The influence of bar width on bar forces and fibre shortening in low consistency pulp refining

This paper describes forces in pulp refiners and their influence on fibre shortening. We show that force per bar length obtained by a simple calculation from the Specific Edge Load (SEL) is a useful parameter to describe threshold force levels below which levels of fibre cutting can be avoided. Our analysis is supported by data from studies on four refiners and in one case by direct measurement of force in an operating refiner. In particular, we show that small bar widths lead to large forces and consequently that SEL must be adjusted to lower levels to avoid fibre cutting. We also speculate on the lower limits of bar width.

Influence of the deformable substrate potential on Shapiro steps in the incommensurate structure

Influence of the deformable substrate potential on Shapiro steps is investigated in the overdamped Frenkel–Kontorova model with incommensurate structure. Both the number and size of Shapiro steps are strongly affected by the deformation of substrate potential. As substrate potential gets deformed, the oscillation of Shapiro steps reduces and the periodicity of oscillation significantly changes. For some value of shape parameter r, the odd and even maxima oscillation forms of harmonic steps still maintain the Bessel-like form. Compared with those in standard potential, the amplitude dependence of harmonic step exhibits obvious difference. The dynamical DC threshold force correspondingly increases with rising the value of r.

Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference ( P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences ( P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.