In the earliest days of American firefighting, cries of fire or ringing church bells summoned volunteers pulling hand-operated pumps. Hand-drawn equipment gave way to steam power, and by the late 19th century, fire signals came from mechanical sirens and horns. Many larger cities installed pull stations and bells, which alerted career firefighters, and many smaller towns and rural areas still used outdoor warning sirens to call out volunteer firefighters well into the 20th century.
After World War II, advances in technology allowed firefighters to communicate and eventually be dispatched by mobile radio. Silver-plated horns gave way to “walkie talkies,” and by the last decades of the 20th century almost all fire departments used radio paging as their primary alerting system.
|(1)The upper housing and vertical fins of Edge Hill Fire
Company’s Federal Signal model SD-10 siren have been removed, revealing the motor and stator, which have been heavily damaged by rust.
(Photos by author.)
Although no longer the front line of fire dispatch, sirens continue to operate in communities across the country. They are invaluable when pager batteries die, cell phones have no service, or firefighters are in weak signal areas, such as inside metal buildings or basements. Sirens are typically activated by outdoor receivers that are far more reliable than portable equipment indoors, and many departments have kept their sirens in service for that reason.
In addition to secondary fire dispatch service, emergency management agencies use sirens for notification of severe weather and industrial emergencies. They are referred to as tornado sirens in many parts of the Midwest and South, tsunami sirens on the West Coast and Hawaii, and hurricane sirens on the East Coast. Nuclear generating stations and other industries also use sirens to warn residents of plant emergencies.
Most emergency management and industrial warning sirens are part of large-area networks that are on a schedule of routine maintenance and testing. Fire sirens, on the other hand, are usually out of sight on hose towers or utility poles and are rarely thought about until they stop working. Although failures are infrequent, most of the fire sirens in service today were manufactured between 1930 and 1980, which equates to anywhere from 30 to 80 years in service. Decades of use and exposure to the elements eventually take their toll, and they require repair or rebuilding.
Mechanical sirens are relatively simple devices consisting of a motor that spins an impeller (sometimes called a chopper) inside a stator. The impeller and stator are manufactured with a series of openings (ports), which alternately open and close as the impeller spins. The flow of air being forced from the center of the impeller outward is interrupted (chopped) as the ports open and close. The pulsating air stream is directed outward by a sheet metal housing, which doubles as a cover to protect the siren from the elements. There are some more complicated variations-including rotation, coding, and supercharging-but most follow this basic design.
|(2) This close-up shows ice damage and scoring on the impeller as it rubs against the inside of the stator.
The first step in servicing a siren, and often the most difficult, is getting it into the shop. Minor repairs can be done in place, but full restoration almost always requires removal. A mechanical siren can weigh in excess of 500 pounds, and a crane is usually needed to get it safely down. A portable hoist can make the job of loading and unloading a little easier after it is on the ground.
Once the siren is in the shop, the repairer removes the siren housings to access the motor, stator, and impeller. Since it is usually constructed of sheet metal, the housing is often damaged and must be repaired or replaced. Repairers may need to fabricate custom housings because many sirens have been out of production for years.
Motors require varying amounts of service, depending on their condition. Motors that have been exposed to the elements or overheated are likely to need rewinding. Bearings often need to be replaced, and single-phase motors will usually need commutator service and brush replacement.
Impellers and stators seldom fail unless exposure to the weather has led to severe damage because of rust or a buildup of ice. In any case, repairers will disassemble, clean, repair (if necessary), and check them for tolerance.
Repairers will address additional equipment such as rotators, superchargers, and coding dampers, if these items are present. Once the motor, housing, stator, and impeller are repaired, repairers will reassemble, test, and paint the siren. They will then install the refurbished siren in the same way it was removed.
|(3) Here the rebuilt 10-horsepower (hp) 230-V three-phase SD-10 motor and blasted and painted impeller are shown.
When Edge Hill Fire Company, located in North Hills, Pennsylvania, began to experience siren troubles, it called its radio service shop for help. When the siren receiver, tone decoder, relays, and timer all checked out fine, it was time to take a look at the siren itself. Edge Hill’s siren is a Federal Signal model SD-10 which dates from the 1950s and was very popular as a Cold War civil defense siren. Fire company members noticed that the siren had been slow to wind up and fast to wind down-signs of potential mechanical damage. The radio service technician used a crane to lift the siren off of the roof while an aerial ladder provided access for technicians to get to the mounting bolts and electrical connections.
Removing the housing from Edge Hill’s SD-10 revealed that water had been entering the siren from the top where the seam had separated. The upper cone-shaped piece had directed the water to the motor and stator, where it severely rusted both components. Additionally, the water got in between the two parts and froze, tilting the motor from a vertical orientation and causing the impeller to rub against the stator. This drag eventually got so bad that it overheated the motor windings and stopped the siren from turning.
Removing the impeller from the motor shaft and stator required a gear puller and a lot of effort. The motor was then unbolted from the stator. Although a good machine shop could have repaired the stator and impeller, it was more economical to salvage these parts from another siren. A rebuilder received the motor and installed new windings and bearings. The motor’s rotor was also dynamically balanced in combination with the impeller. The fire company sent the sheet metal housing to an automotive paint shop for refinishing.
|(4) The repainted SD-10 housing is shown reinstalled. The
remaining housing pieces were added along with new electrical
wiring, flexible conduit, and caulk.
After rebuilding the motor and repainting the housing, reassembling the siren began. The first step was to test fit the impeller and stator and then to paint the stator and motor. This step was followed by assembling and testing the stator, motor, and impeller. A test spin by hand preceded a 230-V 10-second run up.
After verifying proper electrical and mechanical operation of the siren, reassembly continued. The next step was to attach the motor/stator/impeller assembly to the cylindrical base, which is also the air inlet. The housing was then bolted back in place one piece at a time. New electrical wiring and flexible conduit were added. Finally, all of the seams were sealed with silicone caulk to keep as much weather out as possible.
The radio service shop trucked the completed siren back to the firehouse and reinstalled it in the same order shop technicians removed it. The mounting pedestal, which had been removed and painted, received the completed siren and technicians made the electrical connections. They energized the contactor for a quick spin to test the siren. With that first “growl test” successful, they used a signal generator to send out the company’s dispatch tones, and the entire system of receiver, tone decoder, timer, and contactor received a workout.
The tests were successful, and this SD-10 has been back in service for more than a year. This type of rebuild essentially restores a siren to like-new condition, and the Edge Hill SD-10 should provide many years of service.
|(5) When the rebuilt SD-10 siren was installed on its rooftop pedestal, a crane supported the siren while technicians used an aerial ladder to access the mounting
bolts and electrical connections.
You can avoid many siren problems by following a schedule of routine maintenance. The greatest enemies of any mechanical siren are rain, snow, and debris getting into the motor and impeller through a damaged housing. A routine inspection and repair of any small damage in the sheet metal will pay dividends in the long run. Additional maintenance items departments should examine are bearings, pulleys and belts (if present), and motor brushes (found on single-phase motors). Departments need to grease or oil bearings that are not permanently lubricated. Check belts for wear and proper tension, and check brushes for wear and replace them if necessary.
This list is not all-inclusive and, as with all equipment, the manufacturer’s published guidelines are the best sources for maintenance information. Unfortunately, many siren manufacturers have been out of business for years or decades, and preventive maintenance data might be hard to come by. The objective, of course, is to identify and correct small problems while they are still small.
If major repairs do become necessary, rebuilding a siren is usually a viable alternative to replacing it, even though it involves a substantial amount of work. The cost is typically about half that of a new siren, and a properly refurbished siren should last as long as a new one. In addition, a new siren usually won’t have the same sound as the original, and that is often an important consideration in an industry that is as heavy with tradition as the fire service.
CHRISTOPHER BORS is president of Land Mobile Corporation, a Philadelphia, Pennsylvania-area radio communications company that sells and services station alerting systems. He is also deputy chief of McKinley Fire Company in Abington Township, Montgomery County, Pennsylvania, where he began serving in 1981.