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6/99 - Tech Tips

Routine Torque Rod Inspection Prolongs Life

WOODRIDGE, IL - The torque rod has a multi-functional role in a truck's suspension. It controls the driveline angles of the vehicle, restrains rotation of the axle housing while accelerating and braking, and absorbs leaning and cornering forces. It is, however, often overlooked during routine inspections, which may result in universal joint failure, improper traction, bushing failure, and excessive wear.

The upper torque rod will last the life of the walking beam suspension with inspections and timely bushing replacement, according to William Adlaf, manager of technical services at Hendrickson. "It's a relatively inexpensive repair, and it's easy to do. In fact, it is one of the easiest components to service on a suspension," adds Adlaf.

Torque rods have attaching ends designated "straddle mount" or "tapered stud" (see Figure 1). Straddle mount rod end torque attaching fasteners are furnished by the vehicle manufacturer. It is important that the tightening torque of the locknuts be checked during preventive maintenance service. Be sure to follow the vehicle manufacturer's specifications for tightening torque values. Hendrickson recommends using Grade 8 bolts and Grade C locknuts for all straddle mount torque rods. Furthermore, tapered stud rod locknuts should be checked using 175 ft.-lb. to 225 ft.-lb. tightening torque.

All rods need to be inspected for not only looseness, but also for excessively worn bushings. "Worn bushings result in noise as well as a general feeling of looseness," says Adlaf.

In addition to a visual inspection, Adlaf recommends a second way to ensure longevity. In doing so, a lever check can be made by placing a long pry bar under each rod end and applying pressure. In order to check longitudinal torque rods, slowly rock an empty vehicle with power and brakes applied while a mechanic visually inspects the action at both ends.

By inspecting torque rods routinely and replacing or re-bushing them on a timely basis, damage to other suspension components will be avoided and torque rod life will be prolonged.

Hendrickson, with 21 facilities worldwide, is one of the largest international suppliers of truck, tractor and trailer suspensions, lift axles, springs, bumpers, and stamped components to the commercial transportation industry. Having manufactured more than two million truck suspensions, Hendrickson has been meeting suspension needs for more than 70 years. Hendrickson Truck Suspension Systems, a certified QS-9000 supplier, is headquartered at 800 South Frontage Road, Woodridge, IL 60517-4904.