Page 29 - MetalForming February 2012

Proper Prestressing
...of the press tie rods is essential to ensure the proper operation of a straightside press. Most press service manuals explain the correct procedure. It is very important to tight- en each tie rod correctly and equally. An accepted practice throughout the industry has been to prestress press tie rods to 0.0007 in./in. (0.0084 in./ft., or 700 microstrain). The ten- sile stress developed in a mild-steel tie rod at this strain level is approximately 20 ksi. If heat is used to shrink the tie rods, actual stress will be somewhat less once the press members have cooled, due to the elasticity of the members held in compression.
Fig. 2 illustrates the application of a practical shop formula used to obtain the correct tie-rod preload. The constant “4” is universal and is not to be confused with 4 TPI (threads per inch of the tie rods). Stampers should use this formula to determine the amount of rotation needed for each nut (200 deg.); measure the required number of degrees with a pro- tractor and mark it with chalk. If the top nuts require adjust- ment, the tie rods will need support. The tie rods then can be heated and the nuts adjusted.
Proper Heat Application
Presses designed for tie-rod prestressing with a torch have access holes in the columns or uprights through which the flame can be applied to the tie rods. Prior to heating, be
Fig. 2—Stampers can use this practical shop formula to deter- mine the required degree of tie-rod preload.
Deg.=L×T 4
Deg. = 133.25 in. × 6 TPI 4
Deg. = 199.875 deg.
Practical value for shop use = 200 deg.
133.25 in.
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