Various hydraulic, hydraulic/pneumatic, and fully electrical systems with all-electrical actuators are known for active vertical adjustment of a motor vehicle. An active “spring strut” that is suitable for this vertical adjustment has a suspension spring and a preloaded spring which are each separated by an adjustable spring cap. The adjustable spring cap is exposed to positioning forces during vertical adjustment. That is, the adjustable spring cap as needed traverses an axial path dictated by the control, and thus ultimately produces a positioning force on the wheel. The lifting motion of the adjustable spring cap takes place almost exclusively by a ball groove thread drive in which the adjustable spring cap does not rotate during its lifting motion, but remains torsionally strong, while the relatively slender spindle of the ball groove thread drive rotates and is driven by an electric motor which is located in the spring strut itself.
This type of wheel suspension is described by DE 10 2005 001 744 B3 in which there is a preloaded spring located concentrically on the spring strut above the suspension spring; this preloaded spring opposes the spring force of the support spring and thus reduces the adjusting forces which must be applied by an electromechanical actuator or a positioning drive in order to effect adjustment of the level and/or pitching and rolling equalization on the motor vehicle by way of the axially adjustable spring cap. The three-dimensional and functionally reliable arrangement of the positioning drive integrated on the spring strut within the preloaded spring is a problem, however, especially for wheel suspensions with steered wheels in which the wheel requires a corresponding free space in the wheel well and for the wheel suspension elements.
The spindle shown in DE 10 2005 001 744 B3 does have a large bearing base. Valuable installation space is lost, however, by lining up the elongated bearing bush with its lower bearing arrangement. This has an adverse effect on the shock absorber length and on the shock absorber stroke. Moreover, the total weight of the spring strut is increased by the long piston rod which is required in this design and the long bearing bush. The shock absorber plunges into the lower part of the spindle and is cardanically deflected relative to the spindle when driving. This can lead to contact between the oscillating shock absorber and the rotating spindle. This contact between the shock absorber and spindle is further promoted by the piston rod which is elastically supported in its upper end. In order to prevent this contact between the shock absorber and spindle, the radial air gap between the shock absorber and spindle would have to be dimensioned to be correspondingly large. This would, however, lead to a relatively large spindle diameter, as a result of which, however, the rotational mass moment of inertia of the spindle is adversely increased.
The object of the invention is to propose a spring strut arrangement of the generic type which ensures a structurally especially compact and functionally reliable construction.