Effect of viscosity changes in gearbox transmission system

1.Introduction

Gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, and this reduction in speed produces a mechanical advantage, increasing torque. Gearbox lubricants are important in maintaining economic and effective operations. Lubricating oils should reduce wear and friction at the contact surfaces and separate them to make sure applicable operation and avoid failures. Lube oil properties also affect gear dynamics, like vibration, power characteristics and heating. Therefore, continuously monitoring changes in lubricant properties is crucial for avoiding defects at intervals machines and prevent sudden breakdowns. Lubricants in gearboxes are degraded mainly due to oxidation, particle and water contaminations. Both oxidation and water generally increase the lube viscosity.

Power loss sources in a gear transmission relate to various gear parameters such as teeth geometry, specific sliding, and lube properties. Yet the viscosity is a key element in improving gearbox efficiency and performance. Some research shows that the use of a lubricant with a suitable viscosity could save up to 20% of power losses. Power losses in gears can be approximated as follows

P_T = P_fr+P_spl+P_M1+ P_M0+P_sl

Where

P_fr: - denotes gear friction power loss

P_spl: - is gear churning power loss

P_M1: -load-dependent power loss in rolling bearings

P_M0: - denoting power loss in rolling bearings

P_sl: -denoting seal power loss

2. Meshing Details:-

Sr No	Gearbox	Mesh Size	No of Element
1	Gear without clearance	0.45 mm	101840
2	Gear with the clearance	0.45 mm	107707

3. Dynamic Mesh

Dynamic Mesh
Case	Left Gear	Right Gear
X	Y	X	Y
1	0	0	0	0.115
2	0	0	0	0.115
3	-0.06845	0	0.04855	0
4	-0.06845	0	0.04855	0

4. Boundary Condition

#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]*/
}

5. Computational study of the gearbox

Case No	Gearbox Condition	Fluid Combination
1	Without clearance	Air- Water
2	Without clearance	Air - Oil
3	With clearance	Air- Water
4	With clearance	Air - Oil

Case	Adapt Coordinate	Fluid Properties	Adapt Mesh
X min	X max	Y min	Y max	Density	Viscosity
1	-0.05612	0.17112	-0.0675	-0.0375	998.2	0.001003	8878
2	-0.125	0.200	-0.0675	0	871	0.04006889	37168
3	-0.185	0.120	-0.685	0	998.2	0.00103	51005
4	-0.185	0.120	-0.685	0	871	0.04006889	51005

6. Results

Case 1: Animation of air and water phases without clearance.

Case 2: Animation of air and oil phases without clearance.

Case 3: Animation of air and water phases with clearance.

Case 4: Animation of air and oil phases with clearance.

Conclusion

• From the above simulations, we can observe the formation of thin water/oil fluid around the gears when gears rotate. It is more stable for oil than water.
• The splashing of oil is quite less than water as the viscosity of the oil is more than water.
• As the viscosity of water is less sloshing will be more in the gearbox whereas for oil it will be less. And hence using water the hydrodynamic forces generated in the gearbox will be more.