Lubrication system for a gearbox

[avelas31]

Hello

I'm building a machine, that has a reduction gearbox. the engine that move the gearbox runs at 8000 rpm, and the output is going to be something about 3500 rpm (so around 2,3:1 ratio), the orientation of this gear box is vertical, so the small gear (The one that spin with the engine) is at the bottom of the gearbox, and the output gear, which is 2.3 times larger is at the top. so i'm concern about the lubrication of the system. I'm thinking in use a pint (1/8 US gallon), the diameter of the gears are 2.5" and 5.75"

Can the small gear wet the big gear?

Do you guys think that this system can run without a oil pump?

Thank you for your time

Andres Velasquez

Mechanical Engineer

 

[romke]

in theory it could work. however, the speed of the driving gear might well be so high that most of the lubricant is sprayed of before the teeth of the two gears mate. another problem might be a lubricant level that is such that the bottom gear more or less acts like a a kind of "mixer" with foaming and air inclusion in the lubricant as a result.

the viscosity of the gear oil could be determined by the circumferential speed of the gears in the first place, but due to the rpm of the gears a thinner oil might be preferred to avoid foaming/air inclusion.

another factor to consider is the input torque. depending on the load you may get away with a standard AW oil or you may need a thicker EP-oil.

a third factor to take into account is heat dissipation. you are using a small quantity of oil that may well reach a substantial temperature, shortening the life time expectancy of the lubricant.

the easiest way to find out whether it can work at all is to have a test run - with the kind of lubricant calculated based on the input torque. some suggestions can be found here:

http://www.machinerylubrication.com/Read/707/enclosed-gear-drives

 

[avelas31]

romke Thank you for your kindly reply.

I think i'm going to build a test model to see what happen, before send the case to the CNC.

I understand that at high speed, is better to use helical gears, instead of spur gears. do you agree with that?

Do you think that using some kind of gear might help the lubrication issue?

Thank you for your time

Andres Velasquez
Mechanical Engineer

 

[romke]

I do not think that from a lubrication point of view there will be much difference between helical gears in stead of spur gears, although helical gears will run more quiet and maybe due to the longer contact line the load also could be somewhat lower. What will differ is the load on the bearings supporting the shafts where upon the gears are mounted. With helical gears you need to take into account a certain amount of axial loading, that will not be present when using spur gears.

Both types of gears will not be able to ascertain full hydrodynamic lubrication under high load low speed conditions, the chances of getting full fluid film separation will increase with higher speed and lower load. Hence the remark that you might get away with a simple AW oil or need a EP oil, depending on load and speed.