Abstract
The present disclosure relates to a field device of automation technology, including a housing with an opening for accommodating field device components, wherein the housing has at an edge of the opening a first screw thread, a lid for closing the opening of the housing, wherein the lid has at an edge of the lid a second screw thread, wherein the first and second screw threads are embodied to be complementary to one another, so that the housing after closing with the lid in assembly fulfills all Ex-d requirements, wherein the first and second screw threads are embodied as multi-start threads.
Claims
1. A field device of automation technology, comprising: a housing configured to accommodate field device components, wherein the housing includes an opening in a side of the housing and, at an edge of the opening, a first screw thread in an axial direction; and a lid configured to close the opening of the housing in assembly with the housing, wherein the lid includes a side wall defining a second screw thread in the axial direction from an edge of the lid, wherein the first screw thread and the second screw thread are complementary to each other and define a gap length in the axial direction having a flame propagation resistance over the gap length such that the assembly of the housing and lid meet the requirements for explosion protection by explosion-proof and/or flameproof encapsulation as defined by an Ex-d industry standard, and wherein the first screw thread and the second screw thread are each configured as multi-start threads having two or more intertwined threads running in parallel to one another.
2. The field device of claim 1, wherein the first screw thread and the second screw thread are each embodied as four-start threads.
3. The field device of claim 1, wherein the first screw thread and the second screw thread have axially at least five thread crests.
4. The field device of claim 1, wherein the first screw thread and the second screw thread have axially at least eight thread crests.
5. A lid-closing mechanism of a field device of automation technology, comprising: a housing configured to house field device components, the housing including an opening in a housing wall, the opening having an edge from which a first screw thread extends along the wall in an axial direction; and a lid adapted to close the opening of the housing in assembly with the housing, the lid including a side wall having an edge from which a second screw thread extends along the side wall in the axial direction, wherein the first screw thread and the second screw thread are complementary and define a gap length of the opening such that, when the opening is closed by the lid, the field device fulfills requirements of flame propagation resistance over the gap length according to requirements for explosion protection by explosion-proof and/or flameproof encapsulation as defined in Ex-d industry standard IEC 60079-1, and wherein the first screw thread and the second screw thread are each multi-start screw threads having two or more intertwined threads running in parallel to one another.
6. The lid-closing mechanism of claim 5, wherein the first screw thread and the second screw thread are embodied as four-start threads.
7. The lid-closing mechanism of claim 5, wherein the first screw thread and the second screw thread have axially at least five thread crests.
8. The lid-closing mechanism of claim 5, wherein the first screw thread and the second screw thread have axially at least eight thread crests.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:
[0015] FIG. 1 shows a longitudinal section of a housing of a field device of automation technology according to the state of the art;
[0016] FIG. 2 shows a front view of a lid of the invention with four screw threads extending in parallel to form a four-start screw thread for a housing with a four-start screw thread; and
[0017] FIG. 3 shows a front view of a lid and cross-section of a housing with multi-start with two starts.
DETAILED DESCRIPTION
[0018] FIG. 1 shows a longitudinal section of a field device 1 of automation technology according to the state of the art. The field device 1 includes a housing 2 and a lid 6. The housing 2 and the lid 6 are each pot-shaped. The housing 2 is embodied to accommodate field device components 4 and includes, for this purpose, an opening 3. The housing 2 includes in an outer edge region that defines the opening 3. The housing 2 further includes a first screw thread 5 in the outer edge region. The lid 6 includes a second screw thread 7 in an outer edge region that defines an opening of the lid 6. The first screw thread 5 and second screw thread 7 are both single-start threads and are complementary to one another. The housing 2 and the lid 6 may be configured such that, when assembled together, conventional field device 1 may meet all explosion-proof, flameproof requirements. However, because the first screw thread 5 and the second screw thread 7 are both single-start threads, relatively many turns (e.g., revolutions) of the lid 6 to the housing 2 are needed to provide suitable flame penetration resistance over the gap length formed by the housing/lid combination to meet the Ex-d requirement. Consequently, many revolutions of the lid 6 are required to assemble the lid 6 to the housing 2 such that the assembly meets the Ex-d requirements.
[0019] FIG. 2 shows a front view of a lid 16 for a housing 12 of the present disclosure. The lid 16 includes a lid screw thread 17 configured as a multi-start thread and having an axial thread length 11. A multi-start thread consists of two or more intertwined threads running parallel to one another. Intertwining threads allow the lead distance of a thread to be increased without changing its pitch. A double-start thread will have a lead distance twice that of a single-start thread of the same pitch, a triple-start thread will have a lead distance three times longer than a single-start thread of the same pitch, and so on. By maintaining a constant pitch, a depth of the thread, measured from crest to root, will also remain constant. This geometry enables multi-start threads to maintain a shallow thread depth relative to their longer lead distance. Another advantage of a multi-start thread is that more contact surface is engaged in a single thread rotation.
[0020] The multi-start thread of the lid screw thread 17 may be a four-start thread 8, as shown in the detail view of FIG. 2. In such an embodiment, the lid screw thread 17 may have at least eight thread crests over the axial thread length 11. In another embodiment, the axial length 11 may be configured such that the lid screw thread 17 may have at least five thread crests over the axial thread length 11. In alternative embodiments, a lid screw thread 17 may have more or fewer starts, for example, eight starts.
[0021] FIG. 3 shows an exploded view with front view of an embodiment of the lid 16 and a cross-sectional view of a housing 12, each embodied to meet the Ex-d industry requirements in assembly. An embodiment of the lid 16 may include the lid screw thread 17 with a two-start thread 9, as shown in the detail. Such an embodiment of the lid screw thread 17 is complementary to a housing screw thread 15 of a housing 12 formed at an edge of an opening 13 in the housing 12, which housing screw thread 15 is also multi-start. Being complementary to the lid screw thread 17, the housing screw thread 15 may also be a two-start thread. In alternative embodiments, the housing screw thread 15 and the lid screw thread 17 may have more starts yet remain complementary.
[0022] In an embodiemnt, the lid screw thread 17 of the lid 16 and the complementary housing screw thread 15 of the housing 12 may have axially at least five thread crests 10. When the housing 12 is assembled to the lid 16, thereby closing the opening 13, the housing screw thread 15 and lid screw thread 17 define a gap length 18, as shown in FIG. 3. In such assembly, the gap length 18 is configured to provide adequate flame penetration resistance to meet the Ex-d requirements, thereby forming an explosion-proof, flameproof encapsulation for the field device. In at least one embodiment, the pitch of complementary the housing screw thread 15 and lid screw thread 17 may be greater than conventional Ex-d housing assemblies, such as those shown in FIG. 1. By running individual thread starts in parallel with increased pitch, the number of revolutions needed to assemble the housing 12 to the lid 16 to form the explosion-proof, flameproof encapsulation is reduced, thus enabling a faster assembly operation with fewer revolutions. Further, the increased slope of the housing screw thread 15 and lid screw thread 17 at equal tightening torque generates less pre-stress in the assembly joint, which, in turn, allows the lid 16 to be opened more easily. At the same time, the gap length 18 for the flame penetration resistance, and for ignition sparks, may remain the same.
