The present invention relates to a linear actuator preferably for articles of furniture, particularly for lying and sitting furniture of the type which comprises at least one adjustable section.

The acoustic noise level for linear actuators is a rather significant problem particularly in connection with furniture such a lying and sitting furniture, but also height-adjustable desks. This problematic is not only of significance in private homes, it is e.g. also important within the hospital and care sector. The acoustic noise level can e.g. be lowered by means of rubber suspension of the electric motor and transmission or by use of noise-reducing materials in the housing of the actuator. The use of noise-reducing materials for noise-dampening however means that the heat development in the actuator must be taken into account, for which reason it is problematic to dampen the noise by means of noise-reducing materials. Furthermore, actuators are typically so compact that it per se makes it difficult to use noise-reducing materials. In order to reduce the noise it is also possible to optimize the transmission, but this is typically too costly. The farthest you will get is to use e.g. a worm wheel made from plastics in the worm gear, but this raises the subject of the strength of the actuators. The load on actuators can reach 10.000 to 14.000 N, which requires a considerable strength of the actuator. In addition to this, dampening by means of noise-reducing materials, which are also heat insulating, is in conflict with the use of plastic toothed wheels, which cannot withstand high temperatures.

On top of these challenges, the ongoing development in the field of actuators has brought about a demand from the market for actuators in the low price range. Low-cost actuators are as a rule contrary to a low noise level, as one of the means for reducing the costs is to use low- price components, where processing and dimensional tolerances are not paramount.

The purpose of the invention is to provide an actuator, where the problem relating to acoustic noise is sought solved also for low-cost actuators.

This is achieved according to the invention by designing the actuator as stated in claim 1, i.e. in that the actuator comprises a coil spring placed around a cylindrical element in the transmission and where the coil spring is in frictional engagement with the cylindrical element, and where the coil spring has a first and a second end portion and that the actuator comprises a stop with which the coil spring with its first and second end portion can be brought into engagement. The torque, which the coil spring through its frictional engagement with the cylindrical element exerts on this, causes the cylindrical element in the transmission to be secured against unintentional movements as a result of play in the structure and thus the noise level is lowered. The end portions can naturally be given the course, which is most expedient for the structure of the actuator in which the coil spring should be used. In an embodiment the coil spring is shaped such that at least one end portion, preferably both end portions, runs as a tangent to the cylindrical element on which the coil spring is arranged. Further, it is expedient that both end portions of the coil spring are the same length or approximately the same length. Further to it being expedient in terms of manufacturing it also gives the same torque arm and the coil spring can thus immediately be used regardless of how the spring is placed and whether the spindle is subject to compression or tension.

The transmission can be constructed in various ways e.g. as a gear wheel train including a spur toothed gear wheel train or a belt or chain drive.

The cylindrical element in the transmission can be placed where it is most expedient, e.g. where the coil spring is found to have the largest effect on reducing the noise level or a combination of where it would be easiest to place the cylindrical element and where the coil spring moreover is found to have a satisfactory reduction of the noise level. The cylindrical element can e.g. be located in connection with a toothed wheel in the transmission. Expediently, the cylindrical element is constructed as an integrated part of a toothed wheel in the transmission, i.e. it can be cast as a unit.

In an embodiment the transmission consists of or comprises a worm gear with a worm, where the cylindrical element for the coil spring is in connection with the worm wheel. All things considered this gives an expedient transmission . The stop for the end portion of the coil spring can be constructed in various ways. In an embodiment the stop for the first and second end portion of the coil spring is constituted by at least one wall element in the housing. The wall element can be a wall element designed for this purpose or already existing wall elements in the actuator can be sought used. In a closer defined embodiment the stops are constituted by an outer wall of the housing and a second stop is constituted by a partition wall in the housing.

If the end portions of the coil spring engages the stop with its distal end, there is a risk that these will gnaw themselves into the stop, by which the end portion at worst can get stuck to the stop, which counteracts the intended function of the coil spring. Therefore it is expediently arranged such that the end portions of the coil spring do not engage with the distal end against the stop. In an embodiment a length of the distal end of the first and second end portion of the coil spring is bent such that the distal end faces away from the stop, and the coil spring with its end portions comes into engagement with the arched transition to the bent distal end .

The coil spring, however has the disadvantage that it exposes the actuator to a frictional torque regardless of whether the spindle rotates one way or the other, i.e. regardless of whether the actuator should lift a load or lower a load. Thus, the motor should be capable of overcoming this additional frictional torque when the actuator should lift a load. This disadvantage can, however, be turned into an advantage in that, in a further embodiment of the linear actuator, the frictional engagement of the coil spring with the cylindrical element is adapted such that the coil spring, when the current for the motor is interrupted, exerts a torque, such that the spindle unit is prevented from moving. That is, the coil spring has the extra function that it exerts a holding torque to the cylindrical element, which makes the linear actuator self-locking. In some actuators non self-locking spindle units are used in order to achieve a relatively quick adjustment. Non self-locking spindle units have a relatively large thread pitch and thus high speed, but since the thread pitch is larger than the friction, it causes the spindle unit on its own to drive back to its starting position. With the present structure, where the coil spring exerts a holding torque, the linear actuator is in itself self-locking, i.e. it is avoided to build-in a dedicated brake for holding the adjustment element in its place when the current for the motor is interrupted.

It should be noticed that more than one coil spring can be used, if it is found expedient for achieving the desired noise reduction, for instance one coil spring can be used in the first part of a transmission and a second coil spring further along the transmission. It is further understood that the two springs can exert a holding torque, which not separately but collectively make the actuator self-locking.

An embodiment of the transmission comprises a worm gear, where the worm is designed as a continuation of the motor shaft while the worm wheel is positioned on a shaft which extends past the spindle, and where this opposite the spindle is equipped with a worm in engagement with a worm wheel on the spindle. Each of the two worm wheels can be designed with a cylindrical element, on each of which a coil spring is arranged.

A further development of this transmission comprises a further shaft with a third worm wheel in engagement with the first worm, where the engagement is opposite the first worm wheel, and a third worm in engagement with the second worm wheel opposite the second worm, in other words a set of worm wheels/worms is located diametrically opposite the worm in connection with the motor shaft and the worm wheel on the spindle, respectively. The third worm wheel can here also be designed with a cylindrical element for a coil spring.

An example of the invention in the form of a dual actuator will be described more fully below under reference to the accompanying drawing, in which shows a longitudinal section through dual actuator,

fig. 2 shows a detailed view of the drive in one end of the dual actuator,

fig. 3 shows a section in the dual actuator, which shows a motor and transmission in perspective, fig. 4 shows a cross section in the dual actuator through the transmission, which shows the coil spring on the cylindrical element,

fig. 5 shows a detailed view as in fig. 2, but with a different embodiment of the spindle unit with appertaining adjustment element, fig. 6 shows a single actuator shown in perspective from the front end, and

fig. 7 shows an exploded view of the single actuator shown from the rear end.

The dual actuator shown in the drawing comprises a housing 1 with a drive 2 and an opening cover 3 at each end. The covers 3 provide access to a recess 4 in the side walls of the housing for a transverse rotary shaft 5. In association with the recesses 4 there is, as mentioned, a drive 2. In the following only one half of the dual actuator is described, more precisely the right half, as the two halves structurally are identical. The drive 2 comprises an electric motor 6, which through a transmission 7, drives a spindle unit 8, which comprises a spindle 8a, just as the spindle unit comprises a spindle nut 8b, which with a ball bearing 9 is embedded in the housing 1. The transmission 7 comprises a first worm gear 10, where the worm 10a is designed as an extension of the motor shaft. The worm 10a is in engagement with a worm wheel 10b located next to the worm 10a. This worm wheel 10b is mounted on a shaft 11 embedded in the housing. The transmission 7 further comprises a second worm gear 12, where the worm 12a is mounted on the opposite end of the shaft 11 as the worm wheel 10b in the first worm gear 10. The worm 12a is in engagement with a worm wheel 12b. The worm wheel 10b and the worm 12a is with an intermediate tube piece 13 cast as a unit with the shaft 11, which is a through-going steel shaft, which provides strength to the unit. The spindle nut 8b is arranged in the worm wheel 12b, which is designed with a cylindrical part 12c on one side on which the ball bearing 9 for embedding in the housing 1 is mounted.

An adjustment element 14 is secured to the end of the spindle 8a which faces towards an arm 15 on the rotary shaft 5. When the spindle nut 8b is brought into rotation the spindle 8a with the adjustment element 14 will be displaced either in one or the other direction depending on the direction of rotation of the spindle nut 8b.

On the side of the adjustment element 14 there is a fin 14a, which is guided in a channel-shaped guide 16 in each side of the housing 1, and the adjustment element 14 is thus secured against rotation. The front end 14b of the adjustment element 14 is designed as a contact surface for the arm 15 on the transverse shaft 5. With its end 15a the arm 15 loosely engages the end 14a of the adjustment element 14. When the adjustment element 14 is brought into motion towards the end of the housing, it pushes with the front end 14b against the arm 15 on the transverse shaft 5 and causes this to rotate about its longitudinal axis. When the adjustment element 14 is retracted, the load and thus the torque on the shaft 5 will seek to hold the arm 5 in engagement against the front end 14b of the adjustment element 14. In fig. 1 the adjustment element 14 at the right-hand side is shown in its fully retracted position, while the adjustment element 14 at the left-hand side is shown in its fully extended position.

With reference to figs. 3 and 4 a coil spring 17 is arranged on the cylindrical element 12c on the worm wheel 12a for dampening the noise. In fig. 4 the coil spring is depicted in an unbroken line in a position where the spindle 8a stands still or reverses and in a dotted line in a position where the spindle 8a is displaced forwards. In the construction shown in fig. 5, where it is the spindle nut 8b which moves, it corresponds to the spindle nut being displaced outwards or reverses, i.e. is retracted inwards. The coil spring 17 is in frictional engagement with the cylindrical element 12c. The worm wheel 12b and the cylindrical element 12c are cast as one unit of plastic. The frictional engagement of the coil spring 17 with the cylindrical element 12c is determined by the fact that the internal diameter of the coil spring 17 is smaller than the external diameter of the cylindrical element 12c. The smaller the internal diameter of the coil spring 17 is compared to the diameter of the cylindrical element 12c, the larger is the frictional engagement all things being equal (such as the same number of windings, the same thread thickness, the same type of thread on the coil spring etc.) . The coil spring 17 has a first and a second end portion 17a, 17b which are brought into engagement against a first and second stop 18a, 18b depending on the direction of rotation of the worm wheel 12b. A length 17a' , 17b' at the distal end of the first and second end portion 17a, 17b of the coil spring are bent towards one another, such that the distal end faces away from the stop 18a, 18b, which ensures that the coil spring 17 with its end portions 17a, 17b comes into engagement with the stop 18a, 18b with the arched transition ( 19a' ' , 19b' ' ) to the bent distal end. The contact with the arched transition is gentler than with the distal end, where there is a risk that this will gnaw itself into the stop, which is made from plastic. One stop 18a is here constituted by the inner side of an outer wall la in the housing 1, while the other stop 18b is constituted by the side in a cross wall lb of the housing 1. The torque, which the coil spring 17 through its frictional engagement with the cylindrical element 12c exerts on this and thus the worm wheel 12b causes the cylindrical element 12c to retain the transmission against unintended movements as a result of play in the construction, and thus lower the noise level. This applies regardless of whether the adjustment element 14 is displaced outwards or retracted inwards, i.e. whether the transmission and thus the spindle rotates one way or the other. In addition to the positive effect on the noise level the coil spring has another positive effect, namely that the frictional torque, which the coil spring exerts, when the current for the motor is cut off, functions as a holding torque for the spindle unit 8, and thus contributes to making the linear actuator self- locking, i.e. prevent the load on the actuator from bringing the spindle unit 8 into rotation, but retains the spindle 8a and thus the adjustment element 14 in the position it was in when the current for the motor was interrupted .

The drive and the end of the housing at the other end of the dual actuator are constructed similar to the manner described above.

An example of the use of the outlined construction of a dual actuator is typically in beds with an adjustable back rest and leg rest section. The actuator is mounted on the slatted frame by pulling the covers 3 outwards and leading the actuator up until the rotary shafts 5 for the back rest and leg rest section rests in the recesses 4 and the arms 15 on the rotary shafts 5 protrudes down in front of the adjustment elements 14. The covers 3 are closed again by which the actuator hangs on the rotary shafts 5.

The dual actuator shown in fig. 5 of the drawing only differs from the actuator described above by the inverse function of the spindle unit 8, namely in that the spindle 8a with the ball bearing 9 is embedded in the housing 1 and that the spindle nut 8b is arranged in the adjustment element 14. In the adjustment element 14 there is a longitudinal groove for the spindle 8a, and the spindle nut 8b is constructed in the end of the groove which is closest to the worm wheel 12b. The adjustment element 14, which has a square cross-section, is guided in a guideway in the housing 81 and is thus secured against rotation. By rotating the spindle 8a the adjustment element 14 thus moves back and forth depending on the direction of rotation of the spindle. The front 14a, 14b of the adjustment element 14 is designed as a contact surface for an arm 15 on the transverse rotary shaft 5. Also here the end 15a of the arm 15 loosely engages the end 14a of the adjustment element 14. When the adjustment element 14 is brought into motion towards the end of the housing it pushes with the front end 14a against the arm 15 on the transverse rotary shaft 5 and causes this to rotate about its longitudinal axis. When the adjustment element 14 is retracted, the load and thus the torque on the rotary shaft 5 will, as described above, ensure that the arm 15 engages the front end of the adjustment element 14. The invention is above described in connection with a dual actuator, but it is understood that the invention is also applicable for single actuators. In figs. 6 and 7 of the drawing is shown an embodiment of a single actuator. The main components of the actuator comprises a two-piece housing 101 ; 10 la, 10 lb, a mounting console 102, an outer tube 103, which with a rear end is secured to the mounting console 102, and in which a tubular activation element (in technical terms known as an inner tube) 104 is telescopically embedded, a spindle unit 105 with a spindle 105a and a spindle nut 105b, to which the activation element 104 with a rear end is secured, a reversible electric motor 106, a front mounting 107 secured to a front free end of the activation element 104 and a rear mounting 108 secured to the rear end of the housing 101. A cover 103a with a guide for the activation element 104 is secured to the free end of the outer tube 103. A transmission 109, located in the housing, comprises a first worm gear 110 with a worm 110a in continuation of the motor shaft and a worm wheel 110b. The transmission further comprises a second worm drive 111 with a worm wheel 111a mounted with a spline bushing 105c' on a shaft 105c of the spindle 105a and a worm 111b arranged on a shaft 112 embedded in the housing, on which shaft the worm wheel 110b for the first worm gear 110 is likewise mounted. The spindle 105a is with its shaft end embedded in the rear mounting 108 of the actuator with a ball bearing 112. The spindle is driven with the two worm gears 110 and 111, by which the spindle nut 105b and thus the activation element 104 is displaced either forwards or retracted inwards depending on the direction of rotation of the motor 106. For dampening the noise a coil spring 17 is arranged on a cylindrical element 111c on the worm wheel 111a. The coil spring has the same effect as described above in connection with the dual actuator and reference is made thereto. The stops for the outer end of the first and second end portion 17b, 17c of the coil spring are here constituted by the inner side of the wall on the tubular element 102a of the mounting console 102.