Proper Care and Feeding of Furnace Mesh Belts
Mesh belts come in all shapes, sizes, materials and weaves and are used for such diverse applications as case hardening, brazing, sintering and glass-to-metal sealing to name a few. Belts run at temperatures from near ambient to several thousand degrees. They are expected not only to work in the furnaces but also for external part conveyance or in water, oil, brine, polymer and salt quench tanks. Mesh belts are exposed to a multitude of furnace atmospheres ranging from air to pure hydrogen and can be exposed to oxidation, sulfidation, carburization and nitriding. They operate in environments spanning dew points from below -100°F to above +100°F. And most of us expect our belts to perform well beyond normal expectations. How do they survive, and how can we make them last even longer? Let’s learn more.
When operating a mesh-belt conveyor furnace (Fig. 1), the goal should be to maintain a consistent temperature profile and uniform belt loading for any given part number over time. Premature or abnormally short belt life is often the sign of overloading, misapplication, abuse, neglect or ignorance. The vast majority of belt failures are either due to overloading or poor belt-system adjustments. Following some simple advice may extend your current belt life considerably.
Tips to Improve Belt Life1. Know your belt speed by actually timing the belt movement. Do not assume it is what is stated in the instruction manual or what is displayed on a digital indicator. Calibrate the speed control (typically a magnetic or inductive pickup device) as often as your temperature instruments.
2. Determine your belt loading over time. Improperly adjusted vibratory feed systems and weigh scales that do not read correctly are two of the most common causes for improper loading of furnace belts. Load as uniformly as possibly to evenly distribute wear across the bottom of the belt and help to prevent camber and other belt distortion problems.
3. Avoid using side skid plates or belt guide rollers to help belt tracking. These items tend to do more harm than good. Where skid plates must be used, consider coating them with laminated plastic strips. Observe the motion of the belt – it should be smooth, not jerky, and maintain a consistent speed and tracking over time. Remember, a metal mesh belt has flexibility along its length, semi-rigidity across its width and rigidity in its thickness.
4. Work with your equipment or belt suppliers to select a belt weave and alloy best suited for your process and parts. Be aware that some belts using heavier wire or larger crossrods may decrease furnace throughput. Avoid upturned-edge belts whenever possible.
5. Flip and/or reverse the belt at frequent intervals as signs of wear or camber become evident. Camber in a high-temperature belt may be either convex or concave in the direction of travel, or in some cases it may develop with one edge of the belt leading the other edge. It is frequently an indicator of other problems such as uneven loading, uneven cooling, improper drive tension (see #7 below) or a combination of these factors. Only flip and reverse the belt when it is sufficiently flexible to ensure that reversing of the belt does not create a fatigue problem.
6. Be sure that the belt is properly supported over its length. Watch for signs of premature failure such as abnormal distortion of the crossrods, flat spots, deterioration of belt edges, contamination/buildup of foreign residue and tracking problems.
7. Check the tension on the belt frequently or as dictated by production usage. Some systems rely on springs to maintain tension, so check their length. Other systems rely on cylinders. Therefore, verify that plant air pressure does not fluctuate significantly. Remember, check the system for compliance to its design specification.
8. Have your belt drive system analyzed by a furnace manufacturer or belt company at least once a year.
9. Keep loading consistent. Many parts are loaded on the belt in such a way as to leave space along the edges to prevent parts from moving off the sides. This concentrates loading over a smaller square area and may affect belt selection or life.
10. Understand that maintenance is the difference between front-end-drive systems having pinch and tangential rolls (for tracking) and rear-end-drive systems having only pinch rolls.
Other Important ConsiderationsRun belts operating at or near ambient temperature or in quench tanks both unloaded and loaded for a minimum of three revolutions after final belt tracking adjustments and prior to use. Anticipate issues such as the environment (e.g., dirt, chips) or thermal expansion (e.g., austempering) before putting the belt into service.
For high-temperature belts, two frequently overlooked procedures are “break in” and “stress relief.” Take the time to do both. Though opinions differ on how to best perform break in, most agree that, if possible, the furnace should be brought up slowly (<150°C (300°F) per hour) to an operating temperature of 815-870°C (1500-1600°F). The belt should be allowed to complete 2-4 revolutions through the furnace (at least 4-6 hours at temperature with no load) at the lowest practical belt-speed setting. This will ensure proper seating of the spirals and allow for initial movement of the rods. Belt tracking adjustments should be made during and after this procedure.
Stress relief should be performed im-mediately after the break-in procedure is complete. For belts in furnaces operating below 900°C (1650°F), a stress relief at 10°C (50°F) above the normal operating temperature is required with the belt running at a speed to ensure that every portion of the belt reaches this temperature for at least one hour. For belts in furnaces operating above 900°C (1650°F), the furnace should be brought into the 925°C - 955°C (1700°F - 1750°F) range and allowed to complete 1-2 revolutions (at least 1-2 hours at temperature with no load) at the lowest practical belt-speed setting. This will help prevent excessive grain growth and embrittlement in service.
After the stress-relief treatment, the furnace should be brought to normal operating temperature and the belt operated unloaded for as long as practical. Some manufacturer’s recommend 4-6 hours, others up to 100 hours! The more time, the greater the improvement in creep strength.
Proper belt tension (Fig. 2) is also critical, as illustrated by a study conducted by a major belt manufacturer on copper brazing furnaces operating at 1120°C (2050°F). The results are reported to be applicable to all types of high-temperature belts and make the point that every effort should be made to maintain operating tension at the lowest practical level. Either the original equipment manufacturer or the belt supplier has usually predetermined the allowable tension range. If the furnace was not purchased new, it is even more important to know how the drive was sized, where the zero point of loading is and what the proper drum adjustments and belt tension settings are.
Finally, when possible, keep the belt clean (free of oxidation and scale) by running it under protective atmosphere above 760°C (1400°F). In certain situations, such as copper brazing, it has been reported that preoxidizing the belt can extend its service life where spills of copper or other braze alloys attempt to penetrate the surface of the belt.