The present invention relates to a roller mill device for the grinding or refining of granular agricultural food materials. In particular it relates to a roller mill which uses at least one pair of fluted or smooth milling rollers which rotate at mutually different angular speeds.

Roller mills which use one or two pairs of rollers, generally two pairs, in which the rollers rotate in opposite directions at mutually different speeds, are well-known. The use of different speeds between the two rollers has in fact long ago been found to be particularly effective in the effect of grinding or refining cereals and the like. A pair of gears, each supported rigidly to the shaft of one of the two rollers, is generally used to motorize a pair of grinding rollers and to simultaneously set their relative speed. However, the pair of rollers wears down with use, so that in order to keep constant the grinding gap between the surfaces of the two rollers the distance of the axes of the rollers must be adjusted. In practice the axes of the shafts of the rollers must be moved progressively closer as the surfaces of the rollers wear. The fact that the center distance between the grinding rollers is not constant but decreases progressively entails a particularly configuration of the pair of gears supported by the shafts of the rollers in order to avoid the need to change the gears every time the center distance between the rollers is to be adjusted. In practice the pair of gears used has particularly long teeth so that the coupling between the gears may be "loose" on the new

mill, whereas the coupling between the gears becomes increasingly "tighter" and the teeth of the two gears interpenetrate to an increasing depth as the center distance between the rollers decreases. This solution allows to avoid changing the gears at every center distance adjustment, but it has a severe disadvantage: gears with the particular described configuration are extremely noisy and contribute to the forming of hearing damages (hypacusia) and often raise noise levels above the values set by specific laws for operators assigned to these machines. Said gears furthermore require adequate lubrication, are subject to rapid wear and are difficult to find, when they are to be replaced, due to their particular configuration. In order to overcome these disadvantages, a belt transmission, stretched between two pulleys each of which is mounted at a shaft of one of the two rollers, has been proposed in three variations.

According to the first variation, the belt is kept under tension by two other transmission pulleys. According to this first variation, the assembly constituted by the belt and the total of four pulleys is so bulky that it cannot be contained in the normal useful space of a normal mill, so that this solution is not likely to enjoy actual industrial development.

According to a second variation, the belt is kept under tension by a single transmission pulley. This solution does have small and practically acceptable dimensions, but in practice only a small part of the useful power of the mill can be transmitted, and the mill therefore operates at minimum capacity. If the transmitted power is increased to the values normally

used with gear transmission, the belt wears out rapidly and burns.

According to a third variation, illustrated in the patent application No. 22054 A/86 filed in the name of the same Applicant, the roller which rotates faster is driven directly by the motive power and its shaft supports a first pulley which has a set of teeth with grooves which are parallel to the axis of the roller's shaft. The shaft of the roller which rotates slower supports a second pulley which has a set of teeth with grooves which are perpendicular to the axis of the roller's shaft.

However, it has been observed in practice that this third variation, though it constitutes a considerable improvement, allows the transmission of relatively limited powers and therefore is not fully satisfactory.

The aim of the present invention is to overcome the above disadvantages by providing a roller mill which does not entail the presence of noisy gears and allows to transmit all the useful power to the mill without using lubricants.

Another object of the invention is to provide a mill wherein maintenance, adjustment of the center distance between the rollers and replacement of spare parts are particularly economical and simple.

This aim, these objects and others are achieved by the device according to the invention, which comprises: at least one pair of fluted or smooth milling rollers which rotate at mutually different speeds, the roller which rotates faster being driven directly by the motive power, and is characterized in that: the shaft of said roller which rotates faster

supports a first pulley which has a set of teeth with grooves perpendicular to the axis of the shaft of the roller; the shaft of the roller which rotates slower supports a second pulley which has a set of teeth with grooves parallel to the axis of the shaft of the roller; the belt stretched between said pulleys is kept under tension by a third transmission pulley which is adjustable in position. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the device, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a perspective side view of the device according to the invention; figure 2 is a perspective view of the pulleys of figure 1.

With reference to figures 1 and 2, the device according to the invention comprises a pair of milling rollers and preferably two pairs of fluted or smooth rollers which rotate at mutually different speeds. The rollers are not directly visible in the figures, but the shafts 11, 12 of the first pair of rollers and the shafts 13, 14 of the second pair of rollers are shown. Considering only the first pair of rollers from now onward for the sake of simplicity, though the second pair is naturally also motorized in a similar manner, the roller supported by the shaft 12, which rotates faster, is driven directly by the motive power by means of a pulley or a gear, not illustrated in the drawing.

The shaft of the roller which rotates faster

supports a first pulley 15 which has a set of teeth with grooves 16 which are perpendicular to the axis of the shaft 12 of the roller.

The shaft 11 of the roller which rotates slower supports a second pulley 17 which has a set of teeth with grooves 18 which are parallel to the axis of the shaft 11 of the roller.

The belt 19 which is stretched between the pulleys 15 and 17 is kept under tension by a third transmission pulley 20. The position of the third pulley 20 with respect to the position of the other two pulleys 15 and 17 can be adjusted so as to easily control the tension of the belt 19.

The third pulley 20 has a set of teeth with grooves 21 perpendicular to its axis 22.

The center distance between the first pulley 15 and the second pulley 17 is comprised between 210 mm and 310 mm. The center distance between the second pulley 17 and the third pulley 20 is comprised between 230 mm and 300 mm. The ratio between the rp rate of the first pulley 15 and the rpm rate of the second pulley 17 is comprised between 1.1 and 3.5.

The belt 19 has a set of teeth which matches the grooves of the pulleys which it is to engage; in particular, once the belt 19 is mounted on the pulleys 15, 17 and 20, the internal set of teeth is perpendicular to the axes of the pulleys, whereas the outer set of teeth is parallel to the axes of the pulleys.

In practice it has been observed that the device according to the invention allows to achieve the intended aim and objects and in particular allows quiet operation and the transmission of all the useful power of the mill

for long periods of time without requiring particular maintenance .

WHERE TECHNICAL FEATURES MENTIONED IN ANY CLAIM ARE FOLLOWED BY REFERENCE SIGNS , THOSE REFERENCE SIGNS HAVE BEEN INCLUDED FOR THE SOLE PURPOSE OF INCREAS ING THE INTELLIGIBILITY OF THE CLAIMS AND ACCORDINGLY , SUCH REFERENCE SIGNS DO NOT HAVE ANY LIMITING EFFECT ON THE SCOPE OF EACH ELEMENT IDENTIFIED BY WAY OF EXAMPLE BY SUCH REFERENCE SIGNS .