Gearbox efficiency is a measure of the power transmitted by a gearbox, or put another way, the power lost in a gearbox. The difference between input and output power (loss) is due primarily to gear, bearing, and seal friction. However, other factors contribute to losses, such as gear windage and the power to operate a lube pump if the gearbox is so equipped.
The magnitude of loss in a given gearbox depends on many factors, some of which are listed below:
- Number of gears
- Gear type (helical, worm, spur, etc.)
- Gear quality level
- Seal fits
- Bearing type
- Lubricant type and level
- Speeds
- Loads
Calculation of the exact losses is difficult and often turns out to be inaccurate. In fact, two apparently identical gearboxes may have slightly different efficiencies due to tolerances, fits, assembly practices, etc. For this reason, most gearbox manufacturers normally have a “rule of thumb” for estimating losses. The “rule” a given manufacturer establishes is usually based upon a combination of their engineering calculations, testing, and field experience.
Cotta’s rule of thumb for losses is 2% of the rated input HP per loaded gear mesh – “worst case”. For example, an input/idler/output gear set would be estimated at 96% efficient. The reason for this is that there are two loaded meshes, the input-to-idler mesh, and the idler-to-output mesh.
Sample calculation is as follows:
Prime mover HP = 400
Loaded meshes = 2
Loss = 4% (2% * 2 meshes)
Loss = .04 x 400 – 16 HP
16 HP x 2542 = 40.672 Btu/hr
TRAP: Multiple speed gearboxes may have only one loaded mesh; however, there may be several unloaded meshes spinning – sometimes at high speed. This will cause some heat even though no power is being transmitted. Multiple speed transmissions are a special case, and should be discussed with Cotta Engineering.
The losses in a gearbox must be accounted for both in prime mover size and heat rejection. That is, the prime mover must be “oversized” enough to make up the frictional losses. In our example, a maximum of 384 HP can be expected at the load. If 400 HP is required at the load, then the prime mover must deliver 416 HP to the gearbox (400 HP + 16 HP = 416 HP).
Losses in a gearbox manifest as heat and cause the lubricant temperature to rise. Depending upon the design, the gearbox case may dissipate this heat and keep temperatures at a reasonable level. However, as transmitted HP increases, external cooling becomes necessary. The HP losses calculated in our example represents the heat that must be rejected. Heat exchanges should be sized for those values. Cotta application engineers will recommend a heat exchanger if the application indicates over temperature is possible.
If you have questions regarding efficiency, contact Cotta at 608-368-5600, or email sales@cotta.com .
