... We’ve experienced three broken rods in the past seven months, one just above the ram and the other two right below the piston. We are using a Chambersburg hammer installed a few years ago. We are checking all aspects of the hammer to see if it’s square and what might be causing the rods to snap. On the last rod, the end was not polished and we could see the machined grooves (chatter) in the bar — which could have contributed to the breakage. We’ll be making a new rod of Astralloy material and it will be shot-peened.
A: Yours is one of several questions I have had about piston rod failures. Others were mostly smaller hammers — 5,000 to 20,000 tons. You’ve described a classic problem with steam- and air-powered hammers: piston rod failures.
The data sheet indicates that the alloy is closer to AISI 9320 or AMS 6265, and that is a good choice. The slightly higher carbon and Mn contents add to the hardenability. The Ni content is good for toughness. Watch out for too-high hardness — higher than about 38 Rc can result in impact property problems.
You are right to shot-peen and polish the rod, especially just above the boot and just below the piston gland. The circular machining grooves must be polished out to eliminate potential sources for fatigue failures. Be sure to polish in the longitudinal direction.
Remember, too, that harder is not necessarily better.
Checking for ram alignment and hammer tip are good steps. The idea of reducing the diameter by 10-12% for a few inches above the ram boot tends to improve the flexibility of the rod at that location and extend life (after polishing and shot-peening).
I have no solution based on experience for the rod area near the piston, but I advise you to be sure the circular machining marks are polished out longitudinally. Be sure, also, that the rod centerline is concentric and square to the piston. Any slight angularity can be a source of trouble. The gland can be a key to achieving concentricities and squareness, as well.
Be sure to check for ram tipping at the bottom of the stroke because this is where the greatest impact bending loads are experienced in the rods — especially on off-center blows. This can be done with or without dies in the bolster/ram. Remember, the gibs wear out with a fish-tail pattern — loosest at bottom of the stroke. Too many people check alignment with the ram/gib clearances in the up or middle position without checking it at the stroke bottom.
Using a jack to raise the ram alternately from left or right can help identify ram tip.
Do you use segmented gibbing where the lower (impact zone) can be shimmed-in or adjusted separately? If not, this is a good way to correct ram alignment without removing the entire gibs or other parts of the hammer. This worked well in my experience with a long stroke that required a total ram removal to remove the gibs.
For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.
Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.