US2807693A - Potentiometer - Google Patents

Description

Sept. 24, 1957 H. J. SCAGNELLI POTENTIOMETER 2 Sheets-Sheet 1 Filed April 12, 1954 aam qag 1 ma mm m a m 4 S f m w m w fin .T

Sept. 24, 1957 H. J. SCAGNELLI POTENTIOMETER 2 Sheets-Sheet 2 Filed April 12, 1954 INVENTOR flgn'rg f Jcgfmilz' ATTORNEY United States Patent POTENTIOMETER Henry John Scagnelli, Bronx, N. Y., assignor to Ward Leonard Electric Company, a corporation of New York Application April 12, 1954, Serial No. 422,475

8 Claims. (Cl. 20148) This invention relates to plunger operated type of electrical control apparatus and to improvements in the means for apportioning the voltage impressed on the apparatus. It is directed in particular to potentiometer type devices and to means for the fine adjustment of a potentiometer with comparatively small amount of mechanical movement. 3

An object of this invention is to provide a wiping contact between the resistance contact surfaces and the contact, and the proper pressure at the contact point for maintaining good low resistance electrical contact at all contact point positions with minimum of wear.

Another object of the invention is to obtain precise apportionment of the resistance over the range of travel of the contact point and with comparatively small lateral movement between the contacting elements.

Another object of the invention is to provide a potentiometer in which the movable members are light in weight and have little inertia.

A further object of the invention is to provide a potentiometer that is inexpensive to manufacture and dependable in operation.

A still further object is to provide a potentiometer that is durable and resistant to shock.

Other and further objects and advantages will be apparent from the following description and accompanying drawing in which:

Fig. l is a top view of the potentiometer and easing without the cover and illustrating fragmentary views of the hinge and bearing;

Fig. 2 is a sectional view taken along lines 22 of Fig. 1;

Fig. 3 is a sectional view taken along lines 33 of Fig. 1;

Fig. 4 is a fragmentary sectional view of the contact, and

Fig. 5 is a perspective view of the contact side of the hinged member.

In Fig. 1 a rectangular cast aluminum casing 11 is shown with a bottom wall 11a and four side walls 11b, 11c, 11d, lle. A resistor is inside the casing and centrally mounted longitudinally on the bottom wall 11a. Around the resistor 10 a resistance wire 14 is mounted with terminal bands 15, 16 around the respective ends of the wire 14 to provide for the connection of the resistor through the terminals 19 and 20 of the terminal block 67 to an external circuit. The movable contact holder 24 having a contact 23 engaging the row of contact surfaces 141: is shifted by means of the guide plate 40 and the spring loaded reciprocating plunger 31 extending outside the casing through the oil tight sleeve bearing 43 in the wall 110 of the casing.

As illustrated in the sectional view of Fig. 2 the resistor 10 has a hollow generally cylindrical support 30 made of a non-absorbent refractory porcelain to withstand thermal shock. On the outer surface of the support are two'upper fiatsurfaces 30a, 30b, at an obtuse angle to one another and extending lengthwise of the support parallel to the axis. At the top of the resistor and between the surfaces 30a, 30b is a narrow flat surface 300 extending lengthwise between and separating the two upper flat surfaces 30a, 30b. The surface may be flat or rounded. In the preferred embodiment the surface 30c is rounded and the resistance wire 14 is formed around the support and flattened on the surfaces 30a, 30b and curved on the surface 300 to form a narrow row of laterally rounded contact surfaces 14a above the surface 30c. The surface 300 may be flat and the resistance wire flattened on the surface to form flat lateral contact surface sharply defined along the lengthwise edges by the wires in surfaces 30a, 30b. The turns of the resistance wire are spaced to insulate each turn thus spacing the contact surfaces. The contact 23 is at an angle to ride on the top of the wires. The contact surfaces are preferably even to provide for smooth even travel of the contact blade along the resistor.

The bands 15, 16 are made of malleable copper and shaped to conform to the contour of the outer surface of the support 30 and are silver soldered or brazed to the opposite ends of the resistance wire 14 and clamped tightly on the support by rivets 15a, 16a, respectively, secured to normally projecting ends. The terminal band 15 is connected to the terminal 19 by the stranded insulated copper wire 17 which may be soldered to the band 15 and fastened to the terminal 19 by the bolt 19a threaded in the block 67. The terminal band 16 is similarly connected to the terminal 20 by the stranded insulated copper wire 18 which is soldered to the band 16 and fastened to the terminal 20 by the bolt 20a threaded in the block 67.

In Fig. 3 the L- shaped mounting members 12, 13 are fastened to the raised bases 47, 48 by the bolts 47a, 48a and have a shaft 61 threaded at each end and extending through the mounting members 12, 13 and the support to secure the resistor 10 to the bottom wall 11aand space the resistor from the wall 11a. Centering washers 55, 56 are mounted on the shaft 61 and have fingers 62, 63 snugly fitting inside the hollow support to concentrically center the resistor on the shaft 61. The mica washers 57, 58 between the support member 30 and the centering washers insulate the resistor 10 from the mounting members. Bolts 59, 60 are threaded on opposite ends of the shaft 61 and tightened to se curely hold the support in place.

The contact holder 24 is made of brass and has a groove 69 extending along the edge 70 in Fig. 4. A triangular shaped contact 23 made of an alloy of precious metals sets in the groove with the apex directed outwardly and resting on the narrow row of contacts 14a forming an electrical contact point. The sides of the groove are crimped over on the contact 23 and the ends of the contact 23 are soldered to the contact holder 24 to hold the contact in the groove. The contacting edge is preferably straight, and at an angle to the contact surfaces 14a the holder 24 is mounted on the contact plate 38 and secured thereto by bolts 71, 72. The bolts 71, 72 may be mounted on curved slots 74a, 74b, respectively, to permit the. adjustment of the angle between the contact 23 and the row of contacts 14a to vary the length of the path of travel of the contact point or to accommodate the movement of the contact point with the amplitude of movement of the cam 33 and the plunger 31. In those instances where the angle is to remain fixed the holder 24 may be secured to the contact plate by Rivnuts instead of bolts and the lead 25 fastened to the holder by a bolt threaded in the mount. The contact plate 38 is laminated phenolic sheet, insulating the contact holder 24 from the plunger assembly and casing 11. In Fig. 1 a flexible stranded Patented Sept. 24, 1 957v insulated copper wire connects the holder 24 to the terminal 26 of the terminal block 67. The contact point apportions the resistance between the terminals 19 and 26 and terminals 26, 20. The rheostat is connected to external circuits by means of these terminals and the three-conductor cable 22 passing through an oil seal fitting 36 threaded in the boss 36a on the side wall 11a of the casing. The cable 22 has three wires 22a, 22b, 22c connected to the terminals 19, 26, 20 respectively and the apportionment of the resistance is thus transmitted to an external circuit.

For applying the proper contact pressure at the contact point the contact plate 38 is pivotally connected or hinged to the guide plate 40 with resilient means engaging the guide plate and contact plate and pressing the contact 23 against the resistor. The yoke 75 is fastened lengthwise along one edge by the Rivnuts 76 and preferably extends substantially parallel to the resistor. A shaft 77 passes through the arms of the yoke and the end projections of the guide plate 40 to provide an even application of pressure by the torsion spring 34 along the path of travel of the contact ,point. The torsion spring has arms 34a and 34b engaging the contact plate and guide plate respectively. The arm 34b is recessed in a notch 39 in the guide plate 40 and the arm 34a engages the yoke 75 to press the contact 23 into engagement with the resistor.

The guide plate 40 is fastened to the end of the cam plunger 31. A guide shaft 41 slideably fitting into the slot 42 holds the guide plate from rotating movement about the longitudinal axis of the plunger and from lateral movement of the contact 23 resulting from the mounting on the end of the plunger. The shaft 41 is mounted on the wall 11c of the case and extends parallel to the plunger 31. The slot 42 is elongated parallel to the resistor contact surfaces with the sides of the slot snugly engaging the shaft 41 to hold the guide plate 40 from lateral play.

The plunger 31 made of stainless steel is slideably journaled in the bearings 43, 44 which are mounted in the boss 46 and have an O ring 45 therebetween providing an oil tight seal. The bearings 43, 44 are press fitted into the drilled bore in the boss 46. A ring 45a is fitted into the bore 46 between the bearings 43, 44 with a greater inside diameter than the bearings 43, 44 for forming a groove 48 to retain the O ring and recess it with a position of the inner surface exposed above the bearing surface to grip and seal the plunger shaft 31.

The shaft or plunger 31 extends outside of the casing and has a clevis with a roller or cam wheel 32 on the exterior end of the plunger. A cam 33 engages the roller 32 and in cooperation with the helical spring 34 determines the position of the contact point. The spring 34 seats on the outside of the casing around the boss 46 and against the inverted cup shaped centering washer 54. The central portion 54:: forms a centering side slope for positioning coaxially the spring 34 in relation to the shaft 31. The spring seats on the flange 54b and the central portion 54a seats on the clevis 35 to transmit the upward pressure of the cam to compress the spring for returning the cam when the cam pressure is removed. The spring 34 draws the plunger 31 outwardly and seats the guide plate 4%) against the inner surface of the boss 46. The inner end of the plunger 31 is rolled over and fused to the guide plate permitting extended movement of the plunger inside of the casing. The inner and outer end surfaces provide limits to the maximum travel of the plunger 31. The distance of maximum travel may be changed by placing or removing shim washers 35a between the clevis 35 and the cup shaped washer 54. The angle the contact makes with the narrow contact surface formed by the resistance wires may be adjusted so that the contact point travels the length of the resistor 10. The angle may be changed turning the position of the resistor 10' in the casing. The feet of the brackets 12,

13 may have curved slots and the bolts 47a, 48a may be loosened and the angle of the resistor set for the desired contact point travel. The row of contact surfaces may be at an angle to the axis of the resistor and the plunger travel. The contact block is then normal to the plunger travel and at an angle to the contact surfaces engaging several of the surfaces to form a contact edge.

The resistor 10 is insulated from the centering washers 55, 56 by the mica insulating washers 57, 58 and the contact 23 by the contact plate. The terminal block is made of insulating material and the wire connections are insulated from the adjacent casing walls by an insulating fibre sheet 82 in the form of an L-shape with the terminal block secured to the boss or shelf 83 raised from the wall 11a. The wires 22a, 22b, 220 are connected to terminals 19, 26, 20 by means of bolts 19]), 26b, 20b.

The casing is sealed by means of the O ring 45 between the bearings 43, 44, the cable fitting 23 passing the cable 22, and the cover 50 with a neoprene gasket 51 sealing the top of the casing. Thus the interior of the casing may be protected from outside contamination and the contact 23 and the resistor contact surfaces may be maintained free of foreign matter reducing the efliciency or consistency of the electrical contact therebetween.

It is further seen that the cam actuated means moving the contact 23 are simple and rugged. The contact 23 is directly connected with the cam 33 by the contact plate 38, the guide plate 40, and the plunger 31. The change of the position of the cam 33 is reflected in a proportional change in the position of the contact point with a minimum of lost movement from play between the members.

A plat-e 85 for mounting the potentiometer to a panel or board is fastened to the bottom wall 11a by bolts 86 threaded in the bosses 87. The mounting plate has slots 88 for adjusting the potentiometer in a direction along the plunger movement, thus permitting the positioning of the potentiometer in relation to the cam surface so that the travel of the contact point may be set in relation to the cam movement. The length of travel of the contact point may also be varied by adjusting the angle the contact 23 makes with the contact surfaces 14a. The spring 34 holds the contact 23 in engagement with the resistor 10 in any position that the box is mounted without materially diminishing the contact pressure. The casing may be mounted with the resistor 10 above the contact 23. The contact 23 maintains a good electrical contact with contact surfaces.

Instead of the resistor 10 being utilized, an assembly of alternately stacked contact discs and insulating washers may be mounted between the members 12, 13. The contacting discs may be connected to a segment of a remotely located resistor or resistors and as the contact 23 is moved along the narrow row of contacts, greater or less resistance is connected to an external circuit. The pivotal or hinge mounting is utilized to permit the accommodation of the contact 23 to the irregularities in the row of contacts and to maintain a substantially uniform contact resistance.

In the preferred embodiment the contact 23 is straight. Other shapes may be utilized depending on the voltage dividing characteristics desired. The contact may be curved, V-shaped, or saw tooth in shape. The curved contact may have different configurations and the legs of the V-shaped contact may be of different lengths and angles. The segments of the saw tooth contact may be equal in length or different lengths and the segments at different angles.

Various modifications and changes may be made without departing from the scope of the invention.

I claim:

1. A rheostat comprising a casing having a bottom wall and a side wall, a resistive device mounted on said bottom wall, a bearing in said side wall, a plunger shaft reciprocatorily journaled in said bearing to move laterally to said resistive device, a contact assembly actuated by said shaft, the said assembly having a guide plate and a contact plate, said contact plate hingedly mounted on said guide plate and having a contact at an angle to the resistive device, spring means pressing said contact into engagement at a contact point with said resistive device, said guide plate mounted on the end of the plunger shaft to move the contact point along said contact surfaces, and guide means engaging said guide plate to restrict the plunger shaft movement to reciprocatory motion.

2. A rheostat as claimed in claim 1 wherein said bearing comprises two axially spaced bearings with an O-ring therebetween gripping the shaft and engaging the spaced ends of the bearing to seal the shaft from leakage.

3. A rheostat as claimed in claim 2 wherein an annular spacer member is positioned between said axial bearings to space said bearings and having a smaller inside diameter to form a groove.

4. A rheostat comprising a casing having a bottom wall and a side Wall, a resistive device mounted on said bottom wall and having a plurality of contact surfaces in a row successively tapping said resistive device, a conductive blade-like contact at an angle to said row of contact surfaces, guide means in said side wall, means reciprocatorily mounted in said guide means and having a supporting plate integral with said reciprocatory means and extending longitudinally with respect to said resistive device, a plate hingedly mounted on said supporting plate and extending to overlap with said resistive member, said blade-like contact mounted on said hinged plate movable into and out of engagement with said contact surface and spring means mounted on said supporting plate and hinged plate to move with said reciprocatory plates and pressing said bladelike contact at point against said row of contact surfaces on movement of said contact point therealong as the plates are reciprocated.

5. A rheostat as claimed in claim 4 wherein said bladelike contact is in the shape of a curve.

6. A rheostat as claimed in claim 4 wherein said bladelike contact is angular in shape.

7. A rheostat as claimed in claim 4 wherein said bladelike contact extends in approximately a straight line and at an outer angle to the movement of travel of the movable member.

8. A rheostat as claimed in claim 4 wherein the angle of said blade-like contact is adjustable.

References Cited in the file of this patent UNITED STATES PATENTS 1,971,238 Silling Aug. 21, 1934 2,468,344 Pasini et al Apr. 26, 1949 2,548,960 Ekstrom Apr. 17, 1951 2,554,811 Brom-berg et a1 May 29, 1951 FOREIGN PATENTS 208,535 Great Britain Mar. 13, 1924

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