Siemens 680 User Manual download pdf (Page 19)

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Maintenance

Insulated Bearings

One or both bearings may be insulated to prevent

shaft currents from pitting bearing surfaces. The

insulation is located at the joint between the bearing

housing or bracket and the bearing. Insulated

bearings are designated by an instruction plate on

the bearing housing.

Check periodically to be sure the insulation has not

been weakened or destroyed.

The bearing insulation can be checked using an

ohmmeter or circuit test light. For sleeve bearing

motors with one bearing insulated, the shaft must be

raised a few mils at the non-drive end of the motor so

that it is not in contact with the bearing and the shaft

coupling must be parted so that the shaft is not

grounded through the driven equipment. For motors

with both bearings insulated, disconnect bearing

grounding strap before testing for insulation integrity.

Bearing temperature devices must be disconnected

and oil rings must not be in contact with both the

shaft and the bearing.

Antifriction Bearings

Antifriction bearings are selected to give long service

when they are given proper maintenance. Bearing

failure can be caused by too little or too much

lubrication, contamination, excessive bearing load,

improper installation, alignment, or vibration.

The symptoms of antifriction bearing failure are

excessive vibration, noise, and excessive heat

generation. The races and balls should be

periodically inspected for damage. Any damage

requires replacement of the bearing.

Sleeve Bearings

Check sleeve bearings daily to be sure the oil rings

are turning properly. See motor outline drawing to

determine proper oil level. Add oil through the oil ring

sight glass opening or oil inlet pipe, if so equipped.

Be careful not to overfill.

Drain the oil reservoir by removing pipe plug. Clean

and flush with solvent and refill with fresh filtered oil

every three months to one year, depending on

severity of service. Use a high grade turbine oil

having a viscosity of 300-350 SSU at 100°F for units

of 1800 rpm and lower, and 140-160 SSU at 100°F

for machines above 1800 rpm to 3600 rpm.

In addition, seasonal oil changes are desirable if unit

is subject to wide variations in temperature.

Bearing babbitt temperatures that exceed 90°C

(194°F) or a sudden rise in temperature should be

investigated.

Common causes of hot bearings are:

1. Lack of oil.

2. Incorrect viscosity.

3. Poor quality oil.

4. Inoperative oil ring.

5. Misalignment of couplings or bearings.

6. Insufficient bearing clearance.

7. Oil seal rubbing on shaft.

8. Shaft or bushing rough spots.

9. Plugged oil passages on circulating oil systems.

(Note that circulating oil systems include a filter in

the oil line to strain the oil after it leaves the

pump. Check and clean or replace filter when

necessary.)

10. End thrust on bearing face.

Sleeve bearings are bored to an even dimension and

shaft journals are slightly smaller to obtain running

clearance. Side reliefs are provided to distribute oil

axially and reduce friction. During normal operation,

the shaft is supported on an oil film 0.001 to 0.005

inch thick, depending on speed, load and viscosity.

Unless adverse conditions exist which tend to break

down the oil film, metallic contact occurs only during

starting and stopping. Under normal circumstances,

bearing wear is very small.

Oil Rings

Inspect oil level and oil ring operation frequently. Oil

ring operation can be observed through the oil sight

glass. Oil rings should be perfectly round, free of

burrs or rough edges, turn at a constant speed, and

carry a noticeable amount of oil to the top of the

bearing journal. Failure of the oil ring to turn freely

may be caused by:

1. Ring out of round - rings should be round within

0.062 inch.

2. Fouling on a projection of the bearings, bushing,

or housing.

3. Ring not balanced (heavy side will tend to remain

down).

4. Adhesion to guide slot (trapezoidal section

reduces adhesion).

5. Oil too cold, too viscous, or oil level too high.

6. Shaft not level – oil rings tend to bind.

7. Vibration causing oil ring to bounce and slow

down.

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Siemens 680 User Manual Page 19

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