Large Tractor Ladder Latch Design

The boarding ladder attachment for Caterpillar’s D9 through D11 tractors enables operators to access the cab of the machine safely. However, the current actuator driven latching mechanism that secures the ladder in the stowed position fails at low operating hours, creating downtime of the machine and loss of production. This project presents a motor-driven redesign of the latching mechanism to improve reliability and ensure backward compatibility for machines already in the field.

In this design, the access ladder swings up from the ground to the side of the tractor and is secured by two pins that firmly hold the ladder to the latching assembly on the machine. These pins are engaged by custom linkages driven by a geared motor and keyed shaft. This motor removes the linear actuator, the key source of failure from the current design. The redesigned linkages consist of a pivot link that interfaces with a shaft and connecting rods that drive the pins. The new system adds holding pads which adjust to the optimum stopping location of the stowed ladder. It also integrates limit switches with the current system of relays to stop actuation of the latching pins. This, in turn, interfaces with the switches, proximity sensor and rotational actuator that controls ladder movement. The new latching mechanism shows higher natural frequencies and reduction of stresses, ultimately improving structural reliability.