Paper rolls

The balancing of paper rolls have two goals:

To reduce the rotating forces ( caused by the unbalance ) on the supporting bearings To keep the roll straight (on the contrary there is the possibility to brake the paper )

a) Number of balancing planes

Normally paper roll unbalance is caused by a different wall thickness between two opposite lines of the tube , this results as an uniformely distributed unbalance . Almost all paper rolls , even if working away from the natural frequency , are elastic and because of the relatively high original unbalance , with the speed increase bend ; as a consequense the balancing planes are :

2 planes placed at 0,22

l

(with

l

= roll length);

3 planes, the two end planes and a centre third plane .

Only the rigid rolls (presses) are balanced on the two end planes .

b) Balancing speed

The working speed of a paper roll is variable and it is elastic ; the consequence is that the balancing conditions shall be verified on all the working range , up to the maximum service speed . The paper roll is balanced at a speed compatible with the machine

Pn

² value (relatively low speed ) , then the speed is increased up to the maximum service speed and the bending value is recorded .(if the dynamic run out or bending does not increase with the speed it means that the balancing conditions do not change ).

The use of a bending (run out ) device makes it possible to balance a paper roll at high speed with a machine not eccessively rigid (low

Pn

² value) and has the advantage of directly measuring the paper roll dynamic run out which is a parameter today required.

c) Paper rolls classification

The following table classifies paper rolls according to the required balancing specifications Number of balancing

d) Unbalance tolerance

The required balancing tolerance for new paper rolls corresponds to ISO 2.5.

For reconditioned rolls quality G = 6.3.is required .

The next formula calculates quickly the acceptable residual unbalance (quality 2.5) as a function of the main roll data and of its maximum service speed which is , in the most of cases , specified in meters per minute .

wheree:

2 ,

U

1 = Admitted unbalance [gr] referred to the roll inside radius

D

e= Roll external diameter [mm]

D

i= Roll inside diameter [mm]

P

= Roll weight [kg]

V

= Maximum service roll speed [m/min]

The accepted bending value (Dynamic Run out) is also calculated by using the same ISO formula which defines the residual acceptable rotor eccentricity as a function of the max. service speed .

For quality grade G = 2.5 , the acceptable bending value at the roll centre position is calculated by the next formula :

V E =150 ⋅ D

^e

where:

E

= Acceptable dynamic run out ( in microns ) in the centre position (peak to peak value)

D

e= Outside diameter [mm]

V

= Max service speed [m/min]

If the calculated value for

E

is lower than 40 µm ,the value= 40 µm is given to

E

.

The accepted value for the static bending (mechanic run out or roll straightness ) is about 100 µm.

The roll is considered as balanceable if the total measured unbalance is lower than 1/100its total weight .

CEMB S.p.A. - Via Risorgimento, 9 - MANDELLO DEL LARIO (LC) - ITALY - tel. 0341/706111

e) Flexible rolls balancing procedure

Important notes

1) Particular attention is to be paid when fixing masses on the outside diameter and increasing the speed ..

2) When a big original static unbalance is present it is convenient to reduce it by machining the roll journals out of centre . The eccentricity value can be calculated with the next formula

M

E = U

where:

U

= measured unbalance [gr·mm],

M

= roll mass [kg],

E

= radial eccentricity [µm].

3) When the roll wall thickness is not within the specified values (big difference on two opposite angle positions ), it is convenient to remachine the inside diameter in order to obtauin a more unifor thickness .

4) Before measuring the unbalance , keep the rotor running for some time in order to eliminate any static bending ..

e).1 Two planes balancing

1) Define the two balancing planes at 0,22 l.

2) Pre balance at low speed (200 – 300) the roll by fixing masses on the outside diameter with a standing rope.

3) Increase the speed and , at each step correct the unbalance always in the same planes ..

4) When the speed is over the machine admitted

Pn

² value , the bending measuring pick up is to be used , always acting on the same planes and using ropes to fix the provition al compensation masses .

5) Continue the balancing process up to the service speed untill both the unbalance (below machine

Pn

²

value ),both the dynamic run out (on all the roll working range ) are within the specified tolerances . 6) Remove the outside provisional correcting masses and apply it on the inside by increasing the value

inthe ratio outside/inside diameters . e).2 Three planes balancing

1) Select the three balancing planes ; two at the end sides as near as possible to the journals and the third one at the roll centre position .

2) Pre-balance provisionally , on roll ends ,at low speed (200 – 300 RPM).

3) Spin , at the same low speed , and measure the geometric run out . Record this value and remove it (subtract ) electronically in order to measure only the dynamic run out .

4) Increase the speed , as much as possible with regard to a safety operation ,until the monitored dynamic run out increases to an unacceptable value ..

5) Measure and record the dynamic run out .

6) A known test mass is fixed , by using standing ropes on the outside diameter , at the centre roll position at an angle opposide the measured run out angle . The test mass value can be equal to 40% or 60% the masses used to pre balance at low speed (point 2 ).

7) Spin the roll at the same previous speed ,measure and record the new dynamic run out .The mass to be added , at the roll centre position , in order to compensate the original bending (dynamic run out measured at point 3 ) is calculated with the vectorial method described under paragraph 4.3. Normally the machine software calculates the correcting mass (value and position )to be applied in the centre position .

8) Remove the provisional test mass and add the calculated mass on the outside diameter by using standing ropes .

9) Measure the unbalance and correct it on the two end planes at low speed (the firstly applied masses are normally reduced )

10) Repeat steps 3-9 untill the following conditions are obtained :

– Residual unbalance on the end planes at low speed or at the maximum balancing speed permitted by the machine

Pn

² (see 6.10) is below the accepted tolerance .

– The dynamic run out is within the accepted tolerance on all the service speed range .

f) Fixing the inside correcting masses

Reference is made to the next figure.

1) Calculate the length of the correcting steel rod according to the measured unbalance value and to the inside radius .It is inserted through the holes of roll ends .

2) It is advisable to use square or rectangular shaped rod 40x50 mm.max.

3) If the calculated length is longer than 800 mm ,some 4 mm. Depth notch is to be made on the rod length .

4) By using the real correcting rod placed on the external surface ,mark the position of the fixing bolts .the distance between each bolt is about 200 and 300 mm.

5) Drill and countersink the roll wall ,taking care to prevent chips entering inside .

6) Insert the correcting rod and position its threaded holes in correspondence of the roll wall threaded holes .(to facilitate the operation the roll is supported at its ends and the threaded holes are moved in the lower position ).

7) Fix the rod to the roll wall with at least two threaded bolts.

8) Rotate the roll in order to move the threaded holes to the upper position . 9) Apply LOCTITE on the bolts and screw untill its core is broken . 10) Reduce the protuding part , upset ,shape and tape grind it . Note: Do not weld the bolts to the roll wall..

g) Semicritic speed

When the paper roll is rotating , on a balancing machine ,at a speed equal to the middle of its natural speed , sometimes high vibrations are measured (high dynamic run out values at the centre ).These vibrations have a frequency double the rotating speed , cannot be reduced by adding masses ( balancing ).and disappear by changing the rotation speed ( increasing or decreasing it )

The explanation of the experience is simple : it is sufficient to consider that a misallignment (caused by machine supports, rotor journals, support roller cradles ) or out of round rotor journals can generate a vibration having a frequency double the running speed . If the running speed is exactly equal to half the roll natural speed, a small source of vibration (at double frequency ) equal to the roll natural speed ( resonance conditions ) can excite big vibrations . Paper rolls damping factors are very low ,this explaines why low impulses can cause high vibrations .

Some rolls manufacturers specify a limit to the bending value in semicritic conditions (2° order dynamic run out ) between 700 and 1400 microns. According to our opinition there is not a direct relationship , when running in semicritic conditions , between paper roll behaviour on the balancing machine and on service

CEMB S.p.A. - Via Risorgimento, 9 - MANDELLO DEL LARIO (LC) - ITALY - tel. 0341/706111

In document Balancing theory (Page 146-150)