Presses are commonly found in industrial manufacturing facilities. Presses can work on parts ranging in size from a few millimeters to several feet and may have with capacities of up to thousands of tons. If properly maintained, presses can be used for upwards of 50 years.
Presses serve a large number of manufacturing functions, including stamping, forging, extrusion, and injection molding. Each function requires a large amount of force or pressure to be applied repeatedly. Depending on the operation, a press can act hundreds or even thousands of times per minute.
Fatigue Happens with Every Use
Each time a press applies force or pressure to a workpiece, an opposing force acts on the press itself. Presses typically have heavy iron bodies and frames to provide stiffness and stability. Even though the stress experienced by the frame is within the designed limits, repeated stress can result in small localized cracks that grow over time. This method of material cracking is called fatigue.
Fatigue stress cracking is a form of gradual damage that may be excluded by property insurance policies, but it may be included in boiler and machinery coverage. If fatigue is found to be the cause of a loss, it can impact which policy is in effect, or whether the claim is covered at all.
Even though stress is within designed limits, it can still result in small cracks that grow over time.
When a structural component of a press begins to crack, the press may still be usable. However, it may no longer maintain the precision it originally had, and the parts being produced may be outside specified tolerances. The press may also not be able to withstand the highest forces or pressures that it was originally designed for.
Manufacturers who use presses should institute an inspection program so that cracks can be detected, monitored, and repaired before they result in major machine failures. If a press failure occurs, the insured should be able to document the last time the press was inspected or repaired. It is possible that a crack was already identified before it grew and caused a major issue.
Manufacturers should institute a program to detect, monitor, and repair cracks before they result in major failures.
While fatigue cracking is a gradual process, the failure may appear to be sudden when a crack becomes visible. Other types of cracks can be caused by single incidents, such as if a press is operated incorrectly or damaged by an outside force.
Figure 1 – This photo shows cracks propagating in the ram of a press. There is also evidence of prior repairs in the same area.
If a crack appears suddenly, Amset can assist in determining whether the cause of the damage was fatigue or a more isolated incident. The location of the crack, the presence of other cracks, and evidence of past repairs are some details that can be observed from a direct visual inspection. An examination of the crack surface itself can also reveal whether the crack grew gradually or suddenly. Metallurgical analysis of the cracked material can be used to examine the material at a more detailed level.
Fatigue cracks and other press issues can often be repaired, which typically consists of grinding out the crack area and welding the material back together. Both of these processes can alter the appearance of the crack and material properties of the metal, so it is important to have the press inspected before repairs are carried out.