A pump having an adjustable delivery volume, comprising: a. a pump housing featuring a delivery chamber which comprises a delivery chamber inlet for a fluid and a delivery chamber outlet for the fluid; b. a delivery member , which can be rotated within the delivery chamber, for delivering the fluid; c. an adjusting device i. featuring a setting structure , which is mounted such that it can move back and forth within the pump housing in order to adjust the delivery volume of the pump, ii. and a helical spring (10; 20; 30; 40; 50; 60) for applying a spring force, which acts in a setting direction, to the setting structure , wherein d. the windings of the helical spring (10; 20; 30; 40; 50; 60) are enclosed by an envelope lying on the outside of the helical spring (10; 20; 30; 40; 50; 60), e. wherein the cross-sectional area of the envelope as measured transversely with respect to the longitudinal direction of the spring changes progressively in the longitudinal direction of the spring, characterised in that f. the envelope exhibits a concave profile in relation to the longitudinal direction of the spring over the entire length of the helical spring (10; 40; 50; 60), and the concave profile is concave throughout in the circumferential direction of the helical spring (10; 40; 50; 60), and in that g. the cross-sectional area of the envelope is at a maximum in a central region of the helical spring (10; 40; 50) in the longitudinal direction of the spring. a. a pump housing featuring a delivery chamber which comprises a delivery chamber inlet for a fluid and a delivery chamber outlet for the fluid; b. a delivery member , which can be rotated within the delivery chamber, for delivering the fluid; c. an adjusting device i. featuring a setting structure , which is mounted such that it can move back and forth within the pump housing in order to adjust the delivery volume of the pump, ii. and a helical spring (10; 20; 30; 40; 50; 60) for applying a spring force, which acts in a setting direction, to the setting structure , wherein i. featuring a setting structure , which is mounted such that it can move back and forth within the pump housing in order to adjust the delivery volume of the pump, ii. and a helical spring (10; 20; 30; 40; 50; 60) for applying a spring force, which acts in a setting direction, to the setting structure , wherein d. the windings of the helical spring (10; 20; 30; 40; 50; 60) are enclosed by an envelope lying on the outside of the helical spring (10; 20; 30; 40; 50; 60), e. wherein the cross-sectional area of the envelope as measured transversely with respect to the longitudinal direction of the spring changes progressively in the longitudinal direction of the spring, characterised in that f. the envelope exhibits a concave profile in relation to the longitudinal direction of the spring over the entire length of the helical spring (10; 40; 50; 60), and the concave profile is concave throughout in the circumferential direction of the helical spring (10; 40; 50; 60), and in that g. the cross-sectional area of the envelope is at a maximum in a central region of the helical spring (10; 40; 50) in the longitudinal direction of the spring. <EOS>