Braking system for a lift truck

#The present invention relates to lift trucks in general and, more particularly, to a braking system for a lift truck that permits simplified braking structure, elimination of replacement of brake wear components, improved operator comfort and improved lift truck stability during braking. While the present invention may be utilized on a variety of materials handling vehicles, it will be described herein with reference to a counterbalanced lift truck for which it is particularly applicable and will initially be used.

#Oftentimes, lift trucks, particularly standup lift trucks, utilize a “black and white” or on/off spring applied/electrically released brake arrangement to provide both service brakes (brakes for slowing or decelerating a truck while moving) when an operator makes a pedal brake request, and also for parking brakes (brakes applied to immobilize a stopped truck). In these lift truck braking systems, limited modulation may be available by gradual application of the brake pedal and/or by pulsing the brake pedal to turn the brake on and off to thereby control the stopping distance. Further, the braking force applied by these braking systems typically is the same regardless of fork height and/or load carried by the forks. Furthermore, in these braking systems, during a “fault” condition within the truck control system, even the ability to modulate the brakes by pulsing the brake pedal is not available since full braking torque is applied regardless of whether the brake pedal is operated or not. Unfortunately, when a truck is operating with its forks elevated and particularly when the elevated forks support a load, full braking torque can result in an unstable condition.

#Accordingly, there is a need for an improved braking system that addresses the noted properties in existing lift truck braking systems. Preferably, the improved braking system would provide substantially all service braking using regenerative or regen braking by appropriate control of motors used to drive the truck so that during braking, power is generated in the motors and conducted to the power system of the lift truck. In this way, operating times for the truck could be extended, mechanical braking systems used for parking and backup braking could be simplified and these braking systems normally would not require replacement or adjustment of wear components during the entire useful life of the lift truck. In addition, the mechanical brakes provided for parking and backup braking would be controlled so that an operator of the truck could modulate these brakes to improve stability of the truck during electrical system problems.

#The following description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which:

#FIG. 1 is a perspective view of a sit-down counterbalanced lift truck including the invention of the present application;

#FIG. 2 is a side plan view of the counterbalanced lift truck of FIG. 1;

#Reference is made to FIGS. 1 and 2 which are perspective and side plan views, respectively, of a three-wheel sit-down counterbalanced lift truck 100 for which the invention of the present application will initially be used. While the present invention is described herein with reference to the sit-down counterbalanced lift truck 100, it will be apparent to those skilled in the art that the invention and variations of the invention can be more generally applied to a variety of other materials handling vehicles including, without limitation, a three-wheel standup lift truck 100A shown in FIG. 5. Components that can be used to implement the invention of the present application on the lift trucks 100, 100A are illustrated schematically in FIG. 3 and also are identified in FIGS. 1, 2 and 5 to the extent these components are visible in FIGS. 1, 2 and 5. It is contemplated that other components and component configurations can be used for the present invention so that the invention is not to be limited to these components.

#A closed loop dual traction motor controller 200, sometimes referred to as a traction control module, located within a power unit 102 of the truck 100, 100A contains power electronics to control a left drive motor 202 and a right drive motor 204 independently of each other in both braking and motoring modes of operation. In a working embodiment of the invention, one of a number of commercially available controllers was used for the motor controller 200. The dual traction drive motors, i.e., the left drive motor 202 and the right drive motor 204, are coupled to independent gearboxes 206, 208, respectively, to drive a left drive wheel 210, see FIG. 5, and a right drive wheel 212. In a working embodiment of the invention, the left and right drive motors 202, 204 comprise three phase AC induction motors; however, the present invention is not limited to AC motor technology as will be apparent. The present invention can also be used for lift trucks that are driven by a single drive motor instead of dual drive motors and individual drive controllers can be used for the left and right drive motors 202, 204, if desired. For control of the AC drive motors 202, 204, independent feedback encoders 214, 216 are used to monitor the rotational speed and direction of the AC drive motors 202, 204, respectively.

#A mechanical, spring applied, electrically released first brake 218 is coupled to the left drive motor 202 to provide a percentage or fraction X of a total mechanical braking force 100% or 1 that is used for parking and backup braking of the truck 100, 100A. While X can be within a range of fractions, ⅓ of the total mechanical braking force was used in a working embodiment of a truck including the invention of the present application. A mechanical, spring applied, electrically released second brake 220 is coupled to the right drive motor 204 to provide a fraction Y of the total mechanical braking force 1 that is used for parking and backup braking of the truck 100, 100A. The fraction Y is complementary to the fraction X so that the total or 100% of the braking 1 required for parking and backup purposes is provided when both of the brakes 218, 220 are operated, i.e., X+Y=1. Since X was ⅓ in the working embodiment of the truck noted above, Y was ⅔. Other fractional divisions can be used in the present invention with the selection of appropriate fractions being based, for example, on vehicle geometry and minimizing truck skewing. The spring applied, electrically released brakes can be more directly coupled to wheels mounted on opposite sides of a lift truck as would be required for a lift truck having a single drive motor.