Conventional conveyors, such as conveyor 200 illustrated in FIGS. 1-3, are often constructed to have an upper conveyor belt surface that is located 12 inches above the floor or ground or other surface on which the conveyor 200 is mounted. This standard height makes it easier for conveyors to be integrated into existing production lines, many other elements of which are designed to work with a 12 inch conveyor height. Conveyors having belt surfaces located at greater heights can be provided by mounting a standard-height conveyor on a platform. However, in order to accommodate a belt return and associated tensioning rollers 214 and the conveyor drive shaft 212, it is difficult to make conveyors that are significantly shorter than the standard 12 inch height.
Various known stacking devices 202 are designed to form stacks of material on a conveyor that is mounted on a lift table. A lift table 204 having a platform 205 supporting the conveyor 200 is illustrated in FIGS. 4 and 5. A stacking process begins with the lift table 204 in a raised position (FIG. 4) so that items exiting the stacker 202 fall a predetermined distance onto the conveyor 200 and form a stack (not illustrated). As the stack increases in height, the lift table 204 lowers the stack to maintain the predetermined distance between the discharge end of the stacker 202 and the top of the stack. When the stack has reached a desired height, the lift table 204 is operated to bring the top of the conveyor 200 to the standard 12 inch height so that the stack can be transferred onto an adjacent conveyor 206.
As previously noted, the adjacent conveyor 206 will generally have a load carrying surface 218 located 12 inches above the floor. However, given that the lift table 204 has a minimum height of 8 to 12 inches from its bottom to the top of the platform 205, and that it is difficult to make a conveyor 200 having a height less than 12 inches, the lift table 204 cannot lower a conventional conveyor 200 to the 12-inch height necessary for transferring a stack to the adjacent conveyor 206. Therefore, in conventional facilities using stackers 202 and conveyors 200 mounted on lift tables 204, a pit 208 is often dug in the floor 210 next to the stacker 202 so that the lift table 204 can be mounted in the pit 208. This allows the lift table 204 to lower the conveyor 200 close to, even with, or beneath the level of the floor 210 as necessary to bring the top surface of the conveyor 200 to the 12-inch height and align it with the load carrying surface 218 of the adjacent conveyor 206.
Digging a pit in a plant floor is undesirable for many reasons. First, there is an expense involved with digging and lining the pit and, in some cases, sealing the pit against water leakage. Moreover, after the pit is dug, any change in the layout of the manufacturing or processing line must either work around the location of the pit, or the pit must be filled in and a new pit dug in a new location. This reduces the flexibility of the line an increases the cost of any changes. If the pit is significantly larger than the lift table to be mounted therein, the pit may also pose a safety hazard and require appropriate marking, guards, etc. Alternately, raising the stacking device and all other equipment to a non-standard height of 20 inches, for example, would be cost prohibitive. It would therefore be desirable to provide a conveyor that can be used with a lift table and lowered to a 12 inch height without the use of a pit.