Heavy bombers loom large in our planning for victory. One of the most famous fighting ships in this class and one that has already won laurels in actual combat is the Consolidated B-24, called the Liberator by the British.
The B-24 has seen action both in Europe and in the Pacific war area. Gen. H H Arnold, Chief of the Army Air Forces, credits this type of bomber with the destruction of a number of German submarines. This ship holds the Ferry Command record between Newfoundland and England 2200 mi in 400 min, an average of 330 mph.
The B-24 is built on a. continuous final assembly line. Line assembly is used on most military airplanes that have reached the point of quantity production. But B-24 assembly is more than a line; it is a philosophy of manufacture and its keynote is mobility. Every element that has to do with the process moves with the planes themselves.
The building housing the line is merely a shell designed to shelter the operation. True it has compressed air and electrical outlets, tracks for the carriages to roll along, overhead traveling cranes, racks for parts and templates, and electric motors at the end of each section of the line to move the assembly from station to station. The point is that nothing in the way of essential tooling is fixed to the floor. Given another building of appropriate size and a few essential services, the line could be transferred overnight to a new location.
Some advantages of this philosophy are clear:
The line is oval-shaped; it progresses up one side of the building and doubles back on itself to go down the other.
To obtain space economy, the planes travel along the line at a 45° angle. This arrangement nests them together nicely. The finished plane has a wing span of over 100 ft, but the 45° angle enables it to be moved down the shop in a 74-ft bay and back again in a 116-ft bay besides providing an aisle and storage space. If the planes were carried along abreast, the increased floor space required would have cost at least $360,000.
The electric motors for moving each section of the line are hooked up so the line may be moved either continuously or intermittently. The line is moved intermittently at present. The motors have suitable gear reductions to give the proper speed. For moving the line continuously, the 5-hp motor used for intermittent motion on each section is backed up by a ¾-hp motor with an additional gear reduction to give a comparatively low speed. This second motor operates through the armature shaft and gear reduction of the first, but is connected to it by means of an over-running clutch so that it is inoperative when intermittent movement is used.
Stations one to three, inclusive, unite the three major subassemblies; the wing section, the fuselage nose section and the fuselage tail section. These subassemblies are brought from the Parts Plant by truck on a specially constructed trailer, adjustable to handle any one of the three. The wing section is carried in a cradle consisting of front and rear concave supports, padded to prevent damage in transit.
The fuselage sections are carried on three-wheeled dollies, cross-braced with tie rods previously used for their subassembly. The wheel arrangement provides three point support which avoids any changes in deflection due to the pull of gravity. In fact, the three-point principle is being incorporated into virtually all the new jigs built by Consolidated. It makes the jig self-supporting and mobile independent of floor plates or other means of support.
When used for transporting the fuselage sections, the rear cradle support of the special trailer is swung downward to the roadway on hinges by the removal of two pins. With the trailer in this position, the dolly can be run up on three tracks to bring its rear wheels directly over the trailer axle. The cradle support is then swung clear of the roadway for transportation to the final assembly line.
Other subassemblies, such as outer wing panels, ailerons and rudders, are transported by truck using suitable wooden racks where. required. Units furnished by outside vendors, including engines and propellers, are shipped in by rail or by truck. They can be unloaded directly opposite the station at which they are needed on the line.
Two crane runways run the entire length of the plant, one over each bay. Each runway supports two traveling cranes used for hoisting large subassemblies into place.
The carriage which takes the B-24 through its entire circuit of the line is built up of structural shapes and heavy tubing welded together. It consists of a framework having four vertical posts and eight wheels, one pair under each post. The square thus formed travels along one of its diagonals on three tracks. The diagonally opposite two pairs of wheels that follow each other along the center track are pivoted and flanged. These guide the carriage. The two outer pairs of wheels are fixed and have no flanges.
All the carriages in each section of the line are linked together with tie bars which fix their spacing and move them along when the carriage at the head of the line is pulled forward. For the first section of the line, the carriages are spaced on 46-ft centers, but for the return section this dimension is increased to 65 ft.
The tie rod is of hinged design to take care of the change in center distance. Only a single solid length is used along the first part of the line. The tie rod is disconnected as the carriage leaves this section to be swung around on a semicircular length of track for its return trip. When. the tie rod is reconnected, it is swung on its hinged joint to its full length. A similar semicircular length of track at the end of the final half of the line makes it possible to swing the carriage into the starting station without additional handling.
Subassemblies come to the final assembly line with as much work as possible already completed. Each structure is an integral unit itself, and all wiring, piping, sheaves, and connections are in place. All parts and subassemblies are given their last coat of paint before reaching the line. Retouching is done during lunch hour with a spray gun if necessary.
The first step in assembly is to mount the wing section on the carriage. It is picked up from the trailer by a sling in the traveling crane deposited in the four supporting posts. The wing section is squared in position by means of Z-angle guides with adjustable buttons on all four posts. All posts are adjustable vertically by means of elevating screws used to level the wing section.
The mating jig is lifted in the traveling crane and placed over the wing section. This jig, as its name indicated, locates the wing and fuselage sections in their correct relative positions. Three such jigs are now used and carry through the first three assembly stations. At the third station, the jig is released and carried back by crane to the start of the line to be used over again. The jigs are of welded tubular construction and are rigid enough to be independent of deflection caused by gravity.
With the wing section in place, the nose and tail fuselage sections are wheeled in on their dollies. At each end of the mating jig there are three chain hoists for elevating the fuselage sections into place. A positioning plate topped by a horizontal beam is bolted to locating holes in the bomber’s window mounting bulkhead of the nose section. The forward chain hoist hitches to this beam while a belly band is fastened between the other two hoists on the nose of the jig.
All hoists are movable along trolleys in the jig. That at the forward end is actuated by a pulley through lead screws so that when the nose section is elevated it can be moved back into the wing section. The two other hoists are connected by cables and sheaves to maintain their relative positions on the trolleys, thus keeping the nose section square with the wings.
The same procedure used to hoist the tail section of the fuselage. Here the position of the pulleys is reversed with the single pulley at the rear carrying a straight beam passed through plates attached to the empennage connecting holes. Hoisting and longitudinal movement are carried on as for the nose section. For additional stability, nose and tail fuselage sections are held to the carriage with canvas straps.
The three major subassemblies, positioned on the center line of the fuselage in the mating jig, are then riveted together. Outlet boxes for electricity are sunk in the floor along the entire length of the assembly line. Compressed air lines parallel the two outer tracks with connections spaced at short intervals. The mating jig is wired and piped so that a single connection to each service provides adequate outlets tor all working positions. There are three junction boxes, each with four outlets and three air valves on each of the four sides of the jig. The riveting tools can thus be connected conveniently.
The carriage is similarly wired but carries no air lines since the bulk of the rivets are driven at the first three stations. However, the floor air outlets may be used directly at any station. In moving the carriages from one station to the next, the air and electric lines are disconnected, and wire and hose is cleared from the tracks.
The carriages have upper and lower work platforms upon which wooden flooring is laid. After the wing section is mounted, four auxiliary platforms are hooked to the carriage. These extend outward parallel to and on either side of each wing. Each auxiliary platform has two wheels in line with the direction of travel. A connecting walk between each pair of platforms leaves an inclosed well under each wing so that the landing gear can be installed and operated on the line. The two platforms at the outer corners of the square have hinged flaps which hang down for the first section of the line to give adequate aisle space in the narrower bay.
For the return section of the line, these flaps are swung upward to provide additional working areas, and two more auxiliary platforms are hooked on ahead of the wings to provide the space needed in working on the nacelles.
All platforms are made accessible from the floor by means of light stepladders which hook onto them. The ladders also have wheels so they can be moved along with the carriage.
Portable stands enable the assemblers to reach high points on the plane. These are moved to any convenient location on top of the platforms. Ladders are also used on the platforms so the workers can readily climb to the tops of the wings. At one or two stations, special high floor stands are required to reach the front end of the nose section.
The general plan of assembly is to use the first half of this line to complete all operations on the wing section and fuselage. This work involves a vast amount of detailed fitting needed to coordinate the three subassemblies and to add many small parts that could not be installed in the parts plant. On the second half of the line, major units are added to complete the plane. The additional space required by these units accounts for the greater center to center spacing needed on this part of the line.
While the assembly is still in the mating jig a radius rod is installed on either side between the nose section of the fuselage and the jig posts. These rods keep the assembly square with the jig all along the line. The Z-angle guides used initially for squaring the wing section must be removed further along the line when the wing leading edges are installed. Were it not for the radius rod there would be nothing to prevent the assembly from pivoting on the fixture after the guides are removed.
During the mating operation, bomb-well side panels, bomb racks, upper decks over the wings, catwalk and bulkhead segments are all added.
After the mating jig has been removed, the assembly moves down the first half of the line. The parts needed for the installations in these stations are located on racks along the line in convenient locations.
A simple and convenient method is used to keep a record of parts on hand and to forestall shortages. All the parts needed at a given station are listed on a board hung on the side of the stand. Opposite each part is a hook which carries a series of metal disks with holes in them like Chinese money. The disks are of two colors: red and black. Each represents a part on hand ready for assembly.
The red disks represent the minimum supply, which is the quantity required to last until a new lot of this part can be processed. The black disks indicate quantities in excess of the minimum.
As parts are used, the stock leadman at each station removes the proper number of disks from each hook. When he gets down to the red disks on any part he is warned that he is dipping into his minimum quantity.
At the righthand side of each part are three columns: date below minimum, date listed and date of next supply. As soon as the leadman uses a part that calls for a red disk, he notes it in the first column. The stock chaser inspects these boards daily and records his listing of the parts in the second column. When he determines the status of the next lot of parts in process, he puts its completion date in the third column.
Despite all precautions, shortages will occur. An advantage of this mobile method of assembly is its flexibility, and usually no delay is experienced in adding such parts at a later station. This practice is not desirable, however, as it interferes with precise timing on which the performance of the line is predicated.
Every effort is made to predrill all parts by the use of jigs so that rivet hole drilling during assembly will be kept to a minimum. Predrilling is now done successfully on bomb-well side panels and bomb racks. However, many airplane components, made of sheet duralumin, are flexible and at present, it is not possible to eliminate all drilling required to mate holes during final assembly.
On the first of the second part of the line, the engines, outer wing panel assemblies, stabilizers and rudders are all added.
Fowler flaps and propellers are then installed with all the fittings needed for the wing and rudder control systems. The final stations are used to add the nose inclosure and radio instruments.
The entire side of the building is provided with large doorways so these large units can be brought directly opposite the stations on which they will be used. The 45° angular placing of the planes on the line permits the storage of engines, propellers, empennages, flaps and turrets in the additional space thus obtained.
The underside of the carriage platform is provided with racks on which certain parts can be hung. For instance, engines come into the line already fitted with removable cowl panels. Since it is desirable to keep these panels with their own engines, they are hung under the platform and travel along with the assembly while the engines are being installed. Other detachable parts are kept with the plane in the same manner.
The opposite side of the building paralleling the first half of the line, houses a series of service shops. Among these are an armament room, used for preparing and sighting machine guns, a hydraulic and oxygen testing and inspection laboratory, and instrument department, an electrical test room, a tooling and methods office, several stockrooms and a toolroom where quick adjustments can be given templates and other tools without the red tape and delay that would result were such work sent to an outside department. Cribs, toilets and a first aid room occupy the space between the two halves of the assembly line.
A day foreman and a night foreman supervise the work at each eight stations on the line. Under each, eight leadmen have direct charge of each station. A placard hung on the side of each assembly gives the station number, the name of the leadman in charge and the name of the inspector.
Typifying the lengths to which mobile assembly has been carried is the installation of a portable shop clerk's desk on each carriage. Here the bill of materials and parts lists, as compiled by the tooling and methods department, are kept together with other essential shop records.
When an assembly reaches the end of the line. it is rolled out into the yard. Any final adjustments are made here. Air pumped into the oleo-struts lifts the plane from the carriage. It is then ready for engine and flight tests and for delivery to the Ferry Command.
When the assemblies are transferred from station to station signalmen stand on the wing tips on every eighth plane along the line. They have two flags, red and white. The red flag is the ready to move signal; the white one indicates that the line is in motion. It is an inspiring sight this column of giant bombers in motion at one time. Then the B-24 assembly becomes more than a production line. It is transformed into a parade a parade that will lead to victory.
This article was originally published in the July, 1942, issue of Aviation magazine, vol 41, no 7, pp 98-103, 323, with a tip-in foldout between pp 100-101.
The original article includes 17 captioned photos and 3 diagrams.
Photos are not credited, but are certainly from Consolidated.