US2328697A - Limit switch and control system combination - Google Patents

Description

Sept. 7, 1943. P. s. WHITE 2,328,697

LIMIT SWITCH AND CONTROL SYSTEM COMBINATION- Filed Sept. 5, 1942 -36 3g 32 FIG-1 26 b a l I 34 2 1 22 36 a0 {I p 35 a4 2o 24 a4 FIG-2 52 INVENTOR. PAUL G- Wane.

Patented Sept. 7, 1943 LIMIT SWITCH AND CONTROL SYSTEM COMBINATION Paul G. White, Cleveland Heights, Ohio, assignor to The Electric Controller & Manufacturing Coiinpany, Cleveland, Ohio, a corporation 0 0 Application September 5, 1942, Serial No. 457,464

9 Claims.

This invention relates to the control of electric motors by limit switches and particularly to control systems for apparatus wherein the extent of operation of one motor driven element must be limited with reference to the position of another element which is independently operated by another motor. The invention is. particularly suitable for controlling electrically operated cranes of the boom type, and is hereinafter described as applied to that use.

Electrically operated boom cranes-are generally operated by two independent motors. One of the motors drives the winding drum for the boom hoist cable, and the other motor drives the winding drum for the load or hook hoist cable. The boom hoist cable controls the raising and lowering of the boom and extends from'the boom hoist winding drum over an idler pulley, mounted above the front end of the crane cab, to its point of attachment near the free end of the boom. The other or fixed end of the boom is pivoted at the bottom of the front end of the crane cab. The boom thus may swing in a vertical plane about its pivot as the boom hoist cable is wound on or unwound from the boom hoist winding drum. The load or hook hoist cable extends from the load or hook joist winding drum over an idler pulley at the free end of the boom and extends downward therefrom to a hook or other suitable device to which a load may be attached.

The crane cab is pivotally supported at a point near its center so as to be rotatable in a horizontal plane. To counterbalance the weight of the boom and the force of the load which is transmitted to the crane cab at the pivot point of the boom, the driving motors and winding drums generally are mounted at the rear of the crane cab.

Because of this location of the motors and drums and the arrangement of the boom hoist cable, hook hoist cable, and boom, each of the cables and the boom swing about a different pivot upon raising and lowering of the boom and consequently a difierential in movement for any given point on the boom hoist cable, the hook hoist cable, and the boom results. For example, if the boom is at its uppermost limit and the hook hoist cable is operated so that the hook is slightly below the end of the boom, lowering of the boom causes a take-up in the length of the hook hoist cable between the boom pulley and the hook and jams the hook into the boom pulley. On the other hand, it the boom is horizontal and the hook is drawn to within the usual close distance from the boom pulley and the boom is then raised,

there is an excess of hook hoist cable in the raised position of the boom so that the hook is disposed several feet below the end of the boom. If the boom hoist motor has operated to lower the boom to its lowermost position, and the load hoist motor is operating to raise the hook ,automatic means should be provided to stop the load hoist motor before the hook reaches the pulley at the end of the boom. However, if the hook is stopped by the automatic means at its upper limit of travel when the boom is in its lowermost position, raising of the boom without operating the main hoist motor allows the hook to drop farther from the pulley at the end of the boom since the distance from the hook hoist drum to the boom pulley becomes less. Heretofore in the use of such lim t switches, a choice had to be made between the danger of jamming the hook into the boom pulley upon lowering of the boom or limiting the maximum height 01' lift.

Because of these conflicting factors, an ordinary limit switch driven in the usual manner by the load hoist drum and operable automatically to stop the load hoist motor at the limit of travel of the hook when the boom is in its lowermost position does not permit the hook to rise to its full limit of travel when the boom is in a. raised position. On the other hand, an ordinary limit switch driven in the usual manner by the load hoist drum and adjusted to stop the book just before it reaches the boom pulley when the boom is in a raised position, does not provide protection against jamming by hoisting of the hook when the boom is in a lower position.

One of the objects of the present invention is to eliminate both of these difliculties and make it possible to operate a boom crane without danger or jamming the hook and the boom pulley regardless of the position of the boom and without limiting the maximum lifting height of the crane in anyjnanner other than by the length of the boom itself.

In accordance with the present invention, an ordinary and common limit switch of the rotatable type is driven through a differential gear by both the boom hoist and the hook hoist drums. The apparatus is so arranged that the limit switch is driven in one direction during hoisting of the hook or lowering of the boom and is driven in the opposite direction during lowering of the hook or raising of the boom. Consequently, the operation of the limit switch is compensated for boom position. A limit switch means is thus provided which permits the hook to approach as closely to the boom pulley as is safe. but which has contacts which stop the load hoist motor when the hook approaches too closel to the boom regardless of the position of the boom and which stop the boom hoist motor when the boom is lowered to a position where the boom pulley is approaching too close to the hook. The limit switch contacts also prevent raising of the hook or lowering of the boom while the hook and boom pulley are too close together.

An object of this invention is to provide a simplifled limit switch control for controlling the operation of independently driven elements in accordance with their relative positions.

Another object is to provid a differentially operated limit switch means for preventing independently driven element from approaching too close to each other.

A further object is to provide a driving system for a rotatably driven limit switch of the usual and common type so that the limit switch is operable to stop independently driven elements in predetermined relation to each other.

A more specific object is to provide an improved limit switch control system for a boom crane.

Another specific object is to provide an overhoist limit switch control system for a boom crane which is compensated for boom position.

A further specific object is to provide a limit switch control system for a boom crane which effects stopping of the boom hoist motor if the boom is lowered within a predetermined distance from the hook and which. regardless of the position of the boom, effects stopping of the hook hoist motor if the hook is raised within said predetermined distance from the boom.

Other objects and advantages will become ap parent from the following specification wherein reference is made to the drawing, in which Fig. 1 is a diagrammatic representation of a boom crane of the type to which this invention may be applied, and

Fig. 2 is a very much simplified wiring diagram and diagrammatic representation of the appa ratus of this invention.

Referring to Fig. l, a cab ill of a boom crane I2 is provided with a downwardly extending pivt pin i4 and suitable rollers i6 so that the cab Ill can be centrally supported for rotation in a horizontal plane about a point near its center. A boom Ill has its fixed end pivoted at Ill near the lower front or right hand end of the cab in so that the boom II may be raised and lowered in a vertical plane and rotation of the cab Ill about the support for the pivot pin i4 may swing the boom 08 horizontally;

A boom hoist cable 22 for controlling the raising and lowering of the boom l8 has one end attached near the free end of the boom [8 at 24 and extends to a boom hoist winding drum 30 over an idler pulley 26 supported by a framework 28 extending upwardly from the front end of the cab l0. boom winding drum an which ismounted in the rear or left hand end of the cab iii. A load or book hoist cable 32 has a suitable load attaching device such as a hook 44 fastened to one end and passes over a boom pulley 86 at the free end of the boom 88 to a. load or book hoist winding drum 88 mounted in front of the drum 34 near the rear end of the cab Ill. The winding drums 30 and 38 and the driving mo-L tors therefor are mounted in the rear end of the cab ill to act as a counterbalance for the weight The cable 2-2 is wound on the oi. the boom is and the load being lifted. The boom l8 and the cables 22 and 32 are shown in solid lines to indicate their respective positions when the boom I8 is in the lowermost position and are shown in broken lines to indicate their respective positions when the boom I8 is in the uppermost position.

Referring to Fig. 2, the hook hoist drum 3B is arranged to be driven through suitable gearing 40 by a direct current motor 42 having a suitable energized field winding 44. The boom hoist drum 30 is arranged to be driven through suitable gearing 46 by a direct current motor 43 having a suitable energized field winding 50. Reversible power for the armature winding of the motor 42 is obtained from a suitable direct current source 52 through electromagnetic reversing switches 54 and 56. Reversible power for the armature winding of the motor 48 is obtained from the source 52 through electromagnetic reversing switches 58 and 60. The operation of th switches 54 and 56 is controlled by a master controller 62 and the operation of the switches 58 and 60 is controlled by a master controller 64. The controllers B2 and B4 are shown in their off positions. Power for operating the switches 54, 5 8, 58, and 60 is obtained from the source 52. Although direct current motors have been illustrated, it is obvious that the invention may be used with alternating current motors as well.

Movement of the controller 62 to the right from its off position normally energizes the switch 54 which when energized operates to connect the motor 42 to the source 52 from driving the hook hoistdrum 3B in the direction causing hoisting of the hook 214, and movement of the controller 62 to the left from its off position energizes the switch 56 which when energized operates to connect the motor 42 to the source 52 for driving the hook hoist drum 38 in the direction causing lowering of the book 34. Likewise, movement of the controller 64 to the right from its off position normally energizes the switch 58 which when energized operates to connect the motor 48 to the source 52 for driving the boom hoist druni 30in the direction causing raising of the boom I8. and movement of the controller 64 to the left from its off position normally energizes th switch Gil which when energized operates to connect the motor 48 to the source 52 for driving the boom hoist drum 30 in the direction causing lowering of the boom I8. I

The switch 56 may be energized. at any time upon movement of the controller 52 to the left, but the switches 54, 58, and Eli are energized upon movement of their associated controllers 62 or 64 only if certain limit switches now to be described are in the proper positions. The energizing circuit for the switch 54 can be completed only when contacts 58 of a suitable rotatable type limit switch, such as a cam limit switch 68, are

closed. The limit switch 68 also comprises a rotatable circular cam Ill having a projecting portion 12 which cooperates with a pusher rod 14 to open and close the contacts 86 depending upon the rotated posltionot the cam 10. The energizlng circuit for the switch 58 can only be completed when limit switch contacts 15 are closed. The limit switch contacts 15 are closed when the boom i8 is below its upper safe limit of travel and are open only when the boom Ill is at its upper safe limit of travel. The energizing circuit for the switch 60 can only be completed when contacts Hi and B6 are closed. The contacts 16 are closed when the boom 88 is above its lowermost sai'e limit 01' travel and are open only when the boom I3 is at its lower sate limit of travel. The contacts 15 and 16 may be parts or a hatchway type limit switch such as i commonly used on boom cranes to limit the extent or travel of the boom I 8 independently of the position of the hook 34.

The cam 10 is arranged to be rotatably driven by a driven shaft 18 o! a standard differential gear 83. One driving element of the differential II) is driven by the hook hoist drum 38 through bevel gears 32 and gearing 84. The other driving element or the differential 80 is driven by the boom hoist drum 38 through bevel gears 83 and reversing gearing 38.

When the hook 34 is as close to the boom pulley 36 as possible for maximum safe height of lift and the boom I3 is in its lowermost position, the cam I0 is adjusted on the shaft 13 so that the raised portion 12 engages the rod 14 to hold the contacts 56 open, as shown in Fig. 2. With the limit switch 58 in the open position the motor 42 cannot be energized to raise the hook 34 and the motor 48 cannot be energized to lower the boom l8 due to the open contacts 88 in the energizing circuits for the switches 54 and 6B.

Assume that the boom I8 is in its lowermost position and that the hook 34 has been raised to its upper limit leaving the length a (Fig. 1) of the cable 32 between the hook 34 and the pulley 35. Movement of the controller 64 to the left does not cause energization of the switch 60 because the contacts 65 and 15 are open, and movement of the controller 62 to the right does not cause energization of the switch 54 because the contacts 66 are open. Movement of the controller 84 to the right, however, causes energization of the switch 58 which operates to cause the motor 48 to drive the boom hoist drum in the direction which raises the boom l8. Movement of the boom H! from it lowermost position effects closure of the contacts 16 in a well known manner. Rotation of the drum 3!! in the direction which raises the boom l8 drives the differential 80 in a direction to cause the cam ill to rotate counterclockwise, and when the portion 12 passes from beneath the rod '14, the contacts 65 close. When the boom l8 reaches its upper limit as shown by the dotted lines in Fig. l, the contacts 15 open to deenergize the switch 58 which opens to stop the boom hoist motor 48. Due to the geometric configuration of the cables 22 and 32 and the boom l8. the length of the cable 32 between the hook 34 and the pulley 36 has increased from the length a to the length indicated by b as a result of raising the boom l8. However. closure of the contacts 66 upon raising of the boom l8 permits further hoisting of the hook 34 so that the increase in the length of the cable 32 between the boom pulley 38 and the hook 34 may be taken up by further hoisting of the hook 34.

For example, at any time after the contacts 66 close, movement of the controller 62 to the right causes operation of the switch 54 to encrgize the motor 42 to drive the drum 32 in a direction to hoist the hook 34. Rotation of the drum 32 in a. direction to hoist the hook 34 drives the cam 10 through the differential 8B in a clock- Wise direction, and, if the book 34 overtakes the boom IS, the projecting portion 12 of the cam Hi again strikes th rod 14 to open the contacts 55 to stop the motor 42 and prevent the starting of the motor 43. The hook 34 is thus automatically prevented from approaching within a predetermined distance from the boom pulley regardless of the position or the boom l8, and may be hoisted to its maximum sate height while the boom I4 is in a raised position.

At any time that the hook 34 and boom pulley 34 are far enough apart for the contacts 63 to be closed and the boom i8 is not in its lowermost position determined by the contacts 16, movement of the controller 54 to the left causes energization of the switch 5|] which when enersized closes to cause the boom hoist motor 48 to drive the boom hoist drum 3!! in a direction to lower the boom l3, and the cam 10 is driven in a clockwise direction. The boom l8 may continue to lower until the boom pulley 3B nears the hook 34 at which time the projecting portion 12 of the cam Ill lifts the rod 14 to open the contacts 33 or until the boom I8 reaches its lowermost position and opens the contacts 16.

Likewise, if the hook 34 and boom pulley 35 are far enough apart for the contacts 56 to be closed, movement of the controller 52 to the right causes energization of the switch 54 which operates to cause energization of the motor 42 so that the hook hoist drum 33 is driven in a direction to hoist the hook 34. The drum 3!! when hoisting the book 34 drives the cam Ill in a clockwise direction to move the projecting portion 12 toward the rod I4. When the hook 34 again approaches the boom pulley 36, the projecting portion 12 moves the rod 14 upward to open the contacts 66. Opening of the contacts 66 deenergizes the switch 54 which thereupon opens to deenergize the motor 42 and prevents the reclosure of the switch 60 so that the boom l8 cannot be lowered.

The hook 34 may be lowered at any time regardless oi the position of the boom l8 by moving the controller 62 to the right to energize the switch 56 which, when energized, connects the motor 42 so as to drive the drum 38 in a direction to lower the hook 34. When rotating in this direction, the drum 3!) drives the cam 10 through the differential in a counterclockwise direction so that the projection portion 12 moves to the left away from the rod 14. There is no limit to the extent of lowering of the hook provided by the limit switch 68 since it is assumed that all of the main hoist cable will be paid out before the cam TB has made a complete revolution.

The diiierential connection between the drums 33 and 38 and the limit switch 68 ,compensates the, limit switch 68 for the position of the boom [8. The accuracy of the limit switch compensation provided depends somewhat upon the geometric configuration of the pivot point 20, point of cable attachment 24 on the boom l8, pulleys 36 and 26, and the drums 30 and 38. Suitable gear ratios may be selected toinsure that the accuray of compensation is adequate for any particular installation.

The control system for the motors 42 and 48 has been shown greatly simplified for ease of understanding of the present invention. It is understood that actual control systems would be much more complicated including among other things acceleration resistance control and dynamic braking or other electrical braking connections.

I claim:

1. In a control system for separately operable reversible motors arranged to drive separately operable elements, respectively, a difl'erentially operated limit switch means for the motors and driven differentially in response to the operated positions of said operable elements, said limit erable elements, said limit switch means having contacts operable for controlling the energize tion of said motors when said motors are driving said operable elements toward each other.

3. A control system for separately operable reversible motors arranged to drive separately operable elements, respectively, comprising energizing circuits for said motors, respectively, a difierential gear having driving elements and a common driven element, means operatively connecting the driving elements of said differential gear to said operable elements, respectively, for driving said driving elements in a fixed relation to said operable elements, respectively, a limit switch means operatively connected to the driven element of said differential gear and driven thereby, and said limit switch means being operatively associated with said energizing circuits to control limits of relative positions of said operable elements by control of the energization of the motors.

4. A control .system for separately operable motors arranged to drive separately operable elements, respectively, comprising switching means for controlling the energization of said motors, a differential gear having driving elements and a common driven element, means operatively connecting the driving elements of said differential gear to said operable elements, respectively, for

driving said driving elements in a fixed relation to said operable elements, respectively, a limit switch operatively connected to the driven element of said differential gear and driven thereby, and said limit switch being operatively associated with said switching means to control limits of relative positions of said operable elements by control of the energization of the motors.

5. In a boom hoist, a boom, a boom hoist mechanism connected thereto for hoisting and lowering of the boom, an independently operable load hoist drum, a load hoist cable connected thereto for hoisting and lowering of a load independently of the boom, independently operable motors for said boom hoist mechanism and said load. hoist drum, respectively, a differentially operable limit switch means for the motors and driven differentially in response to the operated positions of the boom and the load hoist cable for maintaining predetermined limits of relative positions of the boom and the load hoist cable.

6. In a boom hoist, a boom hoist drum, a boom hoist cable connected thereto for hoisting and lowering of the boom, an independently operable load hoist drum and a load hoist cable connected thereto for hoisting and lowering of a load independently of the boom, independently operable motors for said drums, respectively, and a diilerentially operable limit switch means for the motors and driven differentially in response to the operated positions of the boom and load hoist cables for maintaining predetermined limits of relative positions of the boom and load hoist cables.

'7. In a. boom hoist, a boom hoist drum, a boom hoist cable connected thereto for hoisting and lowering of the boom, an independently operable load hoist drum, a load hoist cable connected thereto for hoisting and lowering of a load independently of the boom, independently operable motors for said drums, respectively, a limit switch for controlling the operation of one of said motors to limit the range of operation of said boom hoist drum, and a difierentially operable limit switch means for the motors and driven differentially in response to the operated positions of the boom and load hoist cables for maintaining predetermined limits of relative positions of the boom and load hoist cables.

8. In a boom hoist, a boom, a boom hoist mechanism conected thereto for hoisting and lowering of the boom, an independently operable load hoist drum, a load hoist cable connected thereto for hoisting and lowering of a load independently of the boom, independently operable motors for said boom hoist mechanism and said load hoist drum, respectively, a differential mechanism having a. plurality of driving elements and a common driven element, means for driving said driving elements in fixed relation to the operation of the boom and the load hoist cable, respectively, and limit switch means for the motors driven by said common driven element of said differential mechanism for maintaining predetermined limits of relative positions of the boom and the load hoist cables.

9. In a boom hoist, a boom, a boom hoist drum, a boom hoist cable connected thereto for hoisting and lowering of said boom, 2. load attaching device, an independently operable load hoist drum, a load hoist cable connected to said lead hoist drum and said load attaching device and associated with said boom for hoisting and lowering of said load attaching device independently of the boom, independently operable reversible motors for said drums, respectively, a differential mechanism having a plurality of driving elements and a common driven element, means for driving said driving elements in fixed relation to the operation of the boom and load hoist drums, respectively, and limit switch means for the motors and driven by said common driven element of said differential mechanism, said limit switch means including contacts for controlling the operation of said motors to maintain a predetermined minimum distance between said boom and said load attaching device regardless of the hoisted position of said boom.

PAUL G. WHITE.

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