The invention concerns a shoe press for a paper or board machine, said shoe press having a press shoe which together with a counter roll forms an extended nip for a paper or cardboard web and for a flexible circulated belt, and at least one piston-and-cylinder assembly, which is arranged between a supporting beam associated with the frame system of the shoe press and the press shoe and in which hydraulic fluid is supplied to a work- ing chamber, said working chamber communicating with a compartment via a throttle, said compartment being ar- ranged on the outside of the piston-and-cylinder assembly in such a manner that the hydraulic fluid therein exerts directly or indirectly a force upon the press shoe in the direction opposite to that of the force exerted upon the press shoe by the hydraulic fluid in said working cham- ber.

A shoe press as described above is disclosed in DE-C1-195 15 832, the piston in the piston-and-cylinder assembly having said compartment.

This prior-art shoe press suffers from the drawback that lateral forces which in operation affect the press shoe are transmitted to the cylinder jacket and the pis- ton, which may cause a jam between the press shoe'and the piston.

The object of the invention is to eliminate this drawback.

According to one aspect of the invention, the drawback implying that lateral forces which in opera- tion affect a press shoe are transmitted to the cylinder jacket and the piston has been eliminated by a new method

of supporting the press shoe, said method having the characteristic features as defined in claim 1. Advanta- geous embodiments are stated in the subclaims.

Moreover, the object has been achieved by means of a shoe press having the characteristic features as defin- ed in claim 4. Preferred embodiments are stated in the subclaims.

Three embodiments of the invention will now be described in more detail with reference to the accompa- nying drawings, in which Fig. 1 is a cross-sectional view of a first embodi- ment of the shoe press according to the invention, Fig. 2 is a cross-sectional view of a second embodi- ment of the shoe press according to the invention, and Fig. 3 is a cross-sectional view of a third embodi- ment of the shoe press according to the invention.

The cross-sectional views are taken transversely of the machine direction of the paper machine, and it is understood in this specification and in the claims that the press shoe is a single piece while there is at least one, preferably more piston-and-cylinder assemblies and several compartments distributed in the longitudinal direction of the press shoe (cross-direction of the paper machine).

In Figs 1 and 2, reference numeral 1 designates two embodiments of a shoe press and reference numerals 2, 3,4 designate a press shoe, a piston and a cylinder, respectively. The cylinder 4 and the piston 3 which is movingly arranged therein define a chamber 5, to which a hydraulic fluid, e. g. oil, is supplied from a duct 6 in the piston 3 via one or more bores 7 in the piston 3. A radial seal 8 is arranged for the sealing of the chamber 5. The piston 3 is slightly tiltable in the cylinder 4.

Reference numeral 9 designates a horizontal support- ing beam located in the shoe press and fixed to the frame. In the embodiment according to Fig. 1, the piston 3 is fixedly connected to the press shoe 2, and the end

wall of the cylinder 4 rests freely on the beam 9, while in the embodiment according to Fig. 2, the piston 3 is fixedly connected to the beam 9 and the press shoe 2 rests freely on the end wall of the cylinder 4. For said connection, use is made of bolts 10. The flexible press belt of the shoe press is designated 11, and the counter roll of the shoe press is designated 12.

In operation with the working chamber 5 pressurized, a paper web W and the circulated press belt 11 guided by the shoe press 1 pass through the nip N between the press shoe 2 and the counter roll 12, the felt F receiving water from the paper web W.

In the end wall of the cylinder 4 there is arranged a through throttle 14 which opens into a shallow compart- ment or pressure chamber 15 in the end wall of the cylin- der 4. Hydraulic fluid in the chamber 5 may thus escape to this compartment via the throttle 14. The area of the compartment 15 is somewhat larger than the inner area of the cylinder 4, which causes the lifting force exerted upon the press shoe 2 by the hydraulic fluid in the com- partment 15 at an initial stage of the operation to be greater than the force exerted upon the press shoe 2 by the hydraulic fluid in the working chamber 5. This results in the forming of a gap 16 between the annular bottom surface 4'of the cylinder 4 and the beam 9 (Fig. 1) and between the annular top surface 4'of the cylinder 4 and the press shoe 2 (Fig. 2), respectively, through which gap 16 hydraulic fluid constantly flows out of the compartment 15, such that the press shoe 2 floats freely. In operation, there is thus a supporting and lubricating hydraulic fluid film in the gap 16, which can be said to define a hydrostatic bearing between the upper side of the beam 9 and the annular surface 4'in Fig. 1 and between the underside of the press shoe 2 and the annular surface 4'in Fig. 2. A person skilled in the art will, without inventive effort, adapt the diameter of the

throttle 14 and the area of the compartment 15 to each other, thereby obtaining a suitable film thickness.

Fig. 3 illustrates a shoe press having a press shoe 300, a counter roll 312, a circulated press belt 311, a piston-and-cylinder assembly 310 with piston 330 and cylinder 340, and a beam 309 which is fixed to the frame.

The piston 330 is attached to the beam 309 by means of bolts 350. The top face of the movable cylinder 340 adjoins the press shoe 300, and in this top face there is a compartment or hydraulic fluid chamber 301, which is defined by an annular flange with an annular surface 340' on the top of the cylinder 340. The compartment 301 is supplied with hydraulic fluid via a sectional pipe 303 which is attached to the longitudinal side edge 304 of the press shoe 300 and which has an inner duct 306, via a bore 302 formed in the press shoe 300 and having bore portions 302'and 302"which are perpendicular to each other, and an opening 308 which is formed in the sec- tional pipe 303 and constitutes a throttle. For the attachment of the sectional pipe 303 use is made of screws 305 and threaded bores 305'in the press shoe. The working chamber 325 of the piston-and-cylinder assembly 310 is supplied with hydraulic fluid, like in the embodi- ments above, via a duct system 326,327 in the piston 330. The difference over the embodiments above thus is that there is no connection between the compartment 301 and the working chamber 325, but the compartment 301 and the working chamber 325 are separately supplied with hydraulic fluid via the ducts 326 and 327 in the piston 330. This arrangement makes it possible to form a-gap 336 between the underside of the press shoe 300 and the annu- lar top face 340'of the cylinder 340, through which gap hydraulic fluid can flow out of the compartment 301 while forming a hydrostatic bearing, even if, as illustrated, the surface area of the compartment 301 is smaller than the inner area of the cylinder 340. An O-ring seal

between the cylinder 340 and the piston 330 is designated 380.

It will be appreciated that the object of the inven- tion has thus been achieved, and moreover that the seal- ing of the above-mentioned prior-art press piston against the press shoe is unnecessary in the inventive shoe press.

The inventive shoe press is applicable also to calendering operations in a paper or board machine.