Flexible ceramic member having a pre-loaded tensile force applying means

Abstract

An elongated flexible member including a plurality of ceramic segments, each segment having at least two opposite surfaces that are flat and parallel. The segments are aligned in stacked relationship with their flat faces in abutting face-to-face relation and forced toward each other in the direction of their composite length with a force which is sufficient to maintain the segments in compression when subjected to conditions of thermal change and/or flexing of the member during use. A method for making the ceramic member is disclosed.

Description

This application is a continuation-in-part of Ser. No. 273,308, filed July 19, 1972 now abandoned.

This invention relates to elongated flexible ceramic elements, and more particularly to an element of this type which is useful in applications where the element is to be subjected to conditions of thermal change and/or forces tending to bend the element along its length.

The physical properties and/or the chemical inertness of ceramic materials frequently suggest such materials for use in applications wherein the material is to be fabricated into elongated articles such as a ceramic conduit for conveying corrosive chemicals. Not infrequently, such elongated elements are subjected to thermal change or forces, such as vibration or frictional drag, which tend to bend or deflect the element along its longitudinal axis.

Because of the relatively high cost and difficulty of manufacturing ceramic elements in continuous lengths, for example lengths greater than about two feet, ceramic materials have heretofore been generally limited to use in those situations where their relatively high cost is justified in order to obtain the advantages from the physical and/or chemical properties of the ceramic materials. Even in such special situations where ceramic lengths have been employed, it has been important to assure that the elongated elements neither bend nor are subjected to localized stresses, so as to avoid cracking and/or breaking of the elongated element. Consequently, the circumstances under which elongated ceramic elements could be used heretofore have been severely limited.

This invention also relates to systems in which there are at least two members, one of which is movable relative to the other and in frictional engagement therewith. More particularly, the invention relates to such a system in which at least one of the members is an elongated flexible ceramic member having a working or wear surface defining an area of contact between the members and which has improved physical characteristics.

Examples of such systems include the combination of elongated foils in contact with a forming fabric in a Fourdrinier length, L (in inches), and a dimension, h (in inches), across the segments, in the direction of the applied deflective force and a cross-sectional area, A_c, in square inches, the preloading on the tension member, e.g. cable 38, which will impart to the ceramic segments the necessary compressive force that precludes separation of the segments is calcuated using the equation ##EQU1## where: E_c = the modulus of elasticity of the ceramic;

A_c = the cross-sectional area of a ceramic segment in a plane perpendicular to the composite length of the member, in square inches;

d = the maximum anticipated deflection of the member, in inches;

h = the dimension of a ceramic segment in the plane perpendicular to the composite length of the member and in alignment with the direction of the deflective force, in inches;

l = the overall length of the member. , and

L = the length of a ceramic segment, in inches

With reference to Equation (1), it is noted that the initially determined preloading is divided by 2 to give the preloading to be used in tensioning the cable 38. This fact arises because of the manner in which the ceramic segments are stressed when the member is deflected while under compression. More specifically, assuming the cable 38 is disposed midway between the ends of the segment dimension h, when the member is in an undeflected state, the stress on each compressed ceramic segment is the same at any point along the dimension h. When the member is deflected, the stress in that portion of a segment on the outside of the line of curvature (on the outside end of the dimension h) is reduced to zero and the stress in that portion of the same segment on the inside of the line of curvature is doubled. Thus when preloading the aligned segments, the stress imparted to the segments is taken as the average of the stresses along the dimension h when the member is deflected by a maximum amount.

The effect of thermal change upon the member 30 must also be taken into account. Thermal changes occur most frequently by reason of the member 30 being manufactured at a first temperature, room temperature for example, and thereafter encountering a substantially higher operating temperature. In such circumstances, the strain in the cable 38 decreases when its temperature increases by reason of the cable expanding when heated. Expansion of the cable cross-section as well as along its length is of importance. The ceramic also expands when heated, but usually to a lesser extent than the cable, so that there is added to the preload calculated for deflection in accordance with Equation (1), an additional preloading which will compensate for the effect of thermal change upon the cable and the ceramic and provide the desired preloading for accommodating deflection up to a maximum temperature. Such additional preloading of the tension means is calculated using the equation ##EQU2## where: α₃ = the coefficient of thermal expansion of the tension member;

α_c = the coefficient of thermal expansion of the ceramic;

ΔT = degrees of temperature change anticipated, in degrees F;

A_s = cross-sectional area of the tension member, in square inches;

E_s = the modulus of elasticity of the tension member;

A_c = the cross-sectional area of a ceramic segment in a plane perpendicular to the length of the member, in square inches;

E_c = the modulus of elasticity of the ceramic.

Combining Equations (1) and (2) gives ##EQU3## where P is the total preloading of the tension member which will prevent separation of the segments of the member 30 when the member is deflected up to a maximum amount d while at a temperature less than an anticipated maximum temperature. It will be noted that in those situations where the member 30 will not experience a thermal change, ΔT will be zero and P_T [including its equivalent expression in Equation (3)] will be zero and no additional preloading will be required to account for thermal changes.

Thus, in any given situation where the elongated member 30 is to be subjected to deflection forces, it is possible to select a composite which exhibits the desired non-separation of abutting segment faces when the composite is deflected along its composite length. As shown in Equation (1), the preloading force (compressive force) applied to the aligned segments, for any given maximum anticipated deflection and total length of the segmented member, depends upon the length of each individual segment and the dimension h of each segment. Thus, if the deflection capability of a given composite of ceramic segments is less than that which precludes physical separation of the abutting faces of the segments under the anticipated deflection, an adjustment can be made, in many instances, in either the length or width of the individual segments, or in both the length and width. Of course, consideration must be give to the added compression experienced by those portions of the abutting segment faces disposed on the inside of the line of curvature of the deflected composite.

The preloading force exerted upon the ceramic segments is kept below that amount of force which will compress the ceramic material to within about one-half, and preferably to within about 20%, of its maximum compressive strength to insure that localized stresses which may occur within the composite do not exceed such maximum compressive strength with resultant damage to one or more segments. This preferred preloading also provides a substantial margin of safety against damage to the segments by inadvertent overloading of the segments to produce undue deflection. In any event, the preloading of the segments is sufficient to shorten the length of each segment, hence shorten the overall length of the composite. Further, in the preferred preloading, the segments are sufficiently deformed at the interface between abutting segment faces as results in substantial loss of joint identity at such interface. Such deformation is known to occur when the segments are preloaded to between about 15% and 20% of the maximum compressive strength of the ceramic. This substantial loss of joint identity has been found to be important in establishing the working surface on the member in that such allows the composite to be ground to a suitable smoothness. Less preloading is acceptable but at a loss of certainty of achieving the desired properties in the composite. Thus, the preloading of the ceramic segments must be sufficient to maintain the segments abutting when the member is deflected by a maximum amount d but less than that preloading which will compress the ceramic to more than one-half its total compressive strength.

It is understood that in the present discussion each of the segments is substantially identical to each other segment in a given composite. Such is assumed for purposes of simplifying the disclosure. It is not required, however, that all the segments be identical. For example, it may be desirable to provide a segment member which is deflected by different degrees along its length. In such an embodiment, the deflective characteristics of the member will differ in different portions of its length and the segments in each such portion may differ in length from the segments in other portions of the length of the member.

As disclosed, one of the members of the system is movable with respect to the other member. In many embodiments, one member is held stationary while the other member moves thereover in frictional engagement therewith. Similarly, in many embodiments the stationary member wll be the flexible ceramic member 30 described above and will include a leading edge 72 which is initially contacted by the other member as it moves over the ceramic member. In such instances it is important that such leading edge be straight and free of irregularities such as gaps resulting from chipping of the leading edge inasmuch as such irregularities, among other things, hinder or prevent alignment between the two members and create wear points between the moving members.

The segmented member 30, being intended for use in a system where it is in frictional engagement with a further member and there is relative movement between the members, is provided with an elongated smooth working surface 70. This surface 70 extends along the length of the member 30 and defines an extended area of contact between the relatively moving members. Minimum wear of this surface and of the other of the moving members is obtained by maximizing the smoothness of this working surface. This is accomplished by grinding the surface 70 after the segments have been formed into the composite 30 and preloaded as described hereinabove.

In a typical grinding operation the segmented member 30 is anchored on the bed of a grinding machine. A diamond impregnated grinding wheel, preferably of the type having an annular planar grinding surface is used in the grinding process. This grinding wheel is moved into contact with the segmented member with the plane of the grinding surface of the grinding wheel disposed at a slight angle with respect to the plane of the surface to be ground so that only a portion of the rotating grinding surface is in contact with the segments at any given time. Preferably the grinding surface plane is also disposed with respect to the working surface so that grinding of the surface takes place as the annular grinding surface moves onto the surface and little or no grinding takes place as the grinding surface is moving away from the surface being ground.

The rotation of the grinding wheel, when grinding a leading edge of the type shown in FIG. 1, is such that the grinding surface initially contacts the leading edge 72 as the grinding surface moves toward that edge. In this manner, the grinding forces exerted upon the segments are directed inwardly of the segments to aid in preventing chipping of the segments edges during grinding. Preferably, the grinding action at the leading edge is in a direction substantially perpendicular to the leading edge. Variations of greater than about 10 degrees from such perpendicular relationship provide relatively poor edges.

In the grinding operation the compression of the segments in the direction of their composite length maintains the edges of abutting segments in supporting relationship to each other. In addition to this physical support of one segment by its neighbor, the compression in the segments is sufficient to prevent the force of the grinding operation from placing the segment edges in tension as the grinding wheel drags across the segment, thereby enhancing the resistance of the segments to edge chipping during grinding. This results in an improved smoothness of the working surface 70 and is believed to be responsible in part for the good surface finishes obtained by applicant when grinding the segmented member as disclosed herein.

The grinding operation disclosed herein provides surface finishes of the working surface of less than about 20 microinches (AA). This degree of surface smoothness has been found to impart exceptionally good wear characteristics to both moving members of the system. Moreover, edges developed by such grinding procedures are substantially straight lines and substantially free of chipped out portions. Such leading edge also has a smoothness substantially equivalent to the smoothness of the remainder of the working surface 70.

EXAMPLE I

A doctor blade for doctoring a paper web from the surface of a cylindrical dryer shell is made as follows. Such doctor blades normally are deflected by different amounts along different portions of their length due to undulations in the dryer shell across its width. The most severe deflection is chosen and the total deflection capability of the blade is made sufficient to accommodate it. In this Example the length, l, of the chosen deflected portion is 50 inches.

The doctor blade in the configuration illustrated in FIG. 1 is made from one inch long (L) alumina segments (AD-995 from Coors Porcelain Co.) each having a cross-sectional area (A_c) of 0.78 square inches. The dimension (H) (h), the dimension in the direction of the application of the deflective forces, is 0.875 inch. These segments are aligned with their flat parallel faces abutting and compressed in the direction of their composite length by a stainless steel cable of 0.14 square inches cross-sectional area threaded through aligned openings in the segments.

The maximum anticipated deflection of the doctor blade over the chosen 50 inch length, l, is determined to be 0.027 inch and the anticipated thermal change is from 70° F. to 300° F. (ΔT = 230° F). The preloading for the cable which passes through the segments is calculated using Equation (3) as follows: ##EQU4## P = 1579.5 + 2385.19 P = 3964.69 pounds

This preloading imparted a compressive force to the ceramic which is about 1.54% of the 330,000 psi approximate maximum compressive strength for AD-995 alumina. This degree of compression provides for the anticipated deflection, occurring at a temperature of 300° F., without complete relief of the compression in those portions of the abutting segment faces furtherest from the longitudinal axis of the member along which the deflection occurs and, importantly, provides for additional compression of those portions of the abutting segment faces nearest the longitudinal axis of the member as necessary to accommodate the deflection.

The working surface 70 of the segmented member 30 is ground while the member is supported along its entire length on the bed of a grinding machine. A five hundred grit diamond impregnated wheel, having an annular grinding surface, as sold by the Norton Company is employed in the grinding operation. The grinding wheel has a diameter of 10 inches, and is rotated at approximately 3600 revolutions per minute. The wheel is moved along the length of the working surface at a speed between about 10 and 20 feet per minute. The position of the grinding wheel relative to the working surface and its rotational movement is as described above. The grinding operation provides a surface finish of about 20 microinches (AA) with no significant chipping of the leading edge 72.

EXAMPLE II

Another system of the type disclosed herein comprises a foil and a forming fabric of a Fourdrinier papermaking machine. In this system, the elongated foil is disposed beneath the forming fabric and serves to support the fabric and remove water from a slurry of papermaking fibers carried on the fabric. In these functions, the fabric slides over the foil while it is pulled against the foil by suction developed by the foil. There is substantial wear of both the foil and the wire in these systems as known heretofore.

A 200 inch long foil for use in a Fourdrinier papermaking machine is made from 200 one inch long AD-995 alumina segments held in compression by a 0.677 inch diameter stainless steel cable which is passed through an opening located centrally of each segment. Each segment has a cross-sectional area (A_c) of 2 square inches, and a dimension (h) of 2 inches. The maximum anticipated deflection of the foil is 0.5 inch and the anticipated thermal change is from 70° F. to 170° F. (ΔT = 100° F).

Using Equation (3), the preloading for the cable for preventing separation of the segments under such conditions is calculated as follows: ##EQU5## P = 10,746 + 3,557.8 P = 14,304 pounds

The preload force in this example stresses the ceramic to 2.17% of its maximum compressive strength.

This foil is provided with a ground elongated working surface having a smoothness of less than about 20 microinches AA in the manner disclosed herein. In use, the foil exhibits excellent wear qualities and does not exhibit gaps between abutting segment faces. Foils of this type when used in a high speed Fourdrinier papermaking machine do not produce streaks in the paper web formed on the forming fabric moving over the foil, as has been experienced by the prior art segmented foils which develop gaps between abutting segments.

EXAMPLE III

A further system of the type disclosed herein comprises a suction device for use in a papermaking machine known as a Uhle Box. This suction device comprises an elongated trough-like device having an elongated slot extending along its length and opening toward a forming fabric or felt moving thereacross. A suction is developed within the Uhle Box so that the fabric or felt is pulled against the edges of the slot and water or other material is pulled from the fabric or felt into the Uhle Box. The edges of the slot are subjected to relatively great wear forces and the Uhle Box, hence the slot edges, are subjected to substantial deflective forces as the fabric or felt moves across the device in a direction transverse to its length.

A Uhle Box having each of its slot edges made of a flexible ceramic member may be fabricated using the teachings of the invention as follows. Each such slot edge is 200 inches long and made of 1 inch long AD-995 alumina segments held in compression by a stainless steel cable having a cross sectional area of 0.25 square inches which is disposed in aligned openings in the segments. Each segment has a cross-sectional area (A_c) of 0.92 square inch and a dimension (h) of 1.250 inches.

In calculating the preload for the cable, the maximum anticipated deflection is 3 inches and the maximum temperature anticipated during use is 170° F. The temperature at assembly is 70° F, giving a ΔT of 100° F. Using Equation (3) the preload is determined as follows: ##EQU6## P = 18,605 + 1,771.5 P = 20,376.5 pounds

The preload force in this Example stresses the ceramic ot 6.71% of its maximum compressive strength.

That portion of the ceramic member which engages the moving fabric or felt is ground to a surface smoothness of less than 20 microinches AA by the procedures set forth above. As in Examples I and II, the ceramic member exhibits good wear charcteristics and does not develop gaps between the segment faces when deflected by the anticipated maximum amount.

While preferred embodiments have been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but rather, it is intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention as defined in the appended claims.

Claims

1. In a system including at least two members one of which is movable relative to the other and in frictional engagement therewith, such as members in a papermaking system, the improvement wherein the other member is an elongated flexible ceramic element comprising

a plurality of ceramic segments, each having at least two opposite surfaces that are substantially flat and parallel, said segments being aligned with their flat faces in abutting face-to-face relation and in respective planes that are oriented substantially perpendicular to the composite length of said plurality of segments,

tension means anchored to opposite ends of said element and forcing said segments toward each other in a direction along their composite length and substantially perpendicular to their respective parallel faces with a preload force on said tension means, when said latter member is in an undeflected condition, that is at least the force calculated by the equation: ##EQU7## where: P is the preload of said tension means, in pounds;

E_c is the modulus of elasticity of the ceramic material;

A_c is the cross-sectional area of a ceramic segment in a plane perpendicular to the composite length of said elongated ceramic element in square inches;

d is the maximum anticipated deflection of said elongated ceramic element, in inches;

h is the dimension of a ceramic segment in the plane perpendicular to the composite length of said elongated ceramic element and in alignment with the direction of said deflective force, in inches;

l is the overall length of said latter member;

α_s is the coefficient of thermal expansion of said tension means;

α_c is the coefficient of thermal expansion of said ceramic;

Δ_T is the degree of temperature change anticipated, in degrees F.;

A_s is the cross-sectional area of said tension means,

E_s is the modulus of elasticity of said tension means, and

L is the length of a ceramic segment, in inches,

but less than the amount of preload force which will compress said ceramic to over about one-half of its maximum compressive strength,

whereby loading forces exerted upon said elongated ceramic element are directed thereagainst in a direction substantially perpendicular to the longitudinal dimension thereof and deflection of said elongated ceramic element pursuant to such loading forces is compensated for in said compressed segments by further compression of said segments in those portions of the abutting faces thereof disposed along the inside of the line of curvature of said elongated ceramic element and by relief of less than all of the compression in those portions of said abutting faces that are disposed along the outside of said line of curvature of said elongated ceramic element without physical separation of said segments at their abutting faces.

2. An elongated ceramic element in accordance with claim 1 wherein said preload force does not exceed that preload force which will compress said segments to greater than about 20% of their maximum compressive strength.

3. An elongated ceramic element in accordance with claim 1 wherein said tension means forcing said segments toward each other is a nonceramic material.

4. An elongated ceramic element in accordance with claim 1 wherein said ceramic comprises alumina.

5. An elongated ceramic element in accordance with claim 4 wherein said alumina has a purity of greater than about 85%.

6. An elongated ceramic element in accordance with claim 1 wherein said ceramic segments are substantially identical to one another and each has an opening extending between its opposite flat and parallel surfaces, said openings in said segments being in register and said tension means extending therethrough.

7. An elongated ceramic element in accordance with claim 1 wherein each of the abutting flat faces of said ceramic segments is flat to within about 0.0002 inches.