GB2049473A - Roller mill - Google Patents

Abstract

A roller mill has a rotatable central grinding ring (3) with an upper and lower grinding path and is coupled to a universally jointed drive shaft (2) through which it is driven in rotation and so as to be tiltable from the position shown in the drawing. Grinding balls (8,4) are located between the upper and lower grinding paths and respective upper and lower grinding rings (6,5) whereby the central grinding ring is supported by and centred with respect to the upper and lower grinding rings (6,5). The angle of inclination of the axes of rotation of the balls is from 90 DEG to 45 DEG . In use grinding pressure is applied through the upper grinding ring (6), and the drive shaft (2) is substantially free from axial stresses arising from the grinding pressure. <IMAGE>

Description

SPECIFICATION Roller mill the invention relates to a roller mill which utilizes grinding balls.

In roller mills of this kind, used e.g. for finely grinding coal, rocks and earth, coarse-particled material is ground finely between the grinding balls, which generally execute a circular movement relative to a grinding pan and the surface of the grinding pan, whilst the fine particles produced are removed by a current of air and the mixture of dust and air flows through a screen, usually accommodated in the same housing.

To satisfy these requirements, it is known to construct ball race mills having a co-rotating grinding pan, also known as the race or grinding dish, placed on a vertically positioned gear pin, wherein a single row of freely movable grinding balls can describe a circular path, whilst an upper pressure ring which does not co-rotate exerts the required grinding pressure. This principle of the ball-thrust bearing is known as the fuller mill or Peters mill.

The disadvantages of this basic arrangement are the high axial pressure exerted on the vertically positioned power take-off pin of the gear, which means that axial bearings have to be provided, usually integrated in the gear, thus making the gear considerably more expensive.

A construction of the radial bearing type is also described in German Patent application P 2829 679.9.

However, with the single-row arrangement, the grinding capacity limit is still relatively low and does not meet requirernents, which have become more stringent in the mean time.

For each mill, this grinding capacity limit is determined by the large diameter of the grinding path and of the grinding balls obtained with the single-row construction, thus resulting in anon- linear increase in the dimensions and manufacturing costs.

Double-row mills constructed in the manner of axial ball bearings are now known, which have the advantage of eliminating the grinding pressure load on the lower, rotating grinding pan and thus obviate the disadvantages found with the corresponding single-row construction. However, these double-row mills present problems from the point of view of transmitting the torque to the central rotating grinding ring and thus obtaining a satisfactory flow of material to be ground through the mill.

The invention is aimed at improving the doubleand multi-row ball mill so that there are no or reduced compulsive forces on the grinding balls, and no or reduced axial stress on the drive, and so as to obtain a flow of material which is to be ground which is favourable for the grinding and screening processes, in one or more stages, through individual grinding assemblies arranged above one another.

According to the invention, we provide a roller mill comprising; a rotatable central grinding ring means, having an upper grinding ring path and an lower grinding ring path, and being coupled to a universal jointed drive shaft to be driven in rotation therethrough and so as to betiltable; an upper and lower grinding ring fixed against rotation; grinding balls arranged between the upper and lower grinding paths and upper and lower grinding rings respectively whereby the central grinding ring means is supported by and centred with respect to the upper and lower grinding rings; the balls, upper and lower grinding paths and upper and lower grinding rings constituting a double inclined ball race in which the angle of inclination of the axes of rotation of the balls is from 90 to 45 ; the arrangement being such that in use the grinding pressure is applied between the two fixed grinding rings, and the drive shaft is substantially free from axial stresses arising from the grinding pressure.

Preferably, and in orderto compensateforthe weights of the rotating grinding rings and grinding balls in the uniform distribution of the effective grinding pressure, the angle of inclination of the axes of rotation of the grinding balls of the lower race is different from that of the upper race, whilst all the grinding balls have the same diameter.

Preferably, and in order to promote the supply of material for grinding to the grinding balls and to promote the transporting of material for grinding over the effective surfaces of the grinding balls and to the grinding paths, including the outlet, and to ensure that the largest possible proportion of the surface of the grinding balls is involved in the grinding operation, at least one ring may be provided for guiding the material to be ground, said ring or rings resting loosely on grinding balls and being driven by friction so as to rotate with said grinding balls.

Preferably, and in order to be able to vary the grinding pressure according to the grinding requirements and apply this pressure uniformly, at least one fluid operated piston and cylinder arrangement may be provided, coupled to a non-rotating grinding ring and operable to produce grinding pressure.

An exemplary embodiment of the invention is diagrammatically shown in the drawing and ex plained more fully hereinafter.

The single Figure shows a vertical section through a ball race mill of double-row construction, with the grinding process being carried out in two stages. If desired the construction could suitably be modified so as to have three or more vertically spaced races of grinding balls.

The gear 1, which remains completely free from vertical forces resulting from the grinding pressure, transmits the drivetorqueto a central rotating grinding ring 3 in angularly movable manner via a universal-joint shaft 2; grinding ring 3 is guided and centred on the stationary lower grinding ring 5 via the lower grinding balls 4. The grinding pressure applied to the upper, non-rotating grinding ring 6 by thefluid e.g. gas operated piston and cylinder arrangement7 is in turn transmitted by the upper grinding balls8 to the central grinding ring 3, the rotation of which imparts a rolling rotational move ment to all the balls.In the example shown, the material to be ground is first fed to the upper grinding assembly via the feed line 9, whilst the screened material flows via the funnels 10, 11 to the tops of the upper train of grinding balls. The pre-ground material emerging from the upper grinding assembly is partly blown upwardly into the screen, whilst being screened by means of the air nozzle ring 12, and partly guided through the material guide ring 13 to the tops of the lower train of grinding balls. The loose material guide ring 13 is caused to rotate by frictional forces: its movement prevents the material from becoming clogged as it passes between the rings 13 and 3.

After the ground material has passed through the lower grinding assembly it falls into the funnel 14 which adjoins the outlet 15. The ground material received in the funnel 14 is moved towards the outlet 15 by means of paddles 16 rotating with the protective casing 17 of the drive shaft.

The advantages obtained with described embodiments in accordance with the invention are the fact that the overall construction is relatively inexpensive, as the effective grinding surface of the grinding members can be accommodated in a grinding path of small diameter, and the diameter of the grinding balls can be kept small in relation to the total grinding capacity.

The distribution of grinding pressure can be adjusted as required thanks to the inclination of the axes of rotation of the balls, and the balls are securely guided, even when they have become worn, and the direction of flow of the material for grinding is generally vertically downwards. The gears, which can be completely relieved of vertical forces, are light, reliable and inexpensive.

Claims (5)

1. Arollermill comprising: a rotatable central grinding ring means, having an upper grinding ring path and an lower grinding ring path, and being coupled to a universal jointed drive shaft to be driven in rotation therethrough and so as to betiltable; an upper and lower grinding ring fixed against rotation; grinding balls arranged between the upper and lower grinding paths and upper and lower grinding rings respectively whereby the central grinding ring means is supported by and centred with respect to the upper and lower grinding rings; the balls, upper and lower grinding paths and upper and lower grinding rings constituting a double inclined ball race in which the angle of inclination of the axes of rotation of the balls is from 90 to 45 ; the arrangement being such that in use the grinding pressure is applied between the two fixed grinding rings, and the drive shaft is substantially free from axial stresses arising from the grinding pressure.

2. Arollermill according to claim 1,whereinthe angle of inclination of the axes of rotation of the balls of the lower race is different from that of the balls of the upper race, whilst all the grinding balls have the same diameter.

3. A roller mill according to claim 1 or 2, further including at least one ring for guiding material to be ground, said ring or rings resting loosely on grinding balls and being driven by friction, so as to rotate with said grinding balls.

4. A roller mill according to any one of claims 1 to 3, further including a fluid operated piston and cylinder arrangement coupled to a grinding ring fixed against rotation and operable to produce the grinding pressure.

5. A roller mill, substantially as described herein, with reference to the accompanying drawing.