1 Heat dissipation Characteristics

11 Overview

In order to control the oil temperature of the wet multi-disc brake within a certain range, the wet multi-disc brake is designed with a cooling system, which can balance the heat generated by the wet multi-disc brake at a certain time with the heat emitted at the same time. In this way, The oil temperature of the brake can be controlled within a certain range (the oil temperature of the wet multi-disc brake is generally controlled at about 120℃). In the working process of wet multi-disc brake, the continuous heat dissipation process of its shell is an unsteady heat dissipation process. However, if the time period of its braking process is discretized and refined into small intervals, each discrete interval can be regarded as a steady-state heat dissipation process, and then the heat dissipation process can be calculated by using the characteristics of steady-state heat dissipation process.

Establishment of temperature rise model and analysis of overall heat dissipation characteristics

Because the braking heat generated by the wet multi-disc brake at the braking instantaneous cannot be immediately transferred to the oil, but is absorbed by the steel disc, there must be a process for the diffusion of braking heat energy to the oil, which is related to the flow state of the oil, the thermal conductivity characteristics of the friction liner, the heat capacity of the dual steel disc and its heat transfer characteristics, in addition to the external environment temperature and air flow condition. Therefore, the generation of braking heat energy is instantaneous, and the emission of braking heat energy is process. Therefore, the length of working cycle time, the heat dissipation capacity of the wet multi-disc brake housing and the braking intensity have a great influence on the increase of the oil temperature in the wet multi-disc brake.

Because the work consumed by the friction torque between the fixed disc and the friction disc in the brake is equal to the braking energy E of the vehicle, and the braking energy E of the vehicle is equal to the sum of the kinetic energy Ei and the potential energy Ej of the vehicle.

In formula E=∑nii=1Ei∑njj=1Ej=∑nii=112mv2i∑njj=1mghj (1), E -- the total energy generated during vehicle braking, JEi -- the kinetic energy generated during vehicle braking, JEj -- the potential energy generated during vehicle braking, Jvi -- initial speed of vehicle before braking, msg -- gravitational acceleration, ms2hj -- road slope height, mm -- total vehicle mass, kg when self-cooling wet multi-disc brake is working, The mathematical model of temperature rise of internal oil at small time periods can be approximated as 14T (1) E (1) =1T (1) [como (to1-t0) csms (ts1-t0)]ΑaA (to1-ta) (2) in the formula T (1) -- the first small time period t0 -- initial temperature value (ambient temperature) E (1) -- time period T (1) Energy Αa -- total heat transfer coefficient to1 -- oil temperature at the end of the first time period ts1 -- wall temperature of the shell at the end of the first time period ta -- working environment temperature co -- specific heat capacity of oil cs -- specific heat capacity of wet multi-disc brake components mo -- brake oil mass ms -- brake mass at the beginning, T0 = ta; When T (1), to1 is considered to be ts1.

During the time period T (1), because c0, cs and Αa changed little, they could be set as constants.

Similarly, in the NTH time period, the temperature rise of wet multi-disc brake can be approximated as 14T (n) E (n) =1

T (n) [como (Ton-to (n-1))) csms (Tsn-TS (n-1))]ΑaA (TON-Ta) (3) in the formula, the energy generated in T (n) -- NTH time period E (n) -- time period T (n) is considered to be ton=tsn when it is in the NTH time period T (n).

The calculation process of heat transfer coefficient Αa is as follows:

ΑaA (to-ta) =ΑA (tsn-ta) (4)

  In Formula α-Heat transfer coefficient of cold fluid side air and outer wall of wet multi-disc brake housing tsn -- temperature of outer wall of wet multi-disc brake housing 5 -- heat flow of wet multi-disc brake during heat transfer process A -- Heat dissipation area of wet multi-disc brake Αa= Tsn-Ato-Ta αA (5) The characteristic number equation Nu of local heat transfer coefficient of laminar flow x=01332Re12Pr13 (6)

Nux=Αx Κ(7)

Re=w∞x south (8)

In the equation, x -- distance from the leading edge Re -- Reynolds number w∞ -- velocity of air flow, ms MPA -- air physical parameters, m2s κ-thermal conductivity Pr -- Prunt number Αx -- local convective heat transfer coefficient Nux -- local Nusserl number can be derived to be Α=2Αx (9)

Usually the qualitative temperature of air is tγ=tsnta2 (10).

The average speed of a working cycle of the measured vehicles was obtained to be w∞=3ms. According to Appendix 4 of reference [2], the air physical parameters of the south, so named by the authors and Pr, Re, Nux and αX could be obtained. The total heat transfer coefficient Αa at different time periods can be determined by Formula (5) through ta, tsn and to. Therefore, the oil temperature ton of T (n) in any time period can be calculated by the formula (3) through cyclic iteration. The theoretical calculation formula (3) of oil temperature is used within the ignition point of oil, that is, to<[to], [to]=190℃. The calculation process is realized by the program compiled by Matlab.

2. Experimental and theoretical calculation analysis

The physical parameters of the wet multi-disc brake and the working conditions of the loader during the test are as follows:

Loader prototype model ZL50D; Brake oil is GL4 vehicle gear oil; The dynamic friction coefficient of the friction bushing is 0105. The hydraulic pressure of the oil of the brake system is 1MPa, and the pressure is 5MPa when it is stepped to 23 places during braking. Physical parameters: co=1816×102J (kgK), cs=466J (kgK), mo=1013kg, ms=158kg, A=018m2.

In general operation, the empty weight is 16t, the bucket capacity is 4t, the working cycle time is T=35s, the braking times are 4 times, the heavy load speed is 19ms, the empty speed is 19ms; In forced operation, the empty weight is 16t, the bucket capacity is 5t, the working cycle time is T=17175s, the braking times are 2 times, and the forward and backward speed is 215ms under heavy load.

1. Theoretical calculation results and actual temperature data measured at the test site

See 1 for the theoretical calculation results and the actual temperature data measured at the test site.

As can be seen from 1, the error has been basically controlled within the error range of 5 permitted by the project. The error between the theoretical calculation results and the actual measured oil temperature data is due to the limitations of the test conditions at that time, such as rain, wind, braking state and unstable oil performance, the oil temperature difference between the front and side walls of the shell and other reasons.

Oil temperature theoretical calculation formula (3) should be used within the oil ignition point, that is, to<[to], [to]=190℃. When to≥[to], formula (3) is out of its scope of use. The cooling oil temperature of wet multi-disc brake is generally controlled at about 120℃. When the oil temperature exceeds the ignition point of oil, the cooling oil of wet multi-disc brake is in a two-phase state of oil and gas (the characteristics of oil have changed and failed). For the two-phase state of oil and gas, formula (3) is no longer applicable.

3 Discussion

When ton=to (n1), it means that the heat generated by the wet multi-disc brake during braking reaches a thermal balance with the heat emitted by the brake itself, and the oil temperature will no longer rise. Then Formula (3) can be simplified as 14T (n) E (n) =ΑaA (ton-ta). (1) The matching relationship between T (n) and E (n) can be obtained from Formula (1). It can be seen that increasing the working cycle time T and reducing the braking energy E (n) can reduce the oil temperature to and improve the performance and service life of the brake.

Set to=120℃ (the maximum allowable oil temperature recommended for brakes) and ta=20℃ (normal air temperature), the reasonable matching relationship between E (n) and T (n) in wet multi-disc brakes under service conditions can be obtained from Formula (1) as follows: E (n) =400ΑaA

It can be seen that the oil temperature can be kept stable at any reasonable value, such as to=120℃, under the conditions of braking intensity E (n) if the appropriate working cycle time T (the total time of high-speed and heavy-load operation, low-speed and no-load reverse operation, high-speed and low-load reverse operation) is selected.

The relationship of heat transfer coefficient Αa, air convection heat transfer coefficient Αr, oil convection heat transfer coefficient Αo, thermal conductivity Κs of metal shells and thickness s is Αa=1Αr1ΑosΚs (13). It can be seen that the heat transfer coefficient Αa is most influenced by the convection heat transfer coefficient Αr of air with small heat transfer coefficient. Therefore, it is very important to improve the performance of air convection heat transfer. In addition, reducing the dirt on the wet multi-disc brake surface can also improve the heat transfer performance of the wet multi-disc brake. By changing the flow pattern and increasing the velocity, the turbulence intensity can be improved to enhance the heat transfer effect. If the area of the side of the extended heat transfer surface with small heat transfer coefficient is increased, the heat transfer effect can be improved. By using additives to change the physical properties of the fluid inside the wet multi-disc brake, the heat transfer can be enhanced. Changing the shape, size and surface condition of the heat exchange surface can enhance the heat transfer effect of the wet multi-disc brake. The heat transfer surface and fluid of the wet multi-disc brake vibrate by the external force of the vehicle body, which can strengthen the convective heat transfer effect.

4 Conclusion

(1) The established mathematical model for calculating the internal oil temperature rise of the self-cooling wet multi-disc brake is feasible, the calculation results can reflect the thermal characteristics of the internal oil temperature rise of the wet multi-disc brake, and can be used for the prediction and analysis of the internal oil temperature rise of the wet multi-disc brake, and has engineering application value.

(2) The selection of appropriate working cycle time can maintain a reasonable value of oil temperature under the conditions of brake intensity, improve the air convection heat transfer performance or reduce the surface dirt of the wet multi-disc brake can effectively improve the overall heat dissipation performance of the wet multi-disc brake.