© 2010 ProTest engineering
NIP FORCE MEASUREMENTS
The most important questions
Nip force measurements are the most important part of a press nip study. They create the basis for
troubleshooting and optimising your nip process. We will investigate if there is more capacity and
will help you to improve your runnability, paper quality and energy efficiency. Our specialists have
an experience of over 200 individual measured and adjusted nips worldwide.
What is a nip force measurement?
Nip force measurement is a nip condition test package
done in a machine shutdown.
With the measurement, we will define at the same time
front and back side actual linear load [kN/m] as a
function of loading pressures [bar]. This is done by
means of two load cells and two to four pressure
sensors. The force is measured between nip roll bearing
houses. Special load cell brackets are used to hold the
load cells in correct position. Normally, at the same
time with the nip force measurement, we will make a
hydraulic/pneumatic nip loading system function tests
with a multi-channel pressure analysator.
What are the typical findings?
Paper Mill PM1 Some typical findings are:
1. Press
FRONT SIDE BACK SIDE
130
120
130
120
- press nip loading mechanics friction
110 110 exceeds allowed limit and this will have
100 100
a negative effect on actual linear load
level. Nip controllability and sensitivity
90 90
80 80
can be poor. Actual linear load level can
Force [kN/m]
Force [kN/m]
70 70
60
50
60
50
also be wrong because of inadequate
40 40 theoretical calculations, wrong nip
30 30
control software parameters, wrong
operator panel programming or because
20 20
10 10
0
30 40 50 60 70 80 90 100 110 120 130
0
30 40 50 60 70 80 90 100 110 120 130
of problems in the nip loading system
Measured
Pressure [bar]
Measured Theoretical Measured
Pressure [bar]
Measured Theoretical hydraulic/pneumatic components.
Values Load Line Load Line Values Load Line Load Line
- difference between front and back side
due to difference in loading mechanics friction, inadequate theoretical calculations, wrong nip
ProTest engineering Oy PL 2, FI-40271 Palokka FINLAND Tel. +358 45 315 0707 Fax +358 50 780 003 86 www.pte.fi info@pte.fi
control software parameters or problems in the nip loading system hydraulic/pneumatic
components.
- linear load control and pressure rampings can be very unstable because of problems in the control
components. Sometimes other mechanical components, such as suction roll saveall or suction pipe
has a undesired force effect on actual linear load.
- with ProTest engineering specialists and measurement equipment all problems mentioned above
can be found and solved.
What kind of further actions can be done according to the measurement results?
Performed by ProTest engineering, nip force measurements gives immediately information on the
need of further actions such as:
- nip loading system mechanical service (normally includes changing of loading arm pivot pins,
bearing sleeves and loading cylinders/bellows)
- reprogramming of nip loading curves to automation system / operator panels
- adjusting of hydraulic/pneumatic component functions
- changing of problematic components such as pressure control valves
With the measurement results, it is possible to adjust actual linear loads right away, so that nip
loading is correct and even between front and back side.
What kind of measurement system does ProTest engineering have?
ProTest engineering measurement system is
a state-of-the-art full digital force/pressure
measurement system. Digitality means
disturbance free measurements. The system
is high sensitive, very accurate, and it is
calibrated periodically in a certified
laboratory. The software is self made
specially for paper machinery nip analysis
and nip adjustments.
The system allows us to make accurate
measurements from very low (<10 kN/m)
to high (>100 kN/m) linear load levels.
What type of nips it is possible to measure?
ProTest engineering nip force measurement is suitable for all type of nips with external
cylinder/bellow loading. We have measured many different type of press nips, size press nips,
calender nips etc. The measurement is even applicable for coating station beam force measurement
ProTest engineering Oy PL 2, FI-40271 Palokka FINLAND Tel. +358 45 315 0707 Fax +358 50 780 003 86 www.pte.fi info@pte.fi
and reel nip force measurement. The measurement is applicable for both hydraulic or pneumatic
loading systems.
How can we improve paper machine energy efficiency with the nip force measurements?
The dryer section is the highest thermal energy consumer on the paper machine, so optimizing
energy efficiency can help to cut operating costs.
For example, if press section linear loads are found too low, we can adjust them back to original
design values to gain more sheet dryness after press section. This can lead to a significantly lower
steam consumption in the dryer section, or potential to increase machine speed.
Optimal nip operation also usually means better runnability with less sheet breaks and easier tail
threading. This improves the machine efficiency and at the same time energy efficiency.
What is the pay-back period for measurement costs?
There has been cases, where pay-back time for measurement costs is only one day!
In these cases the findings were significantly low nip linear load levels. After the correct
adjustments, we have really improved the dryness after the press section and this leads to higher
energy efficiency. Quite often, nip force measurements pay-back time is less than one year.
See below a example calculation from a paper machine case after nip optimisation. In the
calculation, parameter ”steam consumption per water evaporated” is an estimated value on modern
machines according to TAPPI technical paper 0404-33 ”Dryer section performance monitoring”.
Steam consumption calculation
Dryness at reel [%] 97,3 Steam price [€/t] 38
Dryness after press section [%] 41 Production hours per year [h] 8000
Dryness after press section [%]
(after nip optimization) 41,2
Grammage [g/m2] 80
Web width [m] 6,3
Machine speed [m/min] 1100
Steam consumption per water
evaporated [kg steam/kg h2o] 1,3
Production [t/h] 33,26
Steam consumption [kg/s] 16,49 = 475042 t/a
Steam consumption after nip
optimization [kg/s] 16,36 = 471057 t/a
Steam saved every year 3985,4 t
Money saved every year 151444 €
How often should the nip force measurements to be done for the same machinery?
This is a machine dependent parameter and is effected by many factors like quality of pivot pin
greasing, hydraulic oil purity, environment (water, chemicals, heat) etc. There is no right answer.
ProTest engineering Oy PL 2, FI-40271 Palokka FINLAND Tel. +358 45 315 0707 Fax +358 50 780 003 86 www.pte.fi info@pte.fi
Some mills organize the measurements only after they have some problems and some mills
organize the measurements every 1-2 years just to check that everything is ok. Our opinion based
on a long measurement experience is, that nip force measurements should be done every 2-3 years.
And of course right away, if problems arise. It is very important to act, before problems get worse.
Is it possible to arrange a larger nip study, not just nip force measurements?
Absolutely. Quite often the nip force measurement is a part of a larger nip study. The nip study can
include for example detailed analysis of a loading hydraulic system, nip force measurements and
finally electrical nip profile measurements to adjust the nip CD pressure profile by means of fixed
crownings and deflection compensated rolls.
How many hours of shutdown time does it take to complete nip force measurements?
It is important to understand two different things here: 1) fitting the load cell brackets 2) making
the measurements with possible corrective actions. Normally, the customer fits the load cell
brackets according to ProTest engineering advices. It is hard to estimate this fitting time, but very
often, for example with a three nip press section, it has been between 6-12 hours. If there has been
previous measurements with the same rolls, fitting time is quicker. Completing the nip force
measurements requires normally 2-3 hours per nip including possible corrective actions. This is an
estimation, when everything goes as planned. Note! Because of safety reasons, measurements can
start only when all the brackets have been fit and roll drives locked.
Does nip force measurements require special knowledge?
Yes, a lot. To make safe measurements and
to get the correct results and to be able to do
the right corrective actions requires years of
testing experience with different kind of nip
loading systems.
ProTest engineering specialists have the
best knowledge to complete safe and
reliable measurements.
Where to get more information on testing different kind of machines and positions?
Please check ProTest engineering contact information from our website www.pte.fi.
You can find contact information from this brochure, too. We serve our customers in english and in
finnish.
ProTest engineering Oy PL 2, FI-40271 Palokka FINLAND Tel. +358 45 315 0707 Fax +358 50 780 003 86 www.pte.fi info@pte.fi
