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Author Topic: What causes surging?  (Read 786 times)

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Offline Old Crow

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What causes surging?
« on: May 14, 2018, 11:09:41 pm »
I have an old maroon LMS GP tank; big flanges and brass gears. Yes it's an old clunker but I'm quite fond of it. It has always been a bit arthritic but lately it is "surging" ie suddenly speeding up or slowing down jerkily - certainly not a smooth runner. I've had a lot of success with other sluggish locos by cleaning the commutator and very sparingly oiling the bearings but this old dear is definitely a bit doddery and doesn't seem to be improved.
What causes this surging behavior and does anyone have any solutions?

Offline dannyboy

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Re: What causes surging?
« Reply #1 on: May 15, 2018, 12:12:51 am »
I can not answer your query Old Crow, but I know exactly what your maroon clunker is going through!   ;D
David.
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Offline xm607

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Re: What causes surging?
« Reply #2 on: May 15, 2018, 09:00:11 am »
If its got a plastic main gear, it could be split, or one of the wheels moved on axle, or the motor has one of the winding's going down.
Steve.

Online Snowwolflair

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Re: What causes surging?
« Reply #3 on: May 15, 2018, 09:57:24 am »
Carbon brush deposits clogging the gaps on the commutator.  It builds up and acts as a resistance bridge and the loco slows, then it burns away and it speeds up.  On a really old motor very common as the carbon mixes with oil to form a greasy sludge. 

Best way to fix it is a detail toothbrush and gently scrub the commutator, particularly the gaps between the copper plates.  If tough a little meths will help dissolve but let it dry completely before power is applied as it might catch fire  :)



Offline Dr Al

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Re: What causes surging?
« Reply #4 on: May 15, 2018, 10:39:01 am »
Carbon brush deposits clogging the gaps on the commutator.  It builds up and acts as a resistance bridge and the loco slows, then it burns away and it speeds up.  On a really old motor very common as the carbon mixes with oil to form a greasy sludge. 

It's likely this - however, there should not be oil on the commutator - it's common on old locos that people have sprayed with lubricant through lack of knowledge.  So don't add any after cleaning - the commutator should be completely dry - the carbon brushes are ok without lubricant.

Where it does need lubricated are the bearings on the end of the armature shafts. This is close to the commutator at one end, so care is needed.

Also, if the loco is very old with a silver metallic magnet, replace it with a new grey one from BR Lines - the silver ones get very weak in time, and this causes current consumption (and therefore heating) to go through the roof. As an example, I did this recently to an old GP tank, and the current consumption went down from 300mA to about 125-150mA, just due to the improved magnetic field! Excess heating is the primary cause of armature failure on early models - this helps prevent that.

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

Online Snowwolflair

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Re: What causes surging?
« Reply #5 on: May 15, 2018, 10:46:48 am »
Carbon brush deposits clogging the gaps on the commutator.  It builds up and acts as a resistance bridge and the loco slows, then it burns away and it speeds up.  On a really old motor very common as the carbon mixes with oil to form a greasy sludge. 

It's likely this - however, there should not be oil on the commutator - it's common on old locos that people have sprayed with lubricant through lack of knowledge.  So don't add any after cleaning - the commutator should be completely dry - the carbon brushes are ok without lubricant.

Where it does need lubricated are the bearings on the end of the armature shafts. This is close to the commutator at one end, so care is needed.

Also, if the loco is very old with a silver metallic magnet, replace it with a new grey one from BR Lines - the silver ones get very weak in time, and this causes current consumption (and therefore heating) to go through the roof. As an example, I did this recently to an old GP tank, and the current consumption went down from 300mA to about 125-150mA, just due to the improved magnetic field! Excess heating is the primary cause of armature failure on early models - this helps prevent that.

Cheers,
Alan

Yes new magnet is a very good idea, also I have found PTFE lubricant is better than mineral oil, its less sticky and resists dirt buildup.

Offline Old Crow

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Re: What causes surging?
« Reply #6 on: May 16, 2018, 12:02:31 am »
Many thanks for the replies. Yes! I will have a good clean of the commutator especially the gaps; can I use alcohol to clean it? Not sure I have the skill to change the brushes though.
Interesting the magnet comment. My old Poole  Duchess of Hamilton does tend to run much warmer than any other loco.

Online Snowwolflair

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Re: What causes surging?
« Reply #7 on: May 16, 2018, 08:59:58 am »
Make sure its a pure alcohol not a surgical spirit as sold in chemists which also contains castor oil

Offline Old Crow

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Re: What causes surging?
« Reply #8 on: May 17, 2018, 12:19:25 am »
What I have is Isopropyl - or as I call in "inappropriate" alcohol! Would this be.....appropriate?

Online Snowwolflair

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Re: What causes surging?
« Reply #9 on: May 17, 2018, 12:21:44 am »
Yes that is fine, just keep it away from the paint on the loco body.  :)

Offline Old Crow

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Re: What causes surging?
« Reply #10 on: May 17, 2018, 11:05:13 pm »
Thanks for the tips guys.

Cleaned up my maroon GP tank and very carefully (tiny drop on a pin) oiled the bearings. Yes, it's improved but she's still a bit wobbly on her pins. She must be over 30 years old so what else might one try? (Not the humane killer!) What about new brushes? motor? Can bearings become so lose that the worm doesn't engage the drive gear properly?

On the other hand..... took apart a sluggish 2-6-4 Poole tank. Given how hard it was to separate the body and chassis I guess it's had never had a service and yes, the commutator was pretty filthy. Small stiff brush and isopropyl alcohol and oiled the bearings. Very considerable improvement and the noise level much reduced. Not as good as the later model but very usable now.

How do you change the magnet? It looks to me as though the two sides of the motor that contain it are riveted.


Offline ntpntpntp

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Re: What causes surging?
« Reply #11 on: May 18, 2018, 12:17:30 pm »
When I service a motor I remove the brushes and gently roll them in kitchen paper between my fingers, and clean out the brush holders/slots to remove any carbon dust and grease/oil that may have crept in. Sometimes I gently clean and re-profile the end of the brush. I check the brushes aren't worn too far, check spring tension etc.   It all helps toward optimal motor performance.
« Last Edit: May 18, 2018, 12:22:44 pm by ntpntpntp »
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Offline Dr Al

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Re: What causes surging?
« Reply #12 on: May 18, 2018, 10:41:07 pm »
I think you need to be more specific of exactly the symptoms - wobbly in my interpretation has not much to do with the motor, rather the wheelsets. Check the side rods are straight there.

In terms of magnet, get a new magnet and use the strength of the magnetism to clean lift out the old magnet from between the pole pieces. May need a little assistance to free it using a thin blade. If that doesn't work, then just lever off the pole piece on the side opposite to the rivet heads. Reform the rivet tails with pliers to be cylindrical and just hang the pole piece back on them using them as pegs (like later models) - the magnet will hold them in place.

Only change the brushes if they are actually worn out - changing them otherwise is completely pointless, yet the number of folk that assume "change the brushes" will cure everything is still large. In 20 years, I've seen very few locos where the brushes are actually far enough worn to need replaced.

You'd know if the drive gear wasn't engaging as that would be periodic with the wheel rotation to start with, and get worse with time until the worm was totally worn through.

One other thing to check with the really old early models are the pickup wipers - they were quite stiff originally and if not set right would either exert too much force and add a lot of drag, or be completely off the wheel backs and therefore non functioning.

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

Offline Old Crow

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Re: What causes surging?
« Reply #13 on: May 18, 2018, 10:58:59 pm »
Thanks Al. My limited servicing has improved matters but the GP tank is not really a smooth runner. Symptoms mainly "surging" or "hesitating" momentarily as though someone were applying and releasing a brake. Also sometimes hesitant to start. Don't know about it being related to the motor as it's rather random and more like a momentary electrical disconnect. I'll have a look at the pickups and see if I can clean the wheel backs.

Offline Hailstone

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Re: What causes surging?
« Reply #14 on: May 18, 2018, 11:11:29 pm »
this can also be caused by over tightening the screws that hold the keeper plate, which includes the one that goes to the nut holding down the wire at the front of the motor.

Regards,

Alex

 

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