US3741028A

US3741028A - Self grounding mechanical span adjustment

Info

Publication number: US3741028A
Application number: US00221992A
Authority: US
Inventors: J White
Original assignee: Sybron Corp
Current assignee: ABB Automation Inc
Priority date: 1972-01-31
Filing date: 1972-01-31
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26
Also published as: GB1361954A; ZA7360B; FR2170003B1; IT977040B; DE2304706B2; DE2304706A1; AU5119773A; FR2170003A1; NL7301036A; DE2304706C3; CA964486A

Abstract

Disclosed is a mechanical span adjustment for a force balance variable condition transmitter, having an improved mechanical ground provided by a flat spring member which urges the fulcrum of the adjustable span against the body of the instrument, the spring being deformed beyond its yield point, so that it urges the fulcrum against the body of the instrument with a relatively constant force.

Description

United States Patent 11 1 1 1 3,741,028 White June 26, 1973 [54] SELF-GROUNDING MECHANICAL SPAN 2,765,543 10/1956 Crowell et al 74/424.8 R ADJUSTMENT 2,385,090 9/1945 Lowy 74/424.8 R 3,402,613 9/1968 Neusel et al.... 74/89.l5 [75] Inventor: Jack Mort White, North Chili, NY. 3,407,018 10/1968 Miller 74/89.15 [73] Assignee: Sybron Corporation of Rochester,

Rochester Primary ExaminerCharles J. Myhre I Assistant ExaminerWesley S. Ratliff, Jr. [22] Flled: 1972 Attorney-Theodore B. Roessel [21] Appl. No.: 221,992

[57] ABSTRACT [52] U.S. CI 74/409, 74/89, 74/15, Disclosed is a mechanical span adjustment for a force 741/4243 R balance variable condition transmitter, having an im- [51] Int-Cl. F16h 55/18 pr mechanical ground provided by a flat spring [58] Field of Search 74/89. 1 5, 424.8, member h ch urges the fulcrum 0f the adjustable span 74/409; 177 /17() 230 236 against the body of the instrument, the spring being deformed beyond its yield point, so that it urges the ful- [56] References Cit d crum against the body of the instrument with a rela- UNITED STATES PATENTS tively wmmnt fmce- 2,548,807 4/1951 Morgan et a1. 74/41 9 Claims, 3 Drawing Figures PAIENIEU M26 I975 FIG. '1

FIG. 3

FIG. 2

SELF-GROUNDING MECHANICAL SPAN ADJUSTMENT BACKGROUND OF THE INVENTION The present invention relates to a force balance mechanism, having an adjustable span. Such instruments are fairly common in the process control arts and a typical instrument of this type is illustrated, for example, in U.S. Pat. No. 3,396,374. Briefly, such an instrument is used to convert a variable condition, as, for example, the difference between two fluid pressures, into a first mechanical force. This first force is then balanced by a second mechanical force wherein the balance is accomplished by having the mechanical forces opposed to each other, via a mechanical linkage which executes relatively minor changes in its configuration. With this arrangement, any change in the variable condition is detected as the change in the second mechanical force is required to restore the balance condition. In order to provide adjustment for the span of the instrument, the mechanical linkage has one member pivoted on an adjustable fulcrum, which is attached as rigidly as possible to the body of the instrument, or ground.

In the usual case, the adjustable fulcrum is fixed to a span nut, which in turn is driven by a lead or span screw, wherein the position of the fulcrum is adjusted by rotation of the screw. Since the screw is joumaled to the body of the instrument, or ground, and since the engagement of the screw and nut provides for rotation of the screw, the connection of the fulcrum to the body of the instrument is not completely rigid andthere is play between the various components of the instrument. Because the connection is not rigid, relative motion can take place between the adjustable fulcrum and the body of the instrument, especially when the span screw is rotated. This leads to repeatable or non repeatable erros which'affect the accuracy of the instrument. For example, if the screw moves relative to the body when the'span is adjusted and does not return to its initial or ground position, a greater than normal force applied to the fulcrum, say'during an over-ranging of the instrument,will move the screw back ot its initial position. Thus, if the fulcrum is not able to support the excessive loading without extraneous motion, either repeatable or non repeatable, undesirable errors occur. The patent mentioned above, U.S. Pat. No. 3,396,374, provides one novel means wherein the normal configuration of the instrument, though subject to misalignment, over-range, etc., is more or less automatically restored and wherein the over-range does not damage the mechanism. The present invention provides a method of maintaining the span adjustment mechanism grounded against the body of the instrument and in its normal configuration even when span adjustemtns are made to offset the effects of over-range forces.

SUMMARY OF THE INVENTION The present invention provides a force balance differential pressure transmitter wherein force due to differential pressure exerts a moment on a deflectable bar counterbalanced by an opposing moment exerted on the bar. The fulcrum of the bar is adjustably fixed to the body of the instrument or ground" by placing the fulcrum on a nut, the nut being driven by a screw journaled to the instrument body by a V bearing. A flat spring is positioned between the instrument body and the nut, and is attached to the nut for movement therewith. The force of attachment of the spring to the nut deflects the flat spring beyond its yield point, so that the force generated by deflecting the spring urges the nut and screw against the V bearing with a relatively constant force, the force being transferred to the various parts of the mechanism to force the span screw into the V bearing and the span nut against the span screw. The force of the spring thus provides a simple, direct method of applying a constant relatively large spring force to urge the parts together and in a manner similar to the force applied during calibration or an overrange.

OBJECTS OF THE INVENTION One object of the present invention is to provide a process invariable transmitter as aforesaid, wherein the effect of overrange forces on the transmitter is effectively obviated.

Another object of the invention is to provide a force balance instrument having an adjustable fulcrum wherein the fulcrum and its carrier are'urged against the body of the instrument with a relatively constant spring force.

Still another object of the invention is to provide a force balance instrument having an adjustable fulcrum, wherein a single spring is used to urge both the fulcrum and its carrier against the body of the instrument.

A further object of the invention is to provide a force balance instrument having an adjustable fulcrum wherein the ground of the instrument is maintained during span adjustment and which does not rely on calibration or an over-range condition to restore the ground condition.

A still further object of the invention is to provide a force balance instrument having an adjustable fulcrum which is easier to assemble, eliminates parts, is less costly to manufacture and reliable in operation.

These and other objects, advantages and characterizing features of the present invention will become more apparent upon consideration of the following detailed description thereof, when taken in connection with the accompanying drawings depicting" the same;

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view taken along lines 1-1 of FIG. 3, showing a portion of the force balance instrument;

FIG. 2 is a view taken along lines 22 of FIG. 1; and

FIG. 3 is a top view partly broken away and in section of a portion of the force balance instrument, taken along lines 3-3 of FIG. 2. I

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, FIG. 1 shows a portion of a force balance instrument with an adjustable fulcrum, generally indicated at 10. The instrument basically includes a lever 12, supported on a fulcrum 14. An input force F1 is applied to one end of the lever arm. This input force could, for example, be directly proportioned to a variable condition, such as the difference between two fluid pressures, and can be applied to the lever arm by any conventional means, such as a bellows arrangement or, asisusually the case, by a force beam. Such a beam is illustrated generally in U.S. Pat. Nos. 2,781,665 and 3,600,952. This'input force is then balanced by a mechanical force F2, which can be generated, for example, by a feedback motor. As the force Fl changes, a relatively sensitive detecting device (not shown) detects the changes and creates an opposing change in the balancing force F2, which substantially exactly neutralizes the original change that gave rise to the opposing change. Thus, any changes in the variable condition giving rise to force F1 can be detected and measured by the amount of the change in the opposing balancing force F2 required to neutralize the original change.

In order to adjust the span of the instrument, that is the proportion between F1 and F2, fulcrum 14 is adjustable and for this purpose is carried by a nut 16 driven by a screw 18. The screw is in turn supported in a housing 20 and can be rotated by turning the head 22 of the screw, which is calibrated, for purposes of setting the desired span. Such instruments as herein described are quite common in the art and are illustrated in various United States Patents, including Nos. 3,297,045 and 3,396,374 among others.

As shown in FIGS. 1 and 3, the span screw 18 is journaled at its end in V bearings 24, carried by the top and bottom of housing 20. Suitable lock and

flat washers

26, 28 at the top and bottom respectively prevent vertical motion of the span screw with respect to the housing. Because the span screw is journaled in V bearings, it should be appreciated that horizontal movement of the span screw is possible during span adjustment and overrange conditions as set out hereinbelow.

Fulcrum member 14 is simply a blade having a slightly rounded forward edge. The span lever 12 is pivoted on the blade with one end 30 of the lever being connected by a suitable linkage 32 to the primary of the instrument (not shown), which exerts force F1 on this end of the span lever. The outer end 34 of the span lever is connected by a suitable linkage 36 to a feedback motor (not shown), which exerts balancing force F2. It should be appreciated that both forces F1 and F2 are transmitted to the fulcrum 14 and from there to the span nut, span screw and housing.

Also shown in FIG. 2, a bracket 38 extends outwardly from the span nut and through a slot 40 in the side wall of the housing. The bracket allows the span nut to be driven vertically as the span screw is turned and provides a means for indicating the span setting wherein the end 42 of the bracket acts as a pointer for a scale (not shown) on the housing alongside the slot. Thumbscrew 22 at the top of the housing provides means for rotating the span screw to adjust the position of the fulcrum and bracket.

It should be appreciated that because the various components are capable of relative movement, there is the possibility of any extraneous motion producing repeatable or non repeatable, undesired errors in the readout. Referring to FIG. 3, for example, it should be noted that the span nut moves vertically only when bracket 38 engages a stop formed by one edge or another of slot 40. If the span screw is rotated, say, clockwise for adjustment of the span nut and thereafter rotated counterclockwise for readjustment, a small error will be present in the span setting. This error is caused by the span nut rotating with the span screw until bracket 38 moves from against one edge of slot 40 to the other. One method of reducing this error is to maintain close tolerances between the width of bracket end 42 and the width of slot 40. However, high tolerances add to both manufacturing time and expense.

Because the span screw is journaled in V-bearings, it is possible, during rotation of the screw, that either the top or the bottom of the span screw will ride up a side 46 of the V-bearings and move one end of the screw horizontally with respect to the other. Thereafter, during the operation of the instrument, the parts may slip back to their original position in the V-bearings and in firm contact with the housing only after a calibration run, or over-ranging, that is when greater than normal forces are applied to the fulcrum. Since even small movements of the span screw and nut will change the position of the fulcrum, subsequent readings wouldinclude a small percentage of error.

The present invention, as described hereinbelow, maintains bracket end 42 stopped against one edge of slot 40 regardless of the direction of rotation of the span screw. The invention also provides a simple, direct method of applying a constant, relatively large force to urge the nut, screw and housing together in a manner similar to the calibration or over-range forces to eliminate or reduce the possibility of extraneous motion between these parts during adjustment.

A flat spring 48 is utilized to provide this force. One end of the spring is fixed to the span nut by a retaining screw 50. The other end 52 of the spring (FIGS. 2 and 3) extends outward from the span nut and through a slot 54 in the housing. Thus, the spring exerts a force between the housing and the span nut for urging the nut against the span screw and the span screw against the housing. During assembly, all parts except spring 48 are assembled. The spring is then put into position with end 52 extending through slot 54 and spring retaining screw 50 tightened. The front surface 56 of the nut extends slightly beyond the plane of slot 54, so that the spring becomes bowed as the retaining screw is tightened. This causes spring 48 to act as a lever with the fulcrum being at one edge 58 of the span nut. As retaining screw 50 is tightened further to draw the spring and nut flush together, this lever action exerts a torque onspan nut 16 which causes the span nut to rotate clockwise on span screw 18, until the end 42 of bracket 38 is stopped against the edge of slot 40 as shown in FIG. 3. Further tightening of the retaining screw defelcts flat spring 48 as shown in FIG. 3 to exert the force which holds the span nut, screw and housing firmly together.

It is desirable that the spring force should be as high as possible but not so great as to result in difficulty in turning the span adjustment thumbscrew 22 or excessive wear at the bearing point of the V-bearings 24. In order to limit the spring force, the nut and spring are designed so that when the retaining screw 50 is tightened an amount sufficient to hold the spring flush against the nut, the spring will be deflected to or beyond its yield point. This ensures that the force generated by the deflecting spring will remain relatively constant regardless of any increase in the deflection of the spring. For example, if slot 54 is not parallel to the axis of span screw 18, spring member 50 may deflect more or less as the spring travels alongthe slot during adjustments of the span. If the spring is purposely deformed beyond its yield point during assembly, any changes in deflection of the spring during operation should not substantially change the force exerted by the spring. For the same reason, even though the accumulated errors in manufacturing tolerances may be different for each instrument, the force exerted by a spring 48 in similar instruments should be substantially the same.

Thus, with one piece of structure, namely spring member 50, the span nut is urged against the span screw and the span screw is urged against the housing of the instrument with a relatively constant force, similar in direction to the application of an over-range force. Spring 50 also exerts sufficient torque on the span nut to maintain bracket end 42 engaged against one edge of slot 40 as shown in FIG. 3, regardless of the .direction in which the span screw is turned. This eliminates the need for maintaining close tolerances between the bracket and slot and reduces manufacturing cost. Deforming spring 50 beyond its yield point not only ensures the application of a substantially constant force against the screw and nut over the range of the instrument, but also ensures that a substantially similar force will be exerted by similar springs in instrument assemblies of similar design, regardless of accumulated manufacturing errors. Since the spring member has its end 52 riding in groove 54, the spring is able to move vertically with the span nut to maintain the same force regardless of the position of the nut with respect to the screw.

Having thus described the invention, what is claimed as new is:

1. In an adjustable force balance instrument having an instrument body, an adjustable fulcrum support carried by the body, a fulcrum fixed to the support, a lever pivoted on the fulcrum wherein a force applied to one end of the lever is balanced by a force to the other end, the improvement being means to ground the fulcrum against the body of the instrument comprising a spring member between said fulcrum support and said instrument body exerting a force directly on said fulcrum support, for urging the same in a direction normal to the direction of adjustment and against said instrument body.

2. The invention as set forth in claim 1, wherein said spring member is a flat spring with one end fixed to said fulcrum support and another .end slidably engaged with said instrument body so that said spring moves with said fulcrum support as the same is adjusted with respect to said instrument body.

3. The invention as set forth in claim 2, wherein said flat spring between said fulcrum support and instrument body is deformed beyond its yield point so that the force exerted by said flat spring remains substantially constant regardless of changes in the deformation of said spring as said fulcrum support is adjusted.

4. The invention as set forth in claim 1, wherein said adjustable fulcrum support includes a screw journaled to said housing, a nut on said screw, a bracket engaged against a stop on said housing to prevent rotation of said nut as said screw is turned, the improvement comprising said spring member being arranged between said nut and instrument body to exert a torque on said nut for maintaining said bracket engaged against said stop as said screw is turned clockwise or counterclockwise.

5. In a force balance instrument having an instrument body with upper and lower V-bearings, a screw journaled at its ends in the bearings, a nut on the screw, a fulcrum fixed to the nut whereby rotation of the screw moves the fulcrum axially with respect to the screw to adjust the span of the instrument, a lever pivoted on the fulcrum whereby a first force applied to one end of the lever is balanced by a second force applied to the other end, the improvement being mechanical means to ground the fulcrum against the instrument body comprising a flat spring having one end fixed to said nut and its second end extending out from said nut to slidably engage said instrument body, said spring member exerting a force between said nut and body directed normal to the axis of said screw to urge said nut against said screw and to seat said screw in said V-bearings whereby extraneous movement of said nut with respect to said screw and said screw with respect to said instrument body is prevented.

6. The invention as set forth in claim 5, wherein said instrument body has a slot therein extending parallel to said screw for substantially the full span of the adjustment, said spring member having its second end extending through said slot and engaging one edge thereof so that said spring moves with said nut and maintains engagement with said instrument body over the full span of adjustment.

7. The invention as set forth in claim 6, wherein said flat spring is deformed by its engagement between said nut and slot edge beyond the yield point of said spring, whereby the force exerted by said spring to hold said nut against said screw and said screw against said V- bearings remains substantially constant regardlessof the magnitude of said first and second forces acting on said-fulcrum.

8. The'invention as set forth in claim 5, wherein said nut has a-bracket member engaging stop member on said instrument body to prevent rotation of said nut, said spring member being constructed and arranged to exert a torque on said nut to maintain said bracket engaged against said stop as said screw is rotated clockwise or counterclockwise.

9. The invention as set forth in claim 8, wherein said stop means is an elongated slot in said housing receiving one end of said bracket, said spring member exerting a torque on said nut to maintain said bracket engaged against one edge of said slot as said screw is rotated clockwise or counterclockwise.

Claims

2. The invention as set forth in claim 1, wherein said spring member is a flat spring with one end fixed to said fulcrum support and another end slidably engaged with said instrument body so that said spring moves with said fulcrum support as the same is adjusted with respect to said instrument body.

7. The invention as set forth in claim 6, wherein said flat spring is deformed by its engagement between said nut and slot edge beyond the yield point of said spring, whereby the force exerted by said spring to hold said nut against said screw and said screw against said V-bearings remains substantially constant regardless of the magnitude of said first and second forces acting on said fulcrum.

8. The invention as set forth in claim 5, wherein said nut has a bracket member engaging stop member on said instrument body to prevent rotation of said nut, said spring member being constructed and arranged to exert a torque on said nut to maintain said bracket engaged against said stop as said screw is rotated clockwise or counterclockwise.