Linotype mechanism

by Alvin Garfield Swank; Raymond Means

ELECTRIC POT

The electrical equipment consists of four heaters, a dynamic thermometer, and a unit control panel. The passage of electric current through the windings inside the heaters generates heat which melts the metal in the crucible and raises it to the proper operating temperature, while the dynamic thermometer and the magnetic switch on the control panel operate to keep the metal at this predetermined temperature. These equipments are either 110 to 120 volts or 210 to 220 volts, direct current or alternating current.

The crucible heaters are the same for all voltages; they are connected in “series” for 200-volt to 250-volt circuits and in “parallel” for 100-volt to 125-volt circuits. They are immersed directly in the metal and partially surround the pump-well, heating the metal by direct contact. The heating element or resistor of these heaters is composed of resistant ribbon wound on strips of clear mica. Strips of mica entirely surround the resistor, completely insulating it from the metal parts of the pot. The resistors are protected by strong metal casings which surround them. They are not subject to wear and tear in normal service.

Heating the metal by direct contact from within the metal itself by these heaters, makes a very efficient equipment, as all of the heat generated is immediately transmitted to the metal exactly where it is required, and there is no loss of heat due to faulty conduction. The dynamic thermometer bulb being immersed in the metal adjacent to the heaters and the pump-well, permits of a very close temperature regulation.

As these heaters extend nearly the full height of the metal in the crucible and pass down through the top of the metal, there is no possibility of cracking a crucible. When the metal begins to heat, it is that portion in direct contact with the heaters which first becomes molten. Internal pressure is relieved by this melted pathway and the molten metal will flow to the top of the pot.

It is important that the pot never be filled with metal above the under side of the ring which is cast on the inside of the crucible. If the crucible is filled above this ring, metal may splash over into the heating insulating material and touch the electric terminals, grounding them.

The crucible heater must be entirely covered with metal at all times. If they are not, that portion that is exposed to the air will get very hot and continued exposure will burn them out, destroying them. They are not designed for operation in the air.

Do not pry around the units with a screwdriver or pound on them, if the metal envelope is punctured, molten metal will immediately enter, grounding the element and destroying the unit.

The temperature in the crucible is controlled by an adjustable dynamic thermometer mounted on the side of the pot, and which operates a switch in the control panel, turning the current on and off as needed. The bulb and tube of this thermometer contain mercury and the bulb is immersed in the metal in front of the well of the crucible. The tube is connected to a flattened hollow coiled spring tube. The mercury also passes through this air tight spring tube. On the free end of this coiled spring tube is fastened an insulated pin which operates the contact lever. The contact lever is suspended downward between two metal contact disks. These contact disks and the contact lever are connected to the main line through a magnetic switch mounted in the control panel. The thermometer only carries current long enough for the magnetic switch to connect and then the current is carried directly through the switch.

As the temperature of the metal rises the mercury in the bulb expands causing the coiled spring to uncoil slightly, driving the contact lever against the disk that shuts off the current, allowing the switch to fall open and disconnecting the circuit through the crucible heaters.

As the temperature of the metal falls, the mercury in the bulb contracts allowing the coiled spring to force the contact lever against the disk that turns the current on by connecting the switch to the main line. The variation in the temperature is held within 15 degrees of the average working temperature.

The temperature control is normally set for a maximum of 550 degrees and a minimum of 535 degrees, that is, with normal operation the temperature of the metal will always be between these limits. This is found to give the best all around casting results for average metal. In case it is desired at any time to change the operating temperature, this can be done by turning adjusting screw right-handed for hotter metal, and left-handed for cooler metal. The head of this adjusting screw projects through the right-hand side of the dynamic thermometer cover.

Do not change the position of the control disks, unless badly pitted, as their relative distance is determined at the factory, and very seldom need to be changed.

A good thermometer should always be used when adjusting the screw for proper heat. Guessing will not get the proper results.

With ordinary work, and after the metal is at operating temperature, the current will be on and the crucible heaters generating heat about three minutes, then off and not generating heat about twelve minutes, and will repeat this cycle as long as snap switch is left in the “on” position.