With the growing use of height-adjustable tables the re- quirements are also increased. One of the requirements is a wish for a lower minimum height and a greater maxi- mum height for the tabletop. With lifting columns con- sisting of two sections it is difficult to fulfill the wish. It is furthermore made difficult because the column must also be rigid, which necessitates a not inconsiderable overlap between the two column sections.

This makes it difficult to satisfy the requirement of a low minimum height for the table. In order to meet the requirements lifting columns with three sections are now contemplated, which presents new problems.

Because of different conditions such as mutual friction between the sections and off centre loads on the tabletop, it may occur that the middle section moves at random. For example, it may be seen that it hangs on the inner section and suddenly falls down or moves in jerks or at another speed than the column. Such random movement gives an inharmonious appearance of the column, which is naturally unacceptable from all points of view.

The object of the invention is to totally or partly avoid a random pattern of movement for the individual sections of the column.

According to the invention this is achieved with a column of the previously stated type, which is characterized in that a locking element movable in the longitudinal direc- tion of the column is mounted in a first slidable section, which locking element engages with a second adjacent slidable section such that these two sections are locked together at least in one direction of movement, and when the first section reaches an end position in this direction of movement, the locking element is released, whereby the second section is released so that it can continue in the direction of movement. With this active mutual locking of the sections, a desired sequential movement of these may be produced.

When at the same time a locking element locks the first section in its end position to a another adjacent third section, a correct sequential sliding of the sections is ensured in both directions of movement.

With the increased limits for the outer positions of three section columns there will typically be an interme- diate area within which most tables are adjusted up and down, whilst they will only occasionally be brought into the outer positions, just as it will also be seldom that tables will constantly have to be run into an outer posi- tion. Here it is an advantage that the inner section locks with the middle section, so that the middle section is extended first and thereafter the inner one. In most conditions of operation the column appears very rigid, as it is the middle section that is extended, and if the outer section is extended it will only be partly, so that there is a large overlap and hence great rigidity. Only in rare cases will the inner tube be completely extended, which the column is naturally dimensioned to tolerate.

The column is appropriately constructed so that it is one and the same locking element which locks the first slidable section to the two adjacent sections in the respective positions. This reduces the number of locking elements necessary, which is an advantage with regard to both production and cost.

The locking element may be constructed in different ways, e. g. it may be a pawl or pendulum like element. An espe- cially simple embodiment of the locking element is formed by a ball, a cylinder or a similar element which co-oper- ates with recesses in the respective sections. The locking element may be activated in the respective positions by catch and guide surfaces optionally in combination with a spring action.

To ensure that the locking element is not unintentionally moved out of the locking engagement, the section with the locking element may be fitted with a slide member, which, in one position, holds the locking element in engagement with an adjacent section, and which, in another position, releases the locking element said slide member being activated by the second adjacent section.

Instead of placing the slide member so that it moves in the transverse direction of the column, it has been found to be more suitable to place the slide member in the longitudinal direction of the column. It gives the immediate possibility of using, in a simple way, the end of the other adjacent section to activate the slide member. In this respect, it is expedient that the slide member is spring-loaded to engage with the locking element.

Further features of the invention will appear from the following description and the accompanying drawing of an embodiment of the invention. In the drawing: Fig. 1 shows the column seen from the side in a fully retracted state, fig. 2 shows a longitudinal section of the column in fig. 1 at its lower end, fig. 3 shows the column in a completely extended state, fig. 4 shows a longitudinal section of the column in fig. 3 at the lower end of the middle section, fig. 5 shows a perspective view of the slide member, and fig. 6 shows a perspective view of the lower end of the middle section with the slide member.

The lifting column comprises three mutually telescopi- cally arranged sections, namely an outer stationary section 1, a slidable middle section 2 and an inner slidable section 3. All three sections are based on profile tubes with a rectangular cross-section.

A drive unit 27 is fitted at the top of the inner section 3, comprising an electric motor which drives a spindle unit consisting of an inner spindle 4 and a hollow spindle 5 via a transmission. The hollow spindle is housed in an inner tube 6 attached to a bottom piece 7 in the stationary section 1. A nut for the hollow spindle is provided in the upper end of the tube 6, and a further nut for the inner spindle is provided in the upper end of the hollow spindle. The drive unit causes the inner spindle to rotate to screw itself out of the hollow spindle, whereby the inner section 3 is extended. When the inner spindle pushes against the top end of the hollow spindle, the hollow spindle will begin to rotate and thereby screw itself out of the tube 6. The middle

section will thereby slide out of the stationary section 1. When the direction of rotation of the inner spindle is reversed, the column will slide together in the opposite order.

As mentioned in the introductory part, it is not certain, however, that the sections 1, 2,3 move in this way, which may be caused inter alia by unequal friction in the slide bearings 8-11 between the individual sections (corresponding bearings are provided at the top end of the column), and the friction in the spindle nuts as well as the moment load on the column.

To ensure a desired sequential extension of the column, a locking element in the form of a ball 12 is arranged in a recess in the middle section 2, cf. fig. 2. A bushing 27 is fitted in the recess, which partly facilitates the movement of the ball, but at the same time prevents the ball from unintentionally jamming itself into the gaps 13,14 between the sections 1, 2,3. In the retracted position of the column, the ball 12 is seated in a recess in the innermost section 3. The recess is also fitted with a bushing 15 for the same reasons as are mentioned above. To prevent the ball 12 from falling into the hollow of the column, the opening of the bushing has a diameter which is a slightly smaller than the diameter of the ball. As will appear from the drawing, the ball has a diameter which substantially corresponds to the width of the gaps 13,14 between the sections and the thickness of the sections 2,3.

With reference to fig. 2, a sliding member 16 is arranged in a recess 18 in the middle section 2, said sliding member engaging the outer side of the section with protruding side flanges 17, where they are guided in a

recess 8'a in the slide 8' (fig. 6). The slide member 16 is spring-loaded with a spring 19 which extends between a shoulder 20 on the slide member and the lower edge of the recess 18. The spring is held in place in that it rests in a channel 22 in the slide member (fig. 5). The end of the bolt has a stop 23 thereon in the form of a boss which extends inwards below the end of the inner section 3. In the retracted position of the column, the end of the inner section presses the slide member 16 downwards so that the bushing 27 is exposed.

When the column is retracted, the inner and the middle sections 3,2 are thereby locked together by means of the ball 12. When the column is activated, the inner and the middle sections 3,2 are extended as a locked unit. This also causes the column to appear very rigid.

If the column stops and is retracted again, before the middle section 2 reaches its outer position, the two sections 2,3 will still be locked together and will return as a unit.

When the two sections 3,2 reach the outer position of the middle section 2 as a unit, the locking ball 12 will lie opposite a hole 24 in the outer section 1 and roll into it, thereby releasing the inner section 3, cf. fig. 4.

The diameter of the hole 24 is slightly smaller than the diameter of the ball so that it cannot roll completely out. At the same time the hole is covered with a cover 25. Because the diameter of the bushing 15 in the inner section 3 is smaller than the diameter of the ball, the ball will roll out on its own towards the hole 24, and the vibrations that naturally occur in the leg during the travel will intensify the movement.

When the inner section 3 continues outwards, the slide member 16 is released and is pushed upwards by the spring 19. The end of the slide member is configured with a plate-shaped finger 26, which slides up behind the ball and holds it engaged with the hole 24 in the stationary section 1. This position is the parking or packed-away position for the ball 12.

When the inner section 3 is retracted, its end will tread on the boss 23 and pull the slide 16 downwards, thereby releasing the ball 12 which rolls back into the bushing in the inner section 3. The movement of the slide member is here determined by the edge 8'b in the recess in the slide 8'. Thereby, the inner section 3 is locked together again with the middle section 2.

The movement of the ball is brought about in that the in- ternal diameter of the bushing 15 and of the hole 24 is smaller than the diameter of the ball, whereby it will try to roll back on its own, and the vibrations in the column will encourage this movement.

The invention thus provides a column with at least two movable sections, where a desired sequential extension of the sections is achieved in a simple manner.