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Prime Mover Magazine

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Peter Hart

Chassis rail strength

July 2016

Experience shows that chassis rails, cross members and sub-frames should have a design FoS of 3 or more for hard working trucks and trailers.

First some basics. Metals deform when the stress levels reach tensile Yield Strength, and will break when stresses reach the Ultimate Tensile Strength. Chassis rails on heavy-duty trucks are usually made from mid-strength alloy steel, typically with Yield Strength of 500 - 750 MPa. Sub-frames and attachment brackets are more likely to be made from mild steel (yield = 250 MPa) or a low-strength alloy (yield = 350 MPa) so the material strength is probably about half that of the chassis rails.

All rigid trucks have a body attached to the chassis rails. The body, such as a tray, concrete pump or tank is attached to a sub-frame that in turn is attached to the chassis rails. The body will rarely fit neatly directly onto the chassis rails and so the sub-frame is needed to make a durable connection that is ‘tailored’ to the shapes involved. The sub-frame should also have adequate strength to spread the load so that the chassis rails do not experience point loads. Prime movers also experience point loads where the fifth wheel sits onto the chassis-rail angles.

Image 1 shows a location that experienced significant point loads during tipping. Chassis rail strengthening was needed for about one metre on either side of the lifting bracket. The change of strength at the end of the cylinder attachment bracket was too great.

The strength of the chassis rail and sub-frame can be approximately calculated using the approach shown in Diagram 1. The load and support forces are estimated and drawn on the load diagram. Then the Bending Moment diagram is calculated. Finally the rail stress at any location is calculated by dividing the Bending Moment by the chassis rail Section Modulus. The rail Section Modulus is an engineering factor that depends upon the geometry of the chassis rails including any insert (see VSB 6 Section H). Locations with high Bending Moment (where high stresses occur) will probably need rail inserts (cross hatching).

The Bending Moment diagram is generated by calculating the net area (allowing for positive and negative areas) under the load diagram from the left side to the point of interest.

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