Producing Struts For the P-38

by Wilbur G Wood
Chief Engineer, Aircraft Strut Division,
Menasco Manufacturing Co
This article on the production of landing gear struts for the Lockheed P-38 interceptor is a dramatic illustration of the contribution which can be made to military aircraft pro- duction through subcontracting.

Now rolling off the production lines in rapidly increasing numbers, the Lockheed P-38 interceptor must carry much of the load of defending American forces against bomber attacks. And all of the load in taking off or landing the P-38 is carried by the landing gear, which is of tricycle type. Both main and nose struts for the P-38 landing gear are manufactured by a subcontractor, to Lockheed designs. The P-38 landing gear is characterized by its simplicity and ruggedness. Not only does this assure reliability in operation, and simplify maintenance problems, but it helps speed manufacturing.

Basically there are but two parts to the P-38 main landing gear strut. These are the strut cylinder, which is attached to the airplane's structure; and the piston which operates in the cylinder and which carries the wheel at its lower end. Of course there are numerous auxiliary parts required for full operation. Contrasting with the extreme simplicity of this piston-cylinder combination is the rather common practice abroad of separating the strut into a load carrying member and a shock absorbing unit, resulting in many more parts. Some foreign designers also prefer to use heavy ring springs in the strut to carry compression loads, instead of relying on air pressure, as is standard American Practice.

Manufacture Is Simple But Rigidly Controlled

In order to justify its simplicity of design and manufacture the P-38 type strut must give unfailing service. This requires that manufacturing be conducted with extreme care and accuracy. Such manufacturing care must include precise heat treatment and unusual attention to inspection, in addition to a high quality of machine work. Two examples of operations other than machining will illustrate this. In heat treating and tempering the struts they are handled in special furnace equipment which protects them from room atmosphere and so practically eliminates scale formation. This contributes substantially to close control of dimensional tolerances. A further example is the practice of hard chrome plating the wearing surface of the piston tube. The piston is first turned on a lathe, then ground to close tolerances and finally buffed. Final operations are hard chrome plating to within .002" of specified diameter and buffing.

Importance of machinery to production of the P-38 struts is illustrated by comparing rough and finished weights of the major strut parts. The strut cylinder, a heavy seamless steel chrome molybdenum tube, weighs 168 lb in the rough and only 45 lb when finished, including the weight of two forged lugs which are welded on in the course of manufacture. The piston member is produced from a forging which weighs 126 lb in the rough and only 28 lb finished, including a torque flange which is welded on.

Strut Construction

Since the P-38 landing gear is of tricycle type the main landing gear struts are located in rear of the CG, being attached to the airplane's structure by a fulcrum. The struts are semi-cantilever, with drag braces running from about the mid-point of the strut, and side loads carried by brace struts extending up and outboard from the same mid-point fitting. Retraction is accomplished through a hydraulic cylinder operating on the drag strut, drawing the landing gear backward and up into wheel wells which are fully faired when the gear is retracted. These brace struts and the actuating cylinder are not produced by us, permitting us to concentrate all our production facilities on the strut itself.

There are two main chambers in the strut cylinder, the piston leg operating in the lower cylinder, which is filled with oil, while the upper chamber serves as an air chamber. The piston has a diameter of 4" and a stroke of 10". Between the lower and upper chambers is an orifice plate which also serves as a stop when the piston is fully deflected. Oil fills not only the lower chamber, but extends 5" above the orifice plate into the upper chamber, the balance of the chamber being filled with compressed air. Forcing the oil through this orifice produces a "snubbing" action which works in either direction to take the initial shock, reduce rebound, and produce a smooth-acting shock absorbing cylinder. Reserve oil carried in the upper cylinder is chiefly a safeguard against leakage, to insure against loss of oil which would uncover the orifice and impair proper functioning of the strut.

Overall length of the strut in normal position is 49½" from center of the upper fulcrum to center of the wheel axle. Maximum OD of the strut is 5-3/8". Both the cylinder and piston are of chrome molybdenum steel which is heat treated to a strength of 180,000 psi minimum. Bronze bearings are provided on the upper end of the piston and lower end of the cylinder so that sliding metal to metal contact is bronze against hard chrome plate or steel. Retention of the hydraulic fluid is by means of chevron type packing incorporated in the lower end of the cylinder just above the bearing. Proper alignment of the wheel axle is maintained by torque links connecting the piston with the cylinder.

The piston member is machined from a solid forging which incorporates a knuckle at the lower end. This knuckle is bored out to receive the wheel axle, which is pressed into the forging. The piston is precision machined, ground, and honed to a hollow cylinder closed at the upper end by means of a bronze piston head which also serves as the upper bearing. A torque plate is welded to the knuckle end of the piston, serving as a support for the brake mechanism.

A sleeve is also screwed to the lower end of the cylinder to serve as a stop for the piston when in fully extended position. Below this sleeve is the cylinder packing, the lower bearing, and a retainer nut.

Ruggedness Of Construction

Representative of the best American practice in shock strut, design, the P-38 strut is quite conventional throughout. Its efficiency as a shock absorbing unit is the result of, careful proportioning of the air and oil cylinders, and of the orifice between them, based on long experience and extensive testing. Not only is a strut of this design more readily adaptable to quantity production, and more serviceable in the field, as compared with more complex gears, but it is also less susceptible to damage from gun fire or shell fragments. We know of no case of such a strut having been penetrated by an enemy shell or bullet. In fact, the hardened steel tube of the strut, made strong for its work as a structural member is fully equivalent to the armor plate normally used to protect vital parts of an airplane. And the shape of the strut cylinder would tend to deflect any bullet or fragment that did not strike squarely. Finally, even if the strut were pierced, it would still function serviceably without air pressure.

How Quantity Production Is Achieved

In achieving quantity production of P-38 struts, the subcontractor has built. and equipped a complete new factory designed specifically for this work. This plant is half again as large as the portion of the factory formerly devoted to engine manufacture. Furthermore, under the pressure for all-out strut production, we have converted a large part of the engine plant to strut work.

To illustrate the scope of the manufacturing processes involved, there follows a brief description of the operations performed on the main strut cylinder. The upper end of the strut tube is reduced by swaging to approximately half its original diameter. Then both ends of the tube are rough-faced to approximate length.

The cylinder OD is turned on a special Monarch engine lathe, equipped with a contour plate attachment which controls the tool in cutting 12 different diameters, and tapers between them, along the length of the cylinder. This machine performs both rough and finish cut operations. Rough cut for this operation is about is about 3/16" and the finished cut about 1/32". The lathe has a 22" throw and 72" bed. The cutter is set against the work by means of a cam follower held against the profile bar by an air cylinder. Following this, the upper end of the cylinder is drilled for location of oil and air filler plug.

Two forged fittings are welded to the cylinder with a Lincoln arc welder. On the upper side of the cylinder is a ring fitting with two sets of lugs, one extending forward which carries the lower end of the drag brace strut and the other extending laterally to the outboard side to carry the side brace strut. This forging is slipped over the cylinder and welded into place. The lower fitting is a strap type, extending about halfway around the strut, and carrying lugs for the attachment of the upper torque link. This is welded in place while positioned in a precision jig. The air valve and fluid filler boss is also welded in place near the upper end of the cylinder.

Close metallurgical control is exercised over the welding operation, maintaining constant check on the composition of the metal and welding rod, heat of weld, etc. After tack welding, the cylinders are preheated to 300° F in an automatic electric oven and are then mounted in a power rotated welding jig which may be positioned to provide the proper work angle to secure the best flow of the welding material.

Fourteen Operations in a Single Setting

The interior diameters are bored on a special W F & John Barnes Co machine, in which the work is held stationary and the tool head is moved hydraulically to feed the boring bar through the work. Four different diameters are bored including the air chamber, oil chamber, piston bore and lower packing gland recess which is later threaded. The cylinder is clamped in three places for maximum rigidity, and the boring bar is piloted through the work by a tail-stock bushing, the bar first being passed completely through the work before any cutting starts. A roughing cut of about ¼" is first taken and a finishing cut of about 1/16" follows. Cutting oil is fed to the tool through the boring bar at the rate of about 40 gpm. A total of 14 operations is performed while the work is held in this setup. Following the boring three sets of internal threads are tapped for upper and center plug and lower cylinder sleeve.

Heat Treatment And Assembly

The cylinder is now ready for heat treatment. This treatment, as with other operations, is controlled with great care and precision in order to secure a minimum strength in all cylinders of 180,000 psi. Two groups of electric furnaces are used, one for hardening, the other for drying. Both are maintained at the correct temperature through use of Leeds & Northrup Micromax recording controllers. Natural gas is used to provide the controlled atmosphere and a large hood is used while transferring the parts from the furnace to the quenching tank, to reduce scaling effect. During tempering the cylinder material is brought to a hardness of 39 to 42 Rockwell C, and all machining, except rough turning and boring and internal threading is done at that hardness.

The surfaces of the cylinder are then cleaned by sandblasting following which the assembly is Magnaflux inspected. There follows a lathe operation to finish the tube facing for length and to turn a portion of the cylinder for steady rests for grinding and honing,.The piston recess and packing gland recess are then honed following which all threads are capped to clean.

Two plugs are threaded and sweat soldered into the main cylinders. The upper plug seals the air chamber, while the lower serves as an orifice plate. Both are first tinned then screwed into place, after which the cylinder adjacent to the plug is heated from the outside to a temperature sufficient to melt the solder and then is allowed to cool.

The packing gland chamber is then ground followed by the grinding of three external surfaces to produce specified wall thicknesses. After straddle milling the three sets of lugs previously welded in position in semi-finished condition, the lug holes and the upper fulcrum hole in the cylinder are drilled. The radius is then milled on the torque link lug and the external thread on the lower end of the tube is ground. The principal machining operations are completed by finish boring the upper cylinder end for the Welch plug. After burring and stamping the serial number, the assembly is ready for final inspection.

Inspection Routine

Frequent inspections are made throughout the manufacturing process. The inspection has been put on a production basis without sacrificing the necessary precision. For all dimensional checks this is accomplished through the help of special gages and fixtures which eliminate hand setups and make the operation semi-automatic.

Magnaflux equipment of the most modern type permits rapid examination of the materials themselves to reveal cracks and flaws. All welds are subject to Magnafluxing. A total of eight different Magnafluxing examinations are made on the cylinder alone in the course of its manufacture. All batches of steel are submitted to exhaustive metallurgical analysis. Each set of parts that is heat treated carries a test specimen which is then checked for strength and hardness as a guarantee that the heat treatment is to specification.

After the final inspection, the outside diameter of the cylinder is cadmium plated except the upper end, which fits into the fulcrum recess. The interior chambers are cleaned and boxed and the nameplates sweated on and the unit finally assembled for painting and shipping. Before painting, however, the assembly is filled and tested for leaks and run in on hydraulic presses.

As indicated before, these shock struts are of conventional design and of simplified type. We believe, however, that much of the credit for the unfailing performance and reliability in service must go to the limitless care with which they are manufactured.

This article was originally published in the April, 1942, issue of Aviation magazine, vol 41, no 4, pp 74-77, 200-202.
The original article includes 17 captioned photos illustrating the processes described.
Photos are not credited.