(57) A safety device (24) is provided for use, for example, in conjunction with a double-acting
hydraulic actuator (18) operating a lifting arm. The safety device (24) is interposed
between the actuator (18) and the fluid line (20) along which fluid is supplied to
raise the arm. In the event of accidental rupturing of this fluid line (20), the safety
device (24) operates to prevent uncontrolled lowering of the arm. The device (24)
has a working section which comprises a non-return valve (36) inserted in said line
(20) such as to normally only allow fluid into the actuator (18), and control means
selectively operable to inhibit the non-return valve (36) and permit fluid to exit
from the actuator (18). The control means is itself controlled from a controlling
section of the device (24) in such a manner that the control means only operates in
the presence both of a control pressure signal (Ps) indicating that the arm is to
be lowered, and of normal operating pressures in said line (20).
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