US3786936A

US3786936A - Overhead crane apparatus

Info

Publication number: US3786936A
Application number: US00192026A
Authority: US
Inventors: C Staadt
Original assignee: MASONRY SYST INT
Current assignee: MASONRY SYST INC US; MASONRY SYST INT
Priority date: 1971-10-26
Filing date: 1971-10-26
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22

Abstract

An overhead crane adapted for universal movement in a horizontal plane includes a pair of bridge members which span the width of an industrial plant and have guide wheels adapted to be driven along rails running longitudinally along the sides of the plant, a trolley which extends longitudinally of the plant and is mounted on guide wheels which ride along tracks on the bridge members, the trolley having a hoist stationary relative to the trolley, a hoist movable along the trolley, and an operator''s cab at one end of the trolley whereby the operator is provided with a close unobstructed view of the hoists. A sprocket-chain drive is operative to move the trolley along the bridge members to enable the hoists to be positioned close to the sides of the installation site.

Description

United States Patent [1 1 v [1 1 3,786,936 Staadt Jan. 22, 1974 OVERHEAD CRANE APPARATUS [75] Inventor: Charles R. Staadt, Aurora, C010.

[73] Assignee: Masonry Systems, lnc.,

Westminster, C010.

[22] Filed: Oct. 26, 1971 [211 Appl. No.: 192,026

[52] US. Cl 212/11, 212/74, 212/124 [51] Int. Cl. B66c 17/06 [58] Field of Search. 212/10, 11, 74, 125, 126,124; 105/163 R [5 6] References Cited UNITED STATES PATENTS 2,644,593 7/1953 Miller 212/11 1,835,357 12/1931 Townsend 212/125 X 982,883 l/l9ll Schnabel 212/11 2,063,910 12/1936 Fitch 212/126 X 3,103,395 9/1963 Winberg i. 212/74 3,116,586 l/l964 lngham 212/125 X 977,279 11/1910 De Bay i 2l2/ll 2,957,432 10/1960 Schorel 105/163 R Primary ExaminerAllen N. Knowles Assistant Examiner-Gene A. Church Attorney, Agent, or FirmBurton, Crandell and Polumbus [57] ABSTRACT An overhead crane adapted for universal movement in a horizontal plane includes a pair of bridge members which span the width of an industrial plant and have guide wheels adapted to be driven along rails running longitudinally along the sides of the plant, a trolley which extends longitudinally of the plant and is mounted on guide wheels which ride along tracks on the bridge members, the trolley having a hoist stationary relative to the trolley, a hoist movable along the trolley, and an operators cab at one end of the trolley whereby the operator is provided with a close unobstructed view of the hoists. A sprocket-chain drive is operative to move the trolley along the bridge members to enable the hoists to be positioned close to the sides of the installation site.

11 Claims, 6 Drawing Figures PATENIEBJAN22IQY4 sum 1 or 3 I INVENTOR CHARLES R. STAADT PATENTEB JAN 2 21974 sum 2 0P3 PMENIED JAN 2 2 i974 sum 3 0F 3 OVERHEAD CRANE APPARATUS The present invention relates generally to industrial lifting and transporting equipment and more particularly to a novel and improved overhead crane adapted for universal movement in a horizontal plane overlying the work area of an industrial plant or warehouse.

Customarily prior art overhead cranes are typified by a large rigidly constructed rectangular framework that is mounted on wheels to be driven longitudinally of the industrial plant or warehouse. The framework generally includes two transversely extending bridge members which support driven wheel-carrying trucks at the ends of an elongated longitudinally extending trolley. The trolley is movable laterally of the plant on the bridge members so that a hoist on the trolley is positionable universally in a horizontal plane overlying most of the work area in the plant. The hoist is prevented from being positioned over a strip of the work area along each side wall of the plant because the trucks at the ends of the trolley have long wheel spans, usually about one-half the width of the crane, to prevent the trolley from becoming misaligned between the bridge members. Thus, when the trolley is moved to one side or the other of the plant, the trolley trucks are disposed adjacent theside wall of the plant but the hoist, which is centered with respect to the trucks, of necessity is spaced from the side wall a distance equal to about one-fourth of the total width of the crane, leaving a strip of work area which is not serviceable by the hoist.

The operation of a typical prior art crane is generally controlled from a booth mounted on one of the bridge members at the side of the plant. The control booth thereby moves longitudinally of the plant with the crane but does not have the capacity to move laterally across the width of the plant with the trolley. Inasmuch as the hoist moves with the trolley, the crane operator is seldom advantageously positioned relative to the hoist to have a good close view of the hoist and the objects being lifted and transported by the crane.

Another disadvantage of typical prior art cranes is that they are frequently rigidly put together prior to installation and due to their immense size are often difficult to transport and install in a previously existing plant. It is not infrequent that the roof of the plant must be removed before the crane is installed so that the crane can be lowered through the top of the plant onto supporting rails previously mounted on the interior side walls of the plant.

The overhead crane of the present invention is a very simply constructed and versatile crane adapted to service the entire work area of an industrial plant. The crane is provided with an operator's cab that moves with a pair of hoists in close proximity thereto to give the operator an advantageous bird's-eye view of the hoist operations.

Very basically the crane has two spaced bridge members which have guide wheels on their ends adapted to ride on and be guided by a pair of tracks mounted high on the side walls of a plant. The bridge members are held together by three separate spreader bars which are bolted at their ends to the bridge members and a posttensioned cable system which holds the bridge members and spreader bars in compression. A trolley, extending parallel to the spreader bars, has wheelcarrying trucks at its ends which are adapted to move along tracks on the bridge members. The trolley has a hoist which is movable longitudinally of the trolley, a hoist which is fixed in position on the trolley, and an operators cab that moves with the trolley whereby an operator in the cab will always have an advantageous view of both hoists and the objects which are lifted and transported by the crane.

A reversible motor is provided to drive one wheel at each end of one of the bridge members so that the crane can be moved longitudinally of the plant. A separate reversible motor is provided to drive a sprocket at each end of the trolley. Each sprocket has a stationary chain meshed therewith which is anchored to and supported by the associated bridge member whereby rotation of the sprocket advances the trolley in one direction or the other along the bridge members. The positive chain drive at both ends of the trolley prevents the trolley from becoming misaligned between the bridge members so that long trolley wheel spans are not required at the ends of the trolley to keep the trolley in proper alignment. In fact, in the disclosed embodiment the trolley wheel span is approximately one-eighth of the width of the crane. Therefore, the trucks at the ends of the trolley are provided with relatively short wheel spans whereby the hoists on the trolley can be moved into close relationship to the side walls of the plant, eliminating any work areas in the plant that are not serviceable by the crane.

It will be apparent from the detailed description later that the crane can be broken down into relatively long but narrow bundles which are easily transportable. In addition, the crane can be assembled on the rails in the plant, alleviating any need for removing the roof of the plant or otherwise disassembling the plant to install the crane.

Accordingly, it is an object of the present invention to provide a new and improved overhead crane that is simple in construction so that it can be readily assembled and disassembled for convenient transportation and ready installation.

It is another object of the present invention to provide an overhead crane which is universally movable in a horizontal plane and includes an operator cab that moves along with the hoists on the crane so that the operator is provided with a close and unobstructed view of the hoists and the work objects.

It is another .object of the present invention to provide an overhead crane which is adapted to move very close to the sides of an installation plant so that there is no work area in the plant wherein the hoists are not effective.

Other objects, advantages and features of the present invention will become apparent as the description proceeds, taken in conjunction with the accompanying drawings, in which:

FIG; 1 is a diagrammatic perspective view of the crane of the present invention mounted on a pair of parallel tracks;

FIG. 2 is a top plan view with parts broken away of the crane of FIG. 1;

FIG. 3 is a vertical section taken along line 3-3 of FIG. 2;

FIG. 4 is a vertical section taken along line 44 of FIG. 2;

FIG. 5 is a fragmentary perspective view of one of the drive wheels and sprockets for the crane of FIG. 1; and

FIG. 6 is a section taken along line 66 of FIG. 2.

Referring first to FIGS. 1 and 2, a preferred form of the overhead crane of the present invention is shown. The crane can be seen to have a base framework 24 including a pair of transversely extending

bridge members

26 and 28, three

spreader bars

30, 32 and 34 interconnecting the

bridge members

26 and 28, and a plurality of tensioned cables 36 which hold the bridge members and the spreader bars in compression.

The spreader bar 32 is bolted to the top of the

bridge members

26 and 28 at the centers thereof while the spreader bars 30 and 34 are bolted to associated ends of the bridge members in a manner to be described later. The tensioned cables 36 are criss-crossed in connecting the ends of the bridge member 26 to the central portion of the bridge member 28 and the ends of the bridge member 28 to the central portion of the bridge member 26. The cables 36 are releasably connected to the bridge members to facilitate quick assembly and disassembly of the crane. A very rigid and sturdy framework is thereby provided with the tensioned cables holding the bridge members and the spreader bars in compression so that the framework 'will not give in any direction. It will be apparent that the framework can be easily disassembled merely by releasing the tensioned cables and unbolting the spreader bars from the bridge members.

The

bridge members

26 and 28 are identical and can be seen in FIGS. 3 and 4 to include an elongated I- beam 40 that extends transversely across the width of the plant 22 and has a downwardly opening channeled bar 42 welded to the top thereof. The I-beam 40 and channeled bar 42 are seen to terminate at both ends in web-shaped support plates 44 that protrude from the ends of the l-beam and channeled bar to provide support for trucks 46 having open rectangular frames 46 welded to the underside of the support plate. Each of the trucks 46 also has two longitudinally aligned guide wheels 48 whose axles are journaled in the sides of the frames 46'. The wheels 48 are peripherally grooved to mate with and ride along one of two tracks 50 which extend longitudinally of the side walls 51 of an industrial plant. The tracks 50 are both mounted on I-beam support bars 52 that are secured to brackets 54 anchored to the side walls of the plant. The tracks 50 extend the full length of the plant so that the crane can be moved along the tracks from one end of the plant to the other.

The spreader bars 30 and 34 are bolted to the ends of the trucks 46 to rigidly secure the ends of the

bridges

26 and 28 in predetermined spaced relation.

Referring now to FIGS. 1-3, bridge member 28 is seen to have a support plate 56 welded to the top thereof intermediate its ends. The support plate 56 extends outwardly away from the bridge member and supports on its underside an electric reverisble motor 58 which is operably connected through a clutch 60 to a gear reducer 62. The gear reducer has oppositely directed laterally extending output drive shafts 64 that extend over to and are journaled in brackets 66 (FIG. 2) welded to the tops of the trucks 46 associated with the bridge member 28. The drive shafts are directed over one of the wheels 48 in the associated trucks and each carries a rigidly affixed pinion gear 68 that meshes with a gear 70 keyed to the axle of a wheel 48. It will thus be apparent that the wheels 48 having the gears 70 secured thereto are driven by the motor 58 to advance the crane in one direction or the other along the tracks A trolley is suspended at opposite ends from the

bridge members

26 and 28 so as to be reversibly movable along the bridge members from one side of the plant to the other. The trolley 80 can be seen in FIGS. 14 to include an I-beam 82 that has an upturned arm 84 welded to the end adjacent bridge member 28 which rotatably carries a pair of laterally spaced and aligned guide wheels 86 adapted to roll along and be supported by the lower flange on the opposite side of the bridge I-beam 40 from the wheels 86. Spaced inwardly from the opposite end of the trolley 80 and welded to the trolley I-beam 82 is an upturned arm 92 corresponding to the upturned arm 84. The upturned arm 92 rotatably carries laterally spaced and aligned guide wheels 94 which are adapted to roll along and be supported by the lower flange on one side of the I-beam 40 associated with the bridge member 26. An upturned arm 96, corresponding to the upturned arm 88, is welded to the trolley I-beam 82 at a location speed inwardly from the arm 92 and rotatably carries guide wheels 98 adapted to roll along and be supported by the lower flange on the opposite side of the bridge l-beam 40 from the roller wheels 94. It will thus be apparent that the trolley 80 is guided on the

wheels

86, 90, 94 and 98 along the

bridge members

26 and 28 from one side of the plant 22 to the other. It will be appreciated that the

arms

84, 88, 92 and 96 as viewed from the ends of the plant 22 are relatively short compared to the overall width of the crane so that the trolley I-beam can be positioned very close to the side walls of the plant, which is advantageous for reasons to be set forth later. In the disclosed embodiment, the overall width of the crane and consequently the bridge members is approximately 24 feet and the length of the

arms

84, 88, 92 and 96 only about three feet so that the arms are only about one-eighth as long as the crane is wide.

The trolley I-beam 82 can be seen in FIG. 4 to have a downwardly opening channeled bar 104 welded to the top thereof. The channeled bar 104 has a reversible electric motor 106 bolted to the top thereof near its longitudinal center. The motor 106 is operably connected through a clutch to a gear reducer 108 that has oppositely extending drive shafts 110 which have their outermost ends journaled in the respective upturned arms 88 and between the guide wheels associated with the respective upturned arms. The end of each drive shaft has a drive sprocket 112 keyed thereto which is disposed above, between, and in radial alignment with two idler sprockets 114 rotatably carried by each of the

upturned arms

88 and 96. Each of the

bridge members

26 and 28 has a linked chain 1 l6 asso ciated therewith which is anchored by bolts or other suitable means at its ends to the ends of the associated bridge member and is trained over the idler sprockets 114 and under the drive sprocket 112 so that rotation of the drive sprocket 112 will positively drive the trolley 80 along the respective chains 116 and consequently along the

bridge members

26 and 28. It will thus be apparent that the motor 106 is operable to reversibly move the trolley transversely of the plant 22 along the bridge members.

A hoist 124 is attached to and suspended from the trolley I-beam 82 near the end of the I-beam that underlies the bridge member 28. The hoist is a conventional electrically operated hoist having a hook 126 on a cable 128 that may be lowered and raised by an operator.

A second hoist 130, identical to the hoist 124, is suspended from a roller supported carriage 132 which is adapted to be guided along the trolley l-beam 82 by roller pairs 134 on opposite sides of the I-beam. The hoist 130 is driven along the I-beam 82 by a manually opera ble crank mechanism to be described later.

The end of the trolley l-beam 82 adjacent to the bridge member 26 protrudes beyond the bridge member and supports from its underside an operator cab 140 that is bolted to the I-beam. The operator cab is provided with side panel controls for operating every electrical component of the overhead crane including both the longitudinal and lateral movements of the crane which may be effected simultaneously and the raising and lowering of the hooks on the

hoists

124 and 130. The seat in the cab faces the two hoists and, with the control on the side panels of the cab, the operator is provided with a close and unobstructed view of the hoists and the objects which will be lifted and transported by the hoists. Of course, having the cab mounted on the trolley with the hoists allows the cab and the hoists to move in unison so that the operators view of the hoists will never be obstructed. The electrical control system for the crane is considered conventional so that a detailed description of the control system is not deemed necessary.

As best seen from FIG. 3, the movement of the hoist 130 along the trolley l-beam 82 is effected by a manually operable mechanism which includes a crank 146 rotatably mounted in the top of the operator cab 140 so as to be easily accessible to the operator. A rotatable crank shaft 148 fixed to the crank 146 extends vertically above the top of the cab and has keyed on its end a sprocket 150. An idler sprocket 152 is rotatably mounted on the side of the trolley I-beam 82 near the stationary hoist 124. A linked chain 154, whose two ends are anchored to the hoist carriage 132 at 156, is trained around the

sprockets

150 and 152 so that when the crank 146 is rotated in the cab the sprocket 150 will be rotated causing the chain 154 to move with the hoist carriage 132 in a preselected direction along the length of the trolley. A chain guide, not shown, serves to confine the chain along its longitudinal runs. it will be seen that the hoist 130 may be positioned at any location between the bridge member 26 and the idler sprocket 152 so various sizes and shapes of objects may be picked up with the two

hoists

124 and 130 by varying the spacing therebetween.

In operation, although not shown, a stairway or ladder in the plant may be utilized by an operator to reach the cab. From the cab the operator may drive the entire crane framework 24 longitudinally of the plant by energizing the reversible electric motor 58. In order to position himself and consequently the

hoists

124 and 130 at any selected position laterally of the plant, he may energize the reversible electric motor 106 which drives the trolley 80 laterally of the plant. The above operations. of course. enable the operator to universally position himself and the

hoists

124 and 130 at any desired position in a horizontal plane overlying the work area in the plant. As previously mentioned, the operator can additionally move the hoist 130 longitudinally of the plant along the trolley 80 to vary the longitudinal spacing between the two hoists and facilitate the handling of various sized and shaped work loads; and because of the shortness of the

arms

84, 88, 92 and 96 at the ends of the trolley 80, the hoists can be moved very close to the side walls of the plant. In fact, the hoists can be moved so close to the side walls that the associated hooks on the hoists can be swung into contiguous relationship with the side walls when lowered so that the hoists are effective in the entire work area of the plant.

Another desirable feature of the crane 20 is that it can be assembled on the tracks 50 in the plant without the need of removing the roof of the plant or otherwise disassembling or altering the building structure. The crane can be assembled on the tracks 50 in the plant by raising the

bridge members

26 and 28 so that they extend longitudinally of the plant at a point above the level of the tracks 50, then turning the bridge members 90 until the guide wheels 48 overlie the tracks 50. The bridge members may then be simply lowered onto the tracks. Next the trolley may be mounted on the

bridge members

26 and 28 in a similar manner, placing the

wheels

86, 90, 94 and 98 on the trolley'after it has been positioned. The spreader bars 30, 32 and 34 may then be bolted to the bridge members and the entire structure bound together by post-tensioning the cables 36.

It will thus be appreciated that a simply constructed crane has been described which has the capacity to move universally in a horizontal plane over the work area in an industrial plant or warehouse. The crane is designed to be effective in the entire work area of the plant and has an operator cab that may advance in close proximity to the hoists so that the operator has a close unobstructed view of the hoists and the work loads.

It is therefore to be understood from the foregoing description of a preferred form of the present invention that various modifications and changes may be made in the specific design, construction and arrangement of mechanism employed in the present invention without departing from the spirit and scope of the present invention.

What is claimed is:

1. A crane apparatus adapted to move along a pair of tracks extending in a first direction in an industrial plant comprising in combination a pair of bridge members spaced from each other in said first direction and extending in a second direction normal to said first direction, said bridge members having bridge rollers on both ends for riding on said tracks, a trolley extending in said first direction having trolley rollers adapted to roll along said bridge members, said trolley having a pair of hoists suspended therefrom wherein at least one of said hoists is movable along the trolley in said first direction, first drive means for driving said bridge rollers in said first direction, and second drive means for driving said trolley rollers in said second direction, and an operator cab affixed to said trolley having control means for operating said first and second drive means and said hoist.

2. The crane apparatus of claim 1 further including rigid bar means operably attached to said bridge members to maintain a predetermined spacing between said members.

3. The crane apparatus of claim 1 wherein said first drive means includes a motor and drive shafts operably connected to the motor and to one of the bridge rollers at each end of one of the bridge members whereby the operably connected bridge rollers can be driven to move the crane apparatus along the said tracks.

4. The crane apparatus of claim 3 further including a fixed link chain extending the length of each bridge member and wherein said second drive means includes a motor mounted on said trolley, drive shafts operably connected to the motor, and a drive sprocket carried on each drive shaft that meshes with one of said chains whereby said trolley can be driven along said bridge members.

5. A crane apparatus adapted to roll along a pair of tracks running longitudinally along the side walls of an industrial plant comprising in combination a pair of longitudinally spaced transversely extending bridge members having bridge rollers for riding on said tracks, an elongated longitudinally extending trolley having trolley rollers at both ends adapted to roll along said bridge members, said trolley having a hoist with a vertically movable hook mounted thereon, drive means for moving said crane longitudinally oi the plant and for moving the trolley transversely of the plant, said trolley rollers having a narrow span transversely of the plant whereby when said trolley is moved adjacent one side of the plant the hook on the hoist can be made contiguous with the side wall on said one side of the plant, and a straight fixed length of link chain having opposite ends which are fixed to an associated bridge member and extending along the length of the bridge member, said drive means including a motor mounted on said trolley, two drive shafts operably connected to said motor, and a drive sprocket fixed on each drive shaft for meshing with respective ones of said link chains whereby rotation of said drive sprockets will cause said trolley to be advanced along said link chains on said bridge members.

6. The crane apparatus of claim 5 further including idler sprockets mounted on said trolley adjacent said drive sprockets, said idler sprockets also meshing with said link chains.

7. A crane apparatus adapted to roll along a pair of tracks running longitudinally in an industrial plant comprising in combination a pair of longitudinally spaced transversely extending bridge members, trucks on the ends of said bridge members having longitudinally aligned bridge guide wheels for supporting said bridge members for movement along said tracks, rigid spacer bars connected to said bridge members to hold them in predetermined spaced relationship, first drive means including a first motor, a first pair of drive shafts operably connected to the first motor and to at least one of said bridge guide wheels to drive said bridge guide wheel so that the bridge members can be driven as a unit along said tracks, an elongated longitudinally extending trolley having trolley guide wheels, supporting arms near the ends of the trolley for supporting the trolley guide wheels for movement along said bridge members, a hoist secured to said trolley, a second drive means including a second motor, a second pair of drive shafts operably connected to said second motor, each of said second drive shafts being journaled in one of said supporting arms near the ends of said trolley, a drive sprocket fixed on each of said second drive shafts adjacent said supporting arms, and a pair of elongated straight fixed lengths of link chains having opposite ends anchored at to an associated bridge member and extending along the length of said bridge members, said link chains meshing with associated ones of said drive sprockets whereby rotation of said drive sprockets will cause said trolley to roll along said bridge members.

8. The crane apparatus of claim 7 further including a pair of idler sprockets meshing with each of said chains, said idler sprockets being rotatably mounted on one of said supporting arms at each end of the trolley. 9. The crane apparatus of claim 7 further including a tensioned cable system including cable segments connecting the ends of one of said bridge members to a.

central portion of the other bridge member and cable segments connecting the ends of said other bridge member to a central portion of said one bridge member whereby said bridge members and said rigid spacer bars are held in compression by said cable segments.

10. The crane apparatus of claim 9 wherein said rigid spacer bars and said cable segments are releasably connected to said bridge members.

ll. An overhead crane apparatus adapted to roll along a pair of tracks running longitudinally along the upper portion of the side walls of an industrial plant comprising in combination a pair of longitudinally spaced transversely extending bridge members, trucks on the ends of said bridge members having longitudinally aligned bridge guide wheels for supporting the bridge members for movement along said tracks, rigid spacer bars relcasably connected to said bridge members to hold the bridge members in predetermined spaced relationship, tensioned cables connecting the ends of one of said bridge members to an intermediate portion of the other bridge member and connecting the ends of said other bridge member to an intermediate portion of said one bridge member whereby said bridge members and said spacer bars are held in compression by said tensioned cables, a first motor mounted on said one bridge member, a first pair of drive shafts operably connected to said first motor and extending transvcrsely of the plant in opposite directions, a drive gear on the outer end of each drive shaft operatively connected to one of said bridge guide wheels whereby said first motor is effective to drive said bridge wheels and move said bridge members along said track, an elongated longitudinaliy extending trolley having trolley guide wheels rotatably mounted thereon to ride along and be guided by said bridge members, a second motor mounted on said trolley, a second pair of drive shafts operably connected to said second motor and extending longitudinally of the plant in opposite directions, each of said second drive shafts having a drive sprocket fixed on its end, a pair of elongated link chains anchored to and extending along the length of said bridge members, said link chains meshing with associated ones of said drive sprockets whereby rotation of said drive sprockets will cause said trolley to move along said bridge members, a pair of hoists secured to said trolley with at least one of said hoists movable along the trolley in said first direction, and an operator cab mounted on said trolley to .move therewith, said operator cab including controls for operating said first and second motors and for operating said hoist whereby an operator in the operator cab can move universally with the hoist in a horizontal plane overlying the work area in the industrial plant.

4' i i l t

Claims

5. A crane apparatus adapted to roll along a pair of tracks running longitudinally along the side walls of an industrial plant comprising in combination a pair of longitudinally spaced transversely extending bridge members having bridge rollers for riding on said tracks, an elongated longitudinally extending trolley having trolley rollers at both ends adapted to roll along said bridge members, said trolley having a hoist with a vertically movable hook mounted thereon, drive means for moving said crane longitudinally of the plant and for moving the trolley transversely of the plant, said trolley rollers having a narrow span transversely of the plant whereby when said trolley is moved adjacent one side of the plant the hook on the hoist can be made contiguous with the side wall on said one side of the plant, and a straight fixed length of link chain having opposite ends which are fixed to an associated bridge member and extending along the length of the bridge member, said drive means including a motor mounted on said trolley, two drive shafts operably connected to said motor, and a drive sprocket fixed on each drive shaft for meshing with respective ones of said link chains whereby rotation of said drive sprockets will cause said trolley to be advanced along said link chains on said bridge members.

8. The crane apparatus of claim 7 further including a pair of idler sprockets meshing with each of said chains, said idler sprockets being rotatably mounted on one of said supporting arms at each end of the trolley.

9. The crane apparatus of claim 7 further including a tensioned cable system including cable segments connecting the ends of one of said bridge members to a central portion of the other bridge member and cable segments connecting the ends of said other bridge member to a central portion of said one bridge member whereby said bridge members and said rigid spacer bars are held in compression by said cable segments.

11. An overhead crane apparatus adapted to roll along a pair of tracks running longitudinally along the upper portion of the side walls of an industrial plant comprising in combination a pair of longitudinally spaced transversely extending bridge members, trucks on the ends of said bridge members having longitudinally aligned bridge guide wheels for supporting the bridge members for movement along said tracks, rigid spacer bars releasably connected to said bridge members to hold the bridge members in predetermined spaced relationship, tensioned cables connecting the ends of one of said bridge members to an intermediate portion of the other bridge member and connecting the ends of said other bridge member to an intermediate portion of said one bridge member whereby said bridge members and said spacer bars are held in compression by said tensioned cables, a first motor mounted on said one bridge member, a first pair of drive shafts operably connected to said first motor and extending transversely of the plant in opposite directions, a drive gear on the outer end of each drive shaft operatively connected to one of said bridge guide wheels whereby said first motor is effective to drive said bridge wheels and move said bridge members along said track, an elongated longitudinally extending trolley having trolley guide wheels rotatably mounted thereon to ride along and be guided by said bridge members, a second motor mounted on said trolley, a second pair of drive shafts operably connected to said second motor and extending longitudinally of the plant in opposite directions, each of said second drive shafts having a drive sProcket fixed on its end, a pair of elongated link chains anchored to and extending along the length of said bridge members, said link chains meshing with associated ones of said drive sprockets whereby rotation of said drive sprockets will cause said trolley to move along said bridge members, a pair of hoists secured to said trolley with at least one of said hoists movable along the trolley in said first direction, and an operator cab mounted on said trolley to move therewith, said operator cab including controls for operating said first and second motors and for operating said hoist whereby an operator in the operator cab can move universally with the hoist in a horizontal plane overlying the work area in the industrial plant.