Page 30 - MetalForming February 2012
P. 30
Tie-Rod Prestressing
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Fig. 3—This sectional view illustrates installation of hydraulic tie- rod nuts.
sure to thoroughly clean any oil, grease and debris out of the column and press area. Also, keep portable fire extinguish- ers on hand and post a watchperson to observe the press pit during the heating process. Place mineral fiber insulation between the tie rods and sides of the columns to keep the uprights as cool as possible. This will decrease the amount of heat needed to expand the tie rods and help avoid damage to oil lines and wiring. Beware of wiring located too close to the areas being heated, and disconnect and pull wiring out of the way as needed. Serious accidents and costly damage can result from damaged press wiring caused by careless tie- rod heating.
Heat all four tie rods gradually, moving the flame from one to the other in turn. Allow sufficient time for the con- centrated heat to be conducted along a substantial length of each tie rod; for larger work, shops may require more than one torch. Take care to avoid overheating the work—if a small area of a tie rod receives excessive heat, the materi- al may yield as the tie rod shrinks, resulting in insufficient prestressing. Before returning the press to service, allow the tie rods to cool completely and inspect for proper press alignment.
Alternatives: Electrical Heaters, Hydraulic Nuts
Stampers with presses outfitted with tie rods drilled for electrical tubular heaters (Calrod heaters, for example) can employ these for heating the press elements. Here it is important to have a set of four heaters available, as well as the required electrical current available. Ensure that the holes are free of oil and debris before inserting the heaters and initi- ating the heating process. Exercise patience, as it can take 30 to 60 min. to sufficiently heat and expand the tie rods to per- mit tightening the nuts as required.
Note: Some presses feature hydraulic tie-rod nuts (Fig. 3). The hydraulic nuts are tightened up simultaneously and pressure is applied. Once the annular pistons have lifted, split shims are inserted and the pressure is released. MF
28 MetalForming/February 2012
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