Motors and Generators (part 3b) -- Motor and Generator Maintenance

9.5 Synchronous Motors and Generators

The stator of a synchronous machine requires approximately the same care as the stator of an induction motor. In large-sized synchronous machines, the windings are generally more accessible and this facilitates cleaning.

The rotor field coils of a synchronous machine should be cleaned in the same manner as the field coils on a DC machine. Slow-speed synchronous machines have rotor poles held by the spider with studs and nuts, while in high-speed synchronous machines a dovetail construction is utilized with tapered wedges securing the poles.

Some synchronous machines have the poles bolted to the shaft using bolts through the poles. Some 400-cycle synchronous generators utilize a laminated field structure with coils placed in slots, each tooth representing a pole.

Following is a general maintenance guide for synchronous motors:

During any general overhaul, the nuts on the studs or the wedges for the dovetail poles should be check for looseness. The amortisseur winding should be checked for loose or cracked connections.

In dusty installations where collector ring enclosures are not used, the collector rings and brush holders should be blown off weekly with clean dry air. When oil deposits form on the collector ring or brush holder insulation, it should be cleaned by wiping with a suit able solvent and coated with a high-gloss insulating varnish. When cleaning the brush holders, the brushes should be removed to pre vent their absorbing the solvent.

Coat all insulated surfaces of the brush holders and slip rings with a high-gloss insulating varnish. Caution should be exercised. Do not coat brush contact surfaces of the slip rings.

If the collector rings become eccentric, grooved, pitted, or deeply scratched, this condition can best be corrected by grinding the rings with a rotating-type grinder, with the machine running at rated speed in its own bearing. Fine emery cloth or sandpaper should be used for light scratches on iron or steel rings but not on bronze rings.

Regardless of the method used, rings, should be polished to a high gloss with crocus cloth and oil. After polishing, the rings should be thoroughly cleaned with a solvent to remove all abrasives and foreign materials.

In as much as the wear due to electrochemical action is not the same on both the positive and negative collector rings, it is suggested that the polarity be reversed about every 3 months of operation to compensate for this condition.

Field current specified on the nameplate should not be exceeded for continuous operation.

9.6 Cleaning and Varnishing of Machine Windings

The life of a winding depends upon keeping it in its original condition as long as possible. In a new machine, the winding is snug in the slots, and the insulation is fresh and flexible and has been treated to be resistant to the deteriorating effects of moisture and other foreign matter.

Moisture is one of the most subtle enemies of the machine insulation.

Insulation should be kept clean and dry. Certain modern types of the insulation are inherently moisture proof and require infrequent varnish treatment, but the great majority, if exposed to a damp atmospheric place, should be given special moisture-resisting treatment.

One condition that frequently hastens winding failure is movement of the coils caused by vibration during operation. After insulation dries out, it loses its flexibility. Mechanical stresses caused by starting and plugging, as well as natural stresses in operation under load, sometimes precipitate short circuits in the coils and possibly failures from coil to ground, usually at the point where the coil leaves the slot.

Periodic varnish treatment and curing, correctly done so as to fill all spaces caused by drying and shrinkage of the insulation, will provide an effective seal against moisture and should be a matter of routine electrical maintenance. Varnish treatment and curing of rotating electrical equipment follow a logical procedure.

9.6.1 Cleaning

Some machines are exposed to accumulations of materials, such as talc, lint, or cement dust, which although harmless by themselves may obstruct the ventilation. The machine will then operate at higher temperatures than normal, and the life of the insulation will be decreased. Such materials can sometimes be blown out with clean dry compressed air.

The most harmful types of foreign materials include carbon black, metallic dust and chips, and similar substances that not only impair the ventilation but also form a conductive film over the insulation and increase the possibility of insulation failure. Metallic chips may also work themselves into the insulation because of the ventilation and magnetic fields. When windings are cleaned, inspection should be made for any signs of deterioration.

Epoxy-encapsulated windings, a construction finding increasing favor, are sealed against contaminants. They need little attention other than removing dirt accumulations. The common practice when such windings are damaged is replacement with a new winding.

It is extremely important that all wound stators and rotors be perfectly clean before varnish treatment and curing. Unless all conducting dirt and grease are removed, the varnish treatment will not be fully effective. Also, after varnish treatment, the leakage path caused by conducting materials will be difficult to uncover and remove. Correct cleaning involves the following steps:

Dirt should be removed from all coil surfaces and mechanical parts.

Air vent ducts should be clear. As an alternative, clean, dry air at a pressure of not more than 50 psi may be used. Higher air pressure may damage windings. Do not use air if dust from the machine can damage critical equipment nearby.

As much oil, grease, and dirt as possible should be removed by wiping the windings with clean, dry cloths and then with clean cloths that have been moistened with a solvent recommended by the coil manufacturer. If the original varnish on the windings is cracked, a brush should be dipped in solvent and used to clean all conducting particles from the cracks.

For cleaning, armatures or wound rotors should be placed in a vertical position with the commutator or collector ring end up, and a pressure spray gun with solvent should be used to clean under the collecting device and through vent holes. The same procedure should be repeated with the opposite end up, and then repeated again with the commutator or collector ring end up. Most large DC armatures are ventilated through open commutator risers at the front end. The solvent spray should be directed through these risers to reach the inner surface of the armature coils and inner commutator vee-ring extensions.

Silicone-insulated equipment can be cleaned by the same methods used with other insulation systems. If a liquid cleaner is found to be necessary, the recommendations of the coil manufacturer should be followed.

For windings other than silicone, there are a number of good commercial cleaners on the market. The manufacturer can recommend the one most suitable for the conditions. Plant safety rules concerning the use of flammable and toxic solvent should be observed and followed.

Caution should be exercised to remove all liquid cleaners.

9.6.2 Drying

The wound apparatus should be dried in an oven held at a temperature of 115°C-125°C (239°F-257°F) for 6-12 h or until the insulation resistance becomes practically constant. If a vacuum is used, the drying time may be reduced.

The apparatus should be brought up to temperature slowly because excessive moisture may be present in the windings. If heated rapidly, this moisture may vaporize quickly enough to rupture the insulation.

Before treatment, the apparatus should be cooled to within 10°C (50°F) above room temperature, but never to a temperature lower than 25°C (77°F). If the apparatus is cooled to room temperature and allowed to stand, it will take up moisture quickly. If placed in the varnish at a temperature higher than that specified, the varnish will tend to harden.

9.6.3 Varnish

The selection of varnish is dependent upon the operating conditions to which the motor is subjected; also, the type of environmental conditions (i.e., moisture, corrosion, chemical, abrasion) should be taken into consideration.

Varnish must be compatible with the insulation system with which it is to be used. If it is incompatible, it may not adhere and may not give the desired protection. For most applications, the selection of a general-purpose high bonding, yet resilient, synthetic resin varnish is recommended. The varnish can be either class A, B, or F, depending upon the insulation system rating.

On large AC stators using class A insulation, the use of a flexible asphalt or oleoresinous varnish is suggested; then, if it becomes necessary to lift a coil, the coil will not be destroyed.

Many types of varnishes are available, and when applying the insulating varnish, the recommendation of the manufacturer should be followed with respect to specific gravity, viscosity, and curing cycle for the particular varnish in question. After the varnish has been adjusted to give the desired film build and drainage characteristics, the specific gravity and viscosity readings should be recorded; then at periodic intervals the varnish should be examined for either specific gravity or viscosity, or both, and adjustments should be made to bring it within the original limits.

The units should be cured in a correctly ventilated forced-air circulating oven to remove the solvent vapors. The oven can be either gas fired or electrically heated. Infrared heat can be used if desired.

For the most part, the time and temperature of the cure should follow the varnish manufacturer's recommendations. The time of cure will vary from short bakes of several hours up through 16-24 h, based on the physical dimensions and makeup of the units, and taking into consideration the particular characteristics of the type of varnish that has been applied to the equipment.

Curing temperatures will vary from 75°C to 125°C (167°F to 257°F) for oleoresinous-type varnishes to 135°C to 155°C (275°F to 311°F) for classes B and F varnishes. Silicone varnishes usually require a cure temperature range of 185°C-200°C (365°F-392°F) or higher.

Complete rewinding jobs should receive at least two coats of varnish.

Baking time can usually be reduced on the first or impregnated coat, with an extended period of time used on the final coat. The use of additional coats is based on what is expected of the unit after it is in operation. If severe conditions are to be encountered, multiple-coat systems are recommended.

Also, apparatus such as high-speed armatures should receive multiple coats for the maximum bonding of the conductors. One coat is all that is necessary on older units that have been cleaned up on which no rewind work has been done.

In the case of large stators or rotors where the size is such that dipping is not possible, the varnish must be sprayed on the windings. Old winding surfaces must be completely coated.

For most applications, conventional dip methods are recommended. Other accepted methods are brushing and flooding. However, if the length or depth of the slots is great and the windings tightly packed, it may be necessary to use a vacuum impregnation system.

9.7 Lubrication, Bearings, and Oil Seals

9.7.1 Lubrication

Of all the important items of maintenance, lubrication ranks as one of the highest. Incorrect oiling or greasing will produce as disastrous results as any other type of motor mistreatment.

Excess oil may get into the windings where it will collect dust and other foreign matter. Too much grease in antifriction bearings causes heat and sometimes failure of bearings and may also coat the windings. Most manufacturers furnish data on correct oiling and greasing, and numerous articles have been written on the subject. The important point is to set up a definite lubrication schedule and follow it. Years of experience have demonstrated that it is as bad to use too much as too little oil and grease.

Of equal importance is the type of oil or grease used. In general, the recommendations of the manufacturer or experienced oil companies should be followed. In some cases, for design reasons, manufacturers insist on the use of particular lubricants that have been adopted after exhaustive test by the manufacturer. It will pay to follow these recommendations.

9.7.2 Sleeve Bearings

Some oil-lubricated machines are shipped without oil and, in the case of large machines, the journals are often packed and treated for protection during shipment. The rotating elements may also be blocked to prevent damage to the bearings and journals during shipment. Where lubrication is required, the bearing must be opened, the packing removed, and the journal cleaned and flushed before filling the housing with oil. All motor and generator bearings should be checked for oil before starting up.

The bearings of all electrical equipment should be carefully inspected at scheduled periodic intervals in order to obtain maximum life. The frequency of inspection, including the addition of oil, changing the oil, and checking the bearing wear, is best determined by a study of the particular operating conditions. If makeup oil is required in excessive amounts, an investigation for oil leaks should be started immediately.

The more modern types of sleeve-bearing housings are relatively dust and oil tight and require very little attention, since the oil does not become contaminated and oil leakage is negligible. Maintenance of the correct oil level is frequently the only upkeep required for years of service with this type of bearing.

Older types of sleeve bearings require more frequent inspection and checking for wear, and oil changes should be made more often. Never add oil to bearings when the machine is running.

In most cases, the safe temperature rise for a bearing is considered to be within 40°C above the room ambient.

Small sleeve-bearing motors use either wool packing or fluid wick for transferring the lubricant to sleeve bearings instead of oil-ring lubrication.

Some of these small motors have provision for re-lubrication.

When electrical equipment must operate under extreme differences in air temperatures, the use of a lighter oil may be found desirable during cold weather.

Care should always be exercised in the use of reclaimed lubricating oils.

The filtering operation should be positive and should remove all foreign and injurious matter.

A hot bearing is usually due to one of the following causes:

•No oil.

•Poor grade of oil or dirty oil.

•Failure of the oil rings to revolve with the shaft.

•Excessive belt tension.

•Rough bearing surface.

•Incorrect fitting of the bearing.

•Bent shaft.

•Misalignment of shaft and bearing.

•Loose bolts in the bearing cap.

•Excessive end thrust due to incorrect leveling.

A bearing may become warm because of excessive pressure exerted by the shroud of the shaft against the end of the bearing.

Excessive end thrust due to magnetic pull, with the rotating part being sucked into the stator or field because it extends farther beyond the magnetic structure or field poles at one end than at the other end.

Excessive side pull because the rotating part is out of balance.

If bearing becomes hot, the load should be reduced if possible and lubricants fed freely, loosening the nuts on the bearing cap. If the machine is belt connected, the belt should be slackened. In case relief is not afforded, the load should be removed and the machine kept running slowly, where possible, until the shaft is cool in order that the bearing will not freeze. The oil supply should be renewed before starting the machine again.

A new machine should always be run unloaded or at slow speed for an hour or so to make sure that it operates correctly. The bearings should be carefully watched to observe that the oil rings revolve and carry a plentiful supply of oil to the shaft.

9.7.3 Antifriction Bearings

Ball or roller bearings carry the load by direct contact, as opposed to sleeve bearings, which carry the load on lubricating film. Lubrication is necessary to minimize the friction and generation of heat caused by the balls rubbing on the outer race as they roll over the top or on the retainer of the cage.

Antifriction bearings require considerable care to prevent loss of end clearance, distortion of balls, and marking of races. If too much force is used in pressing the bearing on the shaft, the clearance may be destroyed.

It is recommended that antifriction bearings be heated in a hot bath of clean oil rather than by the use of dry heat. When the bearing is pulled off, with all the stress on the outer race, both races may be damaged, with resultant failure when put back in service. The bearing manufacturer's recommendations should be followed when removing and reapplying this type of bearing.

Bearing manufacturers produce a bearing known as the prelubricated shielded bearing. Several years use of this bearing has demonstrated that, for many applications, no further lubrication is needed. Such bearing construction is usually indicated on the nameplate.

In general, to obtain maximum service, ball-bearing motors should be relubricated at intervals determined by the type, size, and service of the bearing. Many motor manufacturers offer as a guide a table suggesting the intervals between lubrication. These tables show time intervals between greasing that range from 3 months or so for motors operating in very severe service, as in conditions involving dirt or vibrating applications, those where the end of the shaft is hot, or subject to high ambient temperatures, to intervals of up to 3 years for easy service, where motors operate for short periods or infrequently.

The bearing housing is usually arranged to introduce new grease and purge the bearing of old grease, allowing it to discharge through a partially restricted escape port or relief hole. This will, in general, allow filling to the desired degree, which is one-third to one-half full, leaving some space in the housing to allow for expansion of the grease.

It is again stressed that over-greasing can be just as harmful as under-greasing. Over-greasing causes churning and internal friction that can result in heating, separation of the oil and soap, oxidation of the grease, and possible leakage through the retaining seals.

9.7.4 Installation of Oil Seals

The importance of correctly installing an oil seal cannot be overemphasized.

Failure to observe correct installation procedures probably accounts for more cases of the incorrect functioning of oil seals than any other single cause.

To secure the ultimate in satisfactory service, it is recommended that the following precautions be observed.

Correct seal:

It is essential that the seal be the correct size for the installation. Oil seals are made for a specified shaft size. When they are installed on a shaft of a larger diameter, there will be drag, frictional heat, and excessive wear on the sealing element and shaft. When installed on a shaft having a smaller diameter, immediate leakage can occur.

Fluid contact:

The seal should be assembled with the toe or wiping edge of the sealing element pointing toward the fluid to be retained. Exceptions for unusual applications must be by specification in manuals or instructions furnished with the assembly.

Bore:

The bore should be checked for adequate chamfer (30° angle to a minimum depth of 1/16 in.). The bore should be inspected for scratches and all sharp edges removed. The seal outside diameter should be correct for the bore in the assembly. When a leak at the outer edge of either metal or rubber-covered seals is caused by abrasion of the oil seal, it may be directly related to incorrect chamfer on the bore of the use of incorrect installation tools.

Shaft:

The surface of the shaft should be uniform and free from burrs, nicks, scratches, and grooves. The surface finish should be between 10 and 20 µin. and, on a repair job, should be buffed to this thickness with crocus cloth.

Lubrication:

In all cases, a lubricant should be applied to the shaft or to the sealing element of the oil seal. This aids installation and reduces heat buildup during the first few minutes of run. The application of a lubricant to the outer periphery of a synthetic rubber-covered seal will reduce the possibility of shearing or bruising.

Pressing tools:

In pressing the seal into the bore, it is imperative that the correct-sized pressing tool be used to localize the pressure on the face of the seal and in direct line with the side walls of the seal case to prevent damage and distortion to the seal cases during the installation. When a seal must penetrate the bore below the surface, the correct pressing tool should be 1/32 in. smaller than the bore diameter. On installations where the seal is flush with the housing, the correct pressing tool should be at least 1/8 in. larger in diameter, and more if room permits. Care should be taken to avoid hammer blows, uneven pressure on seal surfaces, and cocking of the seal during this operation.

When an oil seal of open channel construction is pressed-i t heel first into the bore, an installation tool will be helpful. The tool is designed to have contact with the inside diameter of the seal case.

Shaft end:

If the seal is to be installed toe first, the end of the shaft should have a 30° by 3/16 in. taper, or an installation tool must be used. If the seal is to be installed heel first, no special precautions are necessary other than to remove burrs or sharp edges from the end of the shaft.

Shaft with keyways and the like:

When an oil seal is installed over the keyway, splines, and the like, an installation thimble should be used with the outside diameter not more than 1/32 in. over the shaft.

Pressure-lubricated bearings

Because of speed and bearing loading, it is necessary to pressure lubricate the bearings on some larger motors and generators. Pressure gauge readings may not show the amount of oil flowing, but machines have a sight oil-flow detector where oil flow may be checked. Orifices in the feed lines may clog, and oil-flow detection devices will protect the bearings.

Bearing insulation:

If the bearing is insulated, care must be taken so that the insulated bearing is not grounded by bearing temperature detectors or relays.

9.8 Brushes

Correct care of brushes, brush rigging, and current-collecting parts is a fundamental necessity if satisfactory performance is to be obtained. Adequate inspection is essential to the maintenance of this equipment and the following points should be observed:

Brush holder box should be adjusted between 1/16 and 1/8 in. from the surface of the commutator.

Care should be taken to see that dirt and particles broken from the edges of brushes or the commutator have not lodged in the face of the brush.

Brushes must be correctly aligned, and the commutator brushes must be correctly staggered, pairs of arms (+ or -) being set alternately.

Brush is affected by such adverse conditions as sparking, glowing, rough commutator, severe chattering, no-load running, overload running, incorrect spring pressure, and selective action.

Brush on a machine that sparks or glows owing to load conditions, off-neutral operation, or an electrical fault in the machine will be burned and pitted near the sparking edge.

Severe chattering of the brush is caused by a high-friction film on the surface of the commutator or by incorrect spring pressure.

Brush chattering due to a high-friction film occurs on machines where there is considerable no-load or light-load running. The characteristic curve of friction versus load current is of such a shape that minimum friction can be obtained at approximately 55 A/in.2 and as load current is either reduced or increased, the brush friction is increased. Accordingly, it is sometimes good practice, when a machine is running at very light loads for a considerable period of time, to lift one or more brushes per arm to bring the brush friction into the desirable range. Cases where the load current is above the normal values are more serious, because the higher currents produce sparking, overheating of the machine and brush chatter simultaneously.

Spring pressure has a direct effect on the riding characteristics of a brush. A common error is to reduce spring pressure for cases where brush wear or marking of the commutator has been observed. This permits the brush to bounce on the commutator, which, in turn, causes sparking and selective action and produces a rough commutator. On the other hand, excessive spring pressure causes brush wear and commutator wear, and usually lowers the electrical contact voltage drop to the point where satisfactory commutation is not obtained. Correct spring pressure should be 2½-5½ lb/in. 2 for industrial service and 5-10 lb/in.^2 for traction service. The lower range on traction work will be found where spring-supported motors are used; axle-hung motors use the higher range.

When checking spring pressure, the action of the brush in the box should be free. Dirt or gummy oil on the brush or in the brush box sometimes causes the brush to stick and in some cases, to completely break the contact between the brush and the commutator.

Commutator wear in various forms is frequently attributed to a brush that is too hard. Actually, the abrasiveness of a brush does not result from its hardness. Some of the most abrasive brushes are soft to the touch or low when measured for scleroscopic hardness. The property in a brush of five grade that causes abrasiveness is controlled by the brush manufacturer, who should be consulted for information as to the relative cleaning properties of the various grades.

9.8.1 Brush Adjustment

The brushes of a new machine are generally adjusted at the factory to the electrically neutral position, and it should not be necessary to change the adjustment. An exception to this rule may occur on large machines where an off-neutral setting is sometimes used to improve commutation. In any case, the method for identifying the correct brush position is given in the manufacturer's instruction book. Various methods may be used for determining the neutral position. The kick method is commonly used as is outlined here.

With all brushes raised from the commutator and the machine standing still, voltages will be induced in the armature by transformer action if the shunt field is excited to about one-half of its normal strength and the field current suddenly broken. It will be found that the induced voltages in conductors located at equal distances to the right and left of the main pole centers will be equal in magnitude and opposite in direction.

Hence, if the terminals of a low-reading voltmeter (5V) are connected to two commutator bars on the opposite side of a main pole and exactly halfway between the centerlines of two main poles, the voltmeter will show no deflection when the field current is broken. The spacing of these commutator bars is, therefore, the correct distance between brushes on adjacent brush arms.

The most practical method of making this check is to make two pilot brushes of wood or fiber to fit the regular brush holder, each brush carrying in its center a piece of copper fitted for line contact with the commutator bar.

With a lead for the connection of adjacent brush arms, the brush rigging may then be shifted slightly forward or backward, as necessary, until breaking the field current produces no deflection on the voltmeter. By noting the position at which no deflection is obtained for each pair of brush arms, the aver age of the positions of neutral thus obtained will give the correct running location for the brushes.

A quick and convenient method of locating the neutral position on a DC motor and shunt fields is to check the speed of the motor in either direction with the same impressed line voltage. The position of the brushes that produces the same speed in either direction under the same voltage conditions is the correct neutral position.

Another shortcut is to take a piece of lamp cord and bend it in the middle, bringing the two ends together. The insulation should be removed for 1/2 in. on each end and the bare wires twisted together, fanning out to form a brush.

When this brush is held so that it spans two bars at the outer end of the commutator and moved with and against the direction of rotation, the point of least sparking at the ends of the wires is the correct location for the centerline of the brushes.

9.9 Balancing

Electrical failures are often ascribed to deteriorated insulation, open circuit, short circuit, and so on, but in many cases, failure of insulation results from mechanical disturbances. Unusual noises in electrical apparatus may be the result of grounds, short-circuited coils, changes in voltage or frequency, rubbing or looseness of parts, vibration, defective bearings, and many other causes.

Any unusual amount of vibration or an increase in machine vibration should be investigated immediately. Common causes of undue vibration, other than imbalance, or bearing wear, dirt accumulation, misalignment, an incorrect or a settled foundation, uneven air gap, parts rubbing the rotating element, sprung shafting, a short-circuited field coil, or imbalanced stator currents in the case of AC machines. These should be investigated before balance weights are added or shifted. If at any time it should be necessary to remove the balance weights, they should be replaced in the same position.

Before disassembling a pole on high-speed machines, the axial position of that pole should be accurately marked so that it can be replaced in the same position. Should it become necessary to replace a field coil, or a complete pole, the balance must be checked.

9.9.1 Need for Balancing

Vibrations produced by unbalanced rotating parts may result in the following:

•Excessive bearing wear

•Noisy operation of the equipment

•Failure of structural parts

•Reduced overall mechanical efficiency

•Vibration of machine parts or the supporting structure

9.9.2 Imbalance Measurement

Imbalance is generally measured in ounce-inches (oz-in.). An imbalance of 1 oz-in. in a rotating body will produce a centrifugal force equivalent to that produced by 1 oz of weight 1 in. from the rotational axis. A rotor weighing 62.5 lb (1000 oz) whose mass center is displaced 0.001 from the rotational axis is 1 oz-in. out of balance.

Only force imbalance is measured by static balancing, which is a single plane correction. The part being balanced is not rotated. Dynamic balancing of a part by rotation is required when there is appreciable axle length because, by this method, force imbalance, moment imbalance, or a combination of both may be measured. This is a two-plane correction.

The balancing process is not complete until corrections have been applied relative to the size and that the exact location indicated by the balancing machine. Corrections for balance may be made by the addition or removal of metal.

9.10 Belts, Gears, and Pinions

9.10.1 Belts

In most industrial organizations, installation, adjustment, inspection, and care of belts is the responsibility of a specially trained individual or group.

The application of belts involves alignment and belt tension, which affect bearing operation. Maintenance personnel must report belt alignments that seem inaccurate, tensions that appear excessive, and splices that look doubtful. Drives having upward belt tension may be questioned. Bearing loads on sleeve-bearing motors should not be against that portion of the bearing where the oil is fed into the bearing. Action should be taken to protect the electrical apparatus when there is evidence of belt-produced static.

9.10.2 Gears and Pinions

Gears and gear trains are among the principal sources of noise and vibration. In designing such mechanisms, the manufacturer strives for the best tooth term to give the least amount of whip and backlash, with the gear center so located that the teeth mesh at the correct pressure points.

It is essential, therefore, that the bearings be so maintained that these gear center distances do not change. Correct lubrication of gears is essential to keep down the wear of teeth. A gear with worn teeth, even though it appears to have considerable life left in it, should be replaced to keep vibration and noise to a minimum.