Patent Application
20140374526
1. Field
Embodiments of the present invention relate to laying heads of the type employed in rolling mills to form hot rolled products into helical formations of rings.
2. Description of Related Art
In a conventional laying head, a stationary support structure contains a hollow quill rotatably supported between axially spaced bearings. The quill is equipped with a guide pathway, which may typically comprise 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 helical 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 100 m/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. Pat. 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. Pat. No. 7,086,783, dual pre-loaded roller bearings have been employed to minimize 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 modern day rolling mills.
As described in U.S. Pat. No. 8,004,136 B2, it also has been proposed to employ hydrodynamic bearings in place of roller bearings. In a typical hydrodynamic bearing, as diagramatically illustrated in
The rotating member forms a single pressure field “P” 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 centerline 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.
Hydrodynamic bearings are a mature technology and the graphical solutions presented by Raimondi & Boyd (A. A. Raimondi and John Boyd, “A Solution for the Finite Journal Bearing and Its Application to Analysis and Design, Parts I, II, III,” Trans. ASLE, vol. 1, no. 1, pp. 159-209, in “Lubrication Science and Technology”, Pergamon Press, New York, 1958) are still widely used for bearing design. The design techniques are valid for specific ranges of Sommerfeld numbers and for bearings with a range of specific geometric relationships. For example, the numerical solutions in the literature solve for specific length-to-diameter (L/D) ratios (as shown in
A number of potential problems are encountered when operating hydrodynamic oil film bearings at high speeds under lightly loaded conditions. For example:
Application of a hydrodynamic oil film bearing to a laying head has all of the above problems. However, given the speeds of operation, whirl is a particularly critical problem, as high rotational speeds and low loads will ensure that the bearing will be running in an unstable condition at all times.
For example, a typical laying head application might require a 600 mm diameter bearing. A conventional hydrodynamic bearing would have an L/D ratio of not less than 0.25 and a typical clearance of 0.60 mm. Assuming a rotating mass of 40 kN or less and a typical oil viscosity of 100 cSt, the bearing would have a predicted peak oil film temperature as a function of speed as follows:
In rolling mills producing wire rod, the helical formation of rings exiting from the laying head is typically deposited in an overlapping pattern on a conveyor. The rings are subjected to controlled cooling while being transported by the conveyor to a reforming station where they are gathered into coils.
During normal mill operation, the speed of the laying head may be controlled to implement so called “wobble” and “tail end speed up” functions. The wobble control function is typically employed with larger product sizes, e.g., 10.0 mm and larger, and serves to cyclically alter the speed of the laying head above and below nominal speed to produce differently sized rings that nest inside each other in the reforming chamber, resulting in a denser coil of reduced height. The tail end speed up function is achieved by accelerating the rotational speed of the laying head once the tail end of the product exits from and is no longer being propelled by the laying head pinch roll.
When implementing the wobble function at the lower operating speeds that are commonly used when processing the larger product sizes, the overall system stability of a hydrodynamic bearing is severely compromised as the load zone is continually shifted from one side to the other of the bearing in response to alternating acceleration and deceleration. The rapid acceleration during tail end speed up is similarly detrimental to bearing stability.
Hydrodynamic oil film bearings have been introduced for use in rolling mill laying heads but have not been widely accepted, likely because of the above described problems.
An objective of the present invention is to provide a rolling mill laying head equipped with a novel and improved hydrostatic oil film bearing that overcomes or at least substantially mitigates the problems associated with mechanical roller bearings and hydrodynamic oil film bearings.
In exemplary embodiments of the present invention, the quill of the laying head is rotatably supported by multiple bearings, with at least the bearing at the delivery end of the laying head being a hydrostatic oil film bearing. Instead of a single pressure field formed passively in response to rotation of the quill, as is the case with a hydrodynamic oil film bearing, the hydrostatic oil film bearing of the present invention provides a plurality of discrete pressure fields 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.
The overall stability of a hydrostatic bearing is not a function of the rotational speed of the bearing, i.e., the hydrostatic bearing does not rely on a speed/geometry dependent wedge to lift and center the rotating mass. Because the inherent design of a hydrostatic bearing allows for centering of the rotating mass regardless of the applied load or speed, the bearing has a significant operational advantage over hydrodynamic bearings, particularly during a wobble cycle.
These and other features and their attendant advantages will now be described in greater detail with reference to the accompanying drawings, wherein:
The clearances between the rotating members and bushings in
With reference initially to
As can be seen by further reference to
In laying head applications, the internal diameter D of the bushing 38 is relatively large, typically ranging from about 500 mm to 1000 mm. The loads are relatively light, with a rotating mass of 40 kw or less. In accordance with embodiments of the present invention, and in order to increase the specific loading, the length L of the bearing is purposely shortened to provide an L/D ratio less than 0.25, with L/D ratios as low as 0.15 being shown by tests to be particularly advantageous.
While there is no basis in theory for the use of such large diameter and narrow hydrostatic oil film bearings, testing has shown that such bearings beneficially reduce heating of the bearing. For example, bearing temperatures were measured during tests of a laying head equipped with a hydrostatic oil film bearing in accordance with an exemplary embodiment of the present invention. The hydrostatic oil film bearing had dimensions comparable to those of the previously described hydrodynamic oil film bearing. Its design was similar to that depicted in
Beneficial reductions in oil consumption and power loss are also to be expected when equipping laying heads with hydrostatic oil film bearings in accordance with exemplary embodiments of the present invention.
A rolling mill producing hot rolled small diameter products, e.g., 5.5 mm rods, runs at very high speeds. In the event of an electrical power failure, and due to the inertia of the rotating components, the mill can take up to 45 seconds or more to “coast down” to zero speed. In accordance with a further exemplary embodiment of the present invention, and in order to insure that the hydrostatic oil film bearing of the present invention remains supplied with high pressure oil during this period of deceleration, an auxiliary supply means serves to store high pressure oil in a stand-by mode. As shown in
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 exemplary embodiments of 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 benefits of lower operating temperatures, reductions in oil consumption and power loss, and relatively low starting torques.
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.
