[0001] This invention relates to bellows actuators.
[0002] It is known to provide arrangements in which a resilient bellows actuator is used
to position a valve element in response to a difference between fluid pressures, the
valve element acting to regulate a control pressure signal. A common use for such
an arrangement is in a fuel control system for a gas turbine engine. A gas turbine
engine fuel control system showing a bellows actuator arrangement as described above
is shown in U.K. Patent 1589967.
[0003] In such an arrangement the bellows actuator is resiliently distorted by a fluid pressure
difference across it. A leak in the bellows will therefore remove or reduce the pressure
difference, and the resilience of the bellows will cause a coupled valve element to
be driven towards one end of its travel. The resulting change in the control pressure
signal may have a disastrous effect on operation of the engine.
[0004] Alternatively a bellows actuator may be used to apply a force dependent on a difference
between fluid pressures, and a leak in the bellows will result in an erroneous force
being applied.
[0005] U.K.Patent application 2037082A discloses a bellows device which includes an additional
diaphragm which becomes effective if the bellows leaks. In this device, however, the
bellows device is required to operate a switch, or similar on/off component, so that
the device is,not required to provide an output movement, or output force, which is
critically dependent on a pressure difference between the inside and outside of the
bellows. In the prior art arrangement, therefore, the diaphragm is not arranged to
be subjected to the same pressure difference as will have existed across the bellows
in normal operation.
[0006] It is an object of the present invention to provide a bellows actuator which continue
to respond in a predictable manner to an applied pressure difference, even if the
bellows develops a leak.
[0007] According to the invention a bellows actuator comprises a bellows unit, a chamber
axially aligned with said unit and sealingly connected thereto, an inlet for admitting
a fluid pressure to said chamber, a piston slidable in said chamber and co-acting
with an end wall of said bellows unit, said piston being responsive to a difference
between the pressure at said inlet and the pressure acting on said bellows unit, and
a restricted flow path between said inlet and said bellows unit.
[0008] An embodiment of the invention will now be described by way of example only and with
reference to the accompanying drawing, which is a longitudinal diagrammatic section
through a bellows actuator.
[0009] The diagram is several times full size and shows a resilient metal bellows unit 10
having a bearing element 11 secured to the outside of its closed end 12. The other
end of the unit 10 is sealingly secured to a cylindrical chamber 13 which is axially
aligned with the bellows and in which a piston 14 is slidable with radial clearance.
The piston 14 includes a stem 15 which is biased into engagement with the inside of
the end 12 of the unit 10 by means of a light compression spring 16. An inlet 17 for
a higher fluid pressure PH opens into the chamber 13 at an end thereof remote from
the bellows unit 11. Apertures 18 in the piston 14 co-operate with the radial clearance
19 to provide a restricted flow path between the inlet 17 and the inside of the unit
10. The effective area of the piston 14 is equal to that of the unit 10.
[0010] In use the actuator is mounted so that the bellows unit 10 is exposed externally
to a lower fluid pressure PL, and the bearing element 11 is secured to a part whose
position is to be controlled, or to which a variable force is to be applied. An increase
in the difference between the pressures PH and PL urges the end 11 of the bellows
unit 10 upwardly, as viewed in the drawing. The restriction provided by the apertures
18 and clearance 19 is such as to offer effectively no resistance to the fluid flow
which occurs as a result of distortion of the bellows unit 10, and there is effectively
no difference between the pressure PH at the inlet 17 and a pressure PX acting on
the bellows unit 10.
[0011] If the bellows unit 10 ruptures the pressure therein falls towards the low pressure
PL and the piston 14 is urged upwardly by a force dependent on the difference between
the pressures PH and PL. This force is transmitted to the end 12 of the bellows unit
10, and hence to the bearing element 11, which continues to be subjected to a force
dependent on this pressure difference.
[0012] In arrangements in which the higher pressure PH is required to be applied externally
of a bellows unit 10 a cylindrical chamber, corresponding to the chamber 13, may enclose
the bellows unit 10 and carry a piston, corresponding to the piston 14, which bears
downwardly on the outside of the closed end of the bellows unit 10. The purpose of
the spring 16 is to prevent free movement of the piston 14 in the absence of a substantial
pressure difference thereacross. In an alternative embodiment the spring 16 may be
dispensed with and the stem 15 of the piston 14 be secured to the end 12 of the unit
10: In a further alternative embodiment the piston 14 may be biased downwardly by
a light compression spring, against an internal stop in the chamber 13.
[0013] A small leak in the unit 10 will cause the pressure PX to have a value between the
pressures PH and PL. The forces applied by the unit 10 and piston 14, 15 separately
will thus vary, but the sum of these forces will always be substantially proportioned
to the difference between the pressures PH and PL.
1. A bellows actuator including a bellows unit (10), a chamber (13) axially aligned
with said unit (10) and sealingly connected thereto, an inlet (17) for admitting a
fluid pressure (PH) to said chamber (13) and a device for providing a mechanical output
in response to said pressure (PH) in the event that said bellows unit (10) develops
a leak, characterised in that said device comprises a piston (14, 15) slidable in
said chamber (13) and coacting with an end wall (12) of said unit (10), said piston
(14, 15) being responsive to a difference between the pressure (PH) at said inlet
(17) and a pressure (PX) acting on said unit (10), and a restricted flow path (18,19)
between said inlet (17) and said bellows unit (10).
2. An actuator as claimed in claim 1 in which said chamber (13) communicates with
the inside of said bellows unit (10).
3. An actuator as claimed in claim 1 or claim 2 which includes means (16) for maintaining
said piston (14, 15) in contact with an end wall (12) of said unit.
4. An actuator as claimed in any one of the preceding claims in which the effective
area of the piston (14,15) is substantially equal to that of the bellows unit (10).
