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Anonymous Poster #1

Girth Gear RCFA's

09/16/2011 10:46 PM

Good Morning,

In May 2011 I experienced a girth gear teeth failure on the joints on one of my Ball Mill and recently I just experienced the more or less the same failure on my other Ball Mill. See Mill Spec and Attched Pictures.

ROM SAG Mill

Mill Type

Grate discharge, dry grinding, ball mill.

Construction

The mill is constructed from a fabricated shell, bolted by

Means of flanges to cast steel ends with integral

trunnions which are supported on hydrodynamic white

Metal bearings.

Drive

The mill is driven by a high-speed electric motor via a

Gearbox, pinion and girth gear. The girth gear is fitted

The mill shell flange at the discharge end.

Process Data

Feed Description : Chrome Ore (80%) + Fine Anthracite (20%)

New Feed Rate : 85 tph

Ore Specific Gravity : 2.0 - 2.4 t/m³

Bond Work Index : 22 kWh/t max

Volumetric Loading : 35% of mill volume

Ball Charge : 30% of mill volume (nominal)

35% of mill volume (maximum)

Feed Size : 10 mm (80% passing)

Product Size : 45 μm (80% passing)

Circulating Load : 0%

Effective Grinding Length : 13543 mm

Inside Diameter : 4000 mm

Length - Flange to Flange : 14000 mm

Plate Thickness :50 mm

Plate Specification :SABS 1431 Grade 300WA

Flange Specification :ASTM-A516 Grade 60

Flange Configuration :Straight

Manholes :3

Head / Trunnion Specification Cast Steel to BS 3100 Grade A2

Main Bearing Size :Ø1700 mm x 810 mm wide

Mill Speed (rpm / % critical) :15.71 rpm / 73%

Motor Speed :994 rpm

Mill Motor Power :3000 kW

Feeder Type :Ø400 mm Bore Cast Pipe

Gearing

Gear Type :Cast Steel Single Helical

Pinion Type :Single Helical Integral to Shaft

Teeth (gear / pinion) : 194 / 21

Normal Module :30

Face Width :660 mm

Pressure Angle :25°

Hardness (gear / pinion) :290 - 330 BHN / 58 - 60 Rc

Helix Angle : 9° 3'

AGMA Service Factors : 1.5

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#1

Re: Girth Gear RCFA,s

09/16/2011 11:04 PM

Maybe the last line is the key? I don't even have a rule-of-thumb guide in front of me, but I think I've seen service factors of around 2.5, rather than 1.5, for ball-mill and similar duties.

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#4
In reply to #1

Re: Girth Gear RCFA,s

09/17/2011 2:00 PM

What service factor would you recomend to our application as per specification provided?

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#10
In reply to #4

Re: Girth Gear RCFA,s

09/17/2011 2:52 PM

I'm thinking 2.5, but my memory is uncertain.

Many manufacturers of drive components have an engineering section in their catalog that suggests service factors depending on running time, shock loading, heavy start-up loads, etc. A ball mill is rather severe in these regards, hence a generous SF.

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#2

Re: Girth Gear RCFA,s

09/17/2011 1:00 AM

What is the recommended 'backless' in the gearing? I suspect following reasons for such failure.

- too low or too high backless.

- eccentric centre line (uneven backless on two ends of gear width).

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#5
In reply to #2

Re: Girth Gear RCFA,s

09/17/2011 2:05 PM

I believe it should be minimum as possible (<0.5mm).

I believe the second reason suspected is the challenge we are facing, but how do I validate that?

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#19
In reply to #5

Re: Girth Gear RCFA,s

09/18/2011 7:30 AM

I believe it should be minimum as possible (<0.5mm).

No, it can not be as minimum possible. It depends on gearing type, size and design, its range should be be available in OEM's "operation & maintenance manual". Normally I found the range between 0.4 to 1.0 mm.

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#9
In reply to #2

Re: Girth Gear RCFA,s

09/17/2011 2:47 PM

GA

(Minor editorial note: it should be "backlash" rather than "backless".)

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#20
In reply to #9

Re: Girth Gear RCFA,s

09/18/2011 7:32 AM

Thanks Tornado, you are right, I wrongly spelled it.

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Anonymous Poster #2
#17
In reply to #2

Re: Girth Gear RCFA,s

09/18/2011 5:22 AM

i'm unsure if back lash would a major factor when the gears are always moving in a forward direction under load. i'm having difficulty seeing a clear image of the gears. clearer photos of ring-pinion gear wear patterns and gear damage would help. a good mechanic familar with heavy equipment are great at diagnosing gear mesh promblems.

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#21
In reply to #17

Re: Girth Gear RCFA,s

09/18/2011 8:17 AM

Yes, its a major factor. Backlash has nothing to do with uni-direction or both direction rotation. For checking backlash, gear teeth at the line of mesh should be visible or approachable. Just move pinon little backward to find a gap between the teeth of gear and pinion, measure the gap (backlash) from both the sides with the help of feeler gauge. It can be measured using dial gauge also with more accuracy but it will give minimum gap. Difference in values on two sides indicate that gear and pinion are not properly aligned.

This can be also found by checking gear contact (another important parameter). It will look like a rough and very thick straight line. Following shall be the interpretations with respect to gear contact:

1. If it is equi-distant from top and bottom of tooth face and also from both the sides (above 60% length of the gear width), then backlash as well as alignment is good.

2. If contact line is only one side of the width (ticker on the edge), it indicate that gear and pinion are not properly aligned. Load will transmit from one side leading to teeth wear and failure.

3. Contact line nearer to tip of the teeth face indicate more backlash. Load will transmit from edge of the teeth (which is less thick) and moment of the force at the root is higher which may result crack & uprooting of teeth.

4. Contact line nearer to root of the teeth face indicate less backlash. Load will transmit from near the root of the teeth. It will result in more noise and faster tooth wear, ultimately failure of teeth.

I hope, you will be convinced that it is a major factor for all gear transmissions.

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#23
In reply to #17

Re: Girth Gear RCFA,s

09/18/2011 4:32 PM

This is all i have.See sheared off Pinion Shatf aswell.

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#25
In reply to #23

Re: Girth Gear RCFA,s

09/18/2011 6:14 PM

This requires a field visit from the mill maker or some person of comparable expertise to analyze the total situation to get it permanently fixed. Mills are very very expensive, and work at a high energy and must be operated and maintained and inspected properly. Fixing a break like this will cost more than 20 years of proper maintenance.

The procedure of inspection and maintenance on daily, weekly, monthly, and annual basis need to be changed with the guidance of the maker of the mill. You either do it right or have a breakdown like this every few years. Which is cheaper? remember, you will also lose a large amount of production, and may have to shut down the mine if you are unable to stockpile muck, and you will have no cash flow from processed minerals during this.

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#27
In reply to #25

Re: Girth Gear RCFA,s

09/18/2011 7:01 PM

I like your post aurizon but if i was the owner i would also be talking to a lawyer to see if the manufacturer can be aggressively encouraged to not just assess the failure and advise on its repair but to also provide financial compensation for some if not all of the repairs.

If the operator didnt overfill or overload the machine then it comes back to design failure with that < 5 year operating life .

$ 100,000 for a new girth gear

$ 20,000 for new gearbox shaft and rebuild labour costs

duplicate the drive with a second gearbox and electric motor to reduce the gear load

lost production costs might be the most significant cost as you say.

what country is the machine in ?

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#28
In reply to #27

Re: Girth Gear RCFA,s

09/18/2011 7:34 PM

HeadsUp, I suspect they may have skimped on maintenance, as per the usual schedule, which has the first line an operator as well as a daily mech inspection who listens every day with care because faults like this usually start slowly and build - unless someone left a tool in the gears or left a port open and rocks entered.

As for lawyers, if this was installed professionally they would have set up a full maintenance and support procedure to back up their warranty. If that process has not been followed and it is easy to show bad maintenance, then lawyers will lose. If a bunch of guys wrested it off a truck and installed it and are now long gone, well the mine gets to do it over, with more care.

As they say, there is never enough time for proper maintenance, but there is always time for fixing it at far greater cost. A lesson may be learned here and people might get dismissed

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#29
In reply to #27

Re: Girth Gear RCFA,s

09/18/2011 8:26 PM

South Africa.

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#35
In reply to #29

Re: Girth Gear RCFA,s

09/18/2011 9:04 PM

who is the manufacturer ?

how much did it cost to repair the damaged girth gear when the first one failed by the way ?

when the first ball mill failed , did the gearbox shaft also snap like the current one ?

thanks and good luck :)

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#38
In reply to #35

Re: Girth Gear RCFA,s

09/18/2011 9:16 PM

FLSMIDTH

About R 3,809,000.00

May 2011:yes It did fail

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#39
In reply to #38

Re: Girth Gear RCFA,s

09/18/2011 9:28 PM

R3,809,000.00, (~$80,000 US$) sounds cheap for that job. That makes me wonder if they are using low grade locally made repair materials instead of the proper imported goods. I understand India has very high duties, so the true materials might cost R15,000,000 with duty and installation and alignment efforts.

You need to find who made the mill, and who made the gearbox, Indian, USA, Europe?? If it is made locally, you can expect this type of repair every 5 years or so forever, so in 20 years you will have spent a lot and lost a lot of production. A properly designed, made and installed mill could easily run for 20 years.

With this type of problem, you might want to equip with some mill spares to reduce the down time

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#40
In reply to #39

Re: Girth Gear RCFA,s

09/18/2011 9:33 PM

Rand not Rupee #29

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#42
In reply to #39

Re: Girth Gear RCFA,s

09/18/2011 9:43 PM

R3 809 000.00~5130237USD and it does not sound cheap to me.

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#43
In reply to #42

Re: Girth Gear RCFA,s

09/18/2011 9:48 PM

No, if it is Rand, it is correctly priced. the $80,000 seemed too low, and that made me think of low grade parts that break fast.

Well, in South Africa they know better than to buy badly made parts, so it must be designed/installed badly

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#44
In reply to #42

Re: Girth Gear RCFA,s

09/18/2011 10:23 PM

US$5M ought to be enough to buy a whole new unit. Something doesn't sound right here.

But just to get an idea of scale, how many millions of tons of rocks need to be crushed to get one ton of diamonds? Or is this about something altogether different?

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#45
In reply to #44

Re: Girth Gear RCFA,s

09/18/2011 10:38 PM

A mill like this is quite costly. What is the tons per day or this mill, and the megawatt rating? In addition, buying new parts, shipping them to South Africa, sending in millwrights to open up the old, remove broken parts and install new parts, followed by a testing period can be very costly. I also understand the the SA government charges high duty on such imports as a cash grab - someone advise here what %, if known.

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#46
In reply to #45

Re: Girth Gear RCFA,s

09/18/2011 10:55 PM

One Mill has maximum production rate of 100 tons per hour of fine particles material at a mesh of 92. Ranges between 2.4 and 2.6MW.End product are Ferrochrome (FeCr).

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#47
In reply to #46

Re: Girth Gear RCFA,s

09/18/2011 10:58 PM

Yes, then the cost is indeed high. Do you have any idea what made it so costly? and how well maintained and inspected is this mill?

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#48
In reply to #46

Re: Girth Gear RCFA,s

09/18/2011 11:21 PM

That being the case, and per my previous comment, I'm less sure helical is the right drive choice. Perhaps they didn't realise they are weaker and should have de-rated the throughput.

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#41
In reply to #38

Re: Girth Gear RCFA,s

09/18/2011 9:39 PM

american company supplied it

i would get analysis as i mentioned of the metallurgy , spectro and heat treatment / hardness testing

then talk to a legal expert

considering the big $$$ i would look for the possibility of applying legal pressure

regardless of it being 5 years old and outside of warranty , a lawyer might see ways to obtain compensation

two identical failures to me say its a design stuff up , my first port of call is get support from FLSMIDTH , i would be looking at dual drive , dual gearbox , dual pinions

or scrap the machines , claim it on insurance and buy better ones

good luck

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#49
In reply to #27

Re: Girth Gear RCFA,s

09/19/2011 7:52 AM

How you can claim financial compensation after 5 years of installation and use of the mills supplied by them, unless it is mentioned somewhere in the contract. For such cases OEM should be contacted first for their advise, they are the best people to suggest corrective action. It appears from the OP that still OEM's opinion is missing. As far as I know FLSMIDTH's record is satisfactory as a EPS contractor.

In my previous posts I have mentioned that improper backlash and alignment (parallelity between axis of gear and pinion) may cause such failures. I also suggested how to check. So far OP has not given, what is recommended range of backlash? It is not clear from OP that whether original settings were disturbed by the user during maintenance. If not disturbed, then foundation sinking or design problem can be suspected as some members commented.

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#51
In reply to #49

Re: Girth Gear RCFA,s

09/20/2011 4:49 AM

my experience has been that even 10 years after sale , financial compensation can be obtained under certain circumstances .

this also applies to liability on the supplier or manufacturer for fault or failure causing injury or death to the end user.

i work in diagnostics , and i am surprised there has been no other comments on metallurgical testing of the failed parts.

i have seen a case where a manufacturer has been unable to obtain a grade of steel they have used in a machine for 20 years due to demand from an emerging market , they have switched to a lower grade and upped the heat treatment to compensate , leading to premature failure and an expensive redo .

diagnostics by elimination still puts metallurgical testing at the top of the list for me.

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#54
In reply to #51

Re: Girth Gear RCFA,s

09/20/2011 9:05 AM

"i am surprised there has been no other comments on metallurgical testing of the failed parts."

#13?

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#56
In reply to #51

Re: Girth Gear RCFA,s

09/22/2011 12:44 AM

GA Headsup, it may be possible in some rare cases where suppliers accept to replace the material (if proved due to faulty workmanship or material) beyond the warranty period, just to keep or boost their image. But compensation to damage may be possible through an arbitration process.

Off late we see some car manufacturers recalling thousands of cars sold to replace some sub-standard parts.

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#30
In reply to #25

Re: Girth Gear RCFA,s

09/18/2011 8:29 PM

This requires a field visit from the mill maker or some person of comparable expertise to analyze the total situation to get it permanently fixed.

Who do you recomend?

Anywhere in the world would be fine,arrangement will be considered.

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#31
In reply to #30

Re: Girth Gear RCFA,s

09/18/2011 8:34 PM

After five years, any original warranties are likely expired. However, the manufacturer (if still in business) may be able to advise on repairs and/or upgrades. I would look to them first, if possible.

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#37
In reply to #30

Re: Girth Gear RCFA,s

09/18/2011 9:12 PM

something to consider , before talking to the manufacturer.

take the damaged tooth to a metallurgist and have it tested for hardness to see if the heat treatment was done properly , and also get a spectroscopic analysis done to see if the material has the correct composition

a spectro analysis costs us around $ 70 - 110 AUD , hardness test we do ourselves with a portable tester or a lab will charge around $ 50 - 100

i have discovered cases before in my diagnostic work where shafts were never heat treated at all because a manufacturer messed up , my report forced them to replace every pin and bearing bush on a machine at their cost. ( big $$ )

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#26
In reply to #23

Re: Girth Gear RCFA,s

09/18/2011 6:48 PM

sheared shaft and teeth ripped out says it is being overloaded or not designed for the density of product being used

when you have new girth gear and shaft manufactured , you might want to look at the feasability of adding a second drive motor and pinion gear so you have two 1500 kW drive motors with their own pinion gear to reduce the load on the teeth.

did the shaft have two support bearings or was it an overhung load ?

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#32
In reply to #26

Re: Girth Gear RCFA,s

09/18/2011 8:36 PM

did the shaft have two support bearings or was it an overhung load ?Yes it had two bearings.

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#3

Re: Girth Gear RCFA,s

09/17/2011 2:50 AM

has the foundations sunk? check gear alighnmenmt.

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#6
In reply to #3

Re: Girth Gear RCFA,s

09/17/2011 2:11 PM

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#7
In reply to #3

Re: Girth Gear RCFA,s

09/17/2011 2:11 PM

Yes it is a misalignment situation, since the pressure distribution across teeth is uneven aswell.

The question is what causes that, since it's on both Ball Mills.

It is worth mentioning that both Ball Mill are less than 5 years in operation.

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#15
In reply to #7

Re: Girth Gear RCFA,s

09/18/2011 2:55 AM

the weight of the mill combined with the compaction factor as the balls tumble to the bottom of the drum adds up too an enormous compaction vibrator that requires specially designed foundations. re-alighnment is probably a waste of time. my quess is that you will have to disassemble the mills, tear out and replace the foundations. contact an engineering co. that is able to take core samples and design the correct foundations

where's captmoosie when we need him?

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#16
In reply to #15

Re: Girth Gear RCFA,s

09/18/2011 4:18 AM

do you think the gears have been mis-alighned since the mills were first installed. if so, this may be an easy fix as long as no foundation movement is involved. lateral and elevation adjustments are fairly simple with slotted anchor bolt holes and elavation shims. hire an engineer and contact a company that's familar with moving heavy machinery.

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#33
In reply to #16

Re: Girth Gear RCFA,s

09/18/2011 8:50 PM

do you think the gears have been mis-alighned since the mills were first installed.Since well its the third time of its occurence,then i would say yes.

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#24
In reply to #15

Re: Girth Gear RCFA,s

09/18/2011 4:43 PM

How do a proove that the foundation actualy sunk.

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#8

Re: Girth Gear RCFA,s

09/17/2011 2:13 PM

It is worth mentioning that both Ball Mills are less than 5 years in operation.

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#18
In reply to #8

Re: Girth Gear RCFA,s

09/18/2011 5:35 AM

is it possible that material is falling on the ring gear. the boken tooth appears to have had swallowed something hard. i'm assuming that you're keeping the gears well greased.

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#11

Re: Girth Gear RCFA,s

09/17/2011 10:55 PM

inspect the bed plate grouting. If the grouting has deteriorated, then the heavy vibration might gradually cause a misalignment.

Ask an experienced millwright, with SAG experience to advise you.

SAG = semi autogenous grinding, which means a mix of rock and some steel balls are allowed to tumble inside the rotating mill and atrit the rock to the point where it can exit. They come in various sizes but some of these machines are among the largest machines made by man. very special and costly knowhow, that you may not find here is needed.

SAG Mils

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#12
In reply to #11

Re: Girth Gear RCFA,s

09/18/2011 12:02 AM

Can deterioration on bed plate result in teeth failing only on the joints as per situation I am facing?

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#13

Re: Girth Gear RCFA,s

09/18/2011 1:35 AM

The nature of a helical gear is the contact point migrates across the tooth progressively, then the next engagement takes over the load.

These cracks appear to be from excessive root stress. I see little bruising or peening associated with shock or excessive backlash. It may be alignment is causing a loss of drive on the tail of the previous tooth, resulting in a shock load on the lead of the next.

It would have to be a very measurable misalignment.

I would think these gears are being over-driven, or have metallurgical issues. However it is definitely related to the right hand side of the gear/engagement.

I would be thinking it terms of checking the alignment, but particularly the material properties across the face and looking for variation in the casting.

If that is a crack forming at the next root, central to the tooth, I'd say it's a 50/50 on over-driving or metallurgy.

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#14

Re: Girth Gear RCFA,s

09/18/2011 1:57 AM

did you measure the alignment of the pinion gear axis to the axis of the girth gear ?

is there any slop in their bearings which could cause misalignment when loaded ?

misalignment may not be evident until load is applied

other question i would look at is flexing of the drum itself where the girth gear is attached , ie , when the loaded mass drops onto the lowermost part of the drum , does the structure flex , and in turn transfer that flex into distortion of the gears

the gears being a hardened material wont rebound and stress cracks will occur

or it could be a combination of both of the above factors

can i see a photo of the end of the drums so i can get an idea of any presence of visual indicators that might show flex lines

look for fissuring in painted surfaces , or fissures and " cracks " in rusted surfaces , or elongation of bolt holes

twisting of the drum is another thing to check for , if support rollers are not taking equal mass between them as raised earlier with mention of sinking foundation pads

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#34
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Re: Girth Gear RCFA,s

09/18/2011 8:57 PM
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#36
In reply to #14

Re: Girth Gear RCFA,s

09/18/2011 9:09 PM

more or less the same

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#50
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Re: Girth Gear RCFA,s

09/20/2011 4:03 AM

How can the startup torque have effect on this situation?

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#52
In reply to #50

Re: Girth Gear RCFA,s

09/20/2011 4:56 AM

yes the start up torque would be a factor to examine too

what kind of controller does the motor have ?

soft starter / variable torque ?

have rotating parts been checked for lockup interference anywhere that might throw a load impulse back through the drive ?

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#53
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Re: Girth Gear RCFA,s

09/20/2011 5:05 AM

Motor starts on Liquid starter then short circuits to direct online.

We conducted a strain gauge test and experienced high startup torques, which I belie they are not contribution much on the girth gear failure.

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#22

Re: Girth Gear RCFA,s

09/18/2011 11:09 AM

The line that is an indication of broken tooth starts 4 times exactly at the transition from flank to bottom of the tooth.

And the bottom seems to be very shiny and with much too sharp transition between flank and tooth thus generating a stress intensity factor of likely 5 or above.

The flank to flank transition has to be a smoothly rounded segment of a circle or ellipse.

It seems to be (by the shiny appearance of the bottom and the location of the shiny contact zone) that the contact is much too close to the bottom.

This may be a fabrication error - too thin teeth, or may be an error of too narrow backlash adjustment - causing contact of the tops of the pinion teeth to the bottoms between any 2 gear teeth.

Regrind all the bottoms between any 2 flanks to remove any contact there and do this very well rounded to lower the stress intensity factor. (In new status if contact zone is as here shown)

If this is not suitable then rework the outer diameter of the pinion so that the contact does not reach out to the bottom between any two teeth. Make sure a very smooth transition of non-contact to contact.

Below the bigger crack there is a smaller one (or newer one) starting also at the transition of (upper in the photo) flank to bottom.

So I conclude: in an attempt to minimise backlash pinion and gear are adjusted too tight. This causes touching of bottoms of gear by tops of pinion. And this causes much too high stress at bottom to flank transition by notch at the transition.

A new gear shall be made with considerably thicker teeth and a perfect transition between any two teeth.

Maybe a misalignment is existing too. To judge about this more photos of this crack showing the situation over the full length of this cracked tooth will be necessary.

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#55

Re: Girth Gear RCFA's

09/20/2011 9:22 AM

The photos clearly indicate massive overload at the right side of the photos - so some misalignment is very probable.

The photos indicate also that all the cracks originate at the zone of highest stress intensity so either material or heat treatment was not ok or the grinding there was not ok.? These areas shall have compression stress - not too likely in grinding.

So there has been massive overload: why and when and how did this happen? Minimum once maybe 4 to 5 times deducted from the fractures.

These fractures are not induced by vibrations but by overload in brittle material.

The broken pinion and other massive damage is not of interest as secondary to tooth failure.

The comments in my above post are still valid.

RHABE

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#57
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Re: Girth Gear RCFA's

09/22/2011 12:50 AM

GA for your both comments.

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#58
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Re: Girth Gear RCFA's

09/22/2011 6:31 AM

regarding the crack propagation beginning at the right side of the teeth.....

this could be due to overload / excessive start up torque causing momentary distortion of the pinion shaft and or distortion of the bearing support frame in relation to the girth gear axis

it doesnt have to be misalignment in the mountings or base

visualise the bending affect as a torque reaction , and you could have a momentary twisting of the pinion assembly pushing away from the girth gear , with the path of least resistance being the shaft pushing downwards by 2 - 3 degrees , being enough to create the fracture origins evidenced in the photos

The gearbox would be a more solid structure and is less likely to yield than the pinion outrigger bearing and its mount.

think about it carefully people :)

visualise it in your mind or do a sketch

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