The present invention relates to aerial lift platform apparatus, and more particularly to such apparatus which is mounted on a chassis having endless tracks or treads which engage the ground.
Aerial lift platform apparatus have been provided in the past, and their use has recently been significantly increasing. Such apparatus is in the form of a self-propelled vehicle having a chassis which is mounted on ground engaging wheels. Smaller units often have three wheels, while larger units, having a vertical reach of between approximately forty feet and one hundred and twenty-five feet, are mounted on four wheels. The chassis supports a rotatable upper works, which is capable of rotating through 360 degrees. A boom is carried by the upper works, having at or adjacent its outer end a workman's platform, usually provided with upstanding walls so as to form a basket-like structure, for purposes of safety. The boom is often extensible, and is caused to rotate through 360 degrees, due to its mounting on the upper works. In addition, the boom may be raised and lowered about a horizontal pivot. The upper works usually contains a prime mover, such as an internal combustion engine, which drives one or more pumps, to thereby deliver power to rotary motors positioned at one or more of the wheels in order to propel the vehicle, and also to deliver hydraulic fluid under pressure to various cylinders, such as for steering and boom extension and retraction. Rotation of the upper works is also effected by a hydraulic motor driven by the pump. All controls are provided in duplicate, so that there is a set of controls accessible to a person standing by the machine, and also a set of controls is provided at the platform, so as to be accessible to the operator or workman in the platform.
An aerial lift platform has been provided, mounted on a tracked vehicle chassis, instead of wheels. There was thereby gained the advantage of permitting operation where the supporting surface was relatively soft, and could not sustain a vehicle of such weight when mounted on wheels. This construction was a known crawler type construction which provided a steel driving sprocket and a steel idler at the ends, and intermediate steel rollers which were rigidly mounted to the chassis. This construction, while suitable for use in terrain of a relatively soft nature, provided no shock absorbing or cushioning for the operator, who could only drive the vehicle from the the operator's platform.
In another embodiment, an aerial lift platform was provided which was mounted on tracks, and was provided with a cab on the vehicle, which cab had controls for moving the vehicle. The operator's platform was not provided with controls for moving the vehicle, so that the operator, when the vehicle was moving, could only be in the cab which was mounted directly on the vehicle. Once the aerial lift platform was set in position, an operator occupied the platform, and by manipulation of the controls in the platform on the boom, was able to rotate the upper works, raise and lower the boom and extend and retract the boom, but could not, as noted, cause the vehicle to move. In this construction, pneumatic tires were provided, for both drive and intermediate or idler wheels, and the idler wheels were mounted on independent spring loaded axles. That is, one or more of the idlers on one side of the chassis could be moved upwardly, or downwardly, relative to a reference point on the chassis, without causing movement of any idler on the opposite side, relative to the same or a corresponding reference point on the chassis.
The aerial lift platform apparatus having rigid suspension also had a cab on the chassis, and thus required two operator's stations, one at the cab and one at the platform, and there was thereby required extra cost; in addition there was the problem that the vehicle could not be moved by the operator in the cab and thus was more expensive from the point of view of personnel cost, and was somewhat less efficient. On the other hand, where the shock absorbing or cushioning type suspension was utilized, including pneumatic tires and spring loaded intermediate wheels, stability was not as great as desired, particularly when the boom was extended over one side. It has been found that with the boom extended over one side, there is produced an over-turning moment tending to tilt the machine and overturn it to the side over which the boom extends. The problem is known to be more severe where there is a shock absorbing or cushioning or so-called "sprung" chassis, and this has been solved in the past, for instance in connection with truck mounted cranes, by the use of outriggers. Thus, the truck mounted cranes have the outriggers in stored, non-use position during movement of the truck mounted cranes from one place to another, but when relatively heavy loads are to be lifted, the outriggers are utilized, and their positioning in effect renders the spring mounting of the chassis on the ground engaging wheels inoperative. That is, during the lifting of loads near the capacity of the truck mounted crane, the spring suspension of the chassis is made inoperative and the supporting of the chassis is by the outriggers, which bypass the spring suspension of the chassis. Of course, with such cranes, it is not possible to move them when the outriggers are positioned.
There are known, also, mobile cranes which are intended to lift and move a load by movement of the entire truck mounted crane. In such units, however, although outriggers are not used, movement of the entire truck mounted crane is effected with the load and the boom centered, and not extending over the side.
Various vehicles are known which utilize tracks, and have pneumatic tires, together with spring-loaded independently movable intermediate or idler wheels. In these constructions, however, there is no boom, and therefore no overturning moment resulting from a boom positioned over the side.