Conveyors: power-driven – Conveyor section – Endless conveyor
Reexamination Certificate
2003-01-07
2004-07-06
Crawford, Gene O (Department: 3651)
Conveyors: power-driven
Conveyor section
Endless conveyor
C198S502400, C198S810010
Reexamination Certificate
active
06758327
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
BACKGROUND OF THE INVENTION
The field of invention is conveyor drive assemblies, and more particularly, conveyor drive assemblies including a shaft driven by at least two drive motors.
Conveyor drive assemblies typically include a rotationally driven shaft which rotationally drives one or more sprockets. The sprockets engage an endless belt or chain to drive the belt. Drive shafts for driving relatively narrow conveyor systems, such as less than fifty-four inches wide, typically are rotationally driven by a single drive motor coupled to one end of the shaft.
Wide endless conveyor chains, such as conveyor systems having belt widths greater than ninety-six inches wide, are also typically driven by multiple sprockets on a single drive shaft. The shaft, however, is often driven by at least two drive motors, such as a first drive motor on one end of the shaft and a gearbox or second drive motor on the opposing end of the shaft to minimize the shaft torque load. Advantageously, minimizing the shaft torque load minimizes the required diameter of the shaft which reduces costs.
The rotational speed of the output shaft of each drive motor driving each end of the drive shaft is typically controlled to synchronize both ends of the drive shaft. When controlling the output shaft rotational speed of each of the drive motors driving the drive shaft ends, unless the output shaft rotational speeds of the drive motors are identical, there will almost always be a discrepancy in the radial positions of each drive shaft end. The differences in the output shaft speeds can result in drive shaft twisting wherein one end of the drive shaft leads the other end. The difference in the radial position of the drive shaft ends increases over time, and can result in unequal loading of the conveyor belt.
Typically, the radial positions of each drive shaft end will diverge until system loading equilibrium is reached. At this equilibrium point, the difference in the applied loading of each drive motor is taken up by balancing torque required to twist the drive shaft, torque required to drive the conveyor chain or belt, and other mechanical losses. In extreme cases, it is possible that one drive motor can lead the other drive motor to the extent that the leading drive motor could back drive the lagging drive motor. Moreover, the difference between the drive shaft end radial positions can cause the conveyor chain to be driven from one side only. This unequal loading can cause catastrophic failures in the conveyor system.
The force applied to the conveyor belt at particular points can be measured to prevent catastrophic failures resulting from unequal loading of the belt. Methods for measuring the force applied to the belt include using load cells which measure the force or measuring the torque applied to each end of the drive shaft. Typically, load cells and torque data are used to limit the conveyor system by shutting down the system if a predetermined load or torque limit is exceeded.
The data provided by the torque measurements or load cells can, however, be used to control the rotational speed of each drive shaft end. In order to use the data, however, many theoretical assumptions must be made to control the rotational speed of the shaft. In addition, it is quite feasible that the top loading of the conveyor belt or chain is not equal across the width. Because of this, the loading can fluctuate continuously, causing a system to never calculate an equilibrium speed.
The drive sprockets on a conveyor chain or belt can also cause fluctuations in torque measurements due to the chordal nature of the chain or belt pitch. The fluctuations can cause the system to continuously adjust the rotational speed of the drive motor output shaft which accelerates the deterioration of the system and increases maintenance costs. Therefore, a need exists for a conveyor drive assembly for wide conveyor systems and method of operation which reliably controls the speed of each end of the drive shaft and minimizes unequal loading of the conveyor belt.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a conveyor drive assembly including an elongated shaft having a first and second end and a method of operating the conveyor drive assembly. The method includes detecting the rotational position of the first end of the shaft, detecting the rotational position of the second end of the shaft, determining the rotational position of one of the first and second ends relative to the other of the first and second ends, and then rotationally driving at least one of the first and second ends of the shaft to maintain the rotational position of one of the first and second ends relative to the other of the first and second ends within a predetermined range. Advantageously, the present invention provides a conveyor drive assembly for use with wide conveyors which reliably controls the speed of each end of the drive shaft and minimizes unequal loading of the conveyor belt.
A general objective of the present invention is to provide a conveyor drive assembly and method of operation which does not rely on measuring the rotational speed or torque of the drive motor output shaft or drive shaft to minimize unequal belt or chain loading. The present invention accomplishes this objective by monitoring and controlling the rotational position of one drive shaft end relative to the other drive shaft end.
Another objective of the present invention is to provide a conveyor drive assembly and method of operation which reliably controls the rotational speed of each end of the drive shaft. This objective is accomplished by directly monitoring the radial position of each shaft end.
The foregoing and other objectives and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention.
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Bolhuis William J.
Fork David B.
Stebnicki James C.
Crawford Gene O
Quarles & Brady LLP
Rexnord Industries, Inc.
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