Are large sirens clutched or is the rotor directly driven?

[q2bman]

I noticed my Federal Model 78 has a clutch to disengage the rotor from the motro for a good coast. Do large siren do this as well?

[Daniel]

I am not aware of any outdoor warning sirens using a clutch to disengage the rotor from the motor, because the rotors are usually heavy enough to continue spinning for some time. The STH-10, for example, takes a couple of minutes to wind down. Many vehicle sirens have this feature, like the Q2B and the CAM Model 4, but many others like the Model 28 do not.

There was a prewar German design which used a single motor and two clutches, driving a 3-port low pitch head for fire calls and a 9-port high pitch head for civil defence use. It was a vertically-oriented siren and only one head operated at a time. I'm not sure whether these went into production, but many two-motor sirens were used in Germany at that time, as well as installations of separate high-pitch and low-pitch sirens. The two pitches were never sounded at the same time, and after the war, the 3-port fire signal fell into disuse. There is a YouTube video of a prewar Elektror 3-port siren if you want to hear this unusual sound. The best description is the sound of wooden planks being fed into a large, industrial planer.

[SirenMadness]

Some large sirens do use clutches; one is the Victory siren. I think that it uses a clutch so that it does not activate as soon as the compressor goes to work.

[Daniel]

I believe that the Chrysler siren's clutch is between the motor and the compressor/chopper to allow the engine to reach full operating speed before engaging. Perhaps the gasoline Allertor had this too, but I don't believe any electric sirens used clutches except the German ones I mentioned earlier.

[Robert Gift]

I presume one must have a clutch anytime a reciprocating internal combustion engine is used to power a siren.
Otherwise, the engine is "lugged".
You would not need a clutch if it were a steam engine!