a Thunderbolt 1000 chopper out of the stator housing. This chopper is from a siren belonging to the town of Valley View. It's identical to the single tone chopper in the siren I restored. There are 5 openings in the chopper wheel unit. Air pulses through these openings, through the slot in the stator housing and out the horn as the chopper spins while the siren is running. The dual tone model 1000T Thunderbolt siren has 2 rows of openings in it's chopper. One row of 5 and one row of 6 openings. Click the photo to see a larger version.