An instrument with a removable end cover

Abstract: A cover lock assembly (20) for a process transmitter (10) having threadably removable end caps (14, 16) and corrugated outer surfaces (48) is provided for engaging the corrugated surfaces (48) by way of a toothed cover lock (44) which is removably affixed to the transmitter housing (12).

Description: The invention relates to an instrument with a removable end cover. The instrument may for example be a pressure transmitter with an end cover threaded for removal. Process instrumentation such as pressure transmitters are known wherein the end covers of the transmitter have outwardly projecting flanges having a bolt hole therein. The end cover is screwed onto the transmitter until tight and the bolt hole is aligned with a threaded bolt hole in the transmitter housing itself. A bolt is then passed through the flange and the bolt hole with the bolt threaded in thereby to lock the flange to the transmitter housing and thus lock the end cover to the same housing preventing the unthreading of the cover until the bolt is removed freeing the flange from the transmitter housing. Although this type of cover prevents the accidental unscrewing of the end caps, the bolt, when removed, is sometimes lost and the cover lock's purpose is thereby defeated. Similarly, the flange is difficult to align with the bolt hole causing either a loose fit between the housing and the end cap or in some cases an overturning of the end cap which may cause the threads on the housing to be stripped in an attempt to align the flange with the bolt hole. Note that the aluminium die casting alloy typically used in this industry for transmitter housings and end caps is indeed sensitive to stripping and galling due to overtorqueing. Other transmitter cover locks are known wherein each cover of the transmitter has a flange which is captured by a flat middle piece and is then screwed to the housing. Again, there is the problem of alignment of the cover lock with the locking piece as well as the possibility of losing the locking piece whenever the assembly is unscrewed. German Patent Application DE-A-29 37 641 discloses an instrument housing having a locking system including a retaining catch co-operating with an outer toothed region on an end cap of the housing. The retaining catch is in the form of a two-armed pivotable lever which acts as a pawl to prevent rotation in one direction. US Patent US-A-2 419 849 discloses a nut lock in which the nut has a knurled surface, and a keeper with a corresponding knurled face co-operates with the nut. The keeper and the housing have holes arranged in a way that allows different positions of the keeper to be selected if the keeper will not register with the knurls on the nut in one position. According to the invention there is provided an instrument having a main body with a rotatably removable end cover and a locking mechanism for locking the rotatably removable end cover to the main body, the locking mechanism comprising a) a circumferential corrugated surface on the rotatably removable end cover; b) a mounting base on the main body of the instrument proximate to the rotatably removable end cover; c) a cover lock mountable to the mounting base; d) means for placing the cover lock into an interfering position with the circumferential corrugated surface on the rotatably removable end cover to prevent rotation of the rotatably removable end cover; and e) means for selectively allowing movement of the cover lock with respect to the main body to align it with a desired portion of the circumferential corrugated surface of the rotatably removable end cover. Preferably the means for placing the cover lock into an interfering position comprises at least one tooth which fits into the circumferential corrugated surface to prevent rotation of the removable end cover. Advantageously the circumferential corrugated surface has a series of raised and valley portions and the means for placing the cover lock into an interfering position comprises a plurality of teeth sized to fit into adjoining valley portions of the circumferential corrugated surface. Preferably the mounting base is provided proximate to the removable end cover and the means for selectively allowing movement of the cover lock with respect to the main body comprises an annular opening along a front portion thereof having a captive screw therein for retaining the cover lock to the main body by affixing the screw into a bore in the main body. The instrument may include strap means for retaining the cover lock proximate to the mounting base whenever the cover lock is removed from the mounting base. The strap means preferably includes a flexible cord connected to a side of the mounting base at one end and to the top of the cover lock at the other end. The mounting base is preferably angled toward the corrugated surface of the end cover and the cover lock has a complementary surface located along the annular opening of the cover lock to guide the interfering end portion of the cover lock into the corrugated surface of the end cover whenever the cover lock is affixed to the mounting base. The invention can provide a transmitter end cap cover lock which is easily alignable with the transmitter end cap, which is easily removable from the transmitter end cap but which will not be easily lost. Another aspect of the preferred embodiment is to provide a circumferentially movable cover lock which is easily alignable with the corrugated end cover. The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Fig. 1 is a top view of a pressure transmitter end cap cover lock embodying the invention; Fig. 2 is an isometric view of the cover lock of Fig. 1 shown as being detached from the transmitter housing and the cover lock mounting base; Fig. 3 is a view of a cover lock strap or umbilical cord shown with respect to mounting areas on the cover lock and the cover lock mounting base; Fig. 4 is a bottom view of the cover lock showing flanged or teeth members used to engage the corrugated surface of the transmitter cover; and Fig. 5 is a side view of the cover lock as shown in Fig. 4. Pressure transmitters are used extensively in various process control environments wherein the atmosphere outside the transmitter could be hazardous and explosive. Accidental removal of the transmitter end covers would expose the electronics and terminals inside the transmitter to the explosive atmosphere and could cause an explosion. Thus, such transmitters must have a safe and secure locking mechanism to prevent the unwanted removal of the transmitter end covers from the transmitter. The preferred embodiment of the present invention provides such a secure cover lock as may be best seen with reference to Figs. 1 and 2. A pressure transmitter assembly 10 is shown to have an outer casing 12 which contains the transmitter components therein with these components being accessible by way of threaded end caps 14, 16. Each end cap 14, 16 has internal threads (not shown) which are complementary to the threads (not shown) on the transmitter outer casing 12. Each end cap 14, 16 may be unscrewed to allow access to the pressure transmitter electronic components located inside the outer casing 12. Along the edges of the outer casing 12, adjacent to the threaded end caps 14, 16 are located end cover mounting bases 18 to which a cover lock 20 is threadably attached to form a cover lock assembly 22. The cover lock assembly 22 has the end cover mounting base 18 formed as a substantially rectangular base with a triangular top portion comprising top surfaces 24, 26 along with a front end surface 28. These surfaces 24, 26 along with front end surface 28, are complementary to respective indentations 30, 32, 34 on the cover lock 20. The mounting base 18 also has a threaded hole 36 located at one end for mounting a strap or umbilical cord which will be described later. Also a threaded hole 40 is located on the surface 24 for mounting the cover lock 20 to the base 18. Surfaces 24, 26, 28 are designed to be at a substantially oblique angle to one another (as are similarly the mating surfaces of the cover lock 20) so that as the cover lock 20 rotates on these surfaces it is constrained against twisting away from the end cap. These surfaces allow the cover lock 20 only one degree of freedom of motion; that is, circumferential rotation about the housing. These surfaces are critical to the end cap 14, 16 locking ability of the overall lock assembly 22 because the single threaded fastener (for convenience and speed of operation) is not sufficient on its own to provide a locking means with integrity to resist end cap 14, 16 loosening influences due to vibration and unauthorized attempts at entry into the housing. The cover lock 20 allows an inherent break-away torque to be designed-in simply by making the cover lock 20 out of a more or less strong and stiff material. The break-away feature will allow the cover lock 20 to fail first in the event of severe untorqueing of the end cap without first loosening the cover lock 20. In this manner, more costly and difficult parts to replace (such as the end cap and transmitter housing) are protected. A best known embodiment is to make the cover lock 20 out of molded thermoplastic 30% glass. The cover lock 20 is thus designed to rotate circumferentially about the housing 12 in order to allow easy alignment to an end cap 14, 16 which has been screwed onto the housing 12 at a proper torque (and therefore a fixed position), and then the cover lock 20 can be threadably secured to the housing thereby locking the end cap 14, 16 to the housing. The cover lock 20 rotates circumferentially on housing pad surfaces 24, 26, 28 and in infinitely positionable on these surfaces with an elongated and curved center slot or annulus 38 to allow a single threaded fastener (for convenience) located on center on the housing to be sufficient for threadably securing the cover lock 20 to the housing regardless of cover lock 20 position. The cover lock 20 has an elongated annulus 38 formed through the top surfaces thereof for retaining a screw 42 therein in a captive manner. The screw 42 has threads compatible to those in the bore of the hole 40 in the mounting base 18 and is used to solidly affix the cover lock 20 to the base 18. The screw 42 may be retained to the cover lock 20 by means of a retaining washer threaded above the threaded portion of the screw 42 which will be located on the other side of the elongated annulus 38 to prevent the screw 42 from falling out of the annulus 38 while allowing it to be freely threaded into the bore 40. The front portion of the cover lock 20 has three tooth-like projections 44 which are used to lock the end covers 14, 16 in place once the cover lock 20 is firmly secured to the base 18. The toothed portions 44 and the retaining washer 46 may be best seen by reference to Figs. 4 and 5 of the drawings. To describe how the cover lock assembly 22 firmly locks the end covers 14, 16 in place it should first be noted that each of the end covers 14, 16 have a corrugated or dimpled area 48 extending along the circumference of each end cover 14, 16 proximate to the opposite end portions of the outer case 12. Although a wide range of pitches of corrugations on the end cap 14, 16 and corresponding cover lock 20 will work, a best known embodiment is suggested at an approximate pitch of 0.2" (5.08mm). Furthermore, in order to reduce the overall size of the lock assembly 22 to a minimum, (for down-sizing purposes) the range of circumferential motion of the cover lock can be limited to plus and minus 0.5 - 1.0 times the pitch distance without loss of function. A best known embodiment is suggested at +/- .6 times the pitch distance from a center position. This will require the cover lock 20 to be moved either to the left or to the right of center in order to engage the corrugations of the end cap depending on where the end cap 14, 16 came to rest when its own final tightening torque was reached. The mounting base 18 is located adjacent the corrugated or dimpled areas 48 but is affixed to the outer case 12. The cover lock 20 is mounted to the mounting base 18 by the screw 42 being threaded into the hole 40. This allows the teeth 44 on the front face of the cover lock 20 to engage the open or valley portions 50 of the dimpled area 48 located in between raised portions 52. Thus, it can be seen that the height of each tooth 44 on the cover lock 20 is approximately equal to the depth of the valley portions 50 adjacent to the elevated or raised portions 52 of the corrugated surface 48. Similarly, the width of each tooth 44 is approximately equal to the width between the raised portions 52. Thus, by means of the elongated annulus 38 on the cover lock 20, no matter where the threaded end cap 14, 16 is threadably located on the housing 12, it is a simple matter to capture three adjacent valley portions 50 on the end cap with the three toothed portions 44 of the cover lock 20 by screwing the cover lock 20 to the mounting base 18 to provide a firm engagement of the teeth 44 into the valley portion 50 of the corrugated surface of each end cap 14, 16. Should an operator desire to remove one of these end caps 14, 16, for example, to perform an internal adjustment of the transmitter assembly, he need only unscrew the screw 42 until the teeth 44 are loose enough to clear the raised 52 and valley portions 50 forming the corrugated circumference 48 on each end cap 14, 16. When this is accomplished, each end cap 14, 16 is free to be rotated and removed. When the operator is through with the internal adjustment of the transmitter assembly 10, the procedure is reversed by threading the end cap 14, 16 back on to the outer case 12 and then screwing down the cover lock 20 to the mounting base 18 to engage the threaded or toothed portions 44 with the valley portions 50 of each end cover 14, 16. As was mentioned earlier, the screw 42 is retained to the cover lock 20 by way of the washer 46 located on the inside portion of the cover lock as may be best seen with reference to Fig. 4. Thus, even if the cover lock 20 is fully removed from the base 18 the screw 42 will be retained thereto and will not be lost when the cover lock 20 must be again re-engaged. To prevent the cover lock 20 from being entirely lost, a strap or umbilical cord assembly 54 is provided which may be best seen with reference to Fig. 3. The strap assembly 54 is comprised of a retaining strap 56 made of a polypropylene or similarly flexible plastic material which has formed openings 58, 60 at opposite ends of the strap 56. One end of the strap 56 is affixed to the cover lock 20 by screwing the strap 56 to an embossed area 62 located on the top surface of the cover lock 20 by way of a screw 64. A washer 66 may be utilized to prevent the removal of the screw 64 by vibrations. Similarly, the other end of the strap 60 is mounted to the mounting base 18 by screwing the end 60 of the strap 56 to the hole 36 by way of a screw 68 and washer 70. This keeps the cover lock affixed to the transmitter housing 12 even in situations where the cover lock is completely removed from its base to allow the easy removal of an end cap 14, 16. Certain modifications and improvements have been deleted herein for the sake of conciseness and readability. However, it will be understood that all such modifications and improvements are intended to be fully covered within the scope of the appended claims. By way of example and not limitation, the strap or umbilical cord assembly 54 could be made integral with one or both of either the cover lock 20 and/or mounting base 18, and molded in a single operation. Due consideration would be given, of course, to the material for such a cord so that excessive bending of same would not cause breakage, leading to the separation of the cover lock 20 from the housing 12.

Claims: An instrument having a main body (12) with a rotatably removable end cover (14) and a locking mechanism for locking the rotatably removable end cover (14) to the main body (12), the locking mechanism comprising a) a circumferential corrugated surface (48) on the rotatably removable end cover (14); b) a mounting base (18) on the main body (12) of the instrument proximate to the rotatably removable end cover (14); c) a cover lock (20) mountable to the mounting base (18); d) means (44) for placing the cover lock (20) into an interfering position with the circumferential corrugated surface (48) on the rotatably removable end cover (14) to prevent rotation of the rotatably removable end cover (14); and e) means (38) for selectively allowing movement of the cover lock (20) with respect to the main body (12) to align it with a desired portion of the circumferential corrugated surface (48) of the rotatably removable end cover (14). An instrument according to claim 1, wherein the means for placing the cover lock (20) into an interfering position comprises at least one tooth (44) which fits into the circumferential corrugated surface (48) to prevent rotation of the removable end cover (14). An instrument according to claim 2, wherein the circumferential corrugated surface (48) has a series of raised and valley portions (52, 50) and the means for placing the cover lock (20) into an interfering position comprises a plurality of teeth (44) sized to fit into adjoining valley portions (50) of the circumferential corrugated surface (48). An instrument according to claim 1, claim 2 or claim 3, wherein the mounting base (18) is provided proximate to the removable end cover (14) and the means for selectively allowing movement of the cover lock (20) with respect to the main body (12) comprises an annular opening (38) along a front portion thereof having a captive screw (42) therein for retaining the cover lock (20) to the main body (12) by affixing the screw (42) into a bore (40) in the main body (12). An instrument according to any one of claims 1 to 4, including strap means (54) for retaining the cover lock (20) proximate to the mounting base (18) whenever the cover lock (20) is removed from the mounting base (18). An instrument according to claim 5, wherein the strap means (54) includes a flexible cord (56) connected to a side of the mounting base (18) at one end and to the top of the cover lock (20) at the other end. An instrument according to claim 4, wherein the mounting base (18) is angled toward the corrugated surface (48) of the end cover (14) and the cover lock (20) has a complementary surface (30) located along the annular opening (38) of the cover lock (20) to guide the interfering end portion of the cover lock (20) into the corrugated surface (48) of the end cover (14) whenever the cover lock (20) is affixed to the mounting base (18).

Inventors: ALDEN JEROME S, ALDEN, JEROME S.
Applicants: INT CONTROL AUTOMATION FINANCE, INTERNATIONAL CONTROL AUTOMATION FINANCE S.A.
Application year: 1993
Main classification: G01D11
IPC codes: B65D55, E05B65, G01D11