Lightweight hydraulic actuator

A lightweight linear hydraulic actuator which is unusually inexpensive to fabricate includes a cylindrical barrel of metal tubing material, a head end gland and a rod end gland fastened to the barrel, a piston and a rod in the barrel with the rod extending through the rod end gland, ports in the sidewall of the barrel on opposite sides of the piston and fitting members having channels in registry with the ports, the fitting members including hoop type attachments around the exterior surface of the barrel and a curved surface mating with the curvature of the barrel. A circular groove is formed in the surface of the barrel around each of the ports and an O-ring seal is located in the grooves sealing against the curved surface of the fitting member. An axial extension forming part of each of the fittings includes a boss receiving a threaded fastener for fastening the fittings to the barrel.

The present invention relates to lightweight linear hydraulic actuators and 
more particularly to such an actuator which is comparatively simple in 
construction and inexpensive to manufacture. 
Usual linear actuators, particularly for operating aircraft control 
surfaces, include a cylindrical barrel which may be cast or forged to 
include a head end with a structure for fastening the actuator to a 
stationary part of the aircraft or other vehicle. Alternatively, the head 
end may include a separate structure fastened to the barrel, but in either 
case the end structure usually will include a passage to allow hydraulic 
fluid to flow into and out of the cylinder. A piston attached to a rod is 
movable in the cylinder. At the opposite, or rod end of the barrel, a 
separate rod end gland is fastened to the barrel and this gland includes 
one or more seals surrounding the rod which passes through the barrel and 
usually also will include a passage to allow hydraulic fluid to flow into 
and out of the cylinder. Such passages may also be cast into the barrel if 
the barrel is cast. Similarly, the piston and rod may be cast or forged in 
one piece, or formed of separate pieces which are then secured together. 
Actuators incorporating such cast or forged components are usually quite 
expensive both because of the cost of castings and forgings but also 
because of the required machining. These costs are further escalated by 
whatever losses or scrap are caused by the machining operations. 
There is a need for a lightweight linear actuator which will perform 
essentially as well as the expensive actuators described above but which 
is substantially less expensive to manufacture. It is known to make the 
cylinder barrel of metal tubing, to make the rod of conventional bar stock 
and to form the piston separately in any desired manner for attachment to 
the rod. These techniques reduce costs. There have been problem areas in 
the placement of the fluid ports, however, except in the cases where the 
geometry of the installation will permit installation of the ports in the 
rod end or head end glands. There is often a desire to have the ports 
parallel to the actuator axis to keep the actuator short and to keep the 
tube connection close to the actuator (to save space) without the 
potential of unthreading the port fitting during actuator use. Also, on a 
tandem actuator installation it is difficult to place all of the porting 
in head ends or rod end glands since these usually require transfer tube 
or quill type connections to a mating valve manifold. 
Given the above described cost saving construction, the port fittings can 
sometimes be welded or brazed in place. On comparatively thin wall tubing, 
this sometimes results in high scrap and rework, at least partially 
because of distorting of the barrel. 
Another known possibility is to add separate outer glands surrounding the 
barrel. This technique leaves much to be desired because the extra glands 
must be capable of withstanding pressure hoop stress and must seal on two 
extra and rather large diameter seals which adversely affects size, weight 
and reliability. 
Applicant has devised a way of adding port fittings to an actuator of the 
type described which avoids the above problems and limitations. A cast or 
forged fitting including a thin hoop support structure surrounds the 
barrel and provides a channel with a threaded connection to a fluid source 
which channel registers with a port in the barrel. A small circular groove 
in the surface of the cylinder surrounds the port and receives an O-ring 
seal which seals against a curved surface of the fitting which mates with 
the curvature of the barrel. A suitable threaded fastener maintains the 
fitting securely in place. A superficial consideration of applicant's 
structure might cause one to think that the localized pressure generated 
force in the seal area would tend to increase the gap between the barrel 
outer diameter and the fitting such that the seal would fail. Applicant 
has found, however, that the localized force and resultant strain on the 
fitting is less than the barrel hoop strain (at the same pressure) if the 
fitting is designed properly Therefore, the initial face seal annular gap 
will tend to decrease with pressure rather than the reverse. Thus the 
fitting can be lightweight, has only one seal (potential leak path) and is 
readily removable and replaceable even with the actuator assembled.

The hydraulic actuator, according to my invention, is shown generally at 
numeral 10, including a barrel section 12 which typically is formed of 
steel tubing material. The barrel is closed at its head end by means of a 
head end gland 14 which is securely fastened to the barrel member 10 by 
means of a feedthrough wire 16. A conventional seal 18 is located in the 
surface of the head end gland and seals against the inside wall of the 
barrel 12. A clevis member 20 which may be a part of gland member 14 is 
provided for attachment of the actuator to a stationary member, not shown. 
The opposite end of the actuator 10 is closed by means of a rod end gland 
22 having an opening 24 through which passes an actuating rod 26 driven by 
a piston 28, which separates the interior of barrel 12 into chambers 29 
and 30. Rod end gland 22 is secured to the inside surface of barrel 12 by 
means of a feedthrough wire 31 and piston 28 is similarly secured to rod 
26 by means of a feedthrough wire 32. Conventional seals 34 and 36, which 
may be O-ring seals, prevent communication across the piston between the 
piston 28 and rod 26 and piston 28 and the inside surface of the barrel 
12, respectively. Similar seals 38 and 40 prevent high pressure fluid 
within chamber 30 from leaking along the surface of rod 26 to the exterior 
of rod end gland 22. 
Chambers 29 and 30 are each supplied with hydraulic fluid under high 
pressure from an external source not shown. Chamber 29 communicates with a 
port 46 through the sidewall of barrel 12 which in turn communicates with 
the interior channel 48 of a fitting member 50. Chamber 30 communicates 
with a port 52 which communicates with a channel 54 in a fitting member 56 
which is similar to fitting member 50. Members 50 and 56 are forged or 
cast as ring-shaped members which surround the barrel 12 and include 
bosses 58 and 60, respectively, which receive screws 62 and 64, 
respectively, which are threadedly engaged with the sidewall of barrel 12, 
and as shown may also penetrate into the head and rod end glands 14 and 
22. A pair of face seal grooves 66 and 68 are formed in the sidewall of 
barrel 12 by electrical discharge machining, multi-axis conventional 
machining, or other suitable method such that they surround the ports 46 
and 52. The fitting members 50 and 56 have internal diameters such that 
they afford approximately 0.05-0.13 mm (0.002 to 0.005 in.) clearance 
around the barrel 12 and grooves 66 and 68 may contain conventional O-type 
ring seals for preventing the high pressure fluid in chambers 29 and 30 
from leaking laterally along the outside surface of the barrel 12. 
As indicated above, it has been determined that the localized force and the 
strain on the fitting members 50 and 56 is less than the barrel-hoop 
strain at the same pressure, therefore, the initial face seal annular gap 
tends to decrease with pressure increase rather than the reverse. A very 
thin O-ring or similar seal can be used, 1.24-1.50 mm (0.050 to 0.060 
inches), more or less, while maintaining this effective seal gap closure. 
Thus, the fitting can be lightweight, has only one seal or potential leak 
path and is a removable and replaceable part even with the actuator 
assembled. The seals in grooves 66 and 68 would normally be of the type 
utilizing an O-ring of elastomeric material having an outer diameter 
backup ring of polytetraflorethylene (Teflon) or similar material. Details 
of such a seal are shown in FIG. 2 wherein groove 66 contains an O-ring 70 
which exerts a radially outward force on a Teflon ring 72 which seals 
against the internal surface of fitting member 50. It will be recognized 
that O-ring 70 and ring 72 are deformed to follow the contour of barrel 
12. 
While only a single embodiment has been shown and described herein, those 
skilled in the art will be aware of modifications within the scope of the 
present invention.