US3364318A

US3364318A - Environment-proof limit switch

Info

Publication number: US3364318A
Application number: US481239A
Authority: US
Inventors: Leander J Bulliet
Original assignee: Babcock and Wilcox Co
Current assignee: Acme Precision Products Inc
Priority date: 1965-08-20
Filing date: 1965-08-20
Publication date: 1968-01-16
Anticipated expiration: 1985-01-16
Also published as: GB1110487A; DE1590142B1

Description

L. J. BULLIET Jan. 16, 1968 ENVIRONMENT, PROOF LIMIT SWITCH 4 Sheets-Sheet 1 Filed Aug. 20, 1965 (ENao mvzzmoa Leander J. Bullief,

T RNE ,1968 L. J. BULLIET ENVIRONMENT, PROOF LIMIT SWITCH 4 Sheets-Sheet 2 Filed Aug. 20, 1965 'FIG.5

L. J. BU LLIET ENVIRONMENT, PROOF LIMIT SWITCH 4 Sheets-Sheet 5 Filed Aug. 20, 1965 FIG. 8

FIG. 9

j MA

FIG. 11

, A 4 5 2 A A O A A O m m m m mm m 3 r: 4 o u V a ,A a o ,6 HH vr A A Tm Q m), 4 U 3 z A I 1 A\ A E w A 6 1 A 4 M A '1 M F6 5 6 ma w .1 {K Mg MU O O flfl 8 f A 3 A 2 4 A8 A A w 1 mm m w m m Jan. 16, 1968 L3. BULLIET 3,364,318

ENVIRONMENT, PROOF LIMIT SWITCH Filed Aug. 20, 1965 4 Sheets-Sheet 4- FIG. 14

.2 16A FIG.12 FIG1 A United States Patent 3,364,318 ENVIRGNMENT-PROOF LIMIT SWITCH Leander .I. Bulliet, Rockford, Ill., assignor to The Bahcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Aug. 20, I965, Ser. No. 481,239 Claims. (Cl. 200-47) This invention pertains to a novel switch, and more particularly to a limit switch constructed for highly accurate reproductive operation and particularly immune to malfunction due to the presence of a wide variety of adverse environmental conditions.

Limit switches have been characterized by limitations in many types of service, particularly when subject to the presence of certain liquid, vaporous or dust-like contaminants of either electrically conductive or electrically insulating nature. Under such circumstances the reproductive accuracy and the ultimate life of the switch has been unsatisfactory. Many attempts have been made to overcome the malfunctions of limit switches under diflicult' environment conditions, but all reasonably successful constructions have been prohibitively expensive, and thus generally impractical.

In the present invention, a limit switch is provided which is generally immune to the environmental difiiculties encountered in service. This is accomplished by utilizing a magnetic type switch with the electrical contacts thereof completely and permanently sealed against contamination by foreign materials. Moreover the external electrical connections of the limit switch are sealed after installation so that they can easily be unsealed for removal or replace ment of a mechanically worn or damaged switching component. The novel switch is further provided with a suitably rugged housing or enclosure to protect the electrical elements against mechanical damage and relatively large contaminants such a chips and shavings or the like. The switch is characterized by a high degree of reproductive accuracy of operation through an extended life, ease of installation without excessive manufacturing expenses.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific object attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a front view of one preferred embodiment of the invention;

FIG. 2 is a side view of the encapsulated electrical elements of FIG. 1;

FIG. 3 is a bottom view of the exterior only of the' electrical encapsulation of FIG. 2;

FIG. 4 is a rear view of the assembly shown in FIG. 1;

FIG. 5 is a sectional side view taken substantially along the section line 5-5 of FIG. 4;

FIG. 6 is an enlarged fragmentary view taken as indicated by 66 in FIG. 5;

FIG. 7 is an exploded isometric view of some of the parts shown in FIGS. 4 and 5;

FIG. 8 is a front view of a modified assembly embodying the invention;

FIG. 9 is a side view of the encapsulated parts of FIG. 8;

FIG. 10 is a bottom view of the outside only of the encapsulation shown in FIG. 9;

FIG. 11 is an isometric view, partly in section, of the encapsulation of FIG. 9;

FIG. 12 is an electrical circuit diagram pertaining to electrical parts shown in FIG. 8;

FIG. 12A is an alternate showing of a portion of FIG. 12;

FIG. 13 is a rear view of the assembly of FIG. 8; and

FIG. 14 is a sectional side view taken substantially along 1414 of FIG. 13.

In FIG. 1, numeral 2 indicates a switching element known in the art as a reed switch. Reed switch 2 includes two flexible ferromagnetic leaves or

reeds

4 and 6 which are sealed into opposite ends, respectively, of a glass envelope 8. The

reeds

4 and 6 are of such length that they overlap axially at about the longitudinal center of the glass envelope. The

reeds

4 and 6 are formed fiat throughout the area of overlap and for some distance beyond this area. However, at the ends, these reeds are circular in cross-section for facility in sealing the glass envelope.

In the area of overlap, the

reeds

4 and 6 are overlaid with a thin film of highly conductive material (usually gold) which assures good conductivity when the reeds are in contact one with the other. Before final sealing, the envelope 8 is evacuated and the air is replaced with a gas which is conductive to eflicient arc quenching and does not promote oxidation or other chemical contamination of the contacting surfaces.

As seen in FIG. 1, the overlap portions of

reeds

4 and 6 are in contact with one another. This is due to the presence of the permanent magnet 10 in relatively close proximity to the reed switch. The poles of magnet 10 are disposed on either side of the area of overlap of the reeds so that a portion of the magnetic lines of force are conducted through the reeds and the area of contact thereof. This magnetic force holds the reeds in mutual contact.

When the magnet is shifted to the left, as shown by the dotted lines in FIG. 1, the poles of magnet 10 no longer embrace the overlap area of the reeds. In this latter case, there are no lines of force tending to hold the reeds in contact.

In the original sealing process, the reeds are so positioned that there is a gap between them in the overlap area. When magnet 10 is placed in the solid line position of FIG. 1, magnetic lines of force appear across the gap between the reeds and create an attractive force. The reeds yield to this force and the contact is established by slight bending of the reeds. When magnet 10 is moved from its effective position, the reeds spring apart reestablishing the original gap.

Thus, if magnet 10 is moved back and forth between its solid and dotted line positions (FIG. 1), the contact between

reeds

4 and 6 is alternately established and broken.

Shown also in FIG. 1 is a second reed switch 12. Reed switch 12 is located behind switch 2 in FIG. 1 and so is shown partly broken away. The position of switch 12 is such that the magnet 10 is effective to close its contacts when in the dotted line position. It is to be understood that the magnet 10 has sufiicient depth (perpendicular to the plane of FIG. 1) to be effective on the reeds of both

reed switch

2 and 12.

It is now clear that moving magnet 10 back and forth between its solid and dotted line positions results in alternate opening and closing of the contacts in

switching elements

2 and 12. In this exemplary arrangement, one contact is open when the other is closed and vice versa.

Several modifications of the switching element arrangement are possible as will be obvious to those skilled in the art. For example, one reed switch could be omitted if only a single throw switch is required. As a further example, both reed switches could be oriented such that both are affected by the magnet when in the same position. This would give two sets of contacts both open or closed at the same time. By increasing the depth of magnet lil, additional reed switches could be accommodated; giving more than two sets of contacts arranged in any desired combination of opened and closed conditions. A still further modification could be effected by locating one or more reed switches so as to be affected by a position of magnet intermediate between the solid and dotted line positions. In the latter case, care must be exercised to assure that magnet 10 moves far enough away from one circuit closing position to open that circuit when closing the next. This is a matter merely of dimensions and of the magnet moving mechanism yet to be described. Finally, reed switches are commercially available which contain more than one pair of contacts within the envelope. Use of such reed switches present further possibilities for contact combinations; all within the spirit of the invention.

Persons skilled in the art will recognize that other specific arrangements of the magnetic circuit can be devised for opening and closing the reed switch contacts in response to movement of a mechanical member. For example, one or more permanent magnets can be placed in stationary relationship to the switch gap or gaps. In that case, the moving member would be a ferromagnetic piece which would act to shunt the magnetic influence from the switch or to conduct that influence to the switch or to perform both functions alternately.

For simplicity and definiteness, subsequent portions of this specification describe the one normally open and one normally closed contact configuration which is most common in the limit switch art. It is also confined to the moving magnet version of actuation.

Use of reed switches inherently provides complete isolation of the contact gap from all exterior contamination. However, the external leads extending from the ends of the envelope must be protected from conductive contaminants and must also be provided with convenient terminals for connection to external wiring.

In FIG. 1 the

reed switches

2 and 12 are shown completely embedded or encapsulated in a shaped or molded block 14 of electrically insulating material which should be impervious to any expected liquid or vaporous contaminant. For convenience of disclosure only, this material is also assumed to be transparent in FIGS. 1 and 2. (In FIG. 2 the magnet and associated mechanism are omitted.) Block 14 is secured to the housing by screws 15 threaded into the housing.

In FIG. 1 are shown four

terminal screws

16, 18, Ztl and 22. As shown in FIG. 2 (for screws and 22), each of these terminal screws is threaded into a metal insert such as 24 and 26 molded into the block 14. The

inserts

24 and 26 are shown fluted or knurled on their external cylindrical surfaces for secure holding against rotation in the molded block.

Each terminal screw is surrounded by a square (or other shape) cavity 28, 3t 32 and 34 from the front surface of block 14 to a depth flush with the front surface of the respective insert such as 24. Leading downward (FIG. 1) from each of the aforesaid cavities is a groove or

wire channel

36, 38, 4t) and 42. Each of these grooves is of the same depth as the corresponding screw cavity and leads through the bottom wall of block 14. The grooves are also shown in FIG. 3 which is a bottom view of block 14 but here treated as opaque to avoid confusing details.

The end leads of reed switch 2; are connected by condoctors 44 and 46 to the inserts associated with screws A 16 and 26, respectively. Likewise, reed switch 12 is connected by

conductors

48 and 54 to the inserts of

screws

18 and 22;, respectively. Connections of these conductors may be made by soldering, brazing or welding, etc. Referring to FIGS. 1 and 2, it is seen that the connections from the reed switches to the terminal screws are completely enclosed in the molded block 14 and are thus shielded from environmental conditions.

As so far described, the terminal screws are exposed to the atmosphere within the limit switch enclosure (yet to be described). Depending upon the environment of application, the mentioned exposure may or may not be acceptable. Assuming that the only contaminant likely to invade the housing is of a non-conducting kind (most oils, for example), the exposure of the terminal screws would'not be objectionable.

If electrically conductive dust, liquids or vapors were present in the environment, the exposed terminal screws would be subject to short circuiting or connecting to the enclosure (grounding) or both.

In the past, much effort has been expended to devise limit switch enclosures which will prevent entrance of dust, liquid and vaporous contaminants. This effort has, of course, been to shield both the contact gap and the terminals from exposure. In general, these efforts have led to both inconvenience of application and expensive sealing devices. As the above description shows, the present invention isolates the most vulnerable elements, the contact gaps, by encapsulation. Shielding of the terminal screws then becomes relatively easy as will now be explained.

External wires to the terminal screws are laid in the

grooves

36, 38, 4t) and 42. In preparing the external wires, care is exercised to leave insulation on the wire up to as near as practical to the terminal screws. Thus, the portion of a wire lying in a groove is mostly or entirely covered with wire installation. After the external wires are installed under the terminal screws, the cavities 28, 3t), 32 and 34 and the

grooves

36, 38, 40 and 42 are filled with any of several available sealing compounds. Choice of the particular sealing compound may be left to the user. However, there are several characteristics which the compound should have in most cases. It should be chemically resistant to expected contaminants. It should adhere well to the wire insulation and also to the material of block 14. This will assure complete sealing of the terminal screw from the atmosphere. Where occasional replacement of the switch, due to mechanical damage or wear, is anticipated, a compound should be chosen which will remain relatively soft compared to the material of block 14. This will permit removal of the sealant by means of a sharp instrument without damaging the general encapsulation. One compound which is useful in a wide variety of conditions is silicone rubber cement.

It is now necessary to describe preferred means whereby the magnet It} is transferred from one of its contact closing positions to the other. This transfer is understood to be desired in response to the position of some moving machine member.

If the magnet 10 is moved directly by the moving member, it is found that extraordinary care must be exercised in guiding the moving member if the switch is to change state (open or close) at exactly the same position of the moving member each time. For example, slight variation in the perpendicular distance from the magnet poles to the reeds will cause significant differences in the longitudinal position for contact action.

In the present invention, the position of the magnet is made noncritical by using a mechanical snap-action for original response to the machine member position. When the snap-action mechanism trips, the magnet is transferred substantially instantaneously from a fully active to a fully inactive position relative to an associated reed switch, or vice versa.

A great variety of snap-action mechanisms have been devised both in the limit switch art and in many other mechanical fields. In the limit switch prior art, snap actions have been used to transfer moving electrical contacts quickly in response to moving machine part positions. For purposes of the present invention, the magnet is snapped from place to place instead of a moving electrical contact.

The particular snap mechanism to be described is one of several which have found wide commercial acceptance in limit switch manufacture. This one is readily adaptable to the purposes of the invention but is not per se a part of the invention. On the other hand, the invention can be practiced by adaptation of other specific snap mechanisms.

Considering FIG. 1 as a front view of the limit switch, FIG. 4 is a rear view.

Referring to FIG. 4, numeral 52 designates a metal housing (usually a die casting). Journalled in this housing, as best seen in FIG. 5, is a main actuating shaft 54. Press fitted to one end of shaft 54 is the driver arm 56. The opposite end of shaft 54 is fitted to one end of the position sensing arm 58.

The end of shaft 54 which carries arm 58 is drilled part way through, longitudinally to form a cavity 60 which is taper threaded internally. Into the walls of cavity 60 is cut a slot 62 which is shown in FIG. 5 and is better seen in FIGS. 1 and 6. The outer periphery of the portion of shaft 54 which enters arm 58 has fine longitudinal serrations 64 which do not show in the drawings except in the enlarged fragmentary view, FIG. 6. The mating hole in arm 58 is internally serrated to complement the serrations on shaft 54. By means of the serrations, the arm 58 can be fitted on shaft 54 in any angular relation consistent with the pitch of the serrations. When a suitable angle for the arm 58 has been chosen, it can be made secure by tightening the tapered and externally threaded plug 66 (FIGS. 1 and 6) which is threaded into the cavity 60. As the plug is tightened, its taper spreads the slot 62 slightly thereby holding arm 58 in place.

The end of arm 58, opposite the shaft 54, is slotted to form a fork which receives the roller 68. Roller 68 is free to rotate about a pin 70 press-fitted in a hole through the fork side members of arm 58.

The purpose of arm 58 is to detect the position of a moving machine member and transmit it to the inner mechanism of the limit switch. This purpose is indicated schematically by the cam 72 shown in FIG. 4. Cam 72 may be assumed to be attached to the machine member (not shown) which is movable as indicated by the double headed arrow. If the cam 72 moves to the left, it will eventually make contact with the roller 68; moving arm 58, shaft 54 and arm 56 counterclockwise as seen in the figure. As will appear, below, there is a limit to the counterclockwise motion of arm 56 without damage to the mechanism. In an actual device, spring means are incorporated in the hub portion of arm 56 to permit further rotation (overtravel) of shaft 54 after motion of arm 56 is blocked. Since the overtravel provision is only incidental to the practical application of the device and is not part of the invention, it is not described here.

As seen in FIG. 4, the lower end of arm 56' is normally held at the leftward limit of its travel by the compression spring 74. One end of spring 74 bears against a side of arm 56. The other end of spring 74 bears against a portion of the housing 52 defined by a recess 76. In the normal position shown, spring 74 is partially compressed to maintain arm 56 firmly at its clockwise limit of rotation.

Clockwise rotation of arm 56 is limited by contact with a portion of the trigger member 78 which is itself restrained against leftward motion by contact with the abutment 79 cast integral with the housing 5'2.

With arm 56 in the position described, a roller 80 bears upon the upper surface of the rocker arm 82 which is press-fitted or pinned to an end of the snap-action shaft 84. The roller is freely rotatable about a pin shaft 86 which is press-fitted in two spaced apart fork portions of a sliding guide block 88. Forceful contact between roller 80 and rocker 82 is maintained by partially compressed coil spring 90. One end of spring bears upon the upper end of block 88 while the other end bears upon the bottom of a blind-drilled axial hole 92 in arm 56. The relation of arm 56, spring 90, block 88 and roller 80 is more specifically shown in the isometric exploded view, FIG. 7.

As shown in FIG. 4, the force of springe' 90 is exerted, through roller 80, upon rocker 82 to the left of the axis of shaft 84. This tends to cause counterclockwise rotation of shaft 84. As seen in FIG. 5, shaft 84 extends through and is rotatable in the wall 92 and a boss 94 of the housing 52. At the end of shaft 84 opposite rocker 82 is pressfitted or pinned a non-ferromagnetic arm 96 which partakes of any rotation of shaft 84. In FIG. 1, the arm 96 is shown limited in clockwise rotation by contact with a pin 98 which is press fitted into a boss 100 east integral with the wall 92 of housing 52. The clockwise limit of arm 96 in FIG. 1 corresponds with the counterclockwise limit of rocker 82 of FIG. 4 because of the front-to-back relation of these views. Thus, the counterclockwise r0- tation of rocker 82 in FIG. 4 is limited by arm 96 and pin 98 in FIG. 1. As further seen in FIG. 1, the earlier described magnet 10 is fixed to the upper end of arm 96 by cementing or otherwise. The non-ferromagnetic composition of arm 96 prevents shunting the field of magnet 10.

If now the shaft 54 is turned counterclockwise (FIG. 4), due to action of cam 72 on arm 58 or otherwise, the lower end of arm 56 will move toward the right; further compressing spring 74. As arm 56 moves, the roller 80 moves upward along the slope of the upper surface of rocker 82; further compressing spring 90. As the line of force of spring 90 reaches and passes coincidence with the axis of shaft 84, the torque on that shaft changes from counterclockwise to clockwise. Thereafter, rotation of shaft 84 is prevented by engagement of the right hand end of rocker 82 against the lower surface of a notch 102 in trigger 104. At the time of reversal of torque on shaft 84, force between arm 96 and pin 98 is relieved (trans ferred to rocker 82 and trigger 104, as aforesaid). During this transition, there may be slight rotation of shaft 84 due to deflection of pin 98 or clearance at notch 102 or both. However, such motion is not sufficient to disturb the previously described function of magnet 10 in maintaining electrical contact between the

reeds

4 and 6.

As the lower end of arm 56 (FIG. 4) continues its motion to the right, it eventually engages an upper end portion of trigger 104, whereupon that trigger is forced to rotate clockwise about a pivot pin 106 fixed in a boss 108 integral with wall 92 of the housing. As trigger 104 moves as described, it compresses return spring 110 based in a cavity 112 of the housing.

As the above described related motions of arm 56 and trigger 104 continue, the compression of spring 90 becomes greater and its point of action moves farther to the right of the axis of shaft 84. Consequently, there is a build-up of restrained clockwise torque relative to the axis of shaft 84. At a definite point in this action, the lower surface of notch 102 leaves the underside of rocker 82. Precisely then, the restraint on the aforesaid torque is removed allowing spring 90 to rotate the rocker 82 and the shaft 84 suddenly clockwise (FIG. 4). In FIG. 1, the above mentioned motion appears as a sudden snap of lever 96 counterclockwise into contact with stop pin 114 fixed in boss 116 integral with wall 92. This transferred position is shown by dotted lines in FIG. 1. Obviously, the snap action of lever 96 carries the magnet 10 with it; resulting in the transfer of electrical conditions in the reed switches which was described earlier.

During the motion of arm 56 to the right (FIG. 4), contact with trigger 78 was relieved but the trigger was,

at first, maintained in substantially its original position by interference by the left end of rocker 82. When the rocker suddenly transferred, this interference was removed by the left end of rocker 82 moving above the lower surface of notch 118 in trigger 78. When this occurred, the spring 129 forced the trigger 78 clockwise sufficiently for the notch 118 to restrain rocker 32 on a subsequent reverse snap.

After the snap action, the various parts of the snap mechanism are disposed substantially as shown in FIG. 13 (except for obvious differences in dimensions).

When the original operating force is removed, spring 74 will return arm 56 to the original position shown in FIG. 4. During this return motion, snap action takes place in reverse to that described. That is, rocker 82 is held in its previously operated position until arm 56 moves trigger 78 aside.

Arm 56 can be held normally in its opposite operative position by placing spring 74 on the other side of arm 56 and having it bear on the left wall of cavity 122. Spring 74- may also be removed entirely if it is desirable to actuate arm 58 in either direction by mechanical means and have it remain so until mechanically actuated in the opposite direction. In the absence of spring 74, arm 56 is maintained in either operated position by detent action of spring 90 forcing roller 30 down the slope of rocker 32.

In FIGS. 1 and is shown a cover plate 124 which is omitted from FIG. 4. In plate 124 is a plurality of countersunk screw holes 126 so spaced as to match with threaded holes 128 in the walls of housing 52 (see FIG. 4). By use of flat headed machine screws, the plate 124 can be attached to the housing to cover the mechanism shown in FIG. 4. The use of flat headed screws countersunk into the cover plate leaves a flat surface (back of FIG. 1; to the left in FIG. 5) for mounting the device. Mounting is further provided for by mounting-screw holes 130 through plate 124. Threaded holes 132 are provided in the housing to receive screws for holding any cover desired to protect the parts shown in FIG. 1 from mechanical damage.

At 134- (FIGS. 1, 4 and 5) is a cast hub or boss, integral with the housing. Through this boss is a hole 136 which is provided with tapered threads for receiving a standard electrical conduit fitting.

When installing the back plate 124 and/or a front cover plate, it may be desirable to include some kind of gasket between the cover and the housing wall to exclude liquids, and dust. However, such precaution is not nearly so vital as in conventional limit switch practice since, as previously emphasized, intruding foreign matter cannot reach any electrical conducting parts. One reason for gasketing might be the presence of sticky or abrasive contaminants which could damage the mechanical parts. The same considerations render unnecessary the careful sealing of conduit opening 136 or the bearing of shaft 54. In short, any closure of openings in the housing need provide only for protection against gross invasion of foreign matter and against mechanical damage. It is clear that the mere provision of a general housing is sufficient to protect the magnetic circuit of magnet against adverse effects of ferrous chips or shavings. The active magnetic field gap is so small compared to dimensions of the housing that any chips resting on the housing cannot adversely affect the desired magnetic function.

All reed switches are generally considered to be in th miniature class of electrical components. Even so, the lengths of the larger of these switches are such as to make the limit switch assembly so far described somewhat larger than is desirable for many applications. Furthermore, the largest standard reed switches have limited current carrying capacity. A limit switch made according to FIGS. 1 through 7, would be larger in dimensions but less in load handling capacity than many types of conventional limit switches. In spite of these facts, the device described is 8 not without utility where the mentioned limitations are not critical.

Now to be described is a modification of the invention which provides more compact construction along with greater load handling capacity.

In the figures to be described, analogous parts are given the same numerals as in the previous figures but with the sufiix a added. In this context, analogous means substantially identical function but with possible difference in specific form or dimension. Non analogous parts are given new numerals.

In FIG. 8, numerals 2a and 12a refer to microminiature reed switches. These switches are of the same configuration and general construction as

reed switches

2 and 12 except that they are considerably less than one half the size in each dimension. These smaller reed switches obviously permit a smaller over all assembly. However, they,

necessarily have much less current carrying capacity. As will appear below, it is possible to take advantage of the smaller dimensions and yet include small amplifyin devices to permit ultimate current switching capacity greater than that of the larger reed switches alone.

As seen in FIG. 8, the reed switches 2a and 12a are molded in an insulating block 14a in such position as to be operatively affected by the field of magnet 10a. The magnet is attached to a non-ferromagnetic arm 96a fixed on shaft 84a. In one rest position, arm 96a is held against the abutment stop 138 which is molded integral with the block 14a. In this position of magnet 10, switch 12a is held in electrically conductive condition and switch 2a i non-conductive. Upon actuation of the snap mechanism, the arm 96a shifts quickly to contact with stop abutment 140 also molded as a part of block 14a. In this second rest position, magnet 10a is effective to render switch 2a electrically conductive and switch 12a non-conductive.

Several types of semi-conductor devices are available which may be switched from a substantially non-conductive to a substantially highly conductive (very high impedance to very low impedance) by means of a switch carrying very low current. Such devices include switching transistors, silicon controlled rectifiers, gate-controlled A.C. power switches, etc. With the additional complication of a filament current source, vacuum tubes could also be adapted to this purpose. The invention is meant to include any suitable means for using a low current capacity switch to control the flow of a much larger current in a substantially on-and-otf manner.

Both for purposes of explanation and for appropriate construction characteristics, the preferred amplifier at this time is the gate-controlled A.C. power switch. This semiconductor device is known in the trade as a Triac and will be so referred to hereinafter. One desirable feature of the Triac is that it switches an A.C. load with but a single control terminal. This is an advantage because the great majority of limit switch applications involve A.C. loads. In case of a D.C. load, a different amplifier would be preferred. In common with most power range semiconductor devices, the load current capacity of the Triac is many times the control current required. Specifically, the Triac can switch a load of several amperes with a few milliamperes of control circuit current.

For simplicity, electrical connections are not shown in FIGS. 8 and 9. FIGURE 12 is a schematic circuit diagram for one of the two switching channels; namely, the one associated with reed switch 2a.

In FIG. 12, numeral 142 designates a Triac which is provided with a load circuit terminal 144 and a gate or control circuit terminal 146. A second load circuit connection is, made to the Triac by soldering or otherwise connecting a conductor to the case of the device as at 148. The two load-

circuit connections

144 and 148 are extended by electrical conductors to the external terminal screws 16a and 20a, respectively.

As shown by broken lines in FIG. 12, terminal screw 29a (see FIG. 8 also) is to be connected to one side of an electrical load which may typically be the coil of an AC. operated electromechanical relay. The other side of this load is connected to one side of an A.C. supply source of voltage and frequency appropriate to the load. The remaining side of the power supply is connected to terminal screw 1611. Thus, the substance of the Triac between

connections

144 and 148 is analogous to a simple'switch. That is, when the impedance between these connections is very high, it corresponds to an open switch and the load is essentially deenergized. When this impedance is nearly zero, it corresponds to a closed switch and the load is energized. The performance of the Triac must be regarded as analogous rather than equivalent to a switch because its internal impedance is never completely zero or infinite.

For the purpose of changing the impedance of the load current path within the Triac, the gate or control terminal 146 is connected to one end of resistor 150. The other end of this resistor is connected to one side of switch 2a. The opposite side of switch is connected to the case connection 148 of the Triac 142. The characteristics of the Triac are such that a closed circuit condition of switch 2a results in low impedance in the load path of the Triac. Conversely, an open circuit at switch 20 produces a substantially open circuit through the Triac to the load.

When switch 2a is closed, there is a flow of current through this switch which is limited in amount mainly by the value of resistor 150. If resistor 150 is of too large a value, the flow of current through the control channel in the Triac will not be suflicient to reduce the load circuit impedance adequately. However, experience has shown that resistor 150 may be so chosen as to produce positive switching of the load circuit without exceeding a conservative current rating for the microminiature reed switch 2a.

In FIG. 12A, Triac 142 is represented by its proposed graphic symbol instead of the pictorial showing used in FIG. 12.. The circuit diagram of FIG. 12 may be modified by substituting this symbol and making the connections indicated by the repeated numerals.

It is to be understood that a separate Triac 152 (FIG. 8) is provided to be controlled by reed switch 1201. The circuit for Triac 152 would be electrically identical to FIG. 12; a separate gate circuit resistor being used in place of resistor 150 and the external connections being made to terminal screws 18a and 22a (FIG. 8).

As shown in FIGS. 8 and 9, the Triacs are completely encapsulated in the insulating block 14a. This construction is necessary to prevent access of foreign matter to the Triacs. However, the encapsulation does impair the escape of heat from the Triacs. Since semiconductor devices are limited in their current ratings by the heat dissipation characteristics of their environment, this encapsulation places a load current restriction on the Triac ratings. However, the normal load ratings of commercial Triacs are so high that this derating still leaves a load current capability considerably in excess of that of the larger reed switches shown in FIG. 1. Specifically, Triacs 142 and 152 may readily be used for controlling the coils of conventional relays regularly used in machine tool control circuitry.

An alternate construction can obviously be provided by omitting the Triacs from the housing of the limit switch and connecting the leads of switches 2a and 12a to the terminals 16a and 20a for one switch and to 18a and 22a for the other. In that case, the Triacs would be located remotely in a separate enclosure such as the general control cabinet of the machine tool. The latter arrangement would permit using full current ratings of the Triacs but would lose the advantage of self-containment as shown in FIG. 8.

Means for sealing the external connections in FIG. 8 are generally similar to those described in reference to FIG. 1, but they are different in detail because of the more compact arrangement. As before, each terminal screw is located in a cavity with a narrow wire groove leading to the bottom of the molded encapsulating block. However, as shown in FIG. 9, the adjacent terminal in serts 24a and 26a (likewise for the inserts under screws 16a and 18a) are placed at ditferent levels in block 14a. By this expedient, the wire leading to screw 20a can pass over screw 22a. Furthermore, the groove 42a for the wire leading to screw 20a has a floor in it above the level of screw 22a. By pulling this upper wire reasonably taut, a space is assured for the entry of sealing compound between the screw 22a and the wire to screw 20a. These relations are further emphasized in FIG. 10 and in the partially cutaway isometric view in FIG. 11.

Mechanism for snapping lever 96:: from one switch closing position to the other is shown in FIGS. 13 and 14. This mechanism is functionally identical to the snap mechanism described in reference to FIG. 4, 5, 6 and 7. The only differences are in dimensions, some of which are made smaller in the more compact assembly. The housing and cover plates are also similar to those previously described except for dimensions and some minor changes in configuration. The principles of protection against environmental contaminants are exactly as previously described.

What is claimed is:

1. A limit switch for controlling an electrical circuit comprising a molded block of electrically insulating material, a member moveable between limits within said molded block, snap-action means for positioning said member in any one of a plurality of rest positions, a magnetic member on said member, electrical switch means encapsulated within said molded block in proximity to and operable by said magnetic member, electrical terminals within said molded block, electric connections imbedded in said molded block connecting said switch means with said electric terminals, and means sealing said electrical connections and electrical connections between said terminals and an external electric circuit to be controlled by said switch.

2. A limit switch for controlling an electric circuit comprising a block of electrically insulating material, an arm mounted at one end on a rotatable shaft and moveable between limits within said block, snap-action means attached to said rotatable shaft for abruptly positioning said arm in any one of a plurality of rest positions, a magnetic member on the other end of said arm, an electrical switch element encapsulated within said block in proximity to and operable 'by said magnetic member, means sealing the electrical connections within said switch and between said switch and an electric circuit to be controlled, and a member having a portion extending beyond said molded block operably connected with said snap-action means to actuate said limit switch.

3. A limit switch comprising a block of electrically insulating material, an arm moveable between limits within said block, snap-action means for positioning said arm in any one of a plurality of rest positions, magnetic means activated by movement of said arm, an electrical switch element encapsulated within said block in proximity to and operable by said magnetic means, terminal screws mounted within corresponding recesses within said block, wire means im'bedded within said block and connecting said switch element with said terminal screws, means defining grooves in said block between the recesses of said terminal screws and the exterior of said block,

' said grooves facilitating the insertion of electrical sealing material in said recesses and grooves when connecting said switch with an electrical circuit actuated by said switch, and housing means enclosing said limit switch.

4. A limit switch according to claim 1 wherein electrical valving means regulates electric flow therethrough responsive to actuation of said electrical switch means, said valving means being encapsulated within said block and electrically connected with said electrical switch means.

3,364,318 1 1 1 2 5. A limit switch according to claim 4 wherein said OTHER REFERENCES electrical valving means comprises a Triac.

Lowry, Voltage Regulated SCR Inverter With Sine R ferences Cit d Wave Output' and Current Limiting, General Electric r Application Note, June 1961, p. 1 UNITED STATES PATENTS Rudisill et a1, Guide to Limit Switches, Product 3,031,385 3/1963 Scott 200-47 Engineering, Nov. 12, 1962, p. 99. 3,198,902 8/1965 Deshautreaux 335207 3,215,864 11/1965 Doyle et al. 307-885 BERNARD A. GlLHEANY, Primary Examiner. 3,247,342 4/1966 Ott et a1. 335-207 2,795,678 6/1957 Mertler 200-168 0

Claims

1. A LIMIT SWITCH FOR CONTROLLING AN ELECTRICAL CIRCUIT COMPRISING A MOLDED BLOCK OF ELECTRICALLY INSULATING MATERIAL, A MEMBER MOVEABLE BETWEEN LMIT WITHIN SAID MOLDED BLOCK, SNAP-ACTION MEANS FOR POSITIONING SAID MEMBER IN ANY ONE OF A PLURALITY OF REST POSITIONS, A MAGNETIC MEMBER ON SAID MEMBER, ELECTRICAL SWITCH MEANS ENCAPSULATED WITHIN SAID MOLDED BLOCK IN PROXIMITY TO AND OPERABLE BY SAID MAGNETIC MEMBER, ELECTRICAL TERMINALS WITHIN SAID MOLDED BLOCK, ELECTRIC CONNECTIONS IMBEDDED IN SAID MOLDED BLOCK CONNECTING SAID SWITCH MEANS WITH SAID ELECTRIC TERMINALS, AND MEANS SEALING SAID ELECTRICAL CONNECTIONS AND ELECTRICAL CONNECTIONS BETWEEN SAID TERMINALS AND AN EXTERNAL ELECTRIC CIRCUIT TO BE CONTROLLED BY SAID SWITCH.