Reliable electrical connections help safety, productivity
Case study: Faulty electrical connectors cause 1000 amp safety issues. Pierced cables create short in automotive manufacturing plant.
The combination of high voltage, hot temperatures, and faulty electrical connections created safety issues for Ford Motor Company at a Canadian auto body plant.
When a welding robot’s arm rotated to weld an auto frame, the cable and electrical connector would rotate with it and severely fatigue the termination over time. When the electrical connector broke, it pierced the 1000 amp power cable, sending sparks through the harsh welding environment.
Because the electrical connection, a standard aluminum cord connector with stainless steel wire mesh grip, was hardwired to the robot, it created a short and halted the line. With high temperatures, sparks, and 1000 amps of electricity in each robot, there was risk of fire and power surges, and of great concern for Ford.
It was compromising the company safety record and a productivity issue when the line shut down for repairs. Because the cable and connection were hardwired into the robot, the machine had to be disassembled to replace the cable, connector, and cord grips. These repairs took up to 24 hours each.
Ford worked with Progressive Tool of Southfield, Mich., designer of the assembly line and responsible for its performance. Progressive Tool contacted Remke Industries of Wheeling, Ill., source for the cord connectors and wire mesh grips.
Research began with installation of the cable grip assembly for the 1000 amp power cable. It became immediately apparent that the connection, a standard aluminum cord connector and stainless steel wire mesh grip, was misapplied. The rigid, stiff materials were designed for repetitive movements. Also, the robots were rotating in ways that strained the connector. The robots performed 180-degree turns, spinning nearly upside down and extending their arms in and out, working on the automobile. The cable termination on the robot required support and protection from the welding environment’s high temperatures and high voltage. Matching the arc-of-bend on the robot was a key to reducing connection strain and preventing it from snapping as the robots rotated.
For installation, requirements focused on pull-out protection to keep the power supply intact during the rotating, repetitive movements of the robot. The inside and outside of the connection needed reinforcing.
A tapered rubber cone that housed a threaded metal cable connector was ideal for the electrically charged welding environment because the rubber wouldn’t crack under heat exposure and doesn’t conduct electricity. The metal sleeve provided stability and reinforcement.
Prototypes were developed and tested by replicating the strain on the connector using a 10-ton hydraulic press. By placing the connection in a fixture, Remke applied downward pressure to ensure the mechanical bond in the metal sleeve would not come apart. That pressure was a big part of why the connector split and pierced the 1000 amp cable. The breaking strength of the custom electrical connector also was tested for repetitive movement.
During the testing phase, Ford and Progressive Tool modified the bushing design to include two cable diameters instead of one so the outer diameter of the cable would more closely match the openings in the bushing and ensure it had less movement during production. Threading also was moved to the inside of the connection, instead of being housed externally, to make it easier to change connectors when needed.
After the design and prototypes were approved, Remke created the overmold tool for manufacturing the new, custom electrical connectors using a two-part process: first creating the tapered rubber cone, and then molding it to a metal sleeve. A more durable, custom electrical connector resulted.
Custom electrical connectors
The old electrical connectors, made of aluminum and stainless steel, were replaced with new ones with a molded, tapered rubber cone to improve cable stability. The new rubber connectors resist hard knocks, splitting, and cracking common with PVC connectors. They also meet the arc-of-bend specs for the cable to eliminate damage from repetitive rotations and eliminate potential cable piercing.
When coupled with a strain relief cord grip made of aluminum or stainless steel, Tuff Flex connectors significantly reduced cable pullout, which improved robot safety and reliability.
Additional small problems persisted. Repetitive rotation of the robot’s arm and cable proved to be too much for a stationary connector. There was so much power going through the connection that when the robot moved, the cable jumped and pulsated.
A custom electrical connector with axial movement was needed to rotate with the cable and robot, and have a clamping face to provide even more stability and long-lasting reliability to prevent pierced power cables.
The new connectors gave Ford a short-term safety solution. New robots using a quick-disconnect system made the junction a pluggable piece that can be interchanged instantly. The swiveling, flexible connectors still are used as replacement parts for existing robots.
– Jen Lorenski, for Remke Industries. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, firstname.lastname@example.org.
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