Motor maintenance and bearings

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Tony S

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Most motor will get by on minimal maintenance their biggest enemy being dirt and oil, mix the two together and the poor thing doesn’t stand a chance.

There’s one thing to remember, it’s a damned site harder to change a motor than keeping it happy with a bit of TLC.

A motor doesn’t like being hot. One of the major causes of burnouts is over heating caused by dirt being allowed to build up. it’s so easy to prevent.

Air intakes, keep them clear of any debris and my favourite, things propped up against them.

Cooling fins, a quick brush over. OK get a damp and dusty atmosphere and you may have to resort to a scrapper (hammer and chisel in some of the places I’ve worked).

High ambient temperatures should be taken care of at the design stage. This is just an example of a boiler house fan where the motor is moved outside.

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If it can’t be avoided then a motor with a higher temperature rating should be used. For hostile environments various types of motors are made, drip proof, indirect cooling, etc.

Slip-ring motors.

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Now these do need a fair amount of looking after. Carbon dust in the slip-ring housing if allowed to build up can cause tracking and flashover. The brushes wear so have to be replaced.

Dust can be taken care of by blowing out with a low-pressure air line or dusting down with a dry paint brush. I prefer the air line as you can get the dust out of the hard to reach places. Wear a particle mask, as the dust is horrible tasting and if you smoke, your fags will taste like they’ve been dipped in sugar. Doesn’t do your health much good either.

Many brushes have a minimum wear line etched on them. Go below this and you run the danger of the copper pigtail coming in contact with the rings. This will ruin the rings. Whether you change them a bit early is up to you but it’s better than too late. If there is no wear line, I’ve found it pays to get an old brush and crush it in a vice so you can see how far the pigtail is embedded. Twice the depth of the pigtail should be your mark. A word of warning, make sure the pressure spring will go deep enough in to the brush box.

This picture from RoB shows what I mean.

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On the subject of the amount of wear before changing a brush. I started work at a new company where it was normal to change brushes when the top of the brush was level with the top of the brush box. Barely a ¼ of the useful length had been used. I tried to get them to see sense and explain they were causing work and unnecessary expense. I gave up, but in the 14 years I was working there I never changed a brush. Even the engineering manager told me to give up trying to convince them, “you’re wasting your breath. That’s what they were shown as lads and you’ll never change them”.

While a motor is running the dust doesn’t cause a problem as there is no voltage between the rings due to them being shorted out in the starter. Starting is when problems will occur. The resistance between the slip-ring is at its maximum giving a high voltage, this is when flashover will occur. Once the motor is turning the resistance is lowered and therefore the voltage.

The slip-rings if in good condition should be a rich chocolate brown colour, this is the patina. It is caused by the interaction of the carbon on the metal ring. Black or bright rings are an indication of usually brush grade or atmospheric problems.

Circumferential scoring if light is OK, if it gets too deep it can prevent brush movement in its box.

Pitting will wear brushes very quickly. Both problems usually mean a slip-ring or motor change. There is a way to avoid changing the rings or motor, but the last time I did it the manager was calling me a suicidal maniac as he legged it off out of the way.

Martindale (as in test meters) make wooden handled comstones. With the motor running at full speed the stone is held against the ring to take the pitting and grooves out. The final polishing is by a Martindale brush seating stone. Morganite make special brushes with a abrasive insert that can also be used for this job. I’ve never seen one as they are made to order and expensive.

Com Stones​

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Brush seaters​

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BTW, it’s a horrible filthy job. The first time I dressed sliprings our spare motor was away for rewinding. I was on my days off but that didn’t stop the phone ringing. The duty electrician had got the fitters to take the drive belts off and raided stores for every brush we had before I arrived. Chris was on the brushgear side while I’d got the com stone on the other side. Until we’d got the worse pitting off we had to stop the motor to change brushes after that Chris was changing bushes with the motor running.

Abrasive insert brush​

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RoB went about it the hard way.

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RoB didn’t have much choice. The middle of the night and the plant is loosing £1000’s a minute in down time, you do what is needed.

After changing a brush it should be “bedded” to the circumference of the ring. This is done with glass paper, not emery! Emery will embed it’s self in to the surface of the brush and removing the rings conductive patina. The glass paper is put between the brush and ring and passed backwards and forwards until 66% of the brush is curved to the same circumference as the ring. If there are multiple brushes on a ring there’s Tony’s easy method of brush seating, don’t bother! The other brushes will take the load as the new brush beds in, but only do this with multiple brush sets, one per ring tops!

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Routine maintenance checks.

Most of the plant’s I’ve worked on would have a week or fortnight shutdown every 6 or 12 months, some would run continuously for 7 years. This would give us the chance to go around and give the motors a bit of TLC. On some plants there would be up to 500 motors to check and with limited manpower the checks may seem somewhat cursory.

A quick check from the MCC if the drive wasn’t isolated for IR to earth and CR. These test were mainly done from the local isolator as 9 times out of 10 the drive would have been isolated for other work. We had records from the date the plant was built. Using these readings any dramatic change would start the alarm bell ringing.

The motor terminal box and local isolator would be opened up for the connections to be checked. This turned out to be a double-edged sword on one plant. The plant had been built in the mid 60’s when aluminium cables were a hell of a lot cheaper than copper. I’d been on at our manager to get us a torque wrench and drivers for years, “no to expensive”. After a while we started to get problems with terminals/lugs burning off. One particular lad would check the terminal was tight by a quick tweek with the wrench, with aluminium it was a recipe for disaster. He’d get a full turn on the nut, the next time another…. You can guess the end result, I got torque wrenches issued! How much the burnt out motors must have cost when compared to the cost of the correct kit, I don’t know.

While at the motor it would be given a clean down. The isolator switch blades and operating mechanism would be lubricated.

Lubrication:

Oil, used for small motors with phosphor/bronze sleeve or journal bearing for very large motors. For some odd reason they were popular for lift motors, I suppose because they are quiet. Some large motors will have white metal (babbit) as the running material. The only thing to say really is to follow the manufacturers instructions as to the oil grade to be used. I’ve come across some weird specifications in the past. Two motors spring to mind each 2500HP, the recommended oil was B44 transformer oil? (Don’t ask me why, but they had been running happily 24/7 for 20 years). Even bigger motors may have oil lubricated bearing with water-cooled jackets around the bearings. One thing to watch for with large sleeve bearings is the oil thrower/pick up rings wearing through. You’re quite happily keeping the oil level correct but it’s going nowhere and doing nothing!

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You may notice on this drawing the shaft has no shoulder to locate it. When a large motor is first run the bolts are left out of the coupling, this allows the motor shaft to find its magnetic centre. Once the centre is found the coupling is assembled with spacers to hold the shaft in position.

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The lug you can see on the top of the bearing saddle is to stop the bearing rotating in the cast iron housing. Oil, phosphor bronze and cast iron offers less drag than phosphor bronze and steel. You can also see the oil reservoirs and oil rings.

Grease.

Here we go for the first argument. Some motor manufactures will claim maintenance free bearings. Yeh right, so why the hell am I mauling my nuts off changing this motor?

Grease is oil held in a suspension medium. There are 1000’s of grades each with its own bit of magic. (The one thing I do know is the moment I go near it I get covered in the bloody stuff)!

If grease nipples are fitted to a motor then a shot of grease every 3 to 6 months won’t go amiss, but use your judgment as to how much. Big bearings 2 or 3 shots, small, ½ a shot. Over greasing is as big a killer as no grease at all. The grease needs room to move about in the bearing, too little and it gets pushed out of the bearing, too much and it clogs in the races, over heats and fries it’s self. 2/3rds full is the usual recommended amount, but again go by the manufacturers recommendations.

This may amuse you, going back years we had an old guy going around the plant doing nothing other than lubrication. He had a schedule to follow but was a bit over enthusiastic. One shot every 3 months became 2 a week. I got called to a motor that was tripping on overload, opened the terminal box and it was full of grease. The entire motor was full. Motors, fans, pumps, conveyors nothing escaped Juffy! So much for preventative maintenance, he must have cost us tens of thousands!

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I’ve done two drawings the first shows a standard set up with ball and roller bearings. The second is a foundry motor, where each bearing is in a separate cassette. Not the easiest to strip down.

Bearing brinelling:

Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the brinell hardness of the surface. It sounds nasty in more ways than one.

When a motor has been stood for a long period of time and subject to vibration the balls/rollers develop flat spots and indent the inner and outer races.

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I first came across this with the start up burner fans on kilns. They ran for a week continuously once in every six years the rest of the time they just sat there doing nothing. Came the time to relight a kiln and I’d IR tested the motor, given it a shot of grease and gave it a clean bill of health. Come start up, it sounded like a bag of bolts, the bearing were shot. Quickest fix was rob a motor off the next kiln along, that was just as bad.

The motors were stood on a solid concrete base but vibration had been travelled down the fan ductwork. After this episode all the start up fan motors were removed once they had done their stint.

Anything stored that has bearings needs to be in a vibration free area and the shaft given a spin every so often. This doesn’t apply only to motors, any rotating machine is prone to brinneling as RoB found out.

Years later with a different company there was a spare 800HP motor sat on the floor of the blower house. My shift were told to swap it with one of the duty motors that was giving trouble, I refused saying the bearings will be knackered. One of the other shifts changed it, the bearings were not healthy 😉

Two guesses who got lumbered with the bearing changes on both motors.

Bearing changes is for another episode.

Motor test bed.

Or the scrap heap challenge.

One company I worked at sent all motors out for repair. I wasn’t happy about this so I offered to do all repairs other than rewinds. (One of the things I wished I’d had the chance to learn). As time went on other shifts started doing repairs in slack times. We enjoyed doing them but none of us was happy about sending them out untested.

I suggested building a test bed, we were given the go ahead so long as it didn’t cost anything! There then started the scrap heap challenge. We had a lot of redundant kit dotted around the works, there was even an abandoned pump house with two 300A feeders.

We scavenged:

All the cable

A Telemecanique 250HP DOL starter panel

A variety of EE CFS switches from 30A to 300A

Getting matching CT’s and ammeters took time

6 way TPN 30A EE Red Spot fuse board

Light fitting, switches, etc.

Even the singles for the panel wiring were scavenged

The pump house had a EE CFS panel with two 300A incomers, bus-section and 17 outgoing feeds. (The total loading on this board was one CU feeding a single 60W lamp). The board got butchered in half. “A” section was kept pretty much as it was with the addition of the “new” red spot board. “B” section had its feed diverted in to the starter, the starter then fed directly on to the bus-bars. One tricky bit was getting the CT’s to fit between the bus-bars and the out going switches.

This is what we finished up with.

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I wanted to test run the 800HP motors that we had changed the bearings on. Under normal circumstances they were started by a closed transition Y∆ starter not DOL. Simple answer, replace the 300A fuses with copper links so the back up fuses were 600A and connect the motor in star. It worked a treat but as per usual everyone legged it before I pressed the start button.

© Tony S

Credited photographs © RoB

 
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