Background Art WO00/34161 discloses overhead conveyor systems of the kind stated above, which are convenient for overhead transport of heavy loads. However, in some situations, such as in the case of upward gradients along the trans- port track, occasional slippage between the drive wheel and the drive shaft has been observed, which results in excessive wear and necessitates premature replacing of the drive wheel.

Summary of the Invention An object of the present invention is to solve the above problems and provide satisfactory propulsion of carriages also in the case of upward gradients along the track.

This and other objects are achieved by a system of the kind stated above, further comprising a spring means for pressing the drive wheel against the drive shaft by spring load, which spring means has a first spring con- stant during an initial depression of the drive wheel, and a second spring constant upon depression of the drive

wheel a specific distance, and wherein a distance between a circumferential surface of the drive shaft and the rail is reduced along segments of the rail in which additional driving force is required, so that along said segments the drive wheel is depressed more than said distance.

This design allows a spring action with a higher spring constant along selected segments, for example along seg- ments with an upward gradient, which allows increased driving force to be derived.

According to a preferred embodiment, the stepwise variable spring action can be achieved by means of a first, long helical spring and a second, short helical spring, which are adapted to press the drive wheel to- wards the drive shaft by spring load. In the first, initial depression of the drive wheel, only the long spring will be compressed, against the action of a first spring force corresponding to the spring constant of said long spring. During continued depression, eventually also the second, short spring will be compressed, against the action of a further spring force. Thus, the total spring force increases in steps in a certain point, which corre- sponds to a stepwise spring constant increase.

The first and the second springs may be concentri- cally arranged inside each other. This reduces the amount of space required as well as the need to modify existing carriages. Preferably, the inner spring, which by virtue of its smaller diameter has a higher spring constant, is shorter and the outer spring longer.

The distance between the rail and the circumferen- tial surface of the drive shaft can be varied in differ- ent ways, for example by varying the radius of the drive shaft along said segments, or by arranging the rail closer to the drive shaft along said segments than along the other portions of the rail.

Brief Description of the Drawings The present invention will now be described in more detail with reference to the accompanying drawings, which for the purpose of exemplification illustrate preferred embodiments of the invention.

Fig. 1 is a perspective view of an embodiment of the overhead conveyor system according to the invention.

Fig. 2 is a front view of a carriage belonging to the overhead conveyor system in Fig. 1.

Fig. 3 shows part of the rail and the drive shaft in Fig. 1 according to a first alternative.

Fig. 4 shows part of the rail and the drive shaft in Fig. 1 according to a second alternative.

Detailed Description of a Preferred Embodiment Fig. 1 illustrates an overhead conveyor system 1 of the type disclosed in WO00/34161, in which the carriages are provided with drive means according to the invention.

The overhead conveyor system shown comprises a rail 2 which consists of two parallel rail elements 3, each rail element consisting of a hollow section of rectan- gular cross-section. The rail 2 is fixed by means of yokes 4 which are arranged spaced from each other on the outsides of the rail elements and which also are respon- sible for suspension of the overhead conveyor system 1.

Above the rail 2 and extending along the same extends a rotatable drive shaft 5, which is drivable with the aid of means (not shown) adapted thereto, such as a belt con- nected to an electric motor. A plurality of carriages 7 are movable along the rail 2. The carriages 7 are posi- tioned between the rail 2 and the drive shaft 5.

The carriages 7 shown consist of a carriage body 8 with four wheels 9 and a guide means 10 (see Fig. 2) adapted to keep the carriage on the rail 2. On the upper side of the carriage body 8, a drive unit 11 is arranged consisting of at least one drive wheel 12, which is in- clined relative to the longitudinal direction of the

drive shaft and which by spring action is pressed towards the drive shaft.

The carriages shown in Fig. 1 have four drive wheels 12, which are arranged in pairs on each side of the drive shaft 5. All wheels are rotatable in suitably parallel planes, i. e. all drive wheels 12 are preferably inclined relative to the drive shaft at the same angle. It goes without saying that the number of drive wheels 12 on each carriage may vary and can be, for example, two or even one.

The dimension of the drive unit 11 and the distance between the rail and the drive shaft are such that when the carriage is positioned under the drive shaft 5, the drive wheels 12 are brought into contact with the circum- ferential surface of the drive shaft 5. During rotation of the drive shaft, the carriage is propulsed along the rail. As described in more detail in said patent speci- fication, the drive shaft may have portions 5a, 5b of varying radius, so that the propulsion of the carriages is interrupted when it is located under small-radius por- tions 5b.

Fig. 2 is a detailed front view (or rear view) of a carriage 7. The drive wheels 12 are adapted to be pressed towards the drive shaft by a spring means 14, which may be mounted on the carriage body 8. In the case where the drive unit comprises several drive wheels, the drive wheels may for practical reasons be mounted on a cradle 13, as shown in Fig. 2, which in turn is supported by the spring means 14.

In the example shown, the spring means 14 comprises two pairs of concentrically arranged helical springs, one pair 15a, 15b of which is illustrated more clearly in Fig. 2. In this case, the outermost spring 15a is longer than the inner spring 15b, which means that initially only the spring 15a is compressed. The spring force of the spring means thus corresponds to the sum of the spring constants of the inner springs.

If the depression of the drive wheels 12 is con- tinued, in particular more than a distance L corre- sponding to the difference between the length of re- spectively the outer spring 15a and the inner spring 15b, the inner spring 15b will also be compressed. In this position, the spring force corresponds to the sum of the spring constants of all four springs.

The distance between the rail and a circumferential surface 6 of the drive shaft varies along different seg- ments of the transport track. This may be achieved in a plurality of ways, for example as shown in Figs 3 and 4.

In Fig. 3, spacing means 16 are arranged under the rail 2 in the yokes 4 located along the segment A for the pur- pose of lifting the rail 2 closer to the drive shaft 5 along this segment A. In Fig. 4, the radius of the drive shaft is greater along the segment A.

Along the segments A, where the distance between the rail 2 and the circumferential surface 6 of the drive shaft is reduced, the inner spring will also be compres- sed, which results in a stepwise increasing spring force.

This increased spring force may be used to move the car- riage past, for instance, upward gradients in the track.

It will be appreciated that the spring means can be designed in various ways within the scope of the appended claims. The long spring may, for example, be located on the outside of the short spring, and the number of spring pairs may, of course, be varied. Moreover, the springs do not have to be concentric but may also be arranged side by side or in any other manner.

It is not even necessary to use a plurality of springs. Instead, one helical spring whose spring con- stant varies along different parts of its length may be used. An example of such a spring is a conical helical spring, or a helical spring with varying material thick- ness.

The person skilled in the art may also choose to use other spring means than helical springs, for instance elastic material or spring steel.

It will also be appreciated that the skilled person can modify a number of details concerning the known parts of the overhead conveyor system, such as the carriages.

Thus, the number of wheels and their location may be varied, or they may be replaced with skid rails or the like. Furthermore, the suspension, the design of the rails, the functioning of the guide means, etc may be modified in any appropriate way according to the intended field of application.