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# Oil flow rate to gearbox

## Oil flow rate to gearbox

(OP)
how to know whether the supplying oil flow rate to gearbox is sufficient or more than sufficient? How to estimate the minimised (optimised) oil flow rate to gearbox?
Are there any empirical relations for oil flow rate based on speed, power and other parameters of the gearbox as per standards?

### RE: Oil flow rate to gearbox

In my industry our circulating gearbox lubrication systems are designed around constant flow. We use positive displacement pumps and put a pressure relief on the downstream side to ensure nothing dangerous occurs.

Even with a gearset, a small oil flow is required to keep them lubricated. Bearings require even less. The commercially available PD pumps provide an order of magnitude or more excess flow, such that knowing the minimum oil flow is unnecessary. In terms of optimization, the excess oil flow is 'wasting' less than one percent in our designs and provides ancillary benefits. Plus it gives us some coverage against operation in extreme conditions.

So what range of RPM, HP, PLV, etc are you trying to cover in this question? Is your oil circulation creating significant energy waste?

### RE: Oil flow rate to gearbox

I don't know the answer to your question, but I ask rhetorically if the oil flow in a gearbox is also a function of cooling in order to get heat generated to the housing and then radiated away.

### RE: Oil flow rate to gearbox

(OP)
Dear geesaman.d,
for example lets say a 50MW gearbox, PLV - 130 m/s, mesh oil flow is around 370 LPM, friction and windage power loss is around 320 kw. windage loss will be higher at high PLVs and higher oil flow rates.
How much oil flow can be reduced? How to arrive at the reduced oil flow?

For high power and high speed gearboxes, what is the ratio of power loss due to Windage, Mesh friction and bearings?

### RE: Oil flow rate to gearbox

That's the opposite end of my experience. I work with much smaller HP and lower PLVs. In my world you can dump about as much oil as you please and it won't make much difference aside from a little energy waste.

Indeed, with those PLVs I expect there will be a sweet spot in oil flow to the critical elements that provides adequate cooling but does not invoke excessive churning losses. I have no experience to offer in that.

### RE: Oil flow rate to gearbox

the amount of oil supply to gears and bearings will depend on how you get the oil towards the spots where lubrication is needed.

when the flow is supplied by a pump you need to get the oil towards the gear teeth just before they engage - which means just enough oil that a more or less suitable lubricating film can be formed and also a supply of oil to the bearings. for the gears a amount of oil just able to coat the matching gear flanks before they engage should suffice - and the flow rate should be chosen such that at the highest speeds that requirement will be met. that requirement should also be met at low start temperatures. in reality, then you will have more oil then needed when starting up, but about the right amount under operating conditions.depending on the operating temperature there may also be a requirement to "shower" more of the gears involved, to prevent too high temperatures. that may however require more pumping power.

for anti friction bearings you will need even less oil; - usually the correct supply rate for gears is larger then that for bearings. the amount of oil supplied to bearings may be decreased by mixing it with pressurised air if you want energy losses to a minimum.

if you use journal bearings, you will make sure they get a ample amount of oil to make sure that a hydrodynamic oil film of sufficient thickness is formed and maintained.

when you decide to use splash lubrication for the gears you will have to decide what oil level to use. sufficient to make sure that the oil is taken to the spot where it is needed, and preferably not more because that may introduce churning losses. those churning losses will also vary with temperature - they increase with temperature since oil becomes thinner.

there may be some rules of thumb for all this, but if you want to optimise you should carry out some experiments, since the optimal amount will depend on the oil type, temperature, load and dimensions of the gears.

### RE: Oil flow rate to gearbox

(OP)
In my case i.e., high power and high speed gears, there is enough lubrication (into-mesh) at gear mesh, I would like to see whether the oil for cooling at out-mesh is sufficient or more than sufficient.
Bearings : journal bearings
lubrication and cooling to gears : oil-jet lubrication (straight to mesh point)
Gears: Helical and Spur gears
The major losses are Windage loss, sliding friction loss, and journal bearing loss.
How to estimate the optimum oil for cooling at out-mesh of the gears.
Any suggestions?

### RE: Oil flow rate to gearbox

What oil temperatures are we talking about? The normal operating temperature of high speed industrial gears results in 90 to 120 degrees C for the oil when highly loaded. If the temperatures you experience are beyond this range you might consider using another way of oil supply or using a oil with a different viscosity. When the temperature is too high, a thinner oil might be the solution. Also the composition of the oil may influence the temperature: gears lubricated with a synthetic oil based on poly-alpha-olefins (PAO)usually reduces the oil temperature in comparison with a standard mineral oil with the same viscosity.

### RE: Oil flow rate to gearbox

to add to romke great response, talk to lubrication sales engineer on the application and type of box. while designing.

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