Frame Conditions

Frame Conditions

General

This analysis does not cover the causes of all possible frame problems. However, it may provide valuable assistance with preparing reports.

Frame damage does not happen without a cause. The cause of damage is not always as apparent as it may be in a collision.

Frame damage may be broken up into the following classifications:

* Collision
* Excessive bending movement
* Localized stress concentration

Repair the damage caused by a collision by using the proper methods and reinforcements.

Excessive bending movement damage is caused by overloading, improper weight distribution, or misapplication of the vehicle. Excessive bending movement damage will occur at different areas on various types of vehicles.

For information on localized stress concentration, refer to Stress Concentration.

Tractors





The maximum bending movement of vehicles used in tractor service is in the area of the leading edge of the fifth wheel. The following conditions may cause damage to the frame of the vehicle:

* Overloading
* Excessive forward fifth wheel setting
* Poor fifth wheel installations
* Severe operating conditions
* Severe braking operations (inertia of certain loads) that induce excessive bending movements in the frame

The damage will start at the edge of the lower flanges and, then, will progress across the frame flange and up the web section of the frame rail. Instances may occur in which the upper or the lower frame flanges buckle.

Straight Trucks





The maximum bending movement occurs in the area near the rear of the cab on vehicles having van or platform bodies. The following conditions may cause damage to the frame of the body:

* Overload
* The balance of a load remaining in the extreme front of the body due to the disbursement of the load in diminishing quantities





The highest tensile stress is applied to the bottom side of the lower frame flange in the operation of both the tractor and the straight truck. However, when dump trucks, for example, are operated with the box raised, the center of gravity of the load will move behind the rear axle centerline. The shifting of the center of gravity results in a change of maximum tensile stress location from the bottom of the lower flanges to the top of the upper flanges. This information may be useful when examining cracks on vehicles used in dump service, vehicles that have been operated at excessive speeds over rough terrain with the dump box raised, and trucks with dump boxes that are too long for their wheelbase.

Stress Concentration

Localized stress concentration failures may result from bending movement stresses. However, the stress levels would not be high enough, in the aforementioned instances, to cause any difficulty without localized stress concentration points. These localized stress concentration points may result from poor body mounting, special equipment, or defective material. They may also occur as a result of high bending loads, coupled with severe torsional loads such as off-road service.

Use wood sills between the main rails and the sub-frame on the body installation in order to ensure a satisfactory load distribution.

Special equipment or accessory installation may cause high stress concentrations due to the method of attachment or the weight of the equipment. Do not drill holes through the flanges. Avoid rapid changes to the section modulus that usually occur when large mounting plates are added for the support of special equipment. Mounting heavy equipment across the flanges or on the web of a side rail may cause enough stress concentration to cause damage at any of the following locations:

* The nearest crossmember
* The bracket
* The other frame stiffener
* Through a nearby hole in the frame flange