GB2144340A - Vertical roller mill - Google Patents

Abstract

A hydraulic-pneumatic accumulator (B) for an hydraulic roller-tensioning cylinder (A) of a vertical roller mill is accommodated in an extension of the cylinder housing (11). The tensioning side of the cylinder piston (12) is connected to the hydraulic side of the accumulator piston (20) via ducts (16,17,18,19). The cylinder housing (11) also accommodates a roller-lifting cylinder (C), the piston rod (9) of which is anchored to tile mill foundation. <IMAGE>

Description

SPECIFICATION Vertical roller mill The invention relates to a vertical roller mill with a grinding table rotatable about a vertical axis and at least one grinding roller rotatable about a stationary shaft, the grinding roller being forced against the grinding table by means of a piston rod of an hydraulic roller-tensioning cylinder, the tensioning side of which is connected to the hydraulic side of an hydraulic-pneumatic accumulator.

A vertical roller mill of the kind described may incorporate several grinding rollers, each being forced against the material layer on the grinding table, which layer is to be ground at a certain grinding pressure provided by the hydraulic pressure in the hydraulic roller-tensioning cylinder.

This pressure is maintained, by the accumulator, substantially constant independently of the position of the piston in the cylinder, i.e. independently of the thickness of the material layer.

The material layer thickness may further vary along the grinding path of the grinding table, and the material layer may contain different grain sizes.

Consequently, the piston of the cylinder is exposed to comparatively rapid reciprocating movements during the rotation of the grinding table, and this calls for quick transfer of hydraulic liquid to and fro between the cylinder and accumulator with resulting comparatively high liquid velocities and accelerations.

High liquid velocities and accelerations cause, in the previously known mills, where the cylinder and accumulator are separated from one another, but interconnected by means of hoses or tubes, rather heavy friction losses owing to the comparatively long connecting passage between the cylinder and accumulator.

This can be compensated to a certain extent by giving the connecting passage a larye cross sectional area which, however, requires large dimensioned hoses and tubes.

Thus, it is the object of the invention to eliminate these drawbacks of the known mills, and according to the invention the object is achieved by mount- ing the accumulator for the hydraulic roller-tensioning cylinder in an extension of the housing of the tensioning cylinder, and by the hydraulic connection between the tensioning cylinder and the accumulator comprising at least one duct in the cylinder housing.

Such a mill has the advantage over the known mill of enabling an extremely short connecting passage between the tensioning cylinder and the accumulator whilst providing the connecting duct or ducts with the desired comparatively large cross sectional areas so that the friction loss upon flow through the connection between the cylinder and accumulator becomes very small. Further, the voluminous and often vulnerable pipe connections outside the cylinder housing and between that housing and the accumulator of the known mill are avoided.

A particularly advantageous hydraulic duct connection with a large cross sectional area between the tensioning cylinder and the accumulator is obtained if the duct comprises an annular chamber between the cylinder housing and an inner cylinder lining of the tensioning cylinder.

A tensioning cylinder of the kind described may be double acting, i.e. the piston may be exposed to hydraulic pressure on both sides. To avoid the disadvantages of cavitation on the side of the piston which is not under pressure and, moreover, a possible diesel effect, i.e. self-ignition of hydraulic liquid vapours at a subpressure on that side of the piston deriving from the fast movements of the piston, it is advantageous that the subpressure side of the piston does not contain hydraulic liquid but air and communicates with the surrounding atmosphere. This, however, means that the tensioning cylinder cannot also be used for lifting a grinding roller from the grinding table, which is desirable, e.g. when starting the mill and when replacing the roller, or repairing the grinding path of the roller or the grinding table.

To overcome this, an hydraulic lifting cylinder for a corresponding roller may be mounted at the end of the cylinder housing which is remote from the tensioning cylinder, the piston rod of the lifting cylinder being connected to the mill foundation. Such a lifting cylinder, the piston of which may be hy draulicaily locked during mill operation, allows lifting of the cylinder housing with the tensioning cylinder and consequently the grinding roller.

An hydraulic-pneumatic accumulator may be constructed in various ways e.g. with an elastic membrane for separating the hydraulic liquid from the gas in the accumulator. An accumulator in the form of a piston accumulator, i.e. with a piston freely movable in a bore, and separating the hydraulic liquid from the gas is, however, particularly suited for mounting in the housing of the tensioning cylinder.

The invention is below described in more detail by way of an example of a mill according to the invention and with reference to the drawing, in which: Figure 1 is a diagrammatical side elevation of part of the mill; and, Figure 2 is an axial section through an hydraulic tensioning cylinder of the mill.

Figure 1 shows a grinding table 1, which is rotatable about a vertical axis, and supported and driven by a unit 2. Rolling on the grinding table 1 are grinding rollers 3 for grinding a layer of material (not shown) on the grinding table. Each roller 3 is mounted on a stationary shaft 4, which is secured to a common central frame 5.

On the outer end of each shaft 4 is pivotally mounted a bracket 6 to which is attached a piston rod 7 of an hydraulic-pneumatic, combined cylinder-accumulator 8 for forcing the roller against the grinding table 1.

The cylinder-accumulator 8 is, via another piston rod 9, pivotally connected to a bracket 10, which is anchored to the mill foundation.

Figure 2 shows in more detail the complete cylinder accumulator 8 comprising a roller-tensioning cylinder A, an hydraulic-pneumatic accumulator B and a lifting cylinder C.

The cylinder-accumulator 8 is mounted in a cylinder housing 11 from the top of which the piston rod 7 of the tensioning cylinder A projects, the rod carrying a piston 12 which is slidable in a cylinder lining 13. The top side of the piston 12 is exposed to an hydraulic liquid which is supplied passed to the cylinder A via a pipe 14. Below the piston 12 is air which communicates with the surrounding atmosphere via a duct 15 through the piston 12 and the piston rod 7.

The hydraulic side of the piston 12 is connected via ducts 16 to an annular chamber 17 between the cylinder lining 13 and the cylinder housing 11. The annular chamber 17 is connected via ducts 18 and 19, to the hydraulic side of the hydraulic-pneumatic accumulator B, which is a piston accumulator of known design with a freely movable piston 20 in a cylinder 21. The gas side or the pneumatic side of the accumulator B incorporates, besides a cavity 22, also an annular chamber 23 surrounding the cylinder 21. The two cavities 22 and 23 are interconnected by means of ducts 24 at the bottom of the cylinder 21.

In the lower part of the cylinder housing 11 is mounted the hydraulic lifting cylinder C, which comprises the piston rod 9, a piston 25 of which is slidably mounted in a cylinder 26. The cylinder 26 forms, together with the cylinder housing 11, an annular chamber 29, which communicates, via at least one duct 30, with the annular chamber 23, and is thereby part of the gas side of the accumulator B.

The lifting cylinder C is double acting as the pis- ton 25 may be acted upon on both sides by liquid pressure supplied at the top via a pipe 27 and at the bottom via a pipe 28.

The cylinder-accumulator 8 operates in the following way: During operation of the mill, the piston of the lifting cylinder C is locked in a set position, e.g. by means of a iocked amount of oil on both sides of the piston 25, or by the piston 25 being pressed into Its top position in the cylinder 26 by the liquid pressure whereby the cylinder housing 11 is axially immobile, but pivotally connected to the mill foundation via the bracket 10.

The liquid pressure on the piston 12 of the tensioning cylinder A is set according to the desired grinding pressure of the corresponding grinding roller 3 which the piston 12controls via the piston rod 7. The grinding pressure is mainly maintained constant due to the fact that the hydraulic conneo tion constituted by the ducts 16, the annular chamber 17 and the ducts 18 and 19 is very short and has a comparatively large cross sectional flow area so that the friction loss in the connection is very small causing the piston 20 of the accumulator B to follow the movements of the piston 12 of the tensioning cylinder A very precisely.

The relatively large gas volume of the accumulator B together with the large compressibility of the gas only cause very small pressure changes during the movements of the accumulator piston 20, and thereby negligible pressure changes of the hydraulic liquid in the tensioning cylinder A.

As the bottom side of the piston 12 communicates with the atmosphere via the duct 15 the cavitation of liquid below the piston 12 and possible diesel effects are avoided as previously mentioned.

If, for some reason, the grinding roller 3 has to be lifted from the grinding table 1, the top side of the piston 25 of the lifting cylinder C is pressurised by liquid introduced through the pipe 27, the bottom side of the piston 25 being relieved simultaneously by withdrawing liquid via the pipe 28. The entire cylinder-accumulator 8 is thus lifted, the piston 12 moving into its bottom position in the cylinder 13, and the roller 3 hence being lifted by the upward movement of the piston rod 7.

The roller is lowered again by relieving the liquid pressure on the top side of the piston via the pipe 27.

Claims (5)

1. A vertical roller mill with a grinding table rotatable about a vertical axis and at least one grinding roller rotatable about a stationary shaft, the grinding roller being forced against the grinding table by means of a piston rod of an hydraulic roller-tensioning cylinder, the tensioning side of which is connected to the hydraulic side of an hydraulic-pneumatic accumulator, characterised in that the accumulator is mounted in an extension of the housing of the hydraulic roller-ten sioning cylinder, and that the hydraulic connection between the tensioning cylinder and the accumulator comprises at least one duct in the cylinder housing.

2. A mill according te claim 1, characterised in that the duct for the hydraulic connection between the tensioning cylinder and the accumulator comprises an annular chamber between the cylinder housing and an inner cylinder lining of the tensioning cylinder.

3. A mill according to claim 1 or claim 2, characterised in that the accumulator is a piston accumulator.

4. A mill according to any one of the preceding claims, characterised in that an hydraulic lifting cylinder for a corresponding roller is mounted at the end of the cylinder housing remote from the tensioning cylinder, the piston rod of the lifting cylinder being connected to the mill foundation.

5. A vertical roller mill, substantially as described with reference to the accompanying drawings.