Indicators of Roll Failure and Weld Roll Design Engineering

Here are some technical insights and tips on how to perform regular roll inspection and maintenance that extends roll life and improves performance by using micrometers, gauges, and a trained eye that can spot areas requiring attention before wear can get out of control.

Roll inspection and maintenance are important to extend roll life and improve performance. Regular inspection with micrometers, gauges, and a trained eye can spot areas that require attention before wear can get out of control. Complete failure should never occur on properly maintained rolls unless outside forces, such as impact or temperature variations, affect them. High-carbon tool steel can become very brittle if it is subjected to long periods of abrasion and heat. This surface cracking can travel into the roll body and propagate, leading to complete failure.

Complete inspection must include all roll surfaces. Even the most basic roll program should include a visual inspection and the use of a blade micrometer to measure root diameter wear. Since the root diameter, or throat, is always in contact with the strip, it is a great indicator of when a roll should be sent for rework. Guidelines for how much wear is allowable should be set up and based on past performance.

The next step for inspection is to check the roll contour, using templates, gauges, or plugs. Round washers are the most common, least expensive and, with a trained eye, can be very accurate. Proper training to read a gauge correctly is important.

Rolls that are overloaded or misaligned can be spotted by the wear pattern in the contour. The cause should be found to determine if the rolls were set up improperly or if there was a possible mill problem. Uneven wear can be caused by a design flaw that did not take into account the type of material to be formed or maximum gauge thickness.

Roll bores and faces that are worn beyond normal limits can indicate shaft problems or, again, setup. The outboard stand must be tight or rocking or slipping will occur that wears the roll faces and bores. A dial bore indicator is the most accurate way of checking the roll bores. A depth micrometer can measure the face wear.

Roll bores and faces should be repaired by chroming, if they wear more than .001 per inch of shaft size. Example: A maximum of .003 in over the high tolerance of a 3.000 in diameter shaft. Roll faces can also be corrected by grinding the entire face of the roll and making a new spacer. This is a more cost-effective way to eliminate wear because if the roll face is worn, so is the spacer. All roll spacers should be regularly checked and replaced, if worn more than .001 per inch of shaft size.

Weld integrity is a common problem that can many times be avoided completely by monitoring the wear on the fin blades. A grooved fin blade cannot properly prepare the strip edges before the welder. Spare blades should be on hand to change out as needed. The blades should not be allowed to wear more than ¼ percent to 1 percent of the gauge thickness. Example: For .030 in thick material, allow no more than .003 in wear. Fin blade maintenance is one of the most overlooked areas in mill setup.

Many weld problems can point back to improper strip preparation. Strip edge condition is especially critical in welding coated materials, to keep the weld zone free from contamination. To complete the roll inspection, roll diameters and widths should be checked. To have a complete record of all dimensions, all rolls should be checked for run-out from bore to contour. This can be done by placing smaller rolls on a tapered arbor or measuring from bore to 0.0. with a micrometer. If any run-out is found, it can indicate a bad shaft or bearing problem (assuming the roll is made correctly in the first place).

A good roll program can stop many problems before they develop. Once a limit is set and held for your own internal quality, then scrap, setup time, and rejects can be kept to a minimum. Tooling should never be used until it reaches the point that you just cannot run product.