US3920096A

US3920096A - Vertical hydraulic ram system for scissors assembly scaffold

Info

Publication number: US3920096A
Application number: US484812A
Authority: US
Inventors: Robert E Fisher
Original assignee: Upright Inc
Current assignee: W R CARPENTER HOLDING PTY Ltd; Upright Inc; Up Right Inc
Priority date: 1974-07-01
Filing date: 1974-07-01
Publication date: 1975-11-18
Anticipated expiration: 1992-11-18

Abstract

A scaffold having a base frame and a platform mounted on a vertically extensible and contractable scissors assembly, the scissors assembly comprising a plurality of ascending and interconnected scissors stages, each stage having spaced-apart pairs of elongated scissors elements pivotally connected at their midpoints. Two spaced-apart hydraulic lift cylinders extending vertically within the scissors assembly from the bottom of the base frame to the bottom of the platform when contracted are connected to frame and lowermost scissors stage to act vertically thereon and cause full upward extension of the platform. A third vertical hydraulic ram disposed centrally of the base frame acts upwardly directly on the platform to aid in the initial elevation of the platform and extension of the scissors stages.

Description

United States Patent Fisher Nov. 18, 1975 1 VERTICAL HYDRAULIC RAM SYSTEM FOR SCISSORS ASSEMBLY SCAF FOLD Inventor:

[52] US. Cl. "182/63; 182/14; 182/148 [51] Int. Cl. E04G 1/22 [58] Field of Search 182/63, 141, 148, 12, 14; 52/109; 254/122 [56] References Cited UNITED STATES PATENTS 2,945,551 7/1960 Annin 182/63 3,472,488 10/1969 Naughton 254/122 3,596,735 8/1971 Denier 182/63 3,827,840 8/1974 52/109 Kistler Primary E.ramirzerReinaldo P. Machado Attorney, Agent, or Firm-Phillips, Moore,

Weissenberger, Lempio & Strabala ABSTRACT A scaffold having a base frame and a platform mounted on a vertically extensible and contractable scissors assembly, the scissors assembly comprising a plurality of ascending and interconnected scissors stages, each stage having spaced-apart pairs of elongated scissors elements pivotally connected at their midpoints. Two spaced-apart hydraulic lift cylinders extending vertically within the scissors assembly from the bottom of the base frame to the bottom of the platform when contracted are connected to frame and lowermost scissors stage to act vertically thereon and cause full upward extension of the platform. A third vertical hydraulic ram disposed centrally of the base frame acts upwardly directly on the platform to aid in the initial elevation of the platform and extension of the scissors stages.

9 Claims, 6 Drawing Figures US. Patent Nov. 18, 1975 Sheet 1 Of4 3,920,096

US Patent Nov. 18, 1975 Sheet 3 of4 3,920,096

FIG 3 FIG 4 VERTICAL HYDRAULIC RAM SYSTEM FOR SCISSORS ASSEMBLY SCAFFOLD BACKGROUND OF THE INVENTION Various types of mobile scissors scaffold devices are presently available on the commercial market. In general these scaffolds have a wheeled frame, a vertically extensible and collapsible scissors assembly on the frame and a work platform carried on the upper end of the scissors assembly. Most of these devices are large in size.

There is a need for a small scissors scaffold of this type which can be more maneuverable, pass through narrow aisles and relatively small doors, fit into small work spaces and be transportable by small trucks or trailers. In addition, such a scaffold must be as inexpensive as possible in order to be competitive with other types of small mobile extensible scaffolds. At the same time, the scaffold should have all of the operational advantages found in larger, more expensive devices. Reduction in size results in less space in the device in which to put the operating components. Special parts may be designed to fit into the smaller space but this will increase the cost. Standard parts of small size may be use but with a decrease in working effectiveness.

It is the primary object of this invention to provide a small scissors scaffold utilizing inexpensive and readily available components, with such components being arranged for maximum efficiency.

It is a further object of this invention to provide such a scaffold having the operational advantages of more expensive scaffolds. In particular, the present scaffold can be operated by a' workman standing on the work platform. From there, he can drive the scaffold forwardly or backwardly, and at different speeds, steering as he goes whether the platform is raised or lowered. When the scaffold reaches its desired destination, the operator can lock the drive wheels against movement without demounting. The operator can then cause the platform to raise to any desired level and can lower the platform.

SUMMARY OF THE INVENTION The present invention is a self-contained device, carrying its own power unit which comprises a hydraulic system for performing the various desired functions,

'the hydraulic system including a hydraulic pump driven by a d.c. motor. Although any d.c. motor could be used, the continuous types have a high amount of coprunning torque. The motor is powered by four standard batteries, such as used in golf carts. These batteries are placed as low as possible outwardly of the centerline of 'the machine so that their weight is used to provide stability to the device.

The hydraulic lift system uses three hydraulic rams which are vertically arranged for best mechanical advantage. One of these rams is located centrally of the frame and acts directly upon the platform to move the platform upwardly so that the scissors stages begin to unfold. At the same time the other two rams act upwardly on the lowermost scissors stage to assist in the initial elevation of the platform. The other two rams then take over, pulling directly upwardly on the scissors elements of the lower stage to cause them and the higher scissors stages to extend upwardly. The use of two left rams, one on each side of the device and closely located to the scissors elements on which they act provides an efficient arrangement which at the same time gives more stability and rigidity to the erected scaffold. I

The two hydraulic lift rams are hydraulically interlocked so that the platform will not tilt even though the workman or the load on the platform shifts around.

The degree of vertical extension of the scaffold is a function of the length of stroke of the lift rams. The 1 present invention utilizes as much of the vertical space as possible below the platform so that correspondingly long rams can be used. In particular, the lift rams are positioned vertically and extend from the bottom of the frame close to the ground, upwardly to just below the platform when it is contracted. Yokes then extend downwardly from the upper end of the rams and are attached to the lower scissors stage so that the rams will act directly only on the lower scissors stage elements.

The lift system is also designed so that power is required only for elevating the platform. On platform descent, the hydraulic fluid from the rams passes through constant-flow devices so that a safe rate of descent will be had regardless of variations in scaffold loading.

Other objects and advantages will be apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings fonning a part of this application, and in which like parts are designated by like reference numerals throughout the same,

FIG. 1 is an elevational side view of a scissors-type mobile scaffold utilizing the present invention and in partially extended position;

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, the mobile scaffold 10 comprises a main frame 11 supported by wheels 12, a multi-stage scissors assembly 13 and a work platform 14 mounted on the upper end of the scissors assembly. By way of example, a prototype of the mobile scaffold described herein has a main base frame 39 inches wide and inches long. The upper surface of the work platform is 41 inches off the ground when contracted and is extendible to 14 feet from the ground.

The illustrated scissors assembly comprises a lower stage 15, an intermediate stage 16 and an upper stage 17. If desired, more or less stages may be provided. As best seen in FIGS. 1 and 3, the lower scissors stage 15 comprises an inner frame made up of longitudinal opposed tubular scissors elements 18. and 19,

cross braces

20, 21 and 22 rigidly connected at their ends to the

scissors elements

18 and 19, and longitudinal

inner braces

23 and 24 extending between and rigidly se- 3 cured to cross braces and 21.

Scissors elements

18 and 19 each have a downwardly extending lug at one end which is pivotally mounted on a pivot pin 23 on the main frame 11, and a clevis 26 on the other end for pivotal connection by pin 27 to the lug 25 of the inner frame of the intermediate scissors stage 16.

The lower scissors stage 15 also includes longitudinal opposed

tubular scissors elements

28 and 29 outwardly of the inner frame. These scissors elements each have a depending lug 25 at one end thereof which carry rollers .30 for longitudinal movement in the channel member .31 fixed to the main frame 11. The scissors elements 27 and 28 each have a clevis 26 on the other end for pivotal connection to the lug 25 of the corresponding outer scissors elements of the intermediate scissors :stage.

Both of the spaced-apart pairs of scissors elements, 18-28 and 1929, are pivotally connected together at their midpoints by transverse horizontal pins 32. The

scissors elements

18, 19, 28 and 29 are also provided on top and bottom thereof with longitudinally extending stiffeners 33 which strengthen the middle of the elernents and also prevent them from contracting to an over-center position.

The intermediate and upper scissors stages are similarly constructed in a manner as thus far described, and corresponding elements of the stages have the same reference numerals applied thereto. With reference to the upper stage, the

outer scissors elements

19 and 29 thereof are pivotally mounted to platform 14 by pivot pins 34, while

scissors elements

18 and 28 have rollers 35 thereon which ride in channels 36 on the bottom of the platform.

Platform 14 includes a frame having a suitable number of longitudinal frame tubes 37 and cross frame tube .38 to support flooring 39. Guard rails 40 extend around the periphery of the platform. A control box 41 is mounted at a convenient place on the guard rails, and is connected by electric cable 42 which extends back and forth along the interconnected scissors elements on one side of the scissors assembly down to the main frame 11.

Main frame 11 has four battery compartments 43 thereon, two of which are seen in FIG. 1, the other two being on the opposite side of the frame. These compartments are as close to the ground as possible, while still providing for sufficient ground clearance, and are located outwardly of the centerline of the machine so that the weight of the batteries therein will increase the stability of the apparatus. The battery weight is not insignificant, since golf cart batteries weigh in the order of 75 pounds each.

Wheels 12 preferably have relatively unyielding solid rubber tires 44, again for stability purposes. As seen in FIG. 4, the axle 45 of the right rear wheel 12 is journaled in

bearings

46 and 47 carried by frame pedestal 48, and is driven by chain 49 and hydraulic motor 50. The left rear wheel 12 is similarly mounted on the other side of frame 11 and is driven by a separate hydraulic motor 51. The two front wheels 12 are conventionally mounted on frame 11, as schematically illustrated in FIG. 5, for right and left turning by movement of bell crank 52 produced in response to actuation of piston rod 53 by the double-acting hydraulic cylinder 54.

Referring to FIG. 2, the lower scissors stage 15 has an upwardly extending yoke 55 secured thereto, one side of the lower end of the yoke being pivotally mounted on pin 32 adjacent scissors element 19 and the other side of the yoke being pivotally mounted on pin 56 which is carried by brace 24 in axial alignment with pin 32. The body of hydraulic ram 60 is secured at its upper end to yoke 55. The piston rod 61 extends downwardly and has a clevis 62 at its lower end which is pivotally connected by pin 63 to the longitudinal frame member 64 situated at the bottom of frame 11. A second hydraulic ram 65 is similarly secured by yoke 66 to the lower scissors stage on the opposite side of the longitudinal centerline of the frame, with its piston rod 67 secured by clevis 68 to the bottom of the frame. A third hydraulic ram 70 is mounted on longitudinal frame member 71 on the bottom of and centrally of frame 11, with its piston rod 72 being extended upwardly against the central frame tube 34 of platform 14. Frame members 73 provide lateral support for ram 70. As may be seen from FIG. 2, the bodies of the vertically disposed rams 60, 65 and 70 extend virtually the entire distance from the botton of the frame to the bottom of platform 14. Since the length of the stroke of the piston rods is dependent on the length of the bodies of the rams, this arrangement enables rams to be disposed vertically, completely under the platform and still have as long a length stroke as possible. Even longer rams could be used if they were to extend up through holes in the platform. However, it is customarily desired that nothing should project through the platform since this would interfere with loading of articles onto the platform when it has been lowered.

It is also to be noted that the lift rams 60 and 65 are upside-down, in that their cylinder bodies are attached to yokes 55 and 66 and their

pistons

61 and 67 act downwardly. This arrangement enables standard commercially available rams to be used without having to provide a lug or clevis on the end of the ram body so that it can be pivotally mounted. A pivotal connection is necessary, since the pins 32 travel through an arcuate path 32' (FIG. 1) as the scissors stages extend and retract. Since the yokes are free to pivot about

pins

32 and 56, the ram body can be directly and rigidly connected to its yoke which will allow the ram body to pivot during extension and retraction of the scissors assembly. Although a pivot lug or clevis on the end of the ram body would only require a few inches of space, for a three-stage scissors assembly every inch of stroke of the lift ram pistons will produce six inches of vertical movement of the platform.

OP ERATION The operation of the mobile scaffold 10 is best considered with reference to the hydraulic and electrical diagrams of FIGS. 5 and 6.

Main switch 75, preferably a key-type switch to prevent unauthorized use of the scaffold, is closed, and offon switch 76 on the control panel 41 is closed, to supply voltage from batteries 77 to bus 78. The operator can now cause the platform 14 to elevate by closing the Up-Down switch 79 against contact 80, thus completing the circuit to the Up solenoid 81. At the same time, a circuit is completed through diode 82 to relay 83, causing relay contacts 84 to close so that the motor 85 is energized by the batteries. As mentioned previously, motor 85 is preferably an inexpensive d.c. motor commonly used in automobiles as a starter motor. Motor 85 drives hydraulic pump 86 to supply fluid pressure to common line 87. Merely by way of example, in the aforementioned prototype, the pump has a flow of 3 gpm at no load and a flow of 2.5 gpm at 2,000

psi pressure. Pressure in line 87 is communicated to the shuttle of spring-loaded valve 88 so that valve 88 will relieve excessive pressure in line 87. In the aforementioned prototype, this is set to operate at 2,600 psi.

Since the Up solenoid 81 has been energized, the shuttle of Up valve 89 has moved to the left (as seen in FIG. 5) allowing fluid from common line 87 to flow therethrough, and then through check valve 90, line 91 and check valve 92 to the lower end of the central kick cylinder 70. At the same time, the fluid flowing through Up valves 89 flows through

check valves

93 and 94 to the upper ends of lift rams 60 and 65. The purpose of the kick ram 70 is to assist in the initial elevation of the platform, so that the scissors elements will move out of parallelism with each other. Piston 72 acts upwardly directly on platform 14, causing it to move upwardly, pulling the scissors elements therewith. During such time, lift rams 60 and 65 also aid in the initial extension. Thus, when most power is needed, all three rams act together so that the pressure in the hydraulic system will be as low as possible, with a resulting increase in hydraulic efficiency. In due course, piston 72 will be fully extended. Thereafter the lift rams 60 and 65 take over, applying an essentially vertical lifting force to the midpoints of the opposed pairs of scissors elements of the lower scissors stage. Through the pivotal connections between the scissors stages, extension of the lowest stage will cause a corresponding extension of the other scissors stages and a consequent raising of the platform 14. Since the pressure fluid is free to flow upwardly through both

check valves

93 and 94, the pressure applied to both

pistons

61 and 67 will be equal at all times during elevation of the platform.

Pressure relief valve 95, responsive to the pressure in line 91, is employed so that if the platform is overloaded in the beginning, valve 95 will open to bleed off pressure in line 91 so that the platform cannot be raised until the overload is removed therefrom. By way of example, in the aforementioned prototype, the valve 95 is set to operate at l,300l ,400 psi, which will lift a load of 1,000 pounds on the platform.

At any time during elevation, or after full elevation, the Up switch 79 can be opened, de-energizing the Up solenoid 81, relay 83 and motor 85. Check

valves

93 and 94 now prevent flow of fluid from

rams

60 and 65 back down through the Up valve 89. Fluid from

rams

60 and 65 could flow through

check valves

96 and 97, but check valve 90 prevents such flow from flowing down through the Up valve 89. Check

valves

96 and 97 also serve to prevent flow from lift ram 60 to lift ram 65 or vice versa, which is important for stability purposes after elevation of the platform. For example, suppose the workman or other load on the platform were to move to the side of the platform above lift ram 60. If the

rams

60 and 65 were in direct fluid communication with each other, the increase in downward force on ram 60 would cause fluid therein to flow to ram 65, causing an undesirable tilting of the platform.

To lower the scaffold, the operator closes the Up- Down switch 79 against the Down contact 98 to energize the Down solenoid 99 of the Down hydraulic valve 100. Motor 85 is not energized. Fluid from the lift rams 60 and 65 can now flow simultaneously through

check valves

96 and 97, and then together through the constant flow device 101 to line 91 and then through the now open Down valve 99 to the return line 102 and thence through filter 103 to sump 104. In due course, platform 14 will come down onto piston 72 of the kick ram 70. The fluid therein will flow out through the constant flow device 105 to line 91, valve and return line 102. The constant flow devices 101 and are adjusted so that the rate of descent is within safe limits regardless of the load on the platform. If desired, a manually operable valve 106 is provided on the main frame, in parallel to Down valve 100, so that the platform can be lowered by a workman at ground level.

The mobile scaffold is also arranged to be driven, either forwardly or rearwardly and at different speeds by actuation of the drive control switch 110 which has a switch member 111 movable from its illustrated neutral position into engagement with one of the

Forward contacts

112, 113, 114 and 115 or with one of the

Reverse contacts

116, 117, 118 and 119.

When switch member 111 engages contact 112, it closes a circuit directly to Bypass 1 solenoid 120, and through diode 121 to Bypass 2 solenoid 122, causing both of the

Bypass valves

123 and 124 to shift to full open position. This enables fluid to flow to the right through the series-connected

hydrualic motors

50 and 51, up through check valve 125, Bypass 2 valve 124, flow restrictor 126, down through check valve 127 and back to motor 50. At the same time, the fluid flowing up through check valve can also flow through line 128, Bypass 1 valve 123, flow restrictor 129 and back through check valve 127 to motor 50. Conversely, fluid may flow to the left through the series-connected

motors

50 and 51, up through check valve 130, Bypass 1 valve 123, flow restrictor 129, down through check valve 131 to motor 50. Such fluid can also flow through line 128, Bypass 2 valve 124, flow restrictor 126, and check valve 131 to motor 50. This arrangement, wherein fluid can circulate through the

motors

50 and 51, in either direction, is referred to as the free-wheeling condition.

The switch member 1 11 can now be advanced to engage contact 113. Current can flow through diode 132 so that both

Bypass solenoids

120 and 121 remain energized. Current also flows through

diodes

133 and 134 and causes the Forward solenoid 135 to be energized, shifting the Forward-Reverse valve 136 to the left. At the same time, current will flow through

diodes

137 and 138 to energize relay 83 to start motor 85 and hydraulic pump 86. Pressure fluid from line 87 can now flow through valve 136 to line 139, to the right through

motors

51 and 50, and then by line 140 back through valve 136 to the return line 87. At the same time, fluid can flow up through the check valve 130, through both

Bypass valve

123 and 124 and the

flow restrictions

126 and 129 and then through check valve 131 to line 140. The flow through the flow restrictors, which are thus connected in parallel with each other and with the

motors

50 and 51, will thus cause a considerable amount of the pressure fluid in line 139 to bypass the motors so that they will drive the scaffold forwardly at a low speed. In the aforementioned prototype, this low, or creep, speed is in the order of 0.5 mph.

Ifthe swith member 111 is now moved to contact 113, a circuit will be completed through diode 141 to energize Bypass 2 solenoid 122.

Diodes

133 and 121 will prevent Bypass 1 solenoid 120 from being energized. Bypass 1 valve 129 thus returns to its position illustrated in FIG. 5. The pressure fluid in line 139 can now bypass

motors

50 and 51 only through the flow restrictor 126. As a consequence, more fluid is forced through

motors

50 and 51 and the forward speed of the scaffold is increased, as for example to 1 mph.

When the switch member 111 is moved to contact ll15, the Forward solenoid and motor 85 remain energized as before. but diode 134 prevents energization of either Bypass solenoid. Thus, all of the pressure fluid in line 139 must flow through

motors

50 and 51, applying full power to the wheels, so that, for example, a full speed of two mph is obtained.

Although the hydraulic pressure to the lift rams is limited by valve 95, the full pressure of the system, as limited by valve 88, is available for driving purposes.

The mobile scaffold can be driven in a reverse direction when desired. Movement of switch member 11 to contact 116 causes the two Bypass solenoids 120 and l21 to be energized, shifting the two

Bypass valves

123 and 124 to open, or freewheeling, position. Movement cf switch 111 to contact 117 energizes the two Bypass solenoids, Reverse solenoid 142 and, through diode l43, relay 83 and motor 85 so that the scaffold is driven at low speed. The Forward-Reverse valve 136 has now been shifted to the right from its illustrated position so that fluid flows in an opposite direction through

motors

50 and 51. Similarly, when switch 111 engages contact l18, only the Bypass 2 valve is actuated and the wheels are driven at intermediate speed. Full reverse power is delivered to the wheels when switch 111 engages contact 119.

When the scaffold has been moved to its desired position, switch member 111 is returned to its illustrated neutral position. At such time, none of the solenoids l20, 122, 135 or 142 is energized. As a consequence, all flow through

motors

54 and 51, in either direction, lS blocked and the wheels are thus locked against movement.

Steering is accomplished by the Right-Left switch l44. When the switch engages contact 145, the Right solenoid 146 is energized and the Right-Left valve 147 llS actuated thereby. A circuit is also completed through diode 148 to relay 83 so that the hydraulic pump 86 is driven. Pressure fluid in line 87 can now flow through the Right-Left valve to steering cylinder 54. Piston 53 moves inwardly into the cylinder to cause the front wheels 12 to turn to the right. Similarly, closing of switch 144 against contact 149 energizes Left solenoid 150 and relay 83, so that pressure fluid in line 87 car tlow through valve 147 and cause extension of piston :53 and left turning of the wheels.

.ln order to prevent fast forward or reverse movement when the platform has been raised, a microswitch 151 llS positioned on the main frame so as to close after a limited amount of upward movement of the platform. As may be seen from FIG. 6, whenever the Forward or

lReverse solenoids

135 or 142 are energized, a circuit will be completed through diode 152 to the microswitch 151. If this switch is closed, as it will be if the platform is raised, Bypass 2 solenoid 122 will be energized. Thus, even if switch 111 is moved to contacts ll or 119 for full speed, the Bypass 2 valve 124 remains open to bypass fluid around

motors

50 and 51 and thus limit the speed of the scaffold.

At times the batteries may run down so that there is insufficient power present to run the hydraulic pump 36. The operator may close switch 153, which will complete circuits through

diodes

154 and 155 to the

Bypass solenoids

120 and 122. Unless the batteries are completely dead, there will still be sufficient power remaining to energize the solenoids so that the

Bypass valves

123 and 124 open, to free-wheeling condition. With the wheels unlocked, the scaffold can be pushed 8 or towed with the hydraulic fluid in the

hydraulic motors

50 and 51 being pumped through the Bypass valves as the wheels turn.

A battery charger 156 is also provided, this charger 5 being located on frame 11 between the forward pair of batteries 77.

Having thus described my invention, 1 claim:

1. An elevating scaffold comprising:

a. a generally rectangular frame having an upper surlO face and lower central support members suspended a substantial distance below said upper surface,

b. a plurality of ground-engaging wheels mounted on said frame, the bottom surfaces of said wheels being a relatively small distance below said lower 5 central support members,

c. an upwardly extensible and downwardly contractable scissors assembly mounted on the upper surface of said frame, said scissors assembly comprising a plurality of pivotally interconnected ascending scissors stages, the lowermost of which is connected to said frame, each stage comprising laterally spaced-apart pairs of rigid elongated scissors elements pivotally connected together at their midpoints,

d. a work platform supported in and connected to the uppermost scissors stage,

e. a first yoke connected to the midpoint of one of said pairs of scissors elements of said lowermost scissors stage and a second yoke connected to the midpoint of the other of said pairs of scissors elements of said lowermost scissors stage, said yokes extending upwardly substantially to the underneath of said platform when said scissors assembly has been fully contracted,

f. a pair of laterally spaced-apart, hydraulically extensible rams, one for each yoke, the lower ends of said rams being pivotally secured to said lower central support members and the upper ends of the rams being secured to the upper ends of said yokes,

g. means for forcing hydraulic fluid simultaneously into and for relieving hydraulic fluid simultaneously from said rams.

2. Apparatus as set forth in claim 1 wherein the bodies of said rams are connected to said yoltes and the pis- 5 tons of said rams are connected to said central support members.

3. Apparatus as set forth in claim 1, and wherein said scissors stages are pivotally connected together so that the scissors elements thereof are in parallelism when said scissors assembly has been fully contracted, and

further including:

h. a third hydraulically extensible ram, said ram being vertically disposed with its lower end supported on said lower central support members and its upper end engageablle with the bottom of said platform centrally thereof, the height of said third ram when fully contracted being substantially equal to the vertical distance from said lower central support members to the bottom of said platform when said scissors assembly is fully contracted,

and wherein said means (g) additionally forces bydraulic fluid into and relieves hydraualic fluid from said third ram.

4. Apparatus as set forth in claim 3, wherein said means (g) comprises:

a hydraulic fluid sump mounted on said frame,

a hydraulic pump having its inlet in fluid communication with said sump,

. 9 an electric motor drivingly connected to said pump, a plurality of batteries electrically connectable to and disconnectible from said motor, said batteries being supported by said frame at a low level above the ground and near each of said wheels, conduit means connecting the output of said pump to each of said three rams and for connecting each of said three rams to said sump,

first valve means in said conduit means for opening and closing said conduit means for flow of hydraulic fluid from said pump to said rams,

second valve means in said conduit means for opening and closing said conduit means for flow of hydraulic fluid from said rams to said sump.

5. Apparatus as set forth in claim 4, wherein said means (g) further includes check valve means in said conduit means for preventing flow of hydraulic fluid between said first and second rams.

6. Apparatus as set forth in claim 4, wherein said means (g) further includes constant flow devices in said conduit means in the path of hydraulic fluid flowing from said rams to said sump. I

7. Apparatus as set forth in claim 6, wherein said means (g) further includes control means for selectively opening, one at a time, one or the other of said first and second valve means, and for electrically connecting said batteries to said motor when said first valve is open.

8. Apparatus as set forth in claim 7, wherein said means (g) further includes check valve means in said conduit means for preventing flow of hydraulic fluid between said first and second rams.

9. Apparatus as set forth in claim 4 wherein said means (g) further includes means for limiting the pressure of hydraulic fluid applied through said conduit means from said pump to said rams. Y

Claims

1. An elevating scaffold comprising: a. a generally rectangular frame having an upper surface and lower central support members suspended a substantial distance below said upper surface, b. a plurality of ground-engaging wheels mounted on said frame, the bottom surfaces of said wheels being a relatively small distance below said lower central support members, c. an upwardly extensible and downwardly contractable scissors assembly mounted on the upper surface of said frame, said scissors assembly comprising a plurality of pivotally interconnected ascending scissors stages, the lowermost of which is connected to said frame, each stage comprising laterally spaced-apart pairs of rigid elongated scissors elements pivotally connected together at their midpoints, d. a work platform supported in and connected to the uppermost scissors stage, e. a first yoke connected to the midpoint of one of said pairs of scissors elements of said lowermost scissors stage and a second yoke connected to the midpoint of the other of said pairs of scissors elements of said lowermost scissors stage, said yokes extending upwardly substantially to the underneath of said platform when said scissors assembly has been fully contracted, f. a pair of laterally spaced-apart, hydraulically extensible rams, one for each yoke, the lower ends of said rams being pivotally secured to said lower central support members and the upper ends of the rams being secured to the upper ends of said yokes, g. means for forcing hydraulic fluid simultaneously into and for relieving hydraulic fluid simultaneously from said rams.

2. Apparatus as set forth in claim 1 wherein the bodies of said rams are connected to said yokes and the pistons of said rams are connected to said central support Members.

3. Apparatus as set forth in claim 1, and wherein said scissors stages are pivotally connected together so that the scissors elements thereof are in parallelism when said scissors assembly has been fully contracted, and further including: h. a third hydraulically extensible ram, said ram being vertically disposed with its lower end supported on said lower central support members and its upper end engageablle with the bottom of said platform centrally thereof, the height of said third ram when fully contracted being substantially equal to the vertical distance from said lower central support members to the bottom of said platform when said scissors assembly is fully contracted, and wherein said means (g) additionally forces hydraulic fluid into and relieves hydraualic fluid from said third ram.

4. Apparatus as set forth in claim 3, wherein said means (g) comprises: a hydraulic fluid sump mounted on said frame, a hydraulic pump having its inlet in fluid communication with said sump, an electric motor drivingly connected to said pump, a plurality of batteries electrically connectable to and disconnectible from said motor, said batteries being supported by said frame at a low level above the ground and near each of said wheels, conduit means connecting the output of said pump to each of said three rams and for connecting each of said three rams to said sump, first valve means in said conduit means for opening and closing said conduit means for flow of hydraulic fluid from said pump to said rams, second valve means in said conduit means for opening and closing said conduit means for flow of hydraulic fluid from said rams to said sump.

6. Apparatus as set forth in claim 4, wherein said means (g) further includes constant flow devices in said conduit means in the path of hydraulic fluid flowing from said rams to said sump.

9. Apparatus as set forth in claim 4 wherein said means (g) further includes means for limiting the pressure of hydraulic fluid applied through said conduit means from said pump to said rams.