BACKGROUND DISCUSSION

L Field, of the Invention

This invention relates to laying heads of the type employed in rolling mills to form a hot rolled product into a helical formation of rings.

2. Description of the Prior Art

In a conventional laying head, a stationary support structure contains a hollow quill rotatably supported between axially spaced bearings. The quill carries a curved guide pipe having an entry end aligned with the rotational axis of the quill, and a curved intermediate section projecting in a cantilever fashion from the quill to an exit end spaced radially from the quill axis. The quill is rotatably driven by known means, with the guide pipe being configured to receive a product at its entry end and to form the product into a continuous formation of rings emerging from its exit end.

Roller bearings are typically employed to rotatably support the quill. Under high speed operating conditions, e.g., when handling products traveling at speeds exceeding lOOm/sec, experience has shown that the roller bearings are prone to producing vibrations that disturb operation of the laying head.

Various schemes have been devised in an attempt at eliminating or at least suppressing such vibrations. For example, as described in U.S. Patent No. 5,590,848, the cantilevered portion of the guide pipe has been shortened in order to increase the overall stiffness of the laying head. Also, as described in U.S. Patent No. 7,086,783, the roller bearings have been preloaded to eliminate operating clearances. Although such design modifications have been proven to be beneficial, they have not adequately addressed the vibration problems which continue to plague the laying heads as they are operated at the ever increasing speeds of modem day rolling mills.

As described in WO2005/084842 Al, it also has been proposed to employ hydrodynamic bearings in place of roller bearings. In a typical hydrodynamic bearing, as schematically illustrated in Figure 3, a rotating member 10 is surrounded by a bushing 12. The rotating member is subjected to an applied load "L", and low pressure oil 16 is introduced between the rotating member and bushing via a recess 17 in the interior bushing surface. The rotating member forms a single pressure field "P" formed as a result of a combination of parameters, including the rotational speed of the rotating member, the applied load, the diametrical clearance between the rotating member and bushing, and the viscosity of the oil. The force integrated from the pressure field exactly balances out the applied load, with the centerlme 18 of the rotating member 10 being offset from the centerline 20 of the bushing 12, resulting in an eccentricity "E" that is a function of the aforesaid, parameters.

Under lightly loaded high speed operating conditions, as is the case with rolling mill laying head, hydrodynamic bearings are known to experience a vibration effect commonly referred, to as "whirl", where the rotating member orbits inside the bushing in a highly undesirable mode. The radius of the orbit is essentially equal to the eccentricity of the bearing under the specific operating conditions.

Also, all hydrodynamic bearings require a higher starting torque to overcome the static friction of the rotating member sitting stationary on the bushing. Once rotation commences, the torque requirement drops dramatically. However, the laying head, motors and drive trains must be sized to deliver the higher starting torques, and this in turn increases costs.

The objective of the present invention is to provide a roiling mill laying equipped, with a novel and improved bearing that overcomes or at least substantially mitigates the problems associated with mechanical roller bearings and. hydrodynamic oil film bearings.

SUMMARY OF THE INVENTION

In accordance with the present invention, the quill of a laying head is rotatably supported, by multiple bearings, with at least one bearing being a hydrostatic oil film bearing. Instead of a single pressure field formed passively in response to rotation of the quill, a plurality of discrete pressure fields are formed by high pressure oil being actively pumped into angularly spaced recesses in the bushing. The recesses are arranged in a manner such that their associated pressure fields urge the quill into concentric alignment with the bushing where it is held during continued operation of the laying head, thus minimizing and. ideally eliminating vibration due to eccentricity. The multiple pressure fields also serve to separate the quill from the bushing surface prior to the start of quill rotation, which makes it unnecessary to provide a drive train with a higher starting torque. These and other features and their attendant advantages will now be described in greater detail with reference to the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 is a diagrammatic view, partially broken away, of a laying head equipped with a hydrostatic oil film bearing in accordance with the present invention;

Figure 2 is a diagrammatic sectional view taken through the hydrostatic oil film bearing on line 2-2 of Figure 1; and

Figure 3 is a diagrammatic cross sectional view of a liydrodynamic oil film bearing.

The clearances between the rotating members and bushings in Figures 2 and 3 have been exaggerated for illustrative purposes.

DETAILED DESCRIPTION

With reference initially to Figure 1, a laying head. 22 comprises a quill 24 rotatable about a central axis "X". A guide pipe 26 is carried by the quill The guide pipe has an entry end 26a aligned with axis X, and a curved intermediate section 26b leading to an exit end 26c spaced radially from axis X. The quill is contained within a stationary support structure 28 and is supported for rotation about axis X by axially spaced bearings 30, 32. Bearing 30 is a conventional roller thrust bearing, and bearing 32 is a hydrostatic oil film bearing in accordance with the present invention. The quill is rotatable driven by a conventional drive train including meshed bevel gears 34, 36 powered by a gear box and motor (not shown).

As can be seen by further reference to Figure 2, the hydrostatic oil film bearing comprises a bushing 38 surrounding the journal surface of the quill 24. A plurality of angularly separated recesses 40 are provided in the interior surface of the bushing. The recesses 40 are connected via supply conduits 42 to a distribution header 44, which in turn is connected to a high pressure oil supply 46. The high pressure oil supplied, to the recesses 40 creates discrete pressure fields 48 acting to urge the quill into concentric alignment with the bushing, where it is held, irrespective of the speed at which the quill is driven, during continued operation of the laying head. Eccentricity is thus eliminated, or at least minimized to tolerable levels. The discrete pressure fields 48 also serve, during start up, to lift the journal surface of the quill from the bushing surface, thus reducing friction and eliminating the need for increased starting torque.

Preferably, at least three recesses 40 are provided in the bushing 38. Preferably, the recesses are equally spaced around the bushing circumference.

In light of the foregoing, it will now be appreciated by those skilled in the art that by employing a hydrostatic oil film bearing in accordance with the present invention, the quill can be maintained, in substantially constant concentric alignment with the bushing, and this can be achieved independently of the speed at which the laying head is being operated. Thus, vibration problems due to whirl in hydrodynamic bearings and clearances in mechanical roller bearings are eliminated or at the very least, significantly minimized to an extent that they no longer impede high speed operation of the laying head. This is achieved with the added benefit of relatively low starting friction.

The foregoing description has been set forth to illustrate the invention and. is not intended to be limiting. Since further modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the scope of invention should be limited solely with reference to the appended claims and equivalents thereof.