Technical: Gearbox Lubrication
Products listed in our TB97-101 bulletin are typical lubricants. Cotta does not recommend any specific manufacturer’s product for use in our gearboxes. Download our Lubrication of Gearboxes bulletin.
Technical: Gearbox Overheating
The most frequent cause of gearbox overheating is oil level and can be either too much or too little. Make sure the level is as specified by Cotta. Too little oil may result in reduced delivery or no delivery to moving parts and cause high friction and heat. Conversely, too much oil increases agitation by gears, bearings, and other moving parts which generates unwanted heat. Avoid the inclination to add more oil than necessary “just in case”. Oil levels are determined experimentally at the Cotta factory. If there are special operating circumstances such as pitch or roll, or, jerky motion that can cause the oil level to change, contact Cotta Engineering to discuss the best way to re-locate the oil level.
Technical: Gearbox Noise
Gearbox noise is sometimes difficult to diagnose without sophisticated vibration measurement tools. However, some general guidelines are as follows:
Gear noise from a deteriorating gear set is usually a high pitch “whine”. This whine is generated by the tooth meshing frequency. For example; an input gear with 30 teeth running at 1800 RPM would have a meshing frequency of 900 HZ. Compare this to the shaft rotation frequency of only 30 HZ (1800/60).
Bearings rotate at input shaft speed or some increasing or decreasing multiple. The rotational frequency of the shaft is several orders of magnitude below the gear mesh frequency. Therefore, the noise produced by bearings as they fail is closer to a “growl” or at least a frequency much lower than gear mesh frequency. In the advanced stages of failure, the balls or rollers may slide producing a screech.
Pinpointing the noise is easier said than done especially on systems without torsionally tuned couplings. Noise transfers via shafts and other rigid members and often sounds as though it is coming from a gearbox when it may actually be transmitted there from another source. A “listening rod” may be used to help locate the noise. Troublesome bearings usually generate excessive heat. Look for hot spots on the gearbox.
When it comes to noise, be certain that the gearbox is the culprit. Many a gearbox has been torn down only to be found in good condition.
Technical: Gearbox Efficiency
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 including the number of gears, gear type (helical, worm, spur, etc.), gear quality level, seal fits, bearing type, lubricant type and level, speeds, loads, and other criteria. Calculation of the exact losses is difficult and often turns out to be inaccurate. I 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. This is because there are two loaded meshes; the input-to- idler mesh, and the idler-to- output mesh.
A sample calculation is as follows:
Prime mover HP = 400
Loaded meshes = 2
Loss = 4% (2% * 2 meshes)
Loss = .04 X 400 = 16HP
16HP 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 + l6 HP loss = 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 about efficiency contact us.
Technical: Gearbox Vibration
Vibration can generally be divided into two categories – linear and torsional. Linear vibration is the type most of us are familiar with. This is the shaking and movement of equipment we can easily feel and hear. As a rule, linear vibrations are related to some type of mechanical problem such as imbalance or misalignment, or, advanced wear of gears or bearings. Linear vibrations tend to get worse as speeds increase. Automobile tire imbalance is a good example of liner vibration. As speed increases, the vibration increases in both frequency and amplitude- in other words – the faster the tire turns the worse the imbalance becomes.
Torsional vibrations are much harder to identify because they often cannot be felt. Torsional vibrations usually exist only at certain speed/load conditions and often “come and go” with speed changes. Torsional vibrations exist to some extent in all rotating systems but are not normally a problem unless the system is driven or “excited” by a frequency source which is at or near the natural frequency of the system (resonance). Systems operating at resonance can generate forces equal to several times the expected forces.
Torsional analyses can predict the speeds at which a system may become resonant allowing designers to “retune” the system with various methods such as torsional couplings and mass dampeners. Torsional analyses are typically performed by the system packager or a torsional coupling manufacturer. The damage from linear vibration is usually no surprise. For example, a severely out of balance pulley could easily fail a shaft bearing – no surprise here.
Torsional vibrations generally cause unexpected damage. For example, a gear may wear or break in a fraction of the time it should have taken. Or, a shaft may break unexpectedly even though it is oversized. When there seems to be no explanation for component failures – consider torsional vibrations. One particularly alarming torsional condition is full reversing in which applied torque actually passes back and forth through zero. If a gearbox is subjected to a full reversing torque, a severe rattle can occur as the gear teeth load and unload. The noise generated by this action can be quite loud. Keep in mind that the gearbox may not be causing the noise. The problem is that a severe torsional condition exists; not a defective gearbox.
If you suspect torsional or linear vibrations as a possible problem, contact Cotta Engineering for a discussion of the problem and possible diagnoses.