| 1. |
- Berglind, Daniel, et al.
(författare)
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The influence of the COVID-19 pandemic on physical activity in Stockholm County : Evidence from time series models of smartphone measured daily steps data spanning over 3 years
- 2024
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Ingår i: Preventive Medicine. - : Elsevier Inc. - 1096-0260 .- 0091-7435. ; 183
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Tidskriftsartikel (refereegranskat)abstract
- It has been reported that physical activity levels decreased during the COVID-19 pandemic. Previous studies often relied on self-reported physical activity, which has low accuracy. Studies based on objectively measured physical activity have had short data collection periods, thereby not allowing the consideration of pre-pandemic levels of physical activity or the influence over the different waves of the pandemic. In this study, we utilize smartphone-measured step data from a nonprobability sample in Stockholm County, Sweden, where measures to limit the spread of COVID-19 differed from those in many other countries. The results are based on 522 individuals and 532,739 person-days with step data spanning from 2019 to 2021. Generalized additive models were fitted for each individual, and meta-regression was used to combine the results from individual models. Daily steps decreased during the first wave but increased during the third wave compared to individual pre-pandemic levels. The decrease in daily steps occurred primarily in young individuals and those with occupations allowing remote work. Individuals of retirement age on the contrary increased their daily steps during the same period. This study reveal that the influence of the COVID-19 pandemic was temporary and that younger age and the possibility of working from home were associated with a decreasing trend in physical activity. •Using mobile phone derived daily steps data over three years.•Negative influence of Covid-19-pandemic on walking steps only exists during the early stage of the pandemic.•Being younger and having the possibility of working from home are associated with fewer steps per day.•Individual modeling with meta-analysis is an ideal method for large scale physical activity studies.
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| 2. |
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| 3. |
- Bäckstöm, Mikael, 1963-, et al.
(författare)
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Essential ski characteristics for cross-country skis performance
- 2008
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Ingår i: ENGINEERING OF SPORT 7, VOL 2. - Paris : Springer. - 9782287094132 ; , s. 543-549
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Konferensbidrag (refereegranskat)abstract
- Winner or trail hog? Much depends on the ski characteristics. The manufacturing of skis is a complicated process involving several materials and different process steps. This gives as a result that every ski obtains unique characteristics such as span curve and bending stiffness etc. For high performance skiers as the member of the Swedish ski team the importance of equal characteristics of each ski in a pair is vital. The process of matching skis to a pair is the process of finding two individual skis with the most similar characteristics. This is traditionally done by hand with simpler equipment. Our measurement system is developed for faster and more accurate ski characteristics assessment. The characteristics do impose the overall performance of the ski. It produces the span curve with very high accuracy and gives a good representation of the pressure distribution over the full length of the ski. The measured characteristics could, in our opinion, also be used in selecting skis for different weather and track conditions. The ski measurement system has been used by the Swedish cross-country team during the last 2,5 years which have resulted in a faster and more accurate matching of skis. In collaboration with the Swedish ski team have also an investigation concerning correlation between ski characteristics and weather and track conditions has been initiated with some preliminary results already obtained.
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| 4. |
- Dahlén, Leon, et al.
(författare)
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A study of two lubricated contacts in a radial piston hydraulic motor
- 2003
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Ingår i: The Eighth Scandinavian International Conference on Fluid Power, Proceedings of the conference, Vol 2.. - Tampere : Tampere techn. University. - 9521509724 ; , s. 931-936
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this study is to analyse two sliding contacts in a radial piston hydraulic motor and study their influence on the leakage and power losses. By the use of commercially available FEM software package the losses in the two contacts, the distributor valve and the piston cam-roller contact, are estimated by simulation and related to experimentally measured losses of the motor under different operating conditions. The estimated losses from the two contacts can not explain the total losses and seems also to be a smaller part of the total losses. As long as full film lubrication is present the power losses from the two studied contacts appears to be relatively small. The hydrostatic balancing effect contributes mostly to the external leakage in the piston cam-roller contact. Measured leakage dependency of speed can be related to the piston cam-roller contacts in the motor. At low speed and high load the piston cam-roller contact might enter mix lubrication regime.
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| 5. |
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| 6. |
- Dahlén, Leon, et al.
(författare)
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Analysis of Two Sliding Contacts Inside a Radial Piston Hydraulic Motor
- 2002
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Ingår i: Proceedings at 5th JFPS International Symposium on Fluid Power, Nov. 12-15, 2002, Nara, Japan.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents an investigation of the torque losses in a hydraulic motor of radial piston type commonly used in industrial hydrostatic transmissions. When the speed of the hydraulic motor is decreased an increase of the torque loss can be measured. In order to investigate the reason for this increase a combination of experimental and theoretical studies is performed with special focus on two sliding contacts-the distributor valve and the piston-cam roller contact. The theoretical analysis of the contacts reveals that the distributor valve contribute more to torque and power losses compared to the piston-cam roller contacts and largely to the external leakage of the motor. At low speeds the piston-cam roller contact will enter the mixed lubrication regime.
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| 7. |
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| 8. |
- Dahlén, Leon
(författare)
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Numerical and experimental study of performance of a hydraulic motor
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this study was to develop an understanding of the fluid properties that influence the efficiency of hydraulic systems in a steady-state, especially components in hydrostatic transmission systems under different running conditions. The aim has also been to investigate and model the sources of losses in hydraulic machines, in order to estimate the losses, taking fluid properties into account. Finally, the technique of optimization has been introduced in order to improve the efficiency of a distributor valve in a radial piston hydraulic motor, Marathon M200. From an experimental field-test, which was performed on a belt conveyer using a hydrostatic transmission system, the overall efficiency of the hydrostatic transmission was compared when using a mineral oil, Shell Tellus TX 68, a synthetic fluid, Mobil SHC 526, and a vegetable fluid, Binol Hydrap II. The experimental field-test showed that vegetable and synthetic fluids improve the efficiency compared to mineral oil. The three fluids have the same viscosity but experimental tests showed that the temperature ()- and pressure() -viscosity coefficient differs between them. It was also found that the pressure-viscosity coefficient () of mineral oil was higher compared to vegetable and synthetic fluids. The proposed steady-state model gives greater accuracy regarding overall efficiency than the Wilson model, when examining fluid properties that differ in other aspects than their viscosity. The study has focused on losses in lubricated sliding contacts within a radial piston hydraulic motor, Marathon M200, and an analysis of the losses has been made using the finite element method (FEM). A FEM software package, Solvia, which takes into account fluid properties, such as temperature- and pressure- viscosity coefficient, heat conduction and specific heat, has been used to simulate the behaviour and to estimate the losses in tribological contact. This approach of simulation has been applied to two different tribological contact within the hydraulic motor: to a journal bearing contact and to a hydrostatic annular multi-recess plane thrust bearing. By using an FEM software package linked to an optimization algorithm, the losses in the tribological contact in a distributor valve were reduced significantly. The study shows that the optimized geometry of the distributor valve in the motor can successfully be improved, with regard to losses, by small changes in the geometry. Combining an FEM software package with the optimization routine offers an effective tool for designers to simulate and improve the efficiency of a hydraulic unit.
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| 9. |
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| 10. |
- Dahlén, Leon, et al.
(författare)
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Numerical optimization of a distributor valve
- 2003
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Ingår i: International Journal of Fluid Power. - 1439-9776. ; 4:3, s. 17-25
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Tidskriftsartikel (refereegranskat)abstract
- In this paper a non-linear optimization method is used to improve the design of a distributor valve. The distributor valve is an important component in a radial piston hydraulic motor, and optimization of the design to minimize power losses is an interesting way to increase efficiency. The main function of a distributor valve is to supply the pistons with a pressurized flow and to return oil during rotation. At the same time the distributor valve acts as an externally pressurized lubricated thrust bearing, in order to separate the rotating parts from the motor case. The bearing acts as a hydrostatic annular multi-recess plane thrust bearing, with different recess pressures. The separating force of the bearing is balanced hydrostatically by the pressure that is applied and springs. Losses will occur in the contact between the parts in the distributor valve, due to friction and leakage. This paper shows that modern optimization methods can be used as an effective tool to create new designs and to modify the existing design of the bearing surface geometry of the distributor. A finite element method has been used to simulate the contact, and the program is linked to an optimization routine to perform the optimization. The results of the optimized design show a significant decrease in power loss, compared to the existing design in the operating range. (23 refs.)
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| 11. |
- Dahlén, Leon, et al.
(författare)
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Vegetable and synthetic hydraulic fluids to improve the overall efficiency of a hydrostatic transmission
- 2001
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Ingår i: Journal of Synthetic Lubrication. - : Wiley. - 0265-6582 .- 1557-6841. ; 17:4, s. 263-276
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of the investigation reported here was to determine whether or not a hydrostatic transmission can act as an alternative to a conventional one, and, if so, to ascertain to what extent the fluid type contributes to an improvement in the efficiency of the hydrostatic transmission. The results presented in this paper are derived from an experimental field test of the hydrostatic transmission of a belt conveyor. This belt conveyor is one part in an ore-transport line at LKAB mining company in Kiruna. The hydrostatic transmission replaced a conventional transmission consisting of an electric motor, a belt drive and a gearbox. The hydrostatic transmission was operated using three different hydraulic fluids: Shell Tellus TX 68, a conventional mineral oil; Mobil SHC 526, a synthetic fluid; and Binol Hydrap II, a vegetable fluid. All fluids have the same ISO viscosity grade, VG 68. The measurements on the transmission show a 3% overall efficiency improvement when using vegetable and synthetic hydraulic fluids compared with the mineral oil. The current at the start of the transmission was reduced by a factor of 6, compared to that of the conventional transmission. The installed 110 kW electric motor was replaced with one of 55 kW. The test also showed that a closer study of all operating conditions, and a selection of components suited to the size of the load, can improve the overall efficiency of the hydrostatic transmission.
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| 12. |
- Kuzmin, Leonid, 1958-, et al.
(författare)
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Comparison of Ski Running Surfaces Machined by Various Stone Grinding Equipments
- 2009
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Ingår i: The Impact of Technology on Sport III. - Melbourne, Australia : RMIT University. - 9781921426391 ; , s. 27-31
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Stone-grinding is an important part of the process of preparing the ski running surface (SRS). The ski base is stone-ground in order to achieve a level surface and to give the SRS a specific pattern texture, depending on the snow conditions, in order to reduce the capillary drag, which is a part of total ski friction. In this study, skis were ground using three different machines (Mantec, Tazzari and Wintersteiger), each with distinctive pattern. The stone-grinding was performed in the same way and by the same operator on each of the machines. The roughness and the hydrophobic characteristics of the SRS produced by the machines were measured. The results of the experiment show that stone-grinding is able to change the magnitude of the capillary drag dramatically, up to 74% in the case studied.
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| 13. |
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| 14. |
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| 15. |
- Tinnsten, Mats, et al.
(författare)
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Projekt Davos : hjälper längdlandslaget att åka fortare
- 2004
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Ingår i: Svensk Idrottsforskning. - Sth. : Centrum för idrottsforskning. - 1103-4629. ; 13:1, s. 25-27
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Tidskriftsartikel (refereegranskat)abstract
- DAVOS – Datorbaserade verktyg för optimering och simulering är ett EU fi nansierat (Mål 1) projekt som handlar om tillämpad teknik och tilllämpad optimering. Projektet omfattar ca 10 personer, ca 20 miljoner kr och kommer att pågå fram till 2004. Projektet drivs av institutionen för informationsteknologi och medier vid Mitthögskolan i Östersund men även personal från institutionen för teknik, fysik och matematik ingår och projektledare är docent Mats Tinnsten. I projektet ingår sex delprojekt varav två har en inriktning mot sport och sportrel aterad utrustning. DAVOS har goda kontakter med den regionala industrin, internationella universitet och forskargrupper, nationella universitet, forskargrupper och projekt. Särskilt goda och givande kontakter har projektet med Nationellt Vintersportcentrum och längdskidlandslaget samt ett antal elitskidåkare som har varit eller är studenter vid någon av teknikinstitutionerna vid campus Östersund. DAVOS handlar alltså om tillämpad teknik och optimering och verksamheten är inriktad på numeriska simuleringar och analyser av tekniska tillämpningar vilket närmast faller inom området maskinteknik och man kan ju fråga sig hur detta kan kopplas till området sport och sportutrustning. På ett alldeles naturligt sätt anser deltagarna i projektet och förhoppningsvis ska detta klarna vid genomläsning av den resterande texten.
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| 16. |
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