Test Apparatus For The Evaluation Of Rolling Lubricants

Abstract

A test apparatus for the evaluation of the lubricity of rolling oils and the interaction of such oils with a variety of metal alloys employed in the construction of rolls. The axis of a driven work roll is placed on a radial line of a disc. The lubricant under test is applied to the region of frictional contact between the roll and the disc. The resultant speed of the disc is controlled by applying a torque to the disc. With a roll of given length, and a known load applied by the roll to the disc, the coefficient of friction is determined by measuring the torque on the disc. This coefficient can thus be evaluated for various degrees of slipping velocity of a roll.

Description

This invention is directed to an apparatus for evaluating the effectiveness of a rolling lubricant at conditions closely approximating those which exist in actual rolling practice. In the process of cold rolling of metal strands (sheet, strip, wire), a rolling oil is applied to the strand as it enters the roll bite. The effectiveness of the oil is of critical importance in determining the mill loads that are required to achieve a desired reduction. A number of test devices (generally employing ball bearings) are now available for evaluating the lubricating properties of these oils at varying load conditions. While the effectiveness of a lubricant at such differing load conditions is a valuable aid, it is also desirable to know how the lubrication properties vary for differing degrees of slip, i.e., the differential velocity which exists between the roll and the strand during actual rolling.

In the process of rolling, the strip is reduced in thickness as it passes between two work rolls rotating at a constant velocity. However, as the material is reduced in thickness, its forward velocity increases proportionately. The relationship between strip thickness and velocity may be seen from the following equation, based on the Law of Continuity of Matter.

v.sub.1 h.sub.1 w.sub.1 = v.sub.2 h.sub.2 w.sub.2 ( 1)

where, v = velocity, h = thickness and w = width, and the subscripts indicate the state at two conditions of time.

Since the change in width which occurs is negligible, w.sub.1 is essentially equal to w.sub.2, the equation may therefore be simplified to:

v.sub.1 = v.sub.2 h.sub.2 /h.sub.1 ( 2)

On the other hand, due to its constant rotational velocity (.omega.), the peripheral velocity (Vr) of the work roll (2.pi.r.omega.) is also a constant. The differential in volocity between the strip and the roll (V.sub.1 - V.sub.r) is generally negative at the entry plane, increases to zero at some point in the bite, commonly referred to as the neutral point, and then increases further until the exit plane is reached.

It is therefore a primary object of this invention to provide an apparatus for the evaluation of rolling lubricants under varying conditions of slipping velocity as well as for varying rolling speeds and rolling loads.

Additional objects and advantages of the instant invention will be more apparent from the following description and claims when read in conjunction with the accompanying drawings in which:

FIG. 1, is a schematic representation of a preferred embodiment of the test apparatus; and

FIG. 2, is a top view of the same apparatus (without support yolk).

FIG. 3 is a specific example of the apparatus, including the associated measuring devices.

Referring to FIGS. 1 and 2, the top half of a four high cold rolling mill, that is, a work roll 10 with its associated back-up roll 11 and support yolk 12, are placed on a disc or table 13, wherein the axis of the work roll is placed on a radial line of the disc. Work roll 10 is driven at constant speed by applying a torque Ti to the roll shaft. The speed of disc 13 is controlled by the application of a torque To, e.g., by use of a thrust or radial bearing (not shown). The torque and speed of the disc may be measured by a transducer and tachometer, respectively. The rolling lubricant is applied at the disc-roll interface by supply header 14. Using this apparatus, it may readily be seen that the slipping velocity varies from a maximum positive value, a distance b from axis of rotation of the disc, to a maximum negative value at a radial distance a from the disc axis. The slip velocities which were actually achieved in the instant test apparatus were compared with slip velocities typically encountered in rolling practice. Although the curves were not identical, they were found to cover substantially the same range of slip velocity. The above test apparatus, with its associated measuring devices is shown in FIG. 3. With respect to this figure, the back-up roll and its support yolk are shown by 21 and 22 respectively. The work roll 23 is driven at constant speed by motor 29, the speed of which is measured by a tachometer 30. The rolling load P is applied to the back-up roll support yolk 22, e.g., by a screw-down mechanism or weights. The speed of the disc 31 is restrained by a thrust bearing 24 and a radial bearing 25. A torque transducer 26 and tachometer 28 measure the disc torque and speed respectively. Desired slipping speed is achieved by braking of the disc spindle 27 to control the ratio of the disc speed to roll speed as measured by tachometers 28 and 30. The torque, To, transmitted from the work roll 23 to the disc 31 is determined from transducer 26 and is used to determine the coefficient of friction, the reason for which is discussed more fully below.

The condition existing in an actual roll bite may better be duplicated in a further refinement of this invention. In cold rolling, the maximum pressure occurs at the point of zero slip velocity (i.e., the axis of disc rotation in the instant apparatus). Thus, in this refined embodiment, the diameter of the roll at the axis of disc rotation is made somewhat larger, tapering uniformly to the edges of the roll. This difference in contour between the roll and disc interface will effect a variation in pressure along the axis of the roll and thereby permits the achievement of any desired pressure function, i.e., so as to more closely duplicate that of the roll bite.

In either embodiment, the coefficient of friction between the roll and disc is then determined from the torque To, measured at the braking of the disc spindle. With reference to FIG. 1, the force (p) per unit of contact length is given by:

p = L/(a + b) (3)

Thus, the tangential force (F) between the roll and disc, per unit of contact length will be:

F = .mu. (L/a + b) (4)

and, the torque developed by this tangential force will then be:

To = .mu. (L/a + b)(a) (a/2)- .mu. (L/a + b)(b) (b/2) (5)

therefore, when a .noteq. b, and transposing the terms of Equation (5), the coefficient of friction (.mu.) will be given by:

.mu. = 2 (a + b) To/L (a.sup.2 + b.sup.2)

As may be seen, with a knowledge of the applied load (L) and the lengths a + b, and by measuring the torque To; the coefficient of friction achieved by use of a particular rolling oil may readily be determined for (i) a variety of conditions of slip, (ii) for different roll surfaces and alloys, and (iii) for diverse rolling speeds and rolling loads. By varying these parameters and the lengths a + b, any rolling condition can be simulated for the evaluation of the lubricating properties of a rolling oil.

Claims

We claim:

1. Apparatus for the evaluation of rolling lubricants which comprises,

table means with a planar surface, said means being adapted for rotation about a first axis perpendicular to said planar surface, and means associated therewith for measuring the torque and rotational speed of said table means,

cylindrical roll means of length (1), and means associated therewith for supporting said roll means in a position whereby a linear region of the surface of said roll means is maintained in frictional contact with said planar surface so that the axis of said roll means orthogonally intersects said first axis, while the mid-length (1/2) of said roll means is not coincident with said first axis,

means for exerting a desired load between said roll means and said table means,

means for effecting the rotation of said roll means, and

means for applying the lubricant under evaluation to said region of frictional contact.

2. The apparatus of claim 1, wherein said means for exerting a desired load include a back-up roll and support yolk and said table means is in the form of a disc.

3. The apparatus of claim 2, wherein the contours of said roll means and said planar surface are slightly mismatched so as to vary the pressure therebetween along the line of frictional contact.

4. The apparatus of claim 3, wherein said roll means is substantially ellipsoidal, so that the roll diameter at the point of intersection with said first axis is greater than the diameter at the ends thereof, to achieve said contour mismatch.