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Automobile Suspension Fastener Integrity
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LID is a Mechanical Engineer, Engineering Manager, Automotive Steering Technology and Design Consultant, Automotive Product Engineering and Manufacturing Consultant with world-class expertise in automotive engineering, design and development of chassis components and systems, development of high-tech energy-efficient steering systems, electrical pumping product development, hybrid automotive vehicle transmissions, fuel cells, and automotive cooling systems.
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A significant fastener issue was identified during the design verification testing of a small, front-wheel drive automobile. The front suspension of this vehicle consisted of a McPherson strut with a combined strut-stabilizer bar providing the longitudinal positioning of the lower control arm and the steering knuckle. The strut-stabilizer bar was mounted in front of the arm, and was attached to the arm through a bushing in a hole in the arm. The problem was that the nut which attached the strut-stabilizer bar to the arm was coming loose, on an intermittent and apparently random basis. This was a 3/4" grade 8 prevailing torque nut and it was threaded onto the machined end of the strut-stabilizer bar. The torque of the joint was reacted by a hardened sleeve and washers, and a machined shoulder on the strut-stabilizer bar. Loosening of this nut would first result in a slight dartiness in the steering of the vehicle, then a clunk in the front end, and ultimately result in loss of steering control. This problem had to be resolved before the vehicle could be released for production.
Initial investigation focused on the torque-tension capability of the threaded joint, and its relationship to the axial loads on that joint. Conclusions were that the joint was "capable", and that it should not loosen during any expected loading cases, if its initial torque exceeded the minimum specification. Test vehicles were set up at the minimum torque, and the intermittent problem continued. Further investigation was focused on the specific vehicle test routines which preceded the looseness in each vehicle, the vehicle loading, and finally, the location on the vehicle of the failure. The only conclusion of this analysis was that every incident of looseness occurred on the right-hand side of the vehicle. There were no cases in which the left-hand nut loosened.
A deep-dive analysis into the differences in loading between the right and left sides of the front suspension revealed the following:
- When the vehicle is traveling forward, braking forces caused the front wheels and suspension to go into jounce (moving up, relative to the body).
- Forward braking forces resulted in increased tension on the strut-stabilizer bar nuts.
- The simultaneous effect of the braking tension and the jounce motion resulted in an unscrewing force on the right-hand side, when the vehicle was traveling forward.
- This simultaneous effect on the left-hand side would be to tighten the joint.
- In effect, the suspension had been designed to unscrew itself on the right-hand side.
The ultimate corrective action was to use a left-hand thread on the right-hand side of the suspension. This would assure that the right side of the suspension would tighten itself, as the left side did. Since the suspension and steering are safety-critical, it was also decided to use an encapsulated epoxy adhesive on the threaded joints. (In today's market, this would be considered overkill.) There was never a field problem with this joint.
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