LINEAR ACTUATOR

Abstract: As a driving unit for lifting columns it is common to use a linear actuator as driving unit. This actuator is of the type where a reversible electric motor drives an actuation rod (5) through a spindle/nut assembly (3, 4). The actuator is placed in the inside of the column, but as the acturator is fairly voluminous a relatively large column cross section is required. Designing the ousing (1) of the actuator with a bearing surface (1f) which stretches at an right angle from the outer tube (2) it can be placed on the top of the column. It is therefor only the spindle (3) of the actuator and the actuation rod (5) and possibly the outer tube (2, which are place in the inside of the column. Thereby the actuator can be used even in very slim columns, Further more the actuator can be designed so that the height of the housing over the end of hte column can be very low. An example is shown of an actuator wiht both single and double stroke lenghts. The areas of application of the actuator are of course not limited ot columns

Description: Linear actuator This invention concerns a linear actuator, in particular as a drive unit for lifting columns, comprising a reversible electric motor with a drive shaft connected to a transmission and where the electric motor and transmission are contained in a housing, a spindle unit, the one end of which is connected to the transmission, a spindle nut on the spindle unit, which spindle nut moves longitudinally on the spindle unit, when this is rotated by the motor, a tube shaped actuation rod connected to the spindle nut and which moves respectively longitudinally out and in depending on the spindle nuts movement and where the actuation rod has an internal cross section larger than the spindles outer cross section so that the actuation rod can sit in over the spindle and which with its other end is designed to be fixed to the construction in which the actuator is to be built . The invention is based upon the type of lifting columns that are used with height adjustable desks, and are based on an electrically driven linear actuator as drive unit. Typically the columns comprises two or three telescopically adjustable tubes. The actuator is in its entirety placed so that it is hidden inside the column, which requires a large cross section of the column. The construction is therefore not suitable for slim legs for desks. Furthermore the actuators length of stroke is proportional with its installation dimension which causes a contrast between the installation dimensions of the column and the desired minimum and maximum height of the column. The achievable maximum and minimum heights with a linear actuator are in reality a compromise where each millimeter of the installation dimension and the stroke length counts. The internal placement of the actuator contributes negatively to the relationship between the minimum and maximum heights. The purpose of the invention is to provide an actuator, which can also be used in connection with slim lifting columns . According to the invention the actuator is characterized in that the housing has a bearing surface which stretches at right angles to the longitudinal axis of the spindle unit and is designed to rest against the construction, preferably the top of a lifting column in which the actuator is to be fitted. This gives many possibilities for the shape of the actuator, so that when it is built into a column it gives a more optimal relationship between the installation dimension and the minimum/ maximum length of the column. The actuator is fitted above the column with only the actuation rod and perhaps the outer tube belonging to it led down into the inside of the column. In this way the actuator can be used in connection with very slim lifting columns. By further designing the actuator so that the motor is placed on a level with or a level under the end of the spindle unit and to the side of it, the height of the actuator in the area over the end of the column can be made low. The motor can either be placed so that it sits parallel with the outer tube or at a right angles to it. The column can always be turned such that the motor sits under the desktop. However it is preferred that the motor is placed such that it sits at a right angle away from the actuation rod and therewith the column, in this way the motor is least visible but also takes up the least room with regard to height. The height over the column can be further reduced by the actuator being so designed such that in this area the transmission is in one plane only i.e. gears wheels, chain or belt drive positioned on the same plane. At the same time the bearing surface can be made relatively large or put another way the motor can be moved a relatively long distance from the longitudinal axis of the spindle unit. This means that it is not necessary to make cut outs in the side of the column in order to give room to the motor housing. Furthermore it is important in relation to that there in the areas in which the lifting columns are usually placed often is a closed frame or another bearing profile under the desk surface . A special problem is stopping the actuator in the end positions, which is typically done with end stop switches. The problem is to establish a clear activation and to find room for the contacts in the construction. The invention solves this by the activating organ for the switches being held in a neutral position between two spring elements and where the activating organ is activated either directly or indirectly in the spindle nuts end positions. An embodiment of the invention will be explained in detail in the following with reference to the accompaying drawing. In the drawing: Fig. 1, shows in the perspective Fig. 2, shows a longitudinal cross section of the actuator, Fig. 3, shows a close up picture of the end stop switches seen from the bearing surface, Fig. 4, shows the same as fig. 3 seen from the side, Fig. 5, shows an embodiment of the actuator with a larger stroke length, Fig. 6, shows a longitudinal cross section of the actuator in a retracted condition. Fig. 7, shows a detailed view of the end of the actuator, and Fig. 8, a detailed view of the fixing of the actuation rod in the housing. As shown the actuator has a housing 1, which in this case comprises two shells la, lb and a lid lc. The two shells la, lb have an assembly plan which runs through the longditudinal axis of the actuators outer tube 2. The two shells la, lb forms two chambers Id, le, one of which, the motor chamber Id containing the motor unit is cylindrical in shape and externally closed. The other chamber le, the assembly chamber appears as a rectangular box which is open from above and can be closed with the lid lc. The underside of the assembly chamber le forms a bearing surface If upon which the actuator can rest on the end of a column. Screw holes lg extends from the bearing surface up through the assembly chamber and up onto the upper side of the lid for securing the actuator to the column with screws. Inside the housing the screw holes are shaped as ducts. The outer tube 2 rests wit one end in the assembly chamber. The tube is fixed with cams in the bearing surface of the chamber, these cams rest in recesses in the tube. Inside the outer tube there is a spindle 3, upon which there is a spindle nut 4. A tube shaped actuation rod 5 is with one end fixed to the spindle nut 4. In the other end of the actuation rod an eye 6, is fixed, by means of the actuation rod can be fixed to the column into which the actuator is to be built. Instead of an eye the actuation rod can be restrained from rotating in another way, such as matching cams and recesses or fins and slits. In the free end of the outer tube 2 there is a bushing 24, which functions as a guide for the actuation rod. The bushing is fixed with a set of spring legs, which with a barb engages a cut out slit 25 in the outer tube. On the end of the spindle there is a ball bearing 7, which is nested in the housing. Just above the ball bearing a large gear wheel 8 is mounted on the spindle. This is fixed against rotation by means of a spline connection as on the end of the spindle there is by riveting mounted a bushing 9 on the outer surface of which one part of the spline connection is shaped whilst the corresponding part is shaped in a cut out in the gear wheel. The spindle is driven by a reversible electric motor 10. An extension of the motor shaft is shaped as a worm which drives a worm gear 11. On the side of the worm gear facing upwards and integrated with it there is a cylindrical portion 12 on which outer side there is a coil spring 13. One end of the screw is fixed in the actuator and the coils run such that they create a braking effect on the cylinder and therewith the worm gear, when the actuation rod pulls inwards and thereby contributes to the self locking effect of the actuator. When the actuation rod drives outwards the spring has no braking effect, cp. the applicants EP 662 573. The worm gear with corresponding spring is fitted to a console 14 on the motors front cover. A short transmission axle 22, which with it's middle part is resting in the housing is with it's one end received in the cylinder 12 on the side of the worm wheel 11 and is therefore rotation inhibited via a splines connection. The other end of the axle is by means of a spline joint connected with a gear wheel 16, which engages the gear wheel 8 on the end of the spindle 3. The two gear wheels 8, 16, are thus located in the same plane and are also identical and as such neutral in their exchange ratio. To stop the actuator in the actuation rods end positions there is in the assembly chamber le at the end of the outer tube a contact set of two switches 15a, 15b. In the outer tube 1 there is in a groove a slider 23 made of a metal strip. The end nearest the free end of the outer tube is bent inwards so that it is pushed by the end of the spindle nut 4. On the other end of the slider there is a plastic means 17 with two contact surfaces 18a, 18b, that can activate the respective switches 15a, 15b. the plastic means 17 has a further contact surface 19, which sits inside the outer tube and which is pushed by the other end of the nut. Furthermore the plastic means 17 has a cam 27, which rests between the ends of two coil springs 20a,20b mounted in the assembly chamber. The ends of the two coil springs are held at a distance from each other by a small wall 21 as a distance keeper. When the actuation rod reaches its outer position the spindle nut comes into contact with the end of the slider, causing the switch 18a to be activated cutting off the current to the motor. On the way back the spring effect will ensure that the slider returns to its neutral position. When the actuation rod reaches its inner position the opposite end of the spindle nut will affect the plastic means on the slider causing the other contact to cut off the current to the motor. The spring effect will also here ensure that the plastic means and the slider return to the neutral position when the actuation rod again moves outwards. The construction ensures that the switches are in the correct position when the actuation rod leaves the end positions also that the slider cannot by accident between the end positions be displaced and activate the end stop switches. It is to be understood that the switches either directly cut off the current to the motor or that they are part of a control circuit with relays cutting off the motor current. To determine the position of the actuation rod the usual solutions with optical or magnetic encoders or potentiometers can be used. With a plug 26 the actuator can be connected to a control box containing a control and current supply. The control box can be connected to the mains and a control panel to activate the actuator. In the drawing Fig. 5-8 an embodiment of the actuator is shown according to the invention with a greater stroke length than the previous embodiment. The motor unit with transmission and housing is as described above, the difference is entirely in the telescopic movement. This is based on a telescopic spindle with an inner and outer part 30a, 30b. The outer part 30b is via a spindle nut 32 in connection with the tube shaped actuation rod 31. When moving outwards the outer part 30b will move outwards on the inner part 30a of the telescope spindle, whilst the actuation rod 31 will move outwards on the outerpart 30b. In this way an almost doubling of the stroke length can be achieved in comparison to the previously described embodiment. To stabilize the shaft 30c of the internal spindle part 30a this is supported by a bushing element 33 resting in the housing. There is also an elastic buffer element 34 and a freely rotating disc 35 to counteract overload when the actuator goes into endstop in the inwards going direction. In the free end of the spindle part 30b, an arbour is secured 36, onto which a coil spring 37 is fitted, this functions as a buffer element in the actuators outwards going direction. To stabilize in the actuators extended position a tube section 38 is secured to the free end of the telescopic spindles inner part 30a. This functions as guidance inside the spindles outer part 30b. The overlapping and the guidance give a good stabilization of the unit comprising of the telescopic spindle and the actuation rod. With the invention an actuator is provided that is suitable as a drive unit to slim lifting columns for tables. It is to be understood that the use of the actuator is not limited to this. Even though in the first instance it is considered that the actuator is turned so that the motor is at the top, there is of course no reason why it cannot be turned so that the motor sits at the bottom. In such cases the motor can be built into the foot of the column. An alternative to the bearing surface If is the upper surface of the housing, in the actual embodiment the lid lc, being designed as bearing surface, i.e. the actuator can be secured with the lid abutting to an overhead part, there could be clamps rising up from below or the column could be secured to the underneath of a frame or tabletop and the actuator could itself in a similar manner be secured to the frame or tabletop.

Claims: Claims : 1. A linear actuator especially as a drive unit for lifting columns comprising a reversible electric motor (10) with a drive shaft connected to a transmission (8,16,22,11) and where the electric motor and transmission are contained in a housing (1) , a spindle unit (3;30a,30b), one end of which is connected to the transmission, a spindle nut (4; 32) on the spindle unit, said spindle nut being axially displaceable on the spindle unit when this is turned by the motor, a tube shaped actuation rod (5; 31) connected to the spindle nut and which respectively slides axially out and in depending on the spindle nuts movement and where the actuation rod has an inner cross section greater than the spindles outer cross section so that the actuation rod can sit in over the spindle and with its other end is designed to be secured to the construction into which the actuator is to be built, c h a r a c t e r i z e d by the housing (1) having a bearing surface (If) which stretches at an right angle to the longitudinal axis of the spindle unit (3; 30a, 30b) and designed to rest against the construction, preferably the top of a column into which the actuator is to be built. 2. An actuator according to claim 1, c h a r a c t e r ! z e d in that the motor (10) being placed at a level within the driven end of the spindle unit (3, 30a, 30b) and to the side of this. 3. An actuator according to claim 1 or 2, c h a r a c t e r i z e d in that the motor (10) being placed parallel to the spindle unit (3, 30a, 30b) and with its drive shaft turned towards the driven end of the spindle. 4. An actuator according to claim 1 or 2, c h a r a c t e r i z e d in that the motor (10) being placed at an right angle to the spindle unit (3, 30a, 30b). 5. An actuator according to claim 3 or 4, c h a r a c t e r i z e d in being shaped such that in an area above the bearing surface (If) there is only transmission (8,16,22,11) in a single plane, e.g. gear wheel, chain or belt drive. 6. An actuator according to claim 5, c h a r a c t e r ! z e d in that the transmission above the bearing surface (If) comprises two gear wheels (8,16) with a large diameter, one (8) of which being secured to the end of the spindle unit (3;30a,30b) and the other (16) via further transmission steps, preferably comprising a worm gear (11) is connected to the motor (10) . 7. An actuator according to one of the claims 1-6, c h a r a c t e r i z e d in that the housing comprises two shells (la, lb) with a joining line through the longitudinal axis of the spindle unit (3;30a,30b), and is divided into two chambers (Id, le) , one (Id) of which contains the motor unit and where a surface in the other (le) forms the bearing surface (If) . 8. An actuator according to claim 7, c h a r a c t e r i z e d in that the other chamber (le) being fitted with a lid (lc) . 9. An actuator according to one of the claims 1-8, c h a r a c t e r i z e d in that in the housing (1) in the area of the bearing surface (If) there are screw holes (lg) for mounting the actuator, e.g. to the top of a lifting column. 10. An actuator according to one of the claims 1-9, c h a r a c t e r i z e d in that it comprises a set of end stop switches (15a, 15b), and an activating means (17) with a contact surface (18a, 18b) for activating the respective switch (15a, 15b), and with an arm resting between two spring elements (20a, 20b) held in mutually spaced relationship with a distance holder (21) , and where the spindle nut (4) in its one end position activates the activating means against the one spring element so activating the one switch and in the other end position activates the activating means against the other spring element so activating the other switch, and that the spring elements between the end positions activate the activation means to a neutral position where the switches are not operative. 11. An actuator according to one of the claims 1, c h a r a c t e r i z e d in that the spindle unit is a solid spindle (3) . 12. An actuator according to one of the claims 1, c h a r a c t e r i z e d in that the spindle unit is a telescopic spindle with an inner spindle part (30a) onto which a hollow spindle is mounted (30b) . 13. An actuator according to claim 11, c h a r a c t e r i z e d in the actuation rod (5) rests telescopically in an outer tube (2), one end of which is secured in the housing. 14. An actuator according to claim 1, c h a r a c t e r i z e d in that a coil spring (37) is fitted to the free end of the spindle unit (3; 30a, 30b) , said coil spring works as a buffer element. 15. An actuator according to claim 12, c h a r a c t e r i z e d in that a guidance e.g. in the form of a tubing (30) is secured to the free end of the inner spindle part (30a) .

Inventors: CHRISTENSEN BRUNO, SOERENSEN SOEREN, CHRISTENSEN, BRUNO, SOERENSEN, SOEREN
Applicants: CHRISTENSEN BRUNO, LINAK AS, SOERENSEN SOEREN, CHRISTENSEN, BRUNO, LINAK A/S, SOERENSEN, SOEREN
Application year: 2001
Main classification: A47B9
IPC codes: A47B9, F16H25