Taking Care of Enclosed Gear Drives --
A Good Case For Preventive Maintenance
There are certain maintenance procedures that are essential to keeping gear drives in proper working order. These procedures are important to know because a good maintenance program will both increase the life of equipment and decrease the chances for a catastrophic failure.
Three ingredients are basic to gear drive maintenance. They include proper support, proper alignment to connected equipment, and proper lubrication.
Let's first discuss the matter of supporting the drive. Many believe that gears supplied in a housing are "automatically" in proper alignment and all that is needed is to bolt the pads or feet to the foundation. Unfortunately, this is not usually true. The housing is designed to locate the bearings in relation to each other, support the shafts and contain the lubricant. It is not infinitely rigid. It depends on a smooth, level structure to uniformly support it and carry the loads from shaft torques into the earth.
Most small and medium sized gear drives will provide proper tooth contact and bearing alignment if permitted to maintain their normal shape. Shimming, to ensure uniform support, is all that is required.
Very large or special drives are often provided with leveling pads to assure flatness of the housing to achieve or maintain good gear tooth contact. In all cases, non-uniform support can cause poor meshing of gears which can significantly shorten the life of the most carefully manufactured gears.
Shaft Alignment
Second is the problem of alignment. In most cases a gear drive is connected to a prime mover of some sort on the input side and the driven machine on the output side. The alignment of the gear reducer with its driving and driven member is extremely important in providing long life.
The effect of good alignment will be to reduce the load on the bearings of the reducer. It is also desirable that the coupling provide for a margin of error in the event that the alignment changes either due to temperature rise of the machinery or deterioration of some sort in the foundation structure.
Good alignment means that shafts are parallel to each other, do not have angular misalignment and are not offset with respect to their centerlines.
The question, how good must alignment be, is not an unusual one. It is strongly recommended that the initial alignment be made significantly better than that specified.
Published misalignment capacities of .020-inch (parallel and angular) for typical shaft couplings are common but every effort should be made to get both parallel and angular within .005-inch.
If the connection on the output side is made with belts or chains, not only should the alignment be checked and made in
accordance with the manufacturer's recommendations, but belt or chain tension must be kept within proper limits. All too often, a gearbox which is slightly undersize will slip the belts connecting it to the motor. This may lead the maintenance department to tighten up the belts. But that may very well be the beginning of a bearing failure or even shaft breakage.
Do not exceed the V-Belt manufacturer's recommendation, whether that be 1/2-inch deflection at the midpoint with a given spring load or any other method of establishing the proper tension. To exceed specified belt tension will not only shorten the belt life, but may also impose excessive loads on the reducer.
In the case of a chain drive, it is good practice to mount the sprockets as close to the gear shaft bearings as possible. The connected shafts must be parallel and sprockets must be in axial alignment. Chain tension should be adjusted to the chain manufacturer's specifications, checked after 100 hours of operation, rechecked after 500 hours, and periodically thereafter.
Proper Lubrication
Third, a gearbox, supported and aligned properly, needs one more ingredient to operate successfully. Lubrication! The single most common cause for failure of gearing or gear drives is probably inadequate lubrication.
Exotic lubricants and expensive additives are not normally required for effective lubrication. Regardless of the package it comes in, plain old mineral oil, with a little rust prohibitive and oxidation inhibitor, is probably the most widely used gear lubricant. However, some E.P.'s and synthetic oils have gained popularity and offer benefits for certain low and high temperature applications. Their only drawback is higher cost.
Viscosity -- A Key Consideration When Selecting Oil Lubricants
The viscosity of a given oil varies with its temperature. Viscosities are usually measured and specified at 100 F (40 C). Today gearboxes can operate at temperatures up to 40 C. It is therefore suggested that the viscosity- temperature curve be checked for whatever lubricant is recommended. It is important to make certain that the viscosity does not drop below 100 SSU at any reasonably expected operating condition. At normal conditions, 200-500 SSU would be better.
Open gears, operating at very slow speeds (15 rpm), may require viscosities as high as 10,000 SSU. An enclosed higher speed drive should never be subjected to that type of tacky gear compound. But it is mentioned solely to indicate the high limit of viscosity and gearing lubricants.
It is probably true that far more gear drives have failed because of gears running with a lubricant that has too low a viscosity, than vice versa.
Another factor to consider is whether the lubricant will distribute itself under cold weather starts. A very viscous lubricant which lies like a solid lump in the bottom of a gear case, and does not get into the bearings and gear teeth, is not a very useful one.
Too Viscous Lubricants Result in Heat Loss
Still another factor which needs consideration, and which will help prevent the selection of too viscous a lubricant, is heat loss that builds up in the gearbox. Generally, the more viscous a lubricant, the more heat will be generated and the higher the temperature that will prevail. Remember too, that the oil at the mesh point is probably at least 50 F. hotter than the sump temperature.
High oil temperatures are not harmful to the metal of the gears, bearings, and housings, but could be hazardous to the life of oil seals as well as to the oil itself.
Here again, a plain mineral oil has excellent stability operating in the 170 to 200 F. range. With many oils that have additives, the additives begin to separate out at these temperatures.
Synthetic oils are equal to, if not better than mineral oils at high temperatures.
The quantity of oil should be as recommended by the gear manufacturer. Too little oil will usually result in starvation and lack of circulation of oil to the bearings and the gear mesh. This is roughly the same as having no oil in the gearboxes at all. The effect of too much oil is usually greater heat losses and higher temperatures in the gearbox. This has the tendency of reducing the viscosity of the working oil and increases the likelihood of oil coming out through the seals, breathers, and other places.
If, due to high ambient temperatures, a gearbox sump temperature exceeds the allowable upper limit, several options are available to reduce the temperature. Cooling fans, either shaft driven or motor driven, oil-to-air exchangers, oil-to-water heat exchangers, or circulating water cooling tubes installed in the sump are effective ways to control oil temperatures.
Gear Drive Inspection
This article has dealt primarily with specific actions which need to be taken to improve the odds that a properly selected gear drive will be trouble free. The one further step which can be taken to prevent unexpected shutdowns and lost production is to periodically inspect gear boxes.
Inspection generally involves the use of two or three senses. First and easiest is sound. This must be done while the drive is operating. Everyone associated with "plant" operations, should be encouraged to listen for and report any change in the sound of a gear drive. Each gearbox has a different noise level and set of predominant frequencies. If a change occurs, further investigation is in order.
Sight is another inspection tool offering potential for diagnosis of a gear drive problem.
Gear teeth exhibit two distinct types of distress caused by two different situations. The most common type of distress is surface deterioration stemming from compressive forces at the tooth mesh. This distress can take the form of pitting, spalling, scoring ridging, rippling or other visible signs of deterioration. These signs are not all alike, and do not necessarily have a common cause. Knowing their peculiarities can therefore give a hint as to what corrective action should be taken.
For example, pitting generally indicates an overload situation. However, this doesn't always mean more torque than anticipated. It could mean less tooth area is carrying the load indicating improper housing support or bearing float settings that are incorrect. Or it could mean a variable load whose average value is within the motor rating but which has peaks and valleys substantially greater than average. A variable load situation is usually cyclic in nature and its predominant frequency can often lead to the source. The excitation might be from the driven equipment or from the gears themselves.
Visual inspections of reduction gears should include an observation of the shaft seals. All contact seals will eventually wear and begin to leak. Abrasive atmospheres hasten the wearing process. Seals must then be replaced to prevent the loss of lubricant, and damage to the gears and bearings.
Non-contact seals, with grease purge capabilities, will last for the life of the gearbox. The only maintenance necessary is to periodically purge with fresh grease.
Poor Lubrication Will Also Lead to Gear Teeth Distress
Inadequate lubrication can often be diagnosed by tell-tale lines up and down the tooth. Discoloration of gearing or bearings is almost always a sign of lack of lubrication.
Marginal lubrication can also result in pitting because oil film does not spread the contact (cushion) over a sufficiently wide area. This can result in metal-to-metal contact in the load zone.
Surface hardened gearing exhibits its own peculiar distress signals, usually associated with a shucking of the hardened layer.
Tooth breakage is a situation almost always associated with an overload. It may be high shock loads which cause portions of a tooth to break out after relatively few cycles. A loose tooth going through the mesh often results in a catastrophic failure.
Sometimes a repeated overload of lower magnitude or light shock will create a fatigue crack. This type of break usually shows semi-circular markings which indicate progress of the crack from its origin as a pit, scratch, inclusion, or machining tear mark.
Usually sound and sight are all that is needed to indicate a problem and point the way to its relief. But sometimes sense of feel can be used to detect surface roughness or waviness of gear teeth, which is usually the result of a severe cyclical disturbance in load cycle. A wire edge is sometimes drawn up at the tip of the tooth, which can be felt even though it is difficult to see. Generally, sense of touch only confirms what has been seen.
Gear drives are not the most glamorous pieces of equipment in paper mills, thus they often do not receive the recognition they deserve for keeping processing operations up and running. And unfortunately, they are often inadequately maintained.
That's unfortunate since inadequate maintenance can result in a catastrophic failure in a gear drive, leading to unscheduled lost production.
But the good news is that an effective maintenance program focusing on support, alignment, lubrication and inspection will go a long way toward keeping gear drives rolling along in good operating condition.
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