|Hydraulic Servo Units
Excerpt from Singer in World War
II, 1939-1945 (Published 1946).
Early in 1942, Officers of the Naval Aircraft Factory, Philadelphia, requested the
Company to consider the manufacture of a Hydraulic Servo Assembly. Because of the many
operational difficulties involved in its manufacture and the experimental nature of the
project, the Navy had been unable to place contracts for this work elsewhere. Although
Elizabethport Works was already engaged in the Director work, this additional project was
The Servo Unit is an accurate hydraulic instrument which operates on an electrical
signal current to produce perfectly controlled powerful mechanical forces for actuating
the various controls of an airplane. It is used both in connection with automatic flight
control instruments on Robot Airplanes and for assisting the pilot in operating the
controls of conventional types of airplanes.
The early work on the Servo Unit at Elizabethport consisted almost entirely of
experimenting and redesign. The "tailor-made" model which the Aircraft Factory
had been producing, one at a time, was the work of experiments and had never been adapted
for production methods. Elizabethport's first effort was, therefore, to redesign the unit
for production and on September 3, 1942, the chief engineer at the Naval Aircraft Factory
approved the Elizabethport design after thorough tests on the bench and in airplanes.
Subsequent production was in accordance with the Elizabethport design.
By the very nature of the experimental use to which these Servo Units have been put, it
has been difficult for either the Navy or the Company to anticipate future requirements
and, consequently, it has been impossible for Elizabethport to set up and bring through a
regular flow of production. Procurements have been placed for small quantities, in nearly
every case to meet some specific urgent requirement. Upon completion of such a lot,
facilities frequently lay idle for some months until additional quantities of Servo Units
were required. Although the project has never been reduced to an orderly production
program following a regular schedule over a long period of time, as in the case of the
Director and the Computer, the total production has reached sizeable proportions, about
3,300 units have been produced.
Most of the difficulties encountered in the manufacture of the Servo Unit can be
attributed to the basic fact that the instrument operates with hydraulic fluid at a
pressure of 150 pounds. This fact makes it absolutely necessary that all parts be machined
with extreme accuracy so that the fits may be tight. As an example of the techniques
employed to obtain these fits we cite the method employed in the assembly of the Solenoid
Control Valve Housing. The aluminum casting is heated to 400 degrees above zero and
beryllium copper collars, previously frozen to 60 degrees below zero, are dropped into its
central canal at accurately measured intervals. Upon return to normal temperatures, the
assembly thus completed becomes as firmly united as if it were one piece.
Considerable difficulty was experienced with porous aluminum castings until the Foundry
developed techniques to correct the defects. Beryllium copper allows were used for many of
the parts and the obtaining of this critical material and the learning of the techniques
necessary to produce parts to the accuracies required presented difficult problems.
Likewise, the operations on the steel tubing for the cylinders, the manufacture of the
pistol shaft assembly, the accurate drilling of seemingly numberless holes in the control
valve body and the manifold, and the machining of the precisely parallel exterior channel
cuts on the manifold, all required the best skills available.
One of the most exactly features of the Servo Unit project was the final testing and
inspection by Company and Navy inspectors. A large proportion of the testing standards
were devised by Elizabethport engineers and the testing equipment was produced within the
factory. It has been most gratifying to find that our rigid inspection and testing
practices were so satisfactory that almost invariably they have been accepted by the Navy
With the major emphasis being placed on the larger Director and Computer programs, the
Servo Unit project has been the responsibility of a small group of Elizabethport employees
who have achieved exceptional results. Undoubtedly, if it had been possible to anticipate
the total quantities which were eventually required, the Navy would have authorized a
proper appropriation for tooling and equipment to program the production in an orderly
fashion. What has been accomplished, however, has been largely done with the make-shift
tooling through the resourcefulness of Elizabethport employees. As far as is known, Singer
is the only supplier of the types of Servo Units manufactured at Elizabethport and the
extreme urgency of the Servo Unit requirements has always been recognized. Singer's
associations with the Aviation Supply Office, the Navy Aircraft Factory in Philadelphia
and the Bureau of Aeronautics in Washington have been on a friendly and cooperative basis
and it is gratifying to learn that the Servo Units produced at Elizabethport have been
regarded by the Navy as highly satisfactory.