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Frictional testing of wood: Initial studies with a new device
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (FSCN – Fibre Science and Communication Network)ORCID iD: 0000-0003-3381-5516
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (FSCN – Fibre Science and Communication Network)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Responsible organisation
2009 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 1, p. 190-196Article in journal (Refereed) Published
Abstract [en]

In the chip refining process used for mechanical pulp production, wood fibers are treated in a flat and narrow gap between rotating plates. The process is very energy consuming and much of the electrical energy supplied to the refiner is transferred to the fiber material through friction forces. Even though friction has been discussed frequently over the years among pulp and paper researchers worldwide and held to be of great importance, little has been proven due to the complexity of the refining equipment and the extreme conditions prevailing during operation. This paper presents a new apparatus for studying the frictional properties of wood, in lab-scale, under chip refining conditions. Friction tests can be carried out in a steam atmosphere under high temperature/pressure with maximum sliding velocity as high as 150 m/s. Initial studies at room temperature showed that the coefficient of friction between spruce wood and smooth steel increased linearly with the moisture content of the wood specimens. Impregnation by wood extractives lowered the friction coefficient for dry wood surfaces sliding at high speed. When tests were performed in a saturated steam environment, pressurized to various degrees, the frictional properties were strongly affected and varied by the temperature of the surroundings � which always has been considered as constants in analytical and numerical models of process operation.

Place, publisher, year, edition, pages
2009. Vol. 42, no 1, p. 190-196
Keywords [en]
friction, wood, mechanical pulping, wood extractives
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-6455DOI: 10.1016/j.triboint.2008.03.009ISI: 000265142800024Scopus ID: 2-s2.0-56249138948Local ID: 4536OAI: oai:DiVA.org:miun-6455DiVA, id: diva2:31494
Available from: 2009-01-28 Created: 2009-01-28 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Frictional studies and high strain rate testing of wood under refining conditions
Open this publication in new window or tab >>Frictional studies and high strain rate testing of wood under refining conditions
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When producing thermomechanical pulps (TMP), wood chips and fiber material are loaded mechanically in a disc-refiner to separate the fibers and to make them flexible. In the process, much of the energy supplied is transferred to the fiber material through cyclic compression, shear and friction processes. Therefore, compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, in this thesis, the compressive and frictional behaviors of wood were investigated under simulated chip refining conditions (i.e., hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental setups were developed and used. The equipment used for compression testing was based on the split Hopkinson pressure bar (SHPB) technique and the friction tester was a pin-on-disc type of tribotester (wear rig). Both pieces of equipment allow a testing environment of hot saturated steam.

 

In the wood–steel friction investigation, the influence of the steam temperature (100-170°C) was of primary interest. The wood species chosen for the friction tests were spruce (Picea abies), pine (Pinus sylvestris, Pinus radiata), and birch (Betula verrucosa). When performing measurements in the lower-temperature region (100-130°C), the friction coefficients registered for the softwoods were generally low and surface properties such as lubrica­tion were suggested to have a great influence on the results; however, in the higher-tempera­ture region (~130 -170°C), the friction coefficients of all investigated wood species were probably determined by bulk properties to a much greater extent. When most of the wood extractives had been removed from the specimens, testing results revealed distinct peaks in friction at similar temperatures, as the internal friction of the different wood species are known to have their maxima at ~110–130°C. One suggested explanation of these friction peaks is that reduced lubrication enabled energy to dissipate into the bulk material, causing particularly high friction at the temperature at which internal damping of the material was greatest. During the friction measurements in the higher-temperature region, the specimens of the different wood species also started to lose fibers (i.e., produce wear debris) at different characteristic temperatures, as indicated by peaks in the coefficient of friction. In refining, the generally lower shives content of pine TMP than of spruce TMP could partly be explained by a lower wear initiation temperature in the pine species.

 

Wood stiffness is known to decrease with temperature, when measured at low strain rates. The results presented in this thesis can confirm a similar behavior for high strain rate compression. The compressive strain registered during impulsive loading (using a modified split Hopkinson equipment) increased with temperature; because strain rate also increased with temperature. Accordingly, the strain rates should determine the strain magnitudes also in a refiner, since the impulsive loads in a refiner are of similar type. Larger strains would thus be achieved when refining at high temperatures. The results achieved in the compression tests were also considered in relation to refining parameters such as plate clearance and refining intensity, parameters that could be discussed in light of the stress–strain relations derived from the high strain rate measurements. Trials recorded using high-speed photography demonstrated that the wood relaxation was very small in the investigated time frame ~6 ms. As well, in TMP refining the wood material has little time to relax, i.e., ~0.04–0.5 ms in a large single disc refiner. The results presented here are therefore more suitable for comparison with the impulsive loads arising in a refiner than are the results of any earlier study. It can therefore be concluded that the modified SHPB testing technique combined with high-speed photography is well suited for studying the dynamic behavior of wood under conditions like those prevalent in a TMP system.

Place, publisher, year, edition, pages
Sundsvall: Mittuniversitetet, 2007. p. 88
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 31
Keywords
Friction, High strain rate testing, Wood, Mechanical pulping, Tribology, Refining, Energy consumption
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-8895 (URN)978-91-85317-64-6 (ISBN)
Public defence
(English)
Supervisors
Available from: 2013-04-16 Created: 2009-05-06 Last updated: 2013-04-16Bibliographically approved

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Svensson, Birgitta A.Nyström, Staffan K.Gradin, Per A.Höglund, Hans

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