Thermostat for Automatic Fire Alarm Systems
One of the problems in automatic electrical fire alarm systems has been the loss of motion caused by the difficulty of fitting the pivot of the thermostat arm and another has been that where two open circuit contacts were located upon one side of the flexible leaf, after the leaf touched the second open circuit contact, and heating still continuing, the leaf would be caused to leave the open circuit contact and so break such circuit. A new thermostat has been designed, which will obviate both of these difficulties.
Referring to the illustrations, Fig. 1 is a view showing the thermostat in position; Fig. 2 is a plan with the cover of the thermostat removed; Fig. 3 is an enlarged plan of one end of the thermostat; Fig. 4 is a sectional fragmentary view of a part of the other end of the thermostat: Fig. 5 is a sectional elevation of the portion shown in Fig. 3; Fig. 6 is an end view of the thermostat; Fig. 7 is a diagram showing how the contact is made; and Fig. 8 is a view of a detail.
In this device, when ready for operation, the usual tube 1 is inserted into the end of the thermostat, such end being theaded into the front end of the end wall 2. The portion of the tube behind the threaded part is thus guided in the unthreaded portion of the end wall. A rod 4 within the tube has its end 5 spherical in form and fits into a correspondingly shaped recess in a member 6 fixed to the thermostat arm 7. the said thermostat arm 7 consisting of a blade of spring material fixed to the shoulder 8 of the wall 3 by screws, 9. Such arm 7 is set in such a position that tension thereof will be exerted against the rod 4 within tube. The member 6 in which the rod engages is fixed to the thermostat arm 7 at such a position on the blade that the travel of the end of the blade will be sufficiently multiplied to provide the required movement and yet be sufficiently sensitive to be acted upon by the rod as such rod moves under the action of the tube. As the tube expands by heat the rod 4 will move backwards and permit the blade 7 under its own tension to move in the direction of the arrow in Fig. 2. The end of the thermostat arm 7 is provided with a spring leaf 10 attached to the end of the blade 7 and is normally held in contact with a closed circuit contact 11 while the open circuit contact 12 projects from a member 13 arranged upon the opposite side of the leaf to that of the closed circuit contact 11.
By arranging the contacts in this way only two contacts are required in this invention, the closed circuit contact takes the place of the second open circuit contact employed in the type referred to and after the leaf has touched the open circuit contact 12 further movement of the hlade 7 in the direction of the arrow will cause the front end of the leaf to bend, as shown in Fig. 7, and its front end again to touch with the closed circuit contact 11 and further movement of the blade will only tend to increase the contact of the leaf against the contacts 11 and 12. The closed circuit contact 11 is mounted in an extension 14 of a terminal 15 insulated from the end wall 16 of the thermostat by plugs 17 of insulating material.
The ends of the wires for the closed circuit an respectively connected to the terminals 15 and 20 and the leaf 10 normally rests against the contact 11 as shown in Fig 3. thus completing the closed circuit from one terminal 14 to the other terminal 20 of the closed circuit through the Made 10 and flexible cord 21. The tube 1 may be of any suitable metal having a greater coefficient of expansion than the rod 4 and as the rod is flxed to the outer end of the tube, the latter, when heat is applied, is caused to expand thereby moving the rod whereby the free end of the arm 7 will travel in the direction of the arrows shown in the Figs. 2 and 3. During this primitive movement of the arm 7. the blade 10 carried by it, will be brought into contact with the member 12, thus completing the electrical circuit in which is included the terminals l9 and 20. this electrical circuit having included in it a suitable signalling device or annunciator. As the heat upon the tube 1 increases a greater expansion takes place causing the further movement of the rod 4. which in turn, increases the pressure upon the arm 7 until the blade 10 contacts with the members 11 and 12 as is illustrated in Fig. 7. The contact member 12 acts as a fulcrum for the blade 10, causing the free end thereof to contact with the member 11 and for any unforeseen reason a circuit is not completed from the terminal 20 to the terminal 19, a circuit will be completed from the terminal 15 through the contact 11 to the contact 12 and to the terminal 19 through the conductors.
The wall of the tube 1 is reduced in thickness for its greater length in order to render it more sensitive to a change of temperature.
The end walls 2 and 16 are made circular so that the whole thermostat may he enveloped by a sleeve 24 which is passed over the tube and butts against a flange 25 upon the end wall 16. A cap 26 is screwed upon the end wall 16 to enclose the terminals projecting from the end wall 16. A hand 27 is adapted to encircle the sleeve 24 and has flanges 28 for securing it to a ceiling or wall. A screw 29 passes through the neck of the hand for tightening the band upon the sleeve and thus securing the thermostat firmly in position (see Figs. 1 and 8). For inspection purposes the screw 29 is relaxed enabling the sleeve to be passed off the base on to the tube 1 (see dotted lines in Fig. 1) and the mechanism of the thermostat inspected without removing the thermostat from its position. The bush 30 in the outer end of the tube 1 is slotted as shown in order to tighten the bush upon the threaded end of the rod in case of wear.
The application for a patent for this new thermostat was filed May 8, 1922, the serial number being 559,426. It is the invention of Thomas S. Dixon, Christchurch, New Zealand, assignor to the Vigilant Automatic Fire Alarm Company, Ltd., of Christchurch. New Zealand. The patent was granted on February 12, 1924, the number being 1,483.787.