Aug 30, 2016
Taking nip impressions is dangerous if not performed properly. Only specially trained personnel should perform nip impressions. In addition, anyone involved in taking a nip impression on this calender must first be thoroughly familiar with all safety aspects of the machine and the contents of the Operating and Maintenance manual.
Any questions or comments regarding the safety aspects of the procedure or the machinery supplied by an OEM should be addressed to the OEM for its clarification or other response before using, maintaining or servicing the machinery.
A nip impression is a mapping of the surface contact of adjacent rolls. Nip impressions are usually taken at installation to ensure that rolls are properly positioned for a uniform nip profile. Nip impressions can also be used to investigate the cause for decreased paper quality. They may indicate the need to re-surface filled or polymer rolls.
There are two kinds of nip impressions, static and dynamic (moving). Both types of nip impressions are taken using a pressure-sensitive type of material about 10" wide and longer than the face width of the roll. The paper must be sensitive to pressures up to, and in excess of, the total nip loading of the calendar.
A static nip impression measures the width of the nip. The width of the nip is a machine direction measurement of how much the rolls flatted as a result of nip loading. It is taken by placing a pressure sensitive strip into the open nip, then closing and re-opening the stack.
The width of the nip is important when determining the loading needs for the type of paper being produced. However, it is not recommended that mill personnel perform this type of nip impression since excessive static loading can damage the rolls, leading to stack vibration problems.
Instead, it is recommended that the mill contact Valmet to conduct a static nip impression. When the analysis is conducted by Valmet, a lower pressure is used, and the results are used to extrapolate the required loading needs when running.
A dynamic nip impression can assist in diagnosing the cause for the following conditions:
inability to wind a good roll of paper (for example: hardness variation)
uneven temperature profile of the calender rolls
A dynamic nip impression should only be taken to resolve problems which cannot be resolved with other methods.
A dynamic nip impression is used to verify nip uniformity in the cross machine direction. A chart of high and low spots across the nip is obtained by passing a pressure sensitive strip through the nip at jog or threading speed, under normal nip loading.
High and low spots around the circumference of the roll can also be mapped using a longer piece of pressure sensitive paper.
|CAUTION: Taking nip impressions is dangerous if not performed properly. To maximize safety, it is recommended that the mill contact Valmet to conduct the dynamic nip impressions.
|CAUTION: Hydrodynamic Controlled Crown rolls (older Beloit Corporation designs) may be damaged while performing dynamic nip impressions due to lack of oil film between shell and load shoe. Dynamic nip impressions should not be performed in nips with hydrodynamic CC rolls.
It is recommended that nip impressions be taken for all nips in the stack. For nips consisting of at least one filled or polymer roll, it is recommended that nip impressions be taken both before and after roll changes. If it is not practical to take all of the recommended nip impressions, at least the top and bottom nip impressions should be taken.
If the top and bottom nips look good, the entire stack will usually be good. However, intermediate roll nip impressions may reveal binding. In a nip relieving system, they may show either a bad cylinder or badly proportioned nip relieving zones.
It is important to have enough trained staff available to perform the nip impressions, and to check the brightness of the nip paper immediately after all of the necessary impressions have been taken. Nip impressions should only be conducted by trained personnel who are familiar with the nip impression procedure as well as the operation of the calender.
|DANGER: Dynamic nip impressions should be taken with great care and caution since they involve moving parts. To prevent accidental injury, follow all instructions and warnings!|
A dynamic nip impression requires the following staff, time, and equipment:
1. Take a roll profile of each soft roll using a roll skate.
2. Check for proper operation of the emergency stop cable located on the front of the threading platform, if so equipped. If there is no E-Stop on the threading platforms (older systems), position an extra operator at the nearest appropriate E-stop button.
3. Check that nip guards are in place and properly attached. If your calender does not have nip guards, check with the calender supplier to determine if guards can be fitted to your application.
4. Clean the nip guards to remove oil, water, or dirt.
5. Change the calender program logic to allow the calender to be loaded at slow speed (50 fpm or less).
|NOTE: Document all program logic changes that are made. The changes will need to be reversed later.|
6. Ensure that calender rolls are clean and dry.
7. Set the calender loading cylinders to zero bias between the tending and drive side to achieve equal loading pressure.
8. Follow the appropriate procedure for your system:
With a fixed crown top and bottom roll, use a pi tape to check and record the crown of the top and bottom rolls.
With a variable crown roll (this includes deflection compensated swimming and zone controlled rolls), set the controls to zero bias to achieve a theoretically flat profile.
9. Cut the length of the nip impression paper to extend at least 18” beyond the roll face.
WARNING: To prevent injury during the nip impression process, it is important that the paper extends at least 18” beyond the end of the face of the roll or the shaft nut. This ensures clearance from the nip when all instructions are followed.
Hands should be kept clear of the nip at all times. All work should be performed beyond the face of the roll or the shaft nut.
10. Mark the far ends of the nip impression paper to identify the tend and drive sides. On the drive side, also include the following information: machine number, nip location, date and time, calender load, for fixed rolls record the top and bottom roll crowns, for variable crown rolls record the loading pressure and record whether the nip impression was taken using existing or new intermediate rolls.
|NOTE: It is best to first take a nip impression of the top nip, followed by the bottom nip, then completing the remaining nips. For nips consisting of at least one filled or polymer roll, it is recommended that nip impressions be taken both before and after roll changes. However, mills often take only post roll change nip impressions.|
|NOTE: When replacing filled or synthetically covered rolls, make sure to use rolls with the appropriate roundness and diameter profile as specified by the roll manufacturer. If the profile of the rolls varies more than the recommended amounts, it may be difficult to achieve the desired nip profile, thus affecting web caliper.|
1. Close the calender, but do not apply loading (minimum PLI).
2. Run calender speed to 20-30 FPM.
3. Load the calender to the average PLI run on the calender.
4. Position calender personnel so that there are 2 people on the ingoing side of the nip to be tested, and 3 people on the outgoing side.
|DANGER: Do not place any body parts, hair, clothing, or equipment in the area in front of the ingoing nip.|
5. Carefully, holding the nip paper taut, and keeping all body parts away from the nip, feed the nip paper into the ingoing nip from both ends of the paper (beyond the face of the roll).
DANGER: USE EXTREME CARE when feeding the nip paper into the nip or removing it from the nip.
It is important that the nip paper is held on the very ends of the paper (outside the face of the roll or shaft nut) when it is fed into the nip. Keep a safe distance from nips when rolls are rotating.
Failure to follow this warning can result in serious injury.
Do not guess at the proper procedure. If there are any questions, call your OEM representative.
6. Carefully remove the nip paper after it passes through the nip. The person located in the center of the outgoing nip should use the roll scraper to remove the paper in the event it sticks to a roll.
|CAUTION: Use a roll scraper to help remove the nip impression paper from the outgoing side of the roll. Never use your hands.|
7. Roll up the nip paper and set it aside until all nip impressions have been taken.
8. Repeat these steps for each nip.
1. As soon as possible after all nip impressions have been taken, check and record the brightness of the nip paper every six inches across the full length of each nip impression.
|NOTE: A darker spot in the paper indicates higher linear pressure in the nip. When the linear pressure is uniform, the paper is evenly dark. A closer study requires brightness measuring (e.g., every 10 cm).|
2. Evaluate the top and bottom nip impressions after the roll change.
3. Make any necessary adjustments to achieve the desired flat nip profile, then take a new nip impression to check the results.
4. Compare the results of the "before" and "after" nip impressions to determine whether the problem is created by the incoming paper or the calender stack.
After all nip impressions have been made, remove the changes which were made to the calender program logic to allow loading at slower speed.
For more information on nip impressions and calender operations, contact your Valmet representative for training or other options.