A nut assembly for a linear actuator

Abstract: A linear actuator comprises a driven spindle (10) of metal having a nut (12) of polymeric material which further is connected to the expelling rod (14) of the actuator. The nut and expelling rod are joined to each other by intermeshing embossing mate for contemporary transfer of axial forces and torques. With a polymeric nut a number of advantages are obtained such as low friction, lifetime lubrication, low noise level, low production and maintenance costs. Furthermore, a possibility is provided for in a simple manner to provide the actuator with a safety nut connected to the main nut (12) with breakable entrainment means.

Description: The present invention relates to a nut assembly for a linear actuator comprising a main nut and a safety nut attached to the main nut by breakable entrainment means which will be broken in case of failure of the main nut, said nut assembly co-operating with a driven spindle and the main nut connected to an expelling rod. Further the invention relates to a linear actuator furnished with such nut assembly A linear actuator is for instance known from EP-B1-0 531 247 and EP-A1-0 571 541. Linear actuators that e.g. is used for raising and lowering movements on hospital beds can be relatively heavily loaded. For instance, can an actuator be loaded with up to 15000 N axially and torques to the size of 25 Nm. A safety nut assembly according to the preamble of claim 1 is known from EP 0 586 326 B1, in said nut assembly the breakable entrainment means are constituted by a separate washer located between the main nut and the safety nut, said washer having projections received in respective holes in the two nuts. In case the main nut is being worn out or otherwise damaged, e.g. by an excessive load then the projections would brake and the safety nut takes over the load. Besides from that the forces must be transferred in a sufficiently secure manner; there are also in certain cases demands for low friction, life time lubrication, low noise level, low production and service costs. Linear actuators are attractive because they easily can be incooperated in an electrically control and also are relatively inexpensive. By the present invention defined by the features of claim 1 it is realized that the main nut, that is to cooperate with the metal spindle, can be made of an elastomeric material for obtaining a lubrication free connection with a minimal friction, and that a safety nut of metal directly connected to the main nut by breakable entrainment means integrally formed with the main nut so that the safety nut follows the main nut as a slave nut, whereby the safety nut takes over the function of absorbing the load in case of failure of the main nut. Due to the low friction of the main nut the safety nut would rotate freely against the main nut. The invention is described in detail in the following with reference to the drawing on which fig. 1 shows a longitudinal sectional view of an actuator according to the invention, and fig. 2 is a perspective view of the nut and the expelling rod of the actuator with a safety nut. As mentioned in fig. 1 of the drawing an longitudinal sectional cutaway view is shown through an actuator that comprises a spindle 10 with a nut 12 of plastic. To the nut is fastened a tubular expelling rod 14. The spindle is driven by an electric motor 16 via a gearing 18. In fig. 2 of the drawing is shown an embodiment having a metal safety nut 32 which as a slave follows the main nut 12 and takes over the load if the main nut fails, e.g. as a result of abrasion. Said safety nut 32 is introduced into a recess 34 in the end of the main nut 12 and connected thereto by entrainment flaps 36 in engagement with slots 38 on the outer side of the safety nut 32. The entrainment flaps 36 and the slots 38 are adapted so that the flaps 36 are broken from the main nut 12 by a predetermined load equal to failure of the main nut. By failure the safety nut 32 takes over the load on the expelling rod 14, and as a result of the thereby originated friction between the nut 32 and the spindle 10, the nut will stick on the spindle and participate in its rotation. The entrainment flaps 36 are thereby broken from the main nut, and the safety nut thereby rotates freely against the main nut, while the bottom of the recess in which the safety nut is embedded acts as a sliding bearing. Subsequently, the actuator will then only be able to return to its home position as the friction between the safety nut and the spindle dissapears when the nut are relieved. The operator will expire a malfunction, which almost always will appear by a sudden stop during loaded operation. A natural reaction will be an attempt to make a return movement, which will be succelfull as the safety nut in that way will be relieved and the friction dissappears completely or partially. A further attempt making an extending movement will simply fail in the same way as before. If the extending rod is driven back to the home position further movements will not be possible. Exploiding that the main nut is made of elastomeric material, a visible sign for the need of servicing the actuator is achieved by simple means, without excessively overloading other parts of the construction, and further that the rest of the construction does not suddenly collapse during operation, which could be fatal e.g. by hospital beds, where the head end of the supporting bottom can be raised and lowered by an actuator.

Claims: A nut assembly for a linear actuator comprising a main nut (12) and a safety nut (32) attached to the main nut by breakable entrainment means (36) which will be broken in case of failure of the main nut, said nut assembly co-operating with a driven spindle (10) and the main nut (12) connected to an expelling rod (14), characterised in that the main nut (12) is made of plastic and the safety nut (32) of metal directly connected to the main nut (12) said breakable entrainment means (36) integrally formed with the main nut and engaging the safety nut (32). A nut assembly according to claim 1, characterised in that the safety nut (32) is embedded in a recess in the end of the main nut (12). A nut assembly according to claim 2, characterised in that the recess having a circular cross-section. A nut assembly according to claim 1, 2 or 3, characterised in that the entrainment means is formed as entrainment flaps (36) in the recess (34) and that the safety nut (32) has corresponding axial slots (38) on the outer side. A nut assembly according to claim 4, characterised in that the entrainment flaps (36) are formed on the side walls of the recess (34) and that the axial slots (38) are shaped on the circumferential side wall of the safety nut (32). A nut assembly according to claim 4 or 5, characterised in that there are two entrainment flaps (36). A nut assembly according to claim 6, characterised in that the two entrainment flaps (36) are located diametrically opposed to each other. A linear actuator comprising a driven spindle (10) having a nut assembly according to any of the claims 1-7.

Inventors: JENSEN BENT, JENSEN, BENT
Applicants: LINAK AS, LINAK A/S
Application year: 1995
Main classification: F16H25
IPC codes: F16H25