1. Field of the Invention
The invention relates to a linear actuator having an emergency release mechanism.
2. The Prior Art
Briefly, a linear actuator consists of a spindle driven by a reversible electric motor through a transmission. On the spindle there is a nut having an activation rod secured thereto. By fixing the spindle nut against rotation, it will move in or out along the spindle depending of the direction of rotation of the motor. With movement of the spindle nut, the activation rod is either extended or retracted.
Depending on the construction in which the actuator is incorporated, the actuator may be equipped with a quick release or emergency lowering function. Quick releases are typically used in actuators for hospital and care beds in order to instantaneously perform a quick adjustment, i.e., lower the back rest section in case the patient on the bed has a heart attack or falls into a coma. Emergency manual operation is typically used for actuators incorporated in patient lifts to make it possible to lower the patient despite a power failure. In this case, it is a question of lowering the patient as gently as possible within a given time.
The present invention relates to emergency lowering and examples of this are, i.e., known from DE 196 47 556 A1 to Rudolf Weiner, where the activation rod is released by pulling out a pin, and in doing so, the activation rod may be manually turned inwards by grasping it. In DK 130 460 to Trioteam A/S, the activation rod is retained by a split bushing which may be loosened by means of an operating hand lever. DE 199 50 689 A1 to Okin discloses a construction having two coupling parts with teeth, which are spring loaded for mutual mesh. By releasing the clutch, the activation rod may manually be rotated inwards. EP 789 665 to Liko discloses a construction with a prestressed friction disc, where the friction is adopted so that the activation rod is secured against rotation during normal operation, but may be overcome with an external torque, i.e., by turning a bushing causing the activation rod to be screwed backwards. EP 1 457 710 A1 to SKF discloses a construction comprising a claw clutch surrounded by a coil spring, functioning as a brake. By rotating the operating handle to a certain angle, the claw clutch is released and the load on the mounting fittings thus causes the spindle nut, and the activation rod connected thereto, to be screwed inwards on the non self-locking spindle, by means of which the claw clutch again engages the retracting spring and thus brakes the further movement of the activation rod. To lower the load, the claw clutch should thus be operated repeatedly. The condition for the mode of operation of this construction is that the spindle/nut is non self-locking, the construction does not function with a self-locking spindle, and it further does not work in an unloaded state, as it is the load which pushes the activation rod backwards.
It is evident that the load permanently rests on the activation rod, and in order to retain this in a random position, the spindle/nut in itself will have to be self-locking, and if a non self-locking spindle is used, the gearing and motor will have to possess the sufficient self-locking ability, alternatively the actuator should comprise a brake. This also causes the resulting force from the load to run through the emergency lowering mechanism and thus contributes to pressing the activation rod backwards, when the emergency lowering mechanism is activated.
Lastly, WO 2007/025550 A1 to Linak discloses a construction with a coil spring, which as a clutch spring with a number of windings, placed tightly around the cylindrical part connected to the front mounting and with an end is secured to the other cylindrical part connected to the activation rod and where a ball bearing is located between the two cylindrical parts. The construction is operated by means of an operating handle designed as a bushing, in which the upper end of the spring, which is located opposite the cylindrical part connected to the front mounting, is secured.
The problem with the known emergency lowering mechanisms is that they both are greatly dependent on load and are sensitive to the friction, which during operation changes from a static friction to a dynamic friction, which again is dependent on temperature, wearing, lubrication etc. This may result in that the lowering occurs at an uncontrolled speed or that it requires so much torque that the operation becomes difficult, or even impossible.
The invention derives from the construction in WO 2007/025550 A1 to Linak, as it is distinguished in its simplicity and few components.
The purpose of the invention is to provide an emergency lowering mechanism which counters the problems of the known constructions, i.e., to provide an emergency lowering mechanism which is simple, only consists of few components, is more or less and independent on load, which has a smooth-running operation process.