Page 28 - MetalForming February 2012

Tie-Rod
Prestressing
If the tie rods on your presses do not keep the machines precisely aligned, dies will not close in precise alignment and part quality will suffer. Here’s sound advice to ensure proper prestressing of tie rods, essential for the proper operation of straightside presses.
BY DAVID ALKIRE SMITH
Fig. 1—A die shoe imprinted with a setup-block mark. This con- dition can be caused by improper heating used to free a press stuck on bottom.
The structure of most but not all top-driven straightside presses is held in compression by four tie rods running from the top of the bed through the columns and through holes in the bed (fabricated-plate presses built without tie-rods are an exception). If these tie rods become
David Alkire Smith, a tool engineer with more than 50 years of experience, is the editor of the “Die Design Handbook” and other engineering reference works; dsmith@smithassoc.com, www.smithassoc.com.
damaged—from press overloading or improper heating used to free a press stuck on bottom—precision will be compromised.
Upper-Shoe Imprinting
If the tie rods do not keep the press precisely aligned, the die will not close in precise alignment. The guide pins alone cannot achieve the slide-level condition needed. How do you know if your press tie rods are loose? By checking the location of the press crown relative to the uprights when the press is cycled at full tonnage.
In addition, when conducting plant audits, finding upper die shoes imprinted on the side of the press (Fig. 1) requires further inquiry. Inspectors can detect this condi- tion visually or by feeling with a finger, and then find out if the press has become stuck on bottom and, if so, then learn how the condition was corrected. If the tie rods on just one side of the press were heated, they will have yielded and will remain loose. This will cause the upper-shoe imprint- ing problem on one side, as well as other problems.
For example, when using a tonnage monitor with column- mounted sensors, the monitor will not provide correct read- ings if the column preload is not maintained. In this case, the monitor will not register full press tonnage. Compressive pre- load on these tie rods holds the press structure together to approximately 200 percent of press capacity. In the event of a large overload, the safety factor will be exceeded and the tie rods will stretch, allowing the crown to lift from the top of the columns. This serves much like a safety valve to protect the other press parts. The amount of force developed in a cata- strophic overload is limited by the yield strength of the tie rods. If the tie rods are stretched but not damaged, they can be retensioned.
26 MetalForming/February 2012
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