jlnsol
Mechanical
- Jan 30, 2013
- 106
Who can help me answering the following question?
Gearboxes have so called breather valves to keep the pressure inside the box at atmospheric level during operation. Would it be possible to get rid of breather valves if we put an expansion system in the box, similar to an expansion tank in a central heating system? Our customer would like a smooth and fully closed (small) gearbox in food machinery.
Here is what I think is going on, please correct me if I am wrong: The free space inside the box is filled with oil and air. During operation the oil gets heated by the worm gear inefficiencies and will expand a little (volumetric expansion). Thus the air volume will get less in the same amount. Since the air is heated also the pressure rises. (following the gaslaw). This pressure acts in the air and on the oil. To avoid shaft seals being blown out or starting to sweat and leak, breather valves come into the picture. These valves can be chosen to open at for instance 1.2 bar.
Here is my example calculation: Free box volume 1 liter. At ambient pressure p1 and temperature T1=20 degr C, the oil volume is 0.8 liter. Thus the air volume V1 is 0.2 liter. In full operation max oil temperature is 100 degr C and the air temperature T2=80 degr C. Volumetric expansion coëfficiënt oil: 0.7*10^-3 /K. The volume of the heated oil is thus 1*(1+(80-20)*0.7*10^-3) = 1.042 liter. So the heated air volume V2 is 0.157 liter. Using the gaslaw the pressure p2 would be p1*V1*T2/(T1*V2) = 1*0.2*80/(20*0.157) = 5.1 bar(!) To get this pressure down to 1.2 bar (a value my customer expects the seals can handle) the heated air volume would need to be 5.1/1.2 = 4.25 times 0.157 = 0.667 liter. So some kind of expansion system should have a volume of 0.51 liter. In relation to the small box itself that is huge and not acceptable.
But…if the box was completely filled with oil (no air at all) an expansion system of at least 0.042 liter would be sufficiënt to pick up the oil expansion. Am I right here? Is this feasible? Any input is very much appreciated.
Gearboxes have so called breather valves to keep the pressure inside the box at atmospheric level during operation. Would it be possible to get rid of breather valves if we put an expansion system in the box, similar to an expansion tank in a central heating system? Our customer would like a smooth and fully closed (small) gearbox in food machinery.
Here is what I think is going on, please correct me if I am wrong: The free space inside the box is filled with oil and air. During operation the oil gets heated by the worm gear inefficiencies and will expand a little (volumetric expansion). Thus the air volume will get less in the same amount. Since the air is heated also the pressure rises. (following the gaslaw). This pressure acts in the air and on the oil. To avoid shaft seals being blown out or starting to sweat and leak, breather valves come into the picture. These valves can be chosen to open at for instance 1.2 bar.
Here is my example calculation: Free box volume 1 liter. At ambient pressure p1 and temperature T1=20 degr C, the oil volume is 0.8 liter. Thus the air volume V1 is 0.2 liter. In full operation max oil temperature is 100 degr C and the air temperature T2=80 degr C. Volumetric expansion coëfficiënt oil: 0.7*10^-3 /K. The volume of the heated oil is thus 1*(1+(80-20)*0.7*10^-3) = 1.042 liter. So the heated air volume V2 is 0.157 liter. Using the gaslaw the pressure p2 would be p1*V1*T2/(T1*V2) = 1*0.2*80/(20*0.157) = 5.1 bar(!) To get this pressure down to 1.2 bar (a value my customer expects the seals can handle) the heated air volume would need to be 5.1/1.2 = 4.25 times 0.157 = 0.667 liter. So some kind of expansion system should have a volume of 0.51 liter. In relation to the small box itself that is huge and not acceptable.
But…if the box was completely filled with oil (no air at all) an expansion system of at least 0.042 liter would be sufficiënt to pick up the oil expansion. Am I right here? Is this feasible? Any input is very much appreciated.
