Speedup Gearbox Efficiency Curve for High Ratio Planetary Gears 1

[CMartel]

Hello,

Trying to get a handle on the bottom end efficiency in particular, but an overall efficiency curve is the objective. Here's the application profile:

Gearbox ratio: 155:1 SPEEDUP (not a reduction)

Stages: 3 stages,
with about 5.37:1 speed multiplication per stage

Gear Type: Planetary

Input Power: 100kW

Nominal max Output Speed: 2730RPM

Load and Speed: Load increases aproximately at same speed as input speed. Thus at 10% of input power, input speed is about 10%, at 30% of input power speed is about 30%.

Question: What would the gearbox's efficiency curve look like? If no curve info possible, what can I expect for losses at 30% input power / 30% input speed?
And at 20% / 20%?

Thx

C. Martel

 

[Matt51]

Depends on lubrication and seals - grease lube, oil lube, what viscosity? Pressure lube, wet or dry sump? Rolling element bearings? Oil film thickness on low speed bearings and gears?

As a pure guess, you can end up anywhere from 50% overall efficiency, up to about 95% efficiency. It all depends on detail design, and design conditions. Oil churning, high speed gears dragging in oil, boundary lubrication of low speed mesh, can drive efficiency down.

Three stage reduction planetaries in winches seem to be advertised at 60-70%. Aerospace reductions at 95%. Wind turbine gear drives, also speed increasers, do a google search, probably something is published.

 

[tbuelna]

It's tough to get a ratio of 5.37:1 in a single planetary stage, unless you use a small number of planet gears (ie. 3 or fewer). This will limit the torque capacity per stage. But of course, 2 or 3 planets always ensures even load sharing among the planets.

I would also recommend designing your gears for recess action if possible. This will help with efficiency. And in general, spurs are more efficient than helicals, but helicals (or double helicals) will have more capacity for a given diameter, since you can use a wider face width. Stay away from worm gears or harmonic drives if you're concerned about efficiency. And if your pitch line velocities are high (>5,000 fpm) watch out for windage losses. They can be significantly more than friction losses.

A well designed, manufactured and assembled 3-stage planetary, with oil lubrication and rolling element bearings, should have an overall efficiency of better than 95%. Even at a ratio of 155:1.

 

[Matt51]

Be very careful with a large ratio speed increasing drive. Do not forget to calculate friction torque at the meshes and bearings. If the sum of friction torques is greater than output torque, you will have a self locking drive.
Some gear drives that work quite well as a speed reducer, would not work as a speed increaser. Self locking worm gear sets are an example.

 

[CMartel]

To Matt51: thx for the feedback & tips
To tbuelna: thx for comments

Firstly I wish to indicate that I am not the design engineer but the Project Manager. I mention this because your expectations of my technical background may not match. Other than project management I am working on custom energy efficiency simulations.

Secondly I wish to give some project evolution to continue the discussion:

a) Feedback from gearbox manufacturers indicating amount of engineering involved in such custom designs (and thus minimum orders) plus long lead times on this type of gearbox (planetary) has made us come back to our original design type of good old spur gears.

b) Further, the gearbox speed increase multiplication has been reduced from 1:155 to 1:45 for two reasons: a component down stream requires somewhat less speed, and, it was decided that the last multiplication stage would be belt for cost, noise and facility of maintenance, as this stage is the one with highest speed, and thus the fastest wear, requiring more servicing.

c) So the gearbox has the same initial input of about 100kW, at about 10.5 RPM, now with multiplication of 1:45. We are exploring the possiblity of doing it with 3 stages of about 1:3.56, with the first two stages being spur gears, and the last being helicoidal (for lower noise). The output speed would thus be about 474 RPM feeding a pulley belt speed up of 1:3.

Again part load efficiency of 30% would be useful. Yes I know, Matt, it depends on a pile of things. But we're dealing with large gearbox manufacturing companies that will look at all this, but they can't necessarily project expected losses at part load. This was the object of my inquiry on this site.

Hope the extra info helps and looking forward in reading you all again.

Cheers,


CMartel

 

[Matt51]

Hi Cmartel,

I would prefer a third gear stage to a belt, for reliability. Your problems will probably not be with the high speed stages, but more likely with the first low speed stage.

If we sort through some of the tradeoffs - carburized and ground, nitrided, through hardened, for gear steel, and make a reasonable selection. Then we can make some initial estimates. Each gear shop will have their preferred way of making gears, and sizes and pricing will vary widely.

Some rough hand calcs indicate - and I apologize for English units, but I am old and hard to train - the first stage might be something like 2 diametral pitch, with 20 pinion teeth and 71 gear teeth. This gives a pinion pitch diameter of 10 inches, and gear pitch diameter of 35.5 inches. The face width is about 8 inches, so L/D ratio of the pinion is not exceeded. The pitch line velocity of the low speed mesh is approximately only 100 feet per minute. From AGMA paper P219.16 by Hans Winter, slow speed wear often occurs below .5 m/s, or just about the speed of this mesh. So low oil film thickness, and potentially wear, will be issues. Interestingly, according to this paper, for through hardened steel gears, the slow speed gears will polish themselves, and it made no difference in ultimate wear whether the gears were used as hobbed, or hobbed and ground. If the gears are case hardened, superfinishing may be beneficial in improving the ratio of oil film thickness to surface finish.

With a first stage of 100 fpm, a second stage of 360 fpm, a third stage of 1300 fpm and a fourth (perhaps) stage of 4600 fpm - the lubrication of the fourth stage, if a gear, and not a belt, requires jet lubrication. The others could be splash lubrication.

So lets assume the three stages you plan, with splash lubrication and a wet sump. The last key variables are the oil viscosity and the bearings. The DN (bore diameter times rpm) of the bearings on the first stage is extremely low meaning low oil film thickness. My preference would be grease packed and preloaded, sealed tapered roller bearings outside the sump, for the first stage. So now we will select the oil based on requirements of the slow speed first stage mesh. So we will assume an AGMA EP industrial oil.

From the MAAG Gear Book, they say most losses at a gear pitch line velocity of less than 100 m/s will be in the bearings. "The total losses in single-stage gears with parallel offfet axes and friction journals are 0.8 to 2.2%, depending on the operating concitions; 2-stage low speed heavy duty planetary gear suffer a total loss of about 1%". My belief is that the rolling element bearings would reduce loss compared to low speed journal bearings, but I have not investigated this.

MAAG has two charts in their book published in 1990, showing efficiency at part power. Their test was at 146 m/s. Much higher than what you have. Fig. 3.20, page 202, shows efficiency of this single mesh gearbox at about 92% at 30% Torque. Their plot is a function of torque, not power. It looks like it is 99% efficient at 100% torque.

Next they have a Fig 3.22 on page 204, for a planetary gearset. At 30% power, it is 96% efficient, again approaching 99% at 100% power. This plot is efficiency vs. power, not torque. Speeds were 122 rpm input, 400 rpm output.

Because you have lower pitch line velocity, my guess is your first stage mesh and bearings will have require thicker oil, and have lower efficiency.

So at 30% torque, lets assume for a first guess, the first stage is 85% efficient, and the subsequent stages are 90% efficient. Then you have .85x.90x.90, or 69% overall efficiency. If you used two planetary stages at 6:1 ratio (could use a common ring gear, same part number sun and planet gears) you would have perhaps .90x.96, or 86%. Key issue appears to be the bearings.

One of the primary industry trends has been for manufacturers to produce a standard line of geardrives, as this is low cost and quick delivery. Some of the large industrial warehouses, such as Kaman and National Industrial? stock gearboxes. It might be worth looking at what they have in stock .