Lift Safety System

The present invention relates to a safety system for multistage portable lifts of the type having extensible outriggers necessary for adequate support when a work platform mounted on the lift is elevated.

It is often convenient for construction and repair service work that materials, equipment and personnel be raised at a work site to elevations in the range of 30 to 40 feet by use of a portable lift that can be wheeled when lowered through doorways, and can be easily transported. For stability when elevated, it is essential that such lifts have outriggers, and it is preferred that the outriggers be easy to move between active and stowed positions. It is also preferred that the outriggers have an arrangement permitting the work platform of the lift to be positioned close to walls and corners. Lifts of this general type are disclosed in U.S. Pat. Nos. 4,015,686 and 4,458,785, for example.

Commonly, such lifts have a mast extension system operated by extension of a hydraulic cylinder which is supplied with pressurized fluid by a pump powered by an electric motor, and having its suction connected to a reservoir tank. The pump, motor, and tank are mounted on the frame of the lift, and suitable controls are provided so that the work platform can be raised and lowered by a person occupying the work platform or at ground level.

However, a need exists for a safety system which prevents raising the work platform unless the outriggers are fully extended and properly positioned in a load bearing condition.

In accordance with the present invention, a special control system is provided for the electric motor used to drive the pump supplying pressurized fluid to the hydraulic cylinder in the mast extension system. Disablement of this control system prevents extension of the mast.

In its preferred embodiment, the control system is disabled whenever the outriggers are not in proper supporting position. Normally open switches are provided in a circuit for the control system. These switches are arranged to be closed by engagement with the outriggers when the outriggers are properly extended. Unless all of these switches are closed, the circuit is disabled, thereby shutting down the source of pressurized fluid for raising the mast.

In the preferred embodiment, the outriggers are slidably mounted in horizontal sleeves and have vertically adjustable jack screws at their outer ends which are engaged by quick-release nut segments. When the outriggers are in their extended positions, they are free to rock upwardly in a narrow range responsive to operation of the jack screws into load-carrying ground engagement taking the load of the lift from its transport wheels. The previously mentioned switches in the circuit are mounted within the sleeves and are arranged to be closed by the outriggers when they rock upwardly. This permits the control circuit to then be activated whenever a manual switches on the work platform are closed by the operator.

FIG. 1 is a front isometric view of a portable lift employing a safety system of the present invention.

FIG. 2 is a second isometric view of the portable lift of FIG. 1.

FIG. 3 is a circuit diagram of a safety system circuit.

FIG. 4 is a sectional, side elevational view of an outrigger switch mechanism with the outrigger in an extended, non-load bearing position.

FIG. 5 is a sectional, side elevational view of the outrigger switch mechanism in an extended, load bearing position.

FIG. 6 is a schematic representation of a hydraulic system controlled by the circuit of FIG. 3.

Referring to FIG. 1 and 2, the lift 20 has a base frame 22 with wheels 23 on which is mounted an extensible mast 24 for lifting a work platform 25. The mast 24 may be of the type shown in U.S. Pat. No. 4,458,785, the disclosure of which is herein incorporated by reference in which (referring to FIGS. 1 and 2 of this disclosure) a stationary base stage 24a is rigidly mounted on the base frame 22, and one or more intermediate telescopic stages 24b and a top stage 24c are mounted forwardly of the base stage. A load platform 25 is slidably mounted at the front of the top stage.

The lowest of the intermediate stages is extended and retracted by action of a double-acting cylinder 26 (see FIG. 6) unit in the conventional manner, and the remaining stages and the load platform are responsive to extension of the cylinder unit by way of a chain and pulley system (also commonly known as a reeving system). Systems of this type are well known to those of ordinary skill in the art and are commonly used with three stage fork lift vehicles. The pressured fluid for the cylinder unit is supplied by a pump 28 driven by an electric motor 30. Fluid from a reservoir tank 31 is circulated by the pump 28 to the cylinder unit 26 via a control valve system 32 which also controls the dumping of fluid from the cylinder unit to the reservoir tank when it is desired to retract the cylinder unit to lower the work platform. A suitable control valve system 32, pump 28, motor 30, and reservoir 31 can be obtained from Fenner Fluid Power Company, Rockford, Ill.

The lift 20 is provided with horizontally extending front and back outriggers 33, 34 arranged to extend right and left, and longitudinal outriggers 35, 36 arranged to extend front and back for stabilization of the lift. These outriggers are slidably mounted in respective sleeves 33a--36a comprising part of the base frame 22. The outriggers have respective screw jacks 37 mounted adjacent their outer ends for leveling the base frame 22 and outriggers as a unit and to take the load from the wheels 23. As shown in FIGS. 4 and 5, the screw jacks are engaged by quick-release nut segments 38 pivotally connected to each outrigger 33-36 and operated by spring loaded levers 39.

The vertical tolerance between the outriggers 33-36 and the inside of the sleeves 33a-36a is purposely selected to be large enough so that the outriggers can rock vertically relative to the upper end 39a and lower end 39b of the sleeves in a narrow range when the outriggers are in the extended position, as shown in FIGS. 4 and 5. Accordingly, when the screw jacks 37 are turned to move downwardly by handles 32a into ground engagement to take a load as shown in FIG. 5, the outriggers responsively rock upwardly about upper end 39a to a limited degree. In accordance with this invention, normally-open outrigger operated switches 43a-d are positioned in the sleeves such as to be closed by engagement with the outriggers only when the outriggers 33-36 are extended and rocked upwardly by operation of the four screw jacks 37 to take the load of the lift. The switches 43a-d are part of a control circuit to be later described.

A control panel 48 is mounted on the stationary base section of the lift, and a control box 49 (shown schematically in FIG. 3) is mounted on the work platform 25.

As shown in the circuit diagram of FIG. 3, the control panel 48 contains a key-operated switch 50, and an emergency push-button stop switch 52 in series for controlling power from a 12 volt source 53. This circuit also includes a motor start relay coil 54 for operating a normally open motor relay 54a, an outrigger relay 56 for opening a normally closed safety relay 56a, an "up" solenoid 58 for operating a normally open hydraulic feed valve 58b (see FIG. 6) for the double-acting hydraulic cylinder 26, a "down" solenoid 59 for operating a normally closed hydraulic return valve 59a, and four green indicator lamps 60-63.

The control box 49 moves up and down with the platform 25 and contains a power switch 64, an emergency stop push-button switch 66, an "up" switch 68, and a "down" switch 70. A flexible cable (not shown) connects the control panel 48 and control box 49 and contains positive lead 72 and branch leads 72a and 72b. Within the control panel 48 the branch lead 72a contains the safety relay 56a and then divides to provide a lead 72c for the "up" solenoid 58 and a lead 72d for the coil 54 of the motor relay 54a. Lead 72b continues to the "down" solenoid 59. The outrigger relay coil 56 in the control panel 48 has parallel branch leads each connecting to a respective "stowed" pole of the outrigger operated switches a-d, and each indicator lamp 60-63 is contained in parallel leads connected to the "extended" poles of each respective outrigger switch.

The control system of FIG. 3 operates by preventing energization of the motor 30 and operation of the "up" valve 58c (and thus raising of the mast 24) unless all of the outriggers 33-36 are fully extended into a load bearing position as shown in FIG. 5.

It will be understood that closing of the keyswitch 50 is necessary to power the control circuit. The procedure for raising and lowering the lift is described as follows. To raise the lift, all of the outriggers must be fully extended into a load bearing position as shown in FIG. 5. All of the outrigger operated switches 43a-d are therefore closed as shown in FIG. 3 and as evidenced by energizing of the four indicator lamps 60-63. The outrigger relay coil 56 thus remains de-energized, thereby leaving the normally closed motor relay 56a closed and making it possible to energize the motor relay 54 by closing the power switch 64 and "up" switch 68. As shown in FIGS. 3 and 6, when safety relay 56a is closed by having the outriggers in the extended position, the "up" solenoid 58 is energized and normally-open "up" valve 58c is moved to a closed position, thereby allowing fluid in high pressure line 100 to be pressurized by the pump 28. Fluid will therefore flow through check valve 102 and fluid control valve 104 to the extending side of double acting cylinder 26. Fluid from the retracting side of the cylinder is relieved by way of fluid relief line 106 to the reservoir 31. Note that fluid can not flow from high pressure line 100 to relief line 106 through normally closed "down" valve 59a because "down" solenoid 59 has not been energized. The operator on the working platform controls this action by holding the power switch 64 and "up" switch 68 in closed positions to energize the pump motor relay 54 and thereby start the motor to lift the platform by extension of the hydraulic cylinder unit. As soon as closing pressure is released on the switches 64, 68 they open, and hence the pump motor stops and the valve 58 opens, thereby stopping elevation of the work platform. Note that whenever any one of the outrigger operated switches 43a-d are closed in the stowed positions (when any one of the outriggers are not in a load bearing position as shown in FIG. 5), the outrigger coil 56 is energized and the safety relay 56a is in the open position. The motor relay coil 54, and thus the motor 30, cannot be energized. In addition, the "up" solenoid 58 remains de-energized, maintaining a fluid path between the pump 28 and the reservoir 31. This prevents the pump from pressurizing the high pressure line 100. In this manner, the system is "fail safe" because the switches 43a-d cannot enter the state shown in FIG. 3 until the outriggers are fully extended into load bearing positions.

When the operator on the working platform desires to lower it, he pushes closed the power switch 64 and the "down" switch 70, thereby energizing the "down" solenoid 59 to cause fluid to return from the cylinder unit to the reservoir tank by way of restrictor 108 in fluid control valve 104. Note that this can be done regardless of the state of switches 43a-d. At all times a person at ground level can push open the emergency switch 52 and de-energize the entire circuit. Then the work platform can be lowered by manually opening "down" valve 59 which can be easily reached at ground level.

The indicator lamps can be arranged on the face of the control panel 48 in a generally rectangular pattern simulating the relative positions of the four outriggers. Hence, if one of the indicator lamps does not light, the operator can immediately determine which outrigger is not in fully extended operating position.