| 1. |
- Baart, Pieter, et al.
(author)
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Contaminant particle migration in a double restriction seal
- 2013
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In: Proceedings of the STLE Annual Meeting and Exhibition 2013, Detroit MI, USA.. - : STLE. ; , s. 125-
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Conference paper (peer-reviewed)abstract
- Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile.In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication
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| 2. |
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| 3. |
- Baart, Pieter, et al.
(author)
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The influence of speed, grease type, and temperature on radial contaminant particle migration in a double restriction seal
- 2011
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In: Tribology Transactions. - : Informa UK Limited. - 1040-2004 .- 1547-397X. ; 54:6, s. 867-877
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Journal article (peer-reviewed)abstract
- Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile. In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication.
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| 4. |
- Green, Torbjörn M., et al.
(author)
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A new method to visualize grease flow in a double restriction seal using microparticle image velocimetry
- 2011
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In: Tribology Transactions. - : Informa UK Limited. - 1040-2004 .- 1547-397X. ; 54, s. 784-792
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Journal article (peer-reviewed)abstract
- A new method to visualize and quantify grease flow in between two sealing lips or, in general, a double restriction seal is presented. Two setups were designed to mimic different types of seals; that is, a radial and an axial shaft seal. The flow of the grease inside and in between the sealing restrictions was measured using microparticle image velocimetry. The results show that grease flow due to a pressure difference mainly takes place close to the rotating shaft surface with an exponentially decaying velocity profile in the radial direction. Consequently, contaminants may be captured in the stationary grease at the outer radius, which explains the sealing function of the grease.
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| 5. |
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| 6. |
- Li, Jinxia, et al.
(author)
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Design of test rig for visualizations of cylindrical shear and pressure driven Couette flow using μPIV
- 2014
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Conference paper (peer-reviewed)abstract
- Couette flow is often encountered in concentric cylinder application such as rheometers etc. Being able to visualize such flows is of interest both from a fundamental point of view to understand the dynamics of complex fluids, but also in specific applications such as lubricants flowing through seal geometries. In this study a concentric cylinder test rig has been designed to visualize Couette flow in both radial and axial direction using micro Particle Image Velocimetry. The rig allows for control of the flow motion; the rotating inner cylinder creates a peripheral flow and an applied pressure in the axial direction creates a pressure driven flow. Thus, a single flow direction or a combination of directions can be analyzed. To demonstrate the technique a flow of a non-Newtonian shear thinning fluid in the form of lubricating grease was investigated and discussed. It is found that it is possible to capture the yield behavior of the grease, with regions of fully and partially yielded flow visible. The influence of temperature creep flow is also presented. Grease with both high and low yield stress are measured and compared could be measured and compared in a pocket with variable size. Furthermore, non-homogeneous effects such as shear banding and wall slip can be visualized. The test rig has thus a high potential to investigate the influence of wall material and wettability between fluids and the housing on the flow and wall slip behavior as long as the fluid is optically transparent.
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| 7. |
- Li, Jinxia, et al.
(author)
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Experimental study of free surface grease flow subjected to centrifugal forces
- 2014
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Conference paper (peer-reviewed)abstract
- In order to improve the understanding of grease flow in various applications such as gears, seals and rolling bearings, the free surface flow of different greases under different running conditions has been investigated. A rotating disc has been used to study grease flow as the grease was subjected to a centrifugal force. The grease flow and mass loss was measured for greases with different rheology on different surfaces and with surface textures. It is shown that the speed at which grease starts to move is mostly determined by grease type and yield stress, while the impact of the surface material and roughness is less pronounced. The mass loss is shown to be influenced both by the rheology of the grease and the surface material
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| 8. |
- Li, Jinxia, et al.
(author)
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Grease free surface flow on a rotating plate
- 2014
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Conference paper (peer-reviewed)abstract
- In order to improve the understanding of grease flow in various applications such as gears, seals and rolling element bearings, free surface flow of different greases under different running conditions has been investigated. A rotating disc has been used to study grease flow as the grease is subjected to a centrifugal force. The adhesion and mass loss was detected for greases with different rheology on different surfaces and surface textures. It is shown that the speed at which grease starts to move is mostly determined by grease type, yield stress and bleeding properties rather than surface material. Also, the surface adhesion is shown to be influenced both by the rheology of the grease and the surface material.
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| 9. |
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| 10. |
- Li, Jinxia
(author)
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Lubricating grease Experiments and modeling of wall-bounded- and free-surface flows
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Lubricating grease is commonly applied to lubricate e.g. rolling bearings, sealsand gears. Grease has some clear advantages over lubricating oil: it is a semisolidmaterial, which prevents it from flowing/ leaking out from the bearingsystem and gives it sealing properties, and it also protects the system fromcontaminants and corrosion. Due to its consistency, lubricating grease has manyadditional advantages over lubricating oil: it does not require pumps, filters andsumps. However, the rheology of grease makes it more difficult to measure andstudy its flow dynamics. This study focuses on the influence of rheology ongrease flow in different geometries involving a straight channel with restrictions,concentric cylinder geometry, and free-surface flow on a rotating disc.To better understand grease flow in bearings and seals, two types of flowrestrictions were applied into the straight channel in order to simulate the flowof grease near a seal pocket. In the case of a single restriction, the horizontaldistance required for the velocity profile to fully develop is approximately thesame as the height of the channel. In the corner before and after the restriction,the velocities are very low and part of the grease is stationary. For the channelwith two flow restrictions, this effect is even more pronounced in the narrowspace between the restrictions. Clearly, a large part of the grease is not moving.This condition particularly applies in the case of a low-pressure gradient andwhere high-consistency grease is used. In practice this means that grease may belocally trapped and consequently old/contaminated grease will remain in theseal pockets.A configuration comprising a rotating shaft and two narrow gap sealing-likerestrictions (also called Double Restriction Seal, DRS) was designed to simulatea sealing contact. Two different gap heights in the DRS have been used tocompare the grease flow. It is shown that partially yielded grease flow isdetected in the large gap geometry and fully yielded grease flow in the small gapgeometry. For the small gap geometry, it is shown that three distinct grease flowregions are present: a slip layer close to the stationary wall, a bulk flow layer,and a slip layer near the rotating shaft. The shear thinning behaviour of thegrease and its wall slip effects have been determined and discussed.Free-surface flow of grease occurs in a variety of situations such as during relubrication and inside a rolling element bearing which is filled to about 30%with grease in order to prevent heavy churning. Here the reflow of lubricant tothe bearing races is a key point in the lubricant film build-up, and centrifugalforces have a direct impact on the amount of available grease. Understanding ofthe free-surface flow behaviour of grease is hence important for theunderstanding of the lubrication mechanism. Adhesion and mass loss aremeasured for greases with different rheology on different surfaces andtemperatures. It is shown that the critical speed at which the grease starts tomove is mostly determined by grease type, yield stress and temperature ratherthan surface material. A developed analytical model covers a stationary analysisof the flow resulting in solutions for the velocity profile of the grease as well as asolution for the thickness of the viscous layer remaining on the disc.
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| 11. |
- Li, Jinxia, et al.
(author)
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Lubricating grease shear flow and boundary layers in a concentric cylinder configuration
- 2013
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In: Proceedings of the 3rd International Tribology Symposium of IFoMM (International Federation for the Promotion of Mechanism and Machine Science), Luleå, March 19-21, 2013.
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Conference paper (peer-reviewed)abstract
- Grease is extensively used to lubricate various machine elements such as rollingbearings, seals, and gears. Understanding the flow dynamics of grease is relevant forthe prediction of the grease distribution for optimum lubrication and the migration ofwear- and contaminant particles. In this study grease flow is visualized using themethod of micro Particle Image Velocimetry; the experimental setup comprises aconcentric cylinder with rotating shaft to simulate the grease flow in a DoubleRestriction Seal (DRS) geometry with two different grease pocket heights. It is shownthat grease may be partially yielded in the large grease pocket geometry and fullyyielded in the small grease pocket geometry. For the small grease pocket geometry, itis shown that three distinct grease flow layers are present: a high shear rate regionclose to the stationary wall, a bulk flow layer, and a high shear rate boundary regionnear the rotating shaft. The grease shear thinning behaviour and its wall slip effectshave been detected and discussed.
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| 12. |
- Li, Jinxia, et al.
(author)
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Lubricating grease shear flow and boundary layers in a concentric cylinder configuration
- 2014
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In: Tribology Transactions. - : Informa UK Limited. - 1040-2004 .- 1547-397X. ; 57:6, s. 1106-1115
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Journal article (peer-reviewed)abstract
- Grease is extensively used to lubricate various machine elements such as rolling bearings, seals, and gears. Understanding the flow dynamics of grease is relevant for the prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. In this study, grease flow is visualized using microparticle image velocimetry (μPIV). The experimental setup includes a concentric cylinder configuration with a rotating shaft to simulate the grease flow in a double restriction seal geometry with two different grease pocket sizes. It is shown that the grease is partially yielded in the large grease pocket geometry and fully yielded in the small grease pocket. For the small grease pocket, it is shown that three distinct grease flow layers are present: a high shear rate region close to the stationary wall, a bulk flow layer, and a high shear rate boundary region near the rotating shaft. The grease shear thinning behavior and its wall slip effects have been identified. The μPIV experimental results have been compared with a numerical model for both the large and small gap size. It is shown that the flow is close to one-dimensional in the center of the small pocket. A one-dimensional analytical model based on the Herschel-Bulkley rheology model has been developed, showing good agreement with the measured velocity profiles in the small grease pocket. Furthermore, wall slip effects and shear banding are observed, where the latter imply that using the assumption of uniform shear in conventional concentric cylinder rheometers may result in erroneous rheological results.
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| 13. |
- Li, Jinxia, et al.
(author)
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Measurements of grease flow in channels with restrictions using μPIV
- 2010
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In: 14th Nordic Symposium on Tribology. - Luleå : Luleå tekniska universitet. - 9789174391244
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Conference paper (peer-reviewed)abstract
- Grease is commonly used to lubricate various machine components such as rolling element bearings, open gears etc. Better understanding of the flow properties of grease will contribute to understanding the lubrication mechanism in bearings and flow in lubrication systems. In an earlier paper Micro Particle Image Velocimetry (μPIV) techniques were used to study the flow in a rectangular channel. The present paper is an extension of this work where restrictions were applied in such a channel, which creates a much more complex velocity field. The grease is seeded with fluorescent particles, which are illuminated by a double-pulsed laser. The test geometries that are used in this study are a channel with one flat restriction and one with two flow restrictions in a similar channel. The stationary grease mass-flow and the two dimensional velocity fields have been monitored for different pressure drops. For the channel with one flat restriction, the flow was measured to be symmetric at the inlet and outlet, and the distance for the flow to fully develop is comparable with the height of the channel; Slow motion was followed near the step corner at the inlet. For the channel with two flow restrictions, the vector profiles show that the maximum velocity appears at the restrictions; In-between the two restrictions, a part of the grease is not moving. This particularly applies to cases with low-pressure drop and where high consistency grease was used.
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| 14. |
- Li, Jinxia
(author)
-
µPIV Measurement of Grease Velocity Profiles
- 2013
-
Licentiate thesis (other academic/artistic)abstract
- Lubricating grease is commonly applied to lubricate e.g. rolling bearings, seals and gears. Grease has some clear advantages over lubricating oil. It is a semi-solid material, which prevents it from flowing/leaking out from the lubricated system and gives it sealing properties, protecting the system against contaminants. Unlike oil, grease has a much more complicated rheology, which makes it more difficult to model and understand grease flow. Grease acts as a lubricant reservoir, and understanding grease flow is essential in order to model and predict how grease is transported within e.g., a rolling element bearing housing, a sealing arrangement or replenishment of a gear mesh. Three greases with different rheological behaviors (NLGI 2 grease, NLGI1 grease and NLGI00 grease) have been used in two kinds of test rigs: a straight channel with different restrictions and a rotating shaft with two narrow gap sealing-like restrictions.In the first test rig two types of flow restrictions were applied into a straight channel in order to simulate flow of grease near a sealing pocket. In the case of a single restriction, the distance required for the velocity profile to fully develop when going from a wide to a narrow gap is approximately the same as the initial height of the channel. In the corner pocket before and after the restriction, the velocity is very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving since the yield stress of the grease is not exceeded. This condition particularly applies to the cases with a low-pressure gradient and where high consistency grease is used. In practice this means that grease is not replaced in such “pockets” and that some aged/contaminated grease will remain there. A test rig comprising of a rotating shaft with two narrow gap sealing-like restrictions (a so called Double Restriction Seal, DRS) was designed to simulate the a labyrinth type of seal. Two different gap heights in the DRS have been designed to compare grease flow. It is shown that partially yielded grease is detected in the large gap geometry and fully yielded grease in the small gap geometry. Grease shear thinning behavior and wall slip effects have been detected and discussed. For the small gap geometry, it is shown that three distinct grease flow regions are present: a slip layer close to the stationary wall, a bulk flow layer, and a slip layer near the rotating shaft.
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| 15. |
- Li, Jinxia, et al.
(author)
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µPIV measurement of grease velocity profiles in channels with two different types of flow restrictions
- 2012
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In: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 54, s. 94-99
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Journal article (peer-reviewed)abstract
- Grease is commonly used to lubricate various machine components such as rolling bearings and seals. In this paper the flow of lubricating grease passing restrictions is described. Such flow occurs in rolling bearings during relubrication events where the grease is flowing in the transverse (axial) direction through the bearing and is hindered by guide rings, flanges et cetera, as well as in seals where transverse flow occurs, for example during so-called breathing caused by temperature fluctuations in the bearing. This study uses a 2D flow model geometry consisting of a wide channel with rectangular cross-section and two different types of restrictions to measure the grease velocity vector field, using the method of Micro Particle Image Velocimetry. In the case of a single restriction, the horizontal distance required for the velocity profile to fully develop is approximately the same as the height of the channel. In the corner before and after the restriction, the velocities are very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving. This condition particularly applies to the cases with a low-pressure drop and where high consistency grease is used. In practice this means that grease is not replaced in such “corners” and that some aged/contaminated grease will remain in seal pockets.
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| 16. |
- Westerberg, Lars-Göran, et al.
(author)
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Free-surface grease flow on a rotating plate
- 2014
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In: Tribology letters. - : Springer Science and Business Media LLC. - 1023-8883 .- 1573-2711. ; 56:2, s. 317-325
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Journal article (peer-reviewed)abstract
- Grease lubrication is traditionally used in a great variety of mechanical systems such as rolling bearings, seals, and gears where it has been shown more advantageous than oil, mainly due to its consistency allowing the grease to stay inside the system and not leak out. Knowledge of the flow dynamics of grease is important for the understanding and prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. Free-surface effects play an important role in rolling bearings and open gears as the configuration normally is filled with about 30 % grease to avoid heavy churning. In this study, an analytical model of the stationary uniform flow on a rotating disc is developed and validated with experiments. The model results in the velocity profile for the flow in the thin fully yielded viscous layer in connection to the surface as well as an expression for the plug flow region on top of the viscous layer. Furthermore, the depth-averaged velocity is derived as is the shear stress value on the plate. From the latter, follows a condition for the grease to start moving and in turn yielding an expression for the viscous layer thickness as a function of the grease yield stress value, grease density, angular velocity, and radial position. In addition, an expression of the layer thickness containing the ratio between the flow rate and the layer width which in turn can account for effects not included in the model such as wall slip and surface adhesion and thus add another degree of freedom into the model. Experiments with two different greases having NLGI grade 1 and 2, respectively, shows it is possible to obtain a good fit with the analytically obtained thickness using the rheological parameters for actual greases.
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| 17. |
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| 18. |
- Westerberg, Lars-Göran, et al.
(author)
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Grease flow in elbow channel
- 2014
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In: Society of Tribologists and Lubrication Engineers Annual Meeting and Exhibition 2014. - : Society of Tribologists and Lubrication Engineers. - 9781634391351 ; , s. 398-400
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Conference paper (peer-reviewed)
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| 19. |
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| 20. |
- Westerberg, Lars-Göran, et al.
(author)
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Understanding grease flow through optical visualizations
- 2012
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Conference paper (peer-reviewed)abstract
- The flow dynamics of a lubrication mechanism is very complex, much due to the complex rheology and composition of the grease. In order to obtain an optimal lubrication, both the initial amount of grease and the position of the grease is highly important as too much grease will contribute to an increased friction, and grease in the wrong place will negatively affect the replenishment through oil bleeding. To understand the flow dynamics of grease hence is highly important for the understanding of the lubrication mechanism. Using micro Particle Image Velocimetry (μPIV) we have in a series of studies investigated the dynamics of grease flow in 2D straight channels with- and without restrictions, and in a full 3D configuration comprising a double restriction seal geometry. Velocity profiles for greases of different thickness have been measured, showing the influence of the grease rheology on the grease flow behaviour. KEYWORDS: Lubricants:Greases, Lubricant Physical Analysis:Non-Newtonian Behavior, Lubricant Physical Analysis:Rheology.
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| 21. |
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