Major maintenance

Fan construction varies enormously from one manufacturer to another. There are however, two types which are characterised by their popularity throughout the industry:

A) Semi-universal construction with bolted on sideplates.

B) Fixed discharge construction with split casing.

Readers are referred to Chapter 8 for fuller details. Suffice it to say that where major maintenance is anticipated, this should be given in the operation and maintenance manuals. It should be noted that axial flow fans usually only have motor maintenance to contend with, when the appropriate instructions should be followed. Many lightweight centrifugal fans (Category 1 accord­ing to ISO 13349, are assumed to be a commodity purchase when replacement may be more appropriate.

Before removing any pieces of equipment, the relative posi­tions of mating parts should be marked to simplify erection.

1 Remove any coupling or drive guards.

2 Remove any shaft guard by unfastening the set screws with spring washers.

3 Remove any vee belts or coupling elements.

4 Disconnect inlet ducting, supporting as necessary.

The removal of the rotating assembly from the casing is de­tailed for semi-universal and fixed discharge fans with taper bushed hubs. Many other types are manufactured, and these are mere

Semi-universal fans

Alternative maintenance methods are possible, dependent on the disposition of the ducting.

1 The complete inlet sideplate with spigot and shaped inlet may be removed giving access to the impeller, which may

Then be removed through the inlet side. To prevent the vo­lute “dropping”, it will need to be chocked up underneath.

2 The complete pedestal and rotating assembly may be re­moved by undoing the fasteners on the drive side plate. Sufficient space will be required behind the fan and it will also be necessary to lift the pedestal over the H. D. bolts. Alternatively the assembly may be slid back if mounted on extended steelwork. Again the casing volute must be sup­ported by chocks.

Fixed discharge fans

1 Disconnect inlet spigot from inlet sideplate unbolting the setscrews. Slide shaped inlet away from impeller until it is clear of the impeller shroud. Support the loose venturi (shaped inlet).

2 Remove both halves of any shaft washer, unfastening the screws.

3 Unfasten the bolts along the horizontal join in the drive sideplate.

4 Unfasten the bolts along the horizontal join in the inlet sideplate.

5 Secure slings through the lifting eyes on top of the casing sideplates.

6 Unfasten the bolts along the horizontal join in the scroll.

7 Disconnect any outlet ducting from the casing, unfasten­ing the bolts on each sideplate and the bolts on top and be­low.

8 Disconnect the tongue from its half of the scroll.

9 The top half of the casing can now be carefully hoisted above the unit and removed.

10 Place slings around shaft adjacent to impeller, on the main bearing side, and adjacent to tail bearing on the impeller side. The impeller must be propped in the casing as it will tend to tip the shaft upwards when the pedestal top halves are removed.

11 Remove top halves of bearing blocks, unfastening the bolts on each pedestal. Now carefully lift the shaft, impel­ler, and roller bearings out of the unit, taking care not to damage any of them. The drive end of the shaft should be held down to avoid tipping.

Note: The instructions given above do not apply fully to fans in Arrangements 4 and 5 where the impellers are mounted directly on the motor shaft extension and there are no separate shafts and bearings.

Removal of impeller from shaft

1 Mount the shaft on suitable supports terminating in hard­wood vee blocks. The coupling /pulley end of the shaft must be restrained from moving upwards underthe tipping action of the impeller. Ensure also that the impeller is clear of the ground.

2 Mark positions of centre bush on shaft and make a note of impeller blade angle relative to rotation and bearings.

3 Slacken off all 3 screws in tapered bush and remove them. Replace 2 of the screws in the “jacking off’ holes and screw in alternately, after oiling the thread and point of grub screw (or thread and under cap of a cap screw), con­tinuing screwing in until the bush is loose in the hub and both bush and hub are loose on shaft. Oil shaft and slide impeller and bush towards inlet end of shaft and remove, taking care not to damage the shaft. Remove key from keyway. When removing impeller it should be supported on nylon or padded slings, between backplate and shroud, and around 4 blades.

Of bearings from shaft

Bevelled side of the nut. Tighten the sleeve nut, checking the clearance frequently until it is reduced by approxi­mately 50%. When mounting bearings with C3 or C4 fit, the reduction in clearance should be less than 50%.

7 Position the shaft in the lower half of each housing. Tighten the nut of the locating bearing sufficiently to posi­tion the sleeve on the shaft, which should be supported so that there is no load on the bearing when finally tightening up.

8 Check the radial clearance between the rollers and the outer ring with a feeler gauge (in C type bearings it is eas­ier to check the clearance at the lower part of the bearing). After tightening the nut, swivel the outer ring of the bearing to see that the inner ring is not expanded too much; over-tightening may cause premature failure.

9 Use special “D” spanners to tighten the nut. Do not use drifts or punches as these mutilate the nut and may dam­age the tab washer and bearing cage. In the case of large roller bearings, the locking washer may be damaged if placed between the nut and the sleeve during the driving up process. It is preferable to fit the washer after driving up, but if this not possible, the friction can be reduced by smearing the contact surfaces and threads with oil to lubri­cate them.

10 After the sleeve has been tightened, bend one tab of the washer into a convenient slot in the nut. To line up a tab and a slot tighten the nut slightly rather than slacken off. The bearing outer ring must not rest on its seating during this operation.

11 Only one fixed or located bearing is used on each shaft, the bearing being positioned axially in the housing by one ortwo locating rings, depending on its type. If two rings are fitted, position one on each side of the bearing; if one ring is used it is fitted on the same side as the nut. Make sure that the same bearing is located as previously.

12 The outer ring of a free bearing should be in the centre of the housing seating.

13 When fitting the impeller pulley or coupling, support the shaft so that blows cannot be transmitted to the bearings.

14 Thumb grease into both sides of the bearing and, in addi­tion, fill the bottom half of the housing. For low speeds the bearing housing may be filled completely. Smear a little grease around the shaft adjacent to the felt seals to assist in lubrication and sealing.

15 After fitting the housing cap check that the shaft does not foul at any point.

Roller bearings

1 Check that bearing parts are thoroughly clean and that ex­isting parts are suitable for further service; look for exam­ple for signs of wear and tear in inner and outer races.

2 Remove all old grease from bearing housings and thor­oughly clean using a clean cloth.

3 Ensure that nitrile seal is in good condition.

4 The instructions given previously (Section 18.6.5.2) for re­moval of split roller bearings should be reversed noting the following points.

5 Clean cartridge bore and lightly oil. Clean the outer race and place in the half cartridge so that all the pairing marks coincide and lubrication hole is in top half of cartridge. On the grooved race GR NTL located bearing, just enter the radial socket head cap screws with washers and very lightly tighten. It is important to fit the washers. Fit the side rods and side screws and very lightly tighten.

The expansion bearing EX NTL has no side rods. Check the shaft diameter.

подпись: bevelled side of the nut. tighten the sleeve nut, checking the clearance frequently until it is reduced by approxi-mately 50%. when mounting bearings with c3 or c4 fit, the reduction in clearance should be less than 50%.
7 position the shaft in the lower half of each housing. tighten the nut of the locating bearing sufficiently to posi-tion the sleeve on the shaft, which should be supported so that there is no load on the bearing when finally tightening up.
8 check the radial clearance between the rollers and the outer ring with a feeler gauge (in c type bearings it is eas-ier to check the clearance at the lower part of the bearing). after tightening the nut, swivel the outer ring of the bearing to see that the inner ring is not expanded too much; over-tightening may cause premature failure.
9 use special “d” spanners to tighten the nut. do not use drifts or punches as these mutilate the nut and may dam-age the tab washer and bearing cage. in the case of large roller bearings, the locking washer may be damaged if placed between the nut and the sleeve during the driving up process. it is preferable to fit the washer after driving up, but if this not possible, the friction can be reduced by smearing the contact surfaces and threads with oil to lubri-cate them.
10 after the sleeve has been tightened, bend one tab of the washer into a convenient slot in the nut. to line up a tab and a slot tighten the nut slightly rather than slacken off. the bearing outer ring must not rest on its seating during this operation.
11 only one fixed or located bearing is used on each shaft, the bearing being positioned axially in the housing by one ortwo locating rings, depending on its type. if two rings are fitted, position one on each side of the bearing; if one ring is used it is fitted on the same side as the nut. make sure that the same bearing is located as previously.
12 the outer ring of a free bearing should be in the centre of the housing seating.
13 when fitting the impeller pulley or coupling, support the shaft so that blows cannot be transmitted to the bearings.
14 thumb grease into both sides of the bearing and, in addi-tion, fill the bottom half of the housing. for low speeds the bearing housing may be filled completely. smear a little grease around the shaft adjacent to the felt seals to assist in lubrication and sealing.
15 after fitting the housing cap check that the shaft does not foul at any point.
roller bearings
1 check that bearing parts are thoroughly clean and that ex-isting parts are suitable for further service; look for exam-ple for signs of wear and tear in inner and outer races.
2 remove all old grease from bearing housings and thor-oughly clean using a clean cloth.
3 ensure that nitrile seal is in good condition.
4 the instructions given previously (section 18.6.5.2) for re-moval of split roller bearings should be reversed noting the following points.
5 clean cartridge bore and lightly oil. clean the outer race and place in the half cartridge so that all the pairing marks coincide and lubrication hole is in top half of cartridge. on the grooved race gr ntl located bearing, just enter the radial socket head cap screws with washers and very lightly tighten. it is important to fit the washers. fit the side rods and side screws and very lightly tighten.
the expansion bearing ex ntl has no side rods. check the shaft diameter.
This is done after the impeller and any coupling element or vee

Belts have been removed, as the bearings help to balance the

Assembly.

Spherical roller adapter sleeve bearings

1 Remove the bearing cap setscrews and take off bearing caps, being careful to note which way round they are fitted, and to which plummer block. Note also which is located before removing rings.

2 Lift out shaft and bearings being careful to use rope slings and not to damage anything.

3 Position shaft in wooden vee blocks on a suitable bench.

4 Carefully prise out tab of washer. Loosen off lock-nut and remove.

5 Bearings may now be removed by carefully tapping down taper of the adapter sleeve.

6 Adapter sleeves may now be removed.

Split roller bearings

1 Remove socket head cap screws and take away both halves of cartridge complete with both halves of the outer race.

2 Remove jointing clips of the roller cage and take away both halves of roller cage complete with rollers.

3 To remove outer race from cartridge housing on expan­sion bearing, remove radial socket head cap screws from each half of housing and slip out race.

4 To remove outer race from cartridge housing on located bearing loosen side screws and their side rods in addition to the radial socket head cap screws.

5 The seals (triple labyrinth type) on both bearings are split in one place and secured by half circlips. These circlips are removed and the seal unwrapped off the shaft.

6 Undo socket head cap screws on each clamping ring and remove both halves of each clamping ring.

7 Finally remove both halves of inner race.

Refitting of new bearings on to shaft

Spherical roller adapter sleeve bearings

1 The bearing should not be taken from its packing until re­quired. Do not remove the protective grease except from the bore which should be wiped with a clean cloth damp­ened with white spirit.

2 Saturate the felt seals in a mixture of two-thirds lubricating

Oil and one-third tallow at 80-85°C.

3 Caps and bases are not interchangeable or reversible — do not mix them.

4 Clean the inside of the housing thoroughly. Press the felt seal into the groove in the base, and cut off the ends flush with the machined face. Pack the remaining felt in the cap groove, and trim off the surplus. Make sure there are no frayed edges to prevent the cap fitting correctly; but do not cut off too much as a gap between the ends will allow for­eign matter to enter.

5 Secure the housing base to a support and check for align­ment. Lightly smear the outer ring seating of the housing with a thick oil containing a rust inhibitor.

6 Remove any sharp edges or burrs from the shaft, adapter

Sleeve, tab washer and nut, then wash them clean and wipe dry. Lubricate the thread and lightly oil the outer sur­face of the sleeve. Mount the adapter sleeve roller bear­ing, tab washer and nut loosely on the shaft as previously. 6

Make sure the concave side of the tab washer faces the

7 Place the two halves of the inner race at the correct posi­tion on the cleaned shaft. A tap may be required to spring the races over the shaft.

8 Fit the clamping rings with the joints at about 90° to the in­ner race joint. This overlap is particularly important for side fitting clamping rings of the grooved inner races to ensure alignment of the lips. There should be approximately equal gaps at both parts of the clamping rings and races.

9 Tighten all socket head cap clamping screws equally us­ing the correct hexagon key and tube extension.

10 Tap down each half of the inner race and clamping rings all round the shaft, interposing a fibre or hardwood block be­tween hammer and bearing parts, and re-tighten screws. Repeat until screws are fully tightened.

11 Check that there is a gap at both joints of the inner race, coat the roller cage with grease, fill the cartridge % full in each half, and grease all seals including the bores of the revolving labyrinth seals. Place the roller cage around the inner race and engage jointing clips.

12 Close cartridge and tighten socket head cap joint screws. Lubricate the spherical seating.

It is strongly recommended that ball or roller bearing replace­ments should be of the same type, diametrical clearance, and

Fit as originally supplied.

Refitting of impeller on to shaft

1 Ensure that the mating tapered surfaces on hub and bush are completely clean and free from oil and dirt. Insert bush into hub so that holes line up.

2 Oil thread and point of grub screw or thread, and under head of cap screws. Place screws loosely in holes thread in hub.

3 Clean shaft and fit side fitting key with top clearance in po­sition in keyway. Fit hub and bush to shaft as one unit and locate in correct position by lining up original marks. Re­member that the bush will nip the shaft first and then the hub will be slightly drawn onto the bush.

4 Tighten screws finger-tight.

5 Using a hexagon torque wrench tighten screws gradually and alternately until all are pulled up correctly. Take care not to exceed recommended screw torques given in Table

4 or damage may result.

6 After the fan has been running under load for a short time, stop and check tightness of screws.

7 Fill empty holes with grease to exclude dirt.

Bush size

Screw size (in)

Wrench torque (Nm)

1210

%

20

1615

%

20

2517

R2

48

3020

%

90

4545

%

192

Table 18.4 Recommended screw torques for fitting impeller

Refitting rotating assembly into unit

18.6.8.1 Semi-universal fans

The reverse procedure to that used for removal should be adopted.

18.6.8.2 Fixed discharge fans

1 Support shaft on nylon or padded slings with one sling as close to impeller as possible on the bearings side and one sling outside tail bearing.

2 Lift shaft assembly carefully, holding coupling or pulley end of shaft down to avoid tipping.

3 Move assembly over unit and lower carefully onto bearing pedestals horizontally. Care must be taken not to damage impeller or shaft or bearings. Ensure that the triple seals slide into their corresponding grooves.

4 Replace top half of pedestal housings and fit socket head cap screws on each pedestal (run shaft for a short period before finally tightening pedestal cap screws to ensure swivel alignment). Remove slings.

5 Fasten tongue to outlet half of scroll using set screws.

6 Bolt up horizontal join in inlet sideplate.

7 Bolt up horizontal join in drive side plate.

8 Bolt horizontal join in scroll: attach both halves of shaft

Washer to drive side plates.

9 Bolt up outlet connections to casing on each sideplate, scroll and tongue.

10 Slide shaped inlet into casing until inlet spigot is flush with casing. Bolt spigot to casing set screws. Check impeller overlap and diametrical clearances. Replace inlet connec­tions.

11 Rotate shaft to check for any obstructions, replace cou­pling element or vee belts.

12 Check that pulley or coupling is in original position relative to shafts (check alignment first and correct as necessary).

13 Replace coupling or drive guard.

14 Replace shaft guard using set screws with spring wash­ers.

Again the instructions given above are simplified for fans to Ar­rangements 4 and 5 where the impellers are mounted directly on motor shaft extensions.

Note: Impeller/inlet venturi alignment is most important for the airflow performance of high efficiency fans. The method outlined below is recommended for determin­ing the optimum position.

Place template (shown shaded in Figure 18.2) against inside face of impeller shroud and adjust impeller or venturi to achieve correct alignment. Repeat at four cardinal points.

Vee belt drives — installation

1 Clean all oil and grease from pulley grooves and bores.

2 Remove any burrs or rust.

3 Reduce the centre distance until the belts can be placed in

The pulley grooves without forcing.

Major maintenance

4 Align the pulleys correctly using a straight edge to ensure that the pulleys are in line and the shafts parallel.

5 Tension the drive using the motor slide rail bolts.

6 Check that the vee belts are correctly tensioned:

A) Measure the span.

B) Apply a force at right angles to the belt at the centre of the span.

C) This force should deflect one belt 16 mm for ev­ery metre of span length. See Figure 18.3.

D) The average value of the force in each belt should be compared with Table 18.5 (in accor­dance with BS 1440:1971 and BS 3790:1985). Belts should initially be tightened to the higher values.

Belt section

Small pulley pcd (mm)

Belt speed

0 to 10 m/s

10 to 20 m/s

20 to 30 m/s

SPZ

95

12 to 18

10 to 16

8 to 14

95

18 to 26

16 to 24

14 to 22

SPA

140

22 to 32

18 to 26

15 to 22

140

32 to 48

26 to 40

22 to 34

SPB

250

38 to 56

32 to 50

28 to 42

250

56 to 72

50 to 64

42 to 58

SPC

355

72 to 102

60 to 90

50 to 80

355

102 to 132

90 to 120

80 to 10

Z

50

4 to 6

A

75

10 to 15

B

125

20 to 30

C

200

40 to 60

D

355

70 to 105

Table 18.5 Correct vee belt tensions: required force N at centre of span for belt speed (To obtain kgf divide N by 10 to give approximate value)

Major maintenance

Figure 18.3 Belt deflection measurement

If the measured force falls within the values given in Table 18.5 the drive tension should be satisfactory. Aforce below the lower value indicates undertensioning. When starting up, a new drive should be tensioned to the higher value to allow for stretch dur­ing the running in period. After the drive has been running a few hours the tension should be re-adjusted to the higher value. The drive should be re-tensioned at regular maintenance inter­vals. Make adequate provision for tensioning the belts during their life.

Couplings and shaft seals

Couplings of the grease-filled type will require to be fully charged with suitable grease after alignment and before start­ing up, and replenished at monthly intervals unless otherwise advised.

Shaft seals of the packed stuffing box type generally need no lu­brication except when lead wool packed, when the box should be completely filled with suitable grease and replenished peri­odically during operation by means of the greaser cap provided. This procedure also applies to labyrinth type glands.

Symptom

Possible cause

Remedial action

Fan won’t start

Power failure Blown fuses Motor single phasing Control switch contacts open

Check all power supplies Check all fuses Check motor and wiring

Check all instrument switches in motor contactor circuit

Low flow

Inlet damper closed Discharge damper closed Excessive system resistance Fan speed very low Impeller partially blocked Wrong direction of rotation

Check all inlet dampers in ducting for correct position Check all outlet dampers in ducting for correct position Check calculations, gas temperature etc.

Check drive, motor frequency and voltage Fan vibrates badly — clean impeller and re-balance Reverse two phases in 3-phase motor

High flow

System resistance lower than specification Fan speed high

Check calculations, throttle discharge damper, reduce fan speed

Check fan speed, check motor frequency, check pulley ratios in vee belt drives

High power consumption

Fan power high Electrical faults

Check air/gas temperature and specific gravity

System resistance low, flow high, check operating conditions

Fan speed too high

Voltage and frequency incorrect, check supply Motor problem, check motor

Fan vibration

Poor alignment

Damaged impeller Bearing problems

Check alignment

Check coupling or vee drive assembly Check baseplate grouting Check ducting

Check vibration harmonics, clean and/or repair impeller, re-balance

Check bearing clearances

Check vibration signature for bearing faults

Motor vibration

Poor alignment Bearing problems

Check alignment and coupling/vee drive assembly

Check baseplate grouting

Check bearing clearances

Check vibration signature for bearing faults

Check motor for loose rotor bars

Table 18.6 Trouble-shooting guide for fans 292 FANS & VENTILATION

The speed of the machine should not be increased beyond that specified for the particular duty for which it was installed, with­out first referring the matter to the manufacturer.

All impellers should be accurately balanced during assembly at the works, but on certain applications, unbalance and conse­quent vibration may develop as a result of a build up of deposits on the impellers. This calls for periodic and thorough cleaning of the impellers to restore the original balance. Corrosion, wear, or damage on certain applications may also cause unbalance.

Where a number of machines are installed with one or more acting as standby it is good practice to use them in rotation or to give the rotors a few turns at regular intervals.

Posted in Fans Ventilation A Practical Guide


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