Feature Article - Silverado Strikes Gold with Water

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Silverado Strikes Gold with Water

To build the frames for the Silverado and Sierra with more strength and less weight, GM Truck Group turned to Magna's Cosma Body & Chassis Systems division in St. Thomas, Ontario, Canada. Cosma handed GM souped-up hydroforming technology—among other things...

By Colleen DeJong, Senior Associate Editor

The Front

Photo A: The hydroforming process:

Tubing is cut to desired length
Tube is pre-bent
The tube is inserted into the die, which is then filled with water and pressurized while the ends of the tube are compressed. Holes are also hydro-pierced during this phase.
Completed frame section ready for assembly.

Photo B:The stamping process:

Blanks are stamped from sheet metal
Section forming of the blanks is done
Holes are pierced and the two sections are fitted together
Seam welding of inner and outer halves is done
Steering gear spacers are welded to frame
Completed frame section ready for assembly.

For the most part, the process Magna uses is like other hydroforming processes. Precut tubing is inserted into a die that is then filled with water and pressurized. What sets it apart is the compression of the ends. This calculated compression serves to prevent thinning on the outer wall of the tube, as well as eliminate wrinkling on the inside wall.

When compared to the previous method of making a frame rail—welding two stamped pieces of metal together—hydroforming the front end of the frame eliminates roughly 350 in. of weld per frame. This also makes it lighter since the 10-mm lip of material previously designed into the stamped blanks for welding no longer has to be included. Since the part isn't stamped, 44 lb. of offal per front module are eliminated, as well.

Along with the obvious weight, material, and cost reductions that come with using this hydroforming process is a not-so-obvious reduction in initial investment. A lot of subassembly systems and welding equipment are eliminated from the process. And the hydroforming process uses pressurized water as the mating die, so Magna only had to produce two dies (upper and lower) instead of four.

The Middle

Photo C: The roll form/draw bend process:

Blank coil is roll-milled into straight-formed rails (lip included)
Rails bent using automated frame bending machine
Holes are pierced and rails are ready for assembly.

Photo D: The conventional stamping process:

Blanks stamped from sheet stock
Edges are bent
Rail sections are formed
Lipped edge is formed
Holes are pierced and rails are ready for assembly.

Cosma had been considering roll forming the rails of the middle portion of the truck's frame for quite some time. However, there was a problem with the upward curve the pieces take toward the rear of the mid-section. Namely, how to make them. With the conventional stamping process, that curve was merely edge bent after the blanks had been stamped, but the same thing couldn't be done with a roll forming process.

The draw bending solution came when one of the senior engineers on the project took his children on a tour of a toy train factory. An introductory movie into the world of trains included a brief clip of the production of train rails for real trains. The clip showed the rails being draw bent to accommodate a curve in the track. This short film got him thinking about the hitch they'd hit with the mid-section and the investigation led to the current method of roll-forming the frame rails, then draw-bending the curve into the section. Good ideas can come from anywhere.

Employing the roll-form/draw-bend process has enabled Magna to eliminate many of the welding and riveting applications that used to accompany production of the frame's mid-section. The result has been a saving of money, time, and weight.

The Rear

Production of the rear section of the truck's frame is done with conventional stamping. The real ingenuity here is that the entire rear section of the frame is designed to be removable. Since the rear portion is most often damaged in accidents and takes the most abuse, engineers decided to make it easier to repair. The entire rear frame section can be detached and replaced more quickly and (one would imagine) at less expense to the truck owner.

Over All

Photo E

Photo E: The Silverado/Sierra frame on the whole has been re-engineered to give the truck more torsional strength in the front of the vehicle, where more twisting and turning with the landscape would occur. Since the weight of the cab and truck bed (and any cargo in it) falls on the mid-section and rear of the frame, both are designed with more stiffness than torsional strength. They are engineered to handle weight placed on it the same way the I-beams of a building would handle the weight of the floor above them.

By finding less than mainstream manufacturing methods to produce what GM feels is the best-engineered truck frame in its class pays off. Not only do these processes save GM time and money, but provide a truck that handles better and gives a more vibration-free ride than its predecessors.

And the thought and ingenuity that went into the redesigned Silverado didn't go unnoticed. It recently received Motor Trend magazine's 1999 truck of the year award.

Hydroforming: The Basics
Hydroforming has been around for about 100 years or so. It's primary use has been in the tube forming arena, but equal portions of equipment evolution and design ingenuity bring hydroforming into the vehicle manufacturing arena more and more with each new model year.
Basically, hydroforming has three steps:
Tubes are pre-bent and pre-cut to approximate configuration
Tubes are inserted into the die and filled with water
The water is pressurized, pushing the tube against the die, forming them to the finished shape.

And that's it. While it seems such a simple process would have found its way into automotive manufacturing on a wider scale, there have been a few hitches along the way that manufacturers have had to overcome. For one thing, the cost of hydroforming equipment, being relatively high, has kept hydroforming applications in vehicle manufacturing to a modest scale. The more "traditional" automotive uses being limited to making engine cradles and support pieces (like for the roof and instrument panel) for passenger vehicles.
Another hindrance has been the forming process itself. Until recently, not much had been done to reduce material thinning and wrinkling that occurs during the forming process. To address this, hydroforming companies have been perfecting more sophisticated means of manipulating the metal during forming.
One such company, Vari-Form (Warren, MI), uses a two-stage sequential pressure process that varies both water and die pressure to slowly reshape the metal to its final configuration. By employing this varying pressure method, pressures overall are lower, causing less stress to the metal. A compression of the workpiece (similar to what Cosma does with the Silverado/Sierra frame) can also be done to further control the materials reaction to the hydraulic pressure. A third stage of pressurization can be added to parts with expanded—or flared—ends. By doing this expansion in a third stage, deformation and strain on the material are reduced.