Simulation of sloshing effect of gearbox lubricant by varying the clearance between the gears
Abstract: The gearbox is a vital component of many industrial applications,because they transport mechanical power and change the speed factor of motors. Gearboxes must work efficiently to get the job done properly and lubrication is vital to this efficiency.
Lubricating oils serve a few purposes, including providing surface-wear protection, preventing corrosion of metal surfaces, and cooling the internal parts of the gearbox.There are many issues that can occur gearbox is not properly lubricated. Surface fatigue is caused by low lubrication on the gear wheel teeth. This issue happens because the surface is not safeguarded from stress that causes pitting on the contact region. Another problem that will arise if the film of the lubricating oil is not maintained is scuffing. This problem happens due to increased temperatures, which cause distress on the surface.
The objective of our present case is to analyse the flow patterns of two different fluids(water and oil) acting as a lubrication oil of a gear box of two different clearances.Volume of fluid multiphase method is considered since multi fluid interaction i.e. Air-water and Air-oil. User defined function (UDF) is used to describe the motion of rotating gears. Dynamic meshing techniques are implemented to mesh domain of moving boundaries(gear profile). Realizable `k-epsilon` turbulence model with enhanced wall function is implemented.
Flow Domain:
Case 1: Water acting as a lubricating oil of a gear box of lower clearance
Case 2: Oil acting as a lubricating oil of a gear box of lower clearance
Case 3: Water acting as a lubricating oil of a gear box of higher clearance
Case 4: Oil acting as a lubricating oil of a gear box of higher clearance
Geometry Preparation:
1. The required 2D geometry is extracted from a 3D geometry containing gears and gearbox casing as shown in figure below.
2. Now, the fluid flow volume is extracted from volume extract option in spaceclaim by selecting the four edges of intels and outlets.
3. After fluid volume extraction, the 3D geometry needs to be split into half in order to extract the 2D geometry of the centre plane.This is done using body split option by selecting the body which to be split,and plane along with body is split,finally the portion of body which is to be removed is selected.
4. Now the face of the fluid volume is copied and pasted on a new spaceclaim geometry by selecting the appropriate plane to which the 2D geometry is pasted.For present case the plane is XY plane.
Mesh:
Case 1 and 2:
Element size: 0.001 m
Mesh element: All triangles method
Number of elements: 28994
Case 3 and 4:
Element size: 0.001 m
Mesh element: All triangles method
Number of elements: 30499
Setup:
General:
Solver: Pressure based
Time: Transient approach
Gravity: -9.81 (m/s2) (Y direction)
Models:
Multiphase: Volume of fluid(VOF) multiphase method(formulation implicit)
Viscous: Realizable `k-epsilon` turbulence model with enhanced wall functions
User defined function:
#include \"udf.h\"
DEFINE_CG_MOTION(right_motion, dt, vel, omega, time, dtime)
{
vel[0] = 0.0;
vel[1] = 0.0;
vel[2] = 0.0;
omega[0] = 0.0;
omega[1] = 0.0;
omega[2] = 2.0e2; /* [rad/s]*/
}
DEFINE_CG_MOTION(left_motion, dt, vel, omega, time, dtime)
{
vel[0] = 0.0;
vel[1] = 0.0;
vel[2] = 0.0;
omega[0] = 0.0;
omega[1] = 0.0;
omega[2] = -2.0e2; /* [rad/s]*/
}
1. User defined functions are loaded in ansys fluent by Function --> Compiled --> Add option.
2. In present case, UDF is used to specify the angular velocity of both gears(left and right) in rad/s along z direction. Right motion is defined by angular velocity of 200 rad/s in positive z direction and left_motion is defined by angular velocity of 200 rad/s in negative z direction.
Mesh models:
1. Dynamic mesh: Smoothing, layering, Remeshing
Smoothing Method: Diffusion (boundary distance diffusion function)
Layering Method: Height based
Split factor: 0.4
collapse factor: 0.2
Remeshing method: Local cell
Parameters-
Minimum length scale(m)=1e-08
Maximum length scale(m)= 0.01
Maximum cell skewness=0.7
Size remeshing interval=5
2. Dynamic mesh zones: (To set the right motion to right gear and left motion to left gear)
Left_motion--> left_gear
right motion --> right gear
Case 1 and 2: Lower clearance
centre of gravity of left gear= (0,0) m
centre of gravity of right gear= (0.115,0) m
Case 3 and 4: Higher clearance
centre of gravity of left gear= (-0.0684485,0) m
centre of gravity of right gear= (0.0485515,0) m
Fluid properties:
1. Air:
Density(kg/m^3) : 1.225
Dynamic viscosity(kg/m-s) : 1.7894e-05
2. Water:
Density(kg/m^3) : 998.2
Dynamic viscosity(kg/m-s) : 0.001003
3. Oil:
Density(kg/m^3) : 871
Dynamic viscosity(kg/m-s) : 0.04006889
Mark/Adapt cells: (in order to patch water/ oil in the lower portion of gear box)
Mark/Adapt cells --> cell registers --> region
Input coordinates: (the coordinates is be input of the region in which the mesh cells are going to be marked, in form of X-max,x-min and Y-max and Y-min)
Case 1 and case 2:
X-min: -0.05612
X-max: 0.17112
Y-min: -0.0675
Y-max: -0.0375
Case 3 and case 4:
X-min: -0.1612
X-max: 0.28112
Y-min: -0.0775
Y-max: -0.0345
Initialization: Hybrid
Patch: After initializing, the marked cells are being patched by water(in case 1 and 3) and Oil(in case 2 and 4) with volume fraction of 1.
Calculation:
Angular velocity, `omega=frac{2piN}{60}`
Hence, number of revolutions per min (RPM)= `frac{6000}{pi}`
Time taken for gear to make one revolution, T= `frac{60*pi}{6000}=0.031415` s
Residuals:
Case 1:
Case 2:
Case 3:
Case 4:
Solution animation:
Case 1:
Time step: 8e-05
Simulation flow time: 0.064879 = two rotations of gear(approx)
Case 2:
Time step: 8e-05
Simulation flow time: 0.064879 = two rotations of gear(approx)
Case 3:
Time step: 8e-05
Simulation flow time: 0.223599 = seven rotations of gear(approx)
Case 4:
Time step: 8e-05
Simulation flow time: 0.223599 = seven rotations of gear(approx)
Discussions:
1. In case of water acting as a lubricating oil, we can observe that water tends to break down to droplets and does not stick to the gear tooth surface after one complete rotation of gear, This is due to low viscosity of water .
2. However in case of oil acting as lubricating oil, we can observe that comparatively more quantity of oil sticks to the surface of gear than water, This is due to oil is more viscous than water.
3. Since the main functionality of a gearbox lubricant is too reduce the surface wear and fatigue loading, more viscous lubricant shound be prefered in this case.
4. As the clearance between the gear is reduced, lubricants tends to get more pressurised,hence tends to break doen as droplets quicker as compared to higher clearance.