An actuator comprises a piston and cylinder in which the piston is biased axially inwardly of the end of the cylinder to an intermediate position by two bias members respectively on axially opposite sides of the piston. Limit means associated with each respective bias member serve to coact between the respective bias member and the cylinder to limit the inward travel of the respective bias member in the cylinder to establish a constant position at which the piston receives no net biasing force from the two bias members. Each limit means may comprise a flanged cup which moves with the inner end of the bias member and a stop surface in the cylinder for engaging the cup flange to limit its inward movement, and thus the inward movement of the bias member.
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