This is a continuation of my recent post, Rollers for the Offset Press Market Part 2, and will provide additional details about offset press rollers.
Cores
The heart of a roller is the core. Core material and concentricity will determine the quality of a roller. For a roller to be concentric, all parts of that roller must be made and kept true to the center. The original center is machined into the core when it is manufactured; the rubber is applied and built to the same center, ensuring the concentricity of the core to the rubber. Cores should be concentric within .001” TIR (total indicated run out), or the press will experience detrimental machine harmonics by generating heat. Unlike a tire that needs balancing, cores cannot be balanced to correct the issue. Rollers that are not concentric are considered defective.
Tolerance of the bearing bore can be a huge issue. The tolerance must be kept close to ensure that the bearings do not spin within the bore. When bearings spin within the bore, the spun bearing generates heat exponentially within the press. Heat can affect the rubber and ink performance resulting in other production issues. If you look at the end of your roller and see that the core has been “pinged” or glued, the bearing will likely spin within the bore.
Bearing bore and heat retention can be equally affected by the quality of the core material. Steel and aluminum have different thermal expansion, which affects the bearing pressed into the bore. This difference causes the steel bearing race to stay closer to size while the aluminum expands and the bearing spins within the bore and introducing heat to the ink train. Removing bearings from aluminum cores and replacing them only worsens the problem. Aluminum, which is extruded, has a .30” Tolerance that can lead to unbalanced rollers and detrimental machine harmonics. Great care must be taken to ensure uniformity of aluminum. In contrast steel has a .001” Tolerance for DOM steel, making it ideal for concentric rollers, effectively mitigating the heat build up related to poor harmonics. If your rollers temperatures are higher than 105°F, then begin the process of examining the cause. The material can contribute to the issues that you may be experiencing.
Bearings
Bearing selection is crucial for the best performance on press. Speeds, heat, and loads must be considered to identify the proper bearings to be used on a press. Seek a consistent bearing with good dimensional tolerances and running accuracy. Consistency of dimensional tolerance and running accuracy depend on high quality steel and low contamination content in the steel metallurgy.
Leading manufacturers of bearings use high quality 52100 bearing steel because it holds form and yields the most value for the challenging applications associated with the high-speed offset industry. Clean steel with low oxygen content and few inclusions increases bearing life under difficult applications such as two-piece decoration. With the high speed and extreme load demands of high-speed offset applications, proper bearing choice will lower operating temperature and provide more consistent production.
Bearings must accommodate load pressures in high-speed offset presses while reducing heat friction and wear in the bearing. A high quality bearing will more than pay for itself by providing lower temperatures and longer run times. A good bearing reduces friction, which reduces power consumption in the press, and lower heat makes all components last longer. Proper seal and grease selection will improve the operation of the roller while reducing heat. Carefully selected nitrile sealing material and grease that stays in place will ensure long operating life of the bearings. If your bearings temperatures are in excess of their specifications, consider a different bearing, seals, and grease.
Conclusion
The heartof any press is the roller train. Rollers are treated as a consumable commodity and subsequently are overlooked. A press roller should run reasonably cool at high speeds provided that the rubber, core, and bearings are designed for the task. Because high-speed offset presses run at very high speeds some misting is expected. Lower pressures and temperatures, however, can minimize misting. High quality bearings and setting a minimal nip pressure will lower temperatures within the press; infrared thermometers will give you solid temperature readings while running. If you are seeing temperatures within the roller train that exceed 110°-115°F, start the process of identifying the problem. It could be a bearing that needs to be replaced, or it could be too much nip pressure causing the problem. Some presses have water-cooled rollers to help control temperatures, and those do a great job. Beware though; cooling towers can mask the presence of bad bearings and excessive nip pressure, so use a thermometer to document temperature readings of the individual rollers. Understanding the dynamics of the rollers within the ink train will allow operators to troubleshoot issues more quickly and accurately leading to less downtime and increased productivity for the operator.
Stay tuned for future post, I will provide more insight and information into offset presses and their components. If you would like more information, you can contact\ our marketing manager, Katie Graham.
If you missed the first two parts, here is part one Rollers for the Offset Press Market!
Be on the lookout for future educational post!