Pinion pittings in open girth gear 1

[rkarthik]

What are the possible causes of pitting in pinions. I am service Engineer and have seen cases where there is no pitting in girth gear but its only on pinions,especially , the pitting starts on the pitch line of the pinion. Its a through hardened pinion.The gear units has spray lubrication which are well monitored.The pittings found were of progressive types.
Secondly do anyone have knowledge regarding cracks in girth gear ?

 

[Carburize]

I suggest you get a copy of a reference document such as AGMA 1010-E95 "Appearance of Gear Teeth - Terminology of Wear and Failure" for a complete review.
One basic reason however, for the appearance of pitting on the pinion teeth before the gear shows any damage is the higher number of cycles to which the pinion teeth are exposed compared to the gear. The pitting process is a fatigue mechanism in which subsurface microcracks initiate as a function of the stress, material cleanliness, lubrication efficiency,number of load cycles etc. If everything else is identical in the gear and pinion material and loading the higher number of load applications will cause the pinion to show pitting before the gear.

 

[Metalguy]

Spot on, Carburize.

Perhaps those pinions should have been designed to be ah, er, carburized to a higher surface hardness than the through-hardened ones.

 

[macmil]

Re ypur item 2 crtacks in girth gear are you talking mill or kiln? And what sort of cracks ? teeth or right through ?
mac

 

[rkarthik]

I am referring mill system. The crack are especially near the root of the gear. I have recently come across two gear where the cracks were predominantly on one side or the gear one in the drive side and the other in non drive side.The misalignment is of minor order approx 0.05 to 0.1 mm. so i feel misalignment is not the reason.
In one gear the one of the tooth got totally cracked and for emergency reasons the gear was gouged and welded and is running at a lower capacity approx 50 % of load.
can you give me some inputs on the same so that i can have a look at them.
In one gear the crack has orginated from the root and has run upto the rim of the gear.

 

[Metalguy]

Hmmm, makes me wonder if you have a heat treatment (quench) cracking problem. I don't ever recall seeing a crack START on the unloaded side, but they sure can FINISH there.

 

[diamondjim]

You have not mentioned the duty cycles that
these have experienced. If it were to happen
early on in the gear life, it would seem strange
or indicate some heavy loading. I understand that
the gear is internal so you probably have a high
contact ratio of 1.4 or greater? What are the
relative hardnesses of the two parts. Was the internal
gear generated by a shaper type cutter or just
milled out? Is the rim thickness more than 1.2 times
the gear whole depths? I assume you have full fillet
radii on the gear and pinions. The phenomenom that you
mentioned about cracks starting in or near the
root is normal or at least the most likely.
You might want to look at recess action gearing and
see if that does not help or alleviate many of your
problems. Much has been written about these. They
are also called long and short addendum systems.
Keep in touch.

 

[rkarthik]

Mr.diamond jim
The gear is of short addendum type. the module is 28 and the addendum is 20mm. I am speaking about various type of gear , but surely off late i have experienced four relatively new gears hardly 3000-4000 hrs clocking. The hardness of gear is around 280 bhn that of pinion is around 340 bhn.
The gears has been generated by hobbing maching. Regarding rim thickness, i will let know after i see the drawing.I dont understand what is meant by internal. I am referring to open girth gear system and no.of tooth of pinion : gear is 28:190.
Now recently after i raised this question i came to know from a plant where kiln gear also has some minor cracks.The details for the same is not with me. Will let know the details as soon i get it, since now iam travelling.waiting for your valuable advice
r.karthik

 

[rkarthik]

Mr.diamond jim
The gear is of short addendum type. the module is 28 and the addendum is 20mm. I am speaking about various type of gear , but surely off late i have experienced four relatively new gears hardly 3000-4000 hrs clocking. The hardness of gear is around 280 bhn that of pinion is around 340 bhn.
The gears has been generated by hobbing maching. Regarding rim thickness, i will let know after i see the drawing.I dont understand what is meant by internal & secondly what is recess action gearing. I am referring to open girth gear system and no.of tooth of pinion : gear is 28:190.
Now recently after i raised this question i came to know from a plant where kiln gear also has some minor cracks.The details for the same is not with me. Will let know the details as soon i get it, since now iam travelling.waiting for your valuable advice. Is anyone experienced in repairing the girth gear and running it. Is there any reference to make regarding the welded girth gears after cracks.
r.karthik

 

[diamondjim]

r,
I applied for a job in a job shop in Canton Ohio
that repaired gears by making a replacement
tooth and then welded it in place. This was
big stuff and the gear quality was probably
more like agma 6. Farmers have been doing
this for years on their own equipment, so it
is hard to knock experience, but then their
is the liability factor. I did not get the job
but found the concept interesting but against
all that has been written in text books.

 

[rkarthik]

I dont understand what is meant by internal in your previous reply. Actually this is a open girth gear arrangement iam talking about & secondly what is recess action gearing. Can u explain me that

 

[diamondjim]

r.karthik,
Recess action gearing is often referred to as long
and short addendum gearing. If the addendum of the
pinion is enlarged by 50 percent, then the addendum
of the gear is decreased by 50 percent.
Any percent may be incorporated or used. It was first
done to balance the strengths of the two parts as
normally the pinion is the weaker member.
If you think about the pinion first coming in contact
with the gear until it contacts the pitchline, it
is working or pushing toward the center distance of the
two parts. This is called approach action. Conversely,
after the pinion makes contact at the gear pitch line
it starts withdrawing from the center distance or pulling
away and this is the recess action. The first is a digging
sliding action and the second a polishing sliding action.

With long and short addendums, you are increasing the sliding
actions. I think is was Dudley who described this as
thinking of using a stick and pushing it in front of you
verses dragging it behind you. The recess action is akin
to the dragging action. I hope this all makes sense to you.

 

[rkarthik]

Mr.diamond jim
Thanks for the exp.
The gear is of T type with the following spec
outside dia - 5808 mm
pcd-5766 mm
root dia -5673mmm
dia at rim - 5520 mm ( at the sides below the root)
dia of rim at the centre ( at where slots for weight reduction start - triangular slots )-5420 mm.
This is a dual gearing with both liftig and pressing side. Here the cracks started at the drive side and its helical type gearing.

 

[diamondjim]

r,
Your whole depth ratio seems excessive for
a hob cut gear. Normally the whole depth
with a nice fillet radius would be 2.25
x the module or 2.25 x 28 = 63mm.
I have seen greater ratios for shaped gears
or for gear hobbed with a protuberance hob
and then ground. Because of the hardness
that you listed, I assume the gears are
not ground. The larger radius with the
2.25 ratio whole depth is desirable to
reduce the stresses in the root. I assume
you have 206 teeth in the gear from the
5768 pitch diameter. The rim thickness is
more than adequate. How do you stop the
system? Stall torque can be in the magnitude
of 2 times the running torque and might explain
the crack on the supposed not running side.
How many teeth in the pinion and what is its
outside diameter? Sometimes an inclusion in
the steel will also cause this phenomenom.
You mentioned welding a pit in one of the teeth.
Did you then relieve the welded area by soft
grinding? Erichello teaches a gear failure
course at many seminars and really has some
good information to take back with you as well
as many pictures of gear failures and their causes.
It was one of the more valuable seminars that I
have attended. You will hear over and over again
lack of lubrication for many failures. This is often
true, but there are so many other factors that
cause the different types of gear failure.

 

[rkarthik]

Mr.Diamond jim
The foll are the datas:
pinion no of teeth - 26
gear no of teeth - 190
helix angle 8 deg 42 min
normal pr. angle - 20
pinion od - 867 mm
The mill is mounted on white metal bearings.The mill cannot have free rotation while stopping like as if its mounted on antifriction bearings ,but there is no brake provided to stop. The mill is not stopped by reducing the speed as the drive system is not designed so. The rpm of the mill is around 17 ie girth gear rpm.
Regarding relief after welding,Grinding was done to match the profile with a template.I couldnt get much information on net regarding Erichello papers.If you could send me the same, it will be greatly helpful.
this is my mail
r.karthik@flsmidth.com

 

[diamondjim]

I found this on the web
Gear Failure Analysis
Gear failures can cost thousands of dollars in lost production and replacement costs. This makes it critical to quickly identify causes of failure and implement solutions to prevent reoccurrence. This workshop teaches how to identify the primary failure mode, determine the root cause, and prescribe remedies to solve gear failures. Your seminar manual is a valuable troubleshooting guide that includes over 100 color photos of gear tooth failures and text describing definitions, mechanisms, and prevention of gear tooth failures. Also included is Robert Errichello's Gear Failure Atlas for fast access to the information needed for gear failure analysis.

Instructor: Bob Erichello, GearTech
Date: Monday, March 31
Time: 8:00 a.m. - 4:30 p.m.
Cost: $425
I do not work for Bob but took his coarse and would
highly recommend and encourage you to attend.
The cost is modest for the good info that you
will get.

 

[teb1]

It has been my experience that most girth gear cracks are usually the results of some abnormal operating condition, not the original design of the gear. Most of the time the bolts attaching the gear to the kiln or mill become loose. This allows the gear to flex more than it should. This results in a crack that starts in the root of the tooth and travels through the rim of the gear into the web and usually all the way through web.

In your first post you requested information about cracking in girth gears. I have been envolved in the repair of several girth gears with different types of cracking. If you are interested in additional information about these repairs please contact me directly at metalock@eatel.net.

 

[PaulJuhnke]

Just became a member. Sorry to be so late in responding to your question.

I’ve worked with girth gear problems in many plants, on various rotary kilns & ball mills (up to 5000 hp). The previous responders are spot on regarding the cause of pitting.

There are some additional methods for correcting pitting on the girth gear regarding carburized pinions and machine in place technology. Had good luck with that procedure on a 4000 hp ball mill over 4 years ago, which saved a $600,000 girth gear reverse, and also saved the back side of the girth gear for the future reverse. Still running well, with low vibration, etc. The service companies involved have done over 50 girth gears over the last 15 years.

Regarding your tooth root cracks: Sounds like you have axial mis-alignment, which will cause alternating side root cracking. We developed a method to shim large girth gears (60 ton - 26’ diameter) to correct axial mis-alignment from 0.120” to within 0.010” total axial run-out. Another service company has an excellent, inexpensive method to look for root cracks in big gears.

Welding gear teeth is possible in many applications. When designing the repair the base material should be thoroughly investigated to better plan the welding. Some older cast gears respond better to mechanical / welded connections using high-strength (100,000 psi) silver solder. Other materials are readily weld-able.

If you need more information, call me at 641-423-9363 with more details about your application. Good Luck, Paul Juhnke, Maintenance Engineering Services

 

[beloka]

Re Pinion pitting
One of the main reasons for mill gear and pinion problems has been incorrect lubricant and/or incorrect application of lubricant. Misalignment of pinion with the gear can make the problem much worse. It is essential to get both lubrication and alignment right to avoid problems.
Gear sets are usually designed on the basis that lubricants will have certain properties. It is always advisable to make sure that the lubricant you are using has the necessary properties. Lubricant suppliers will often recommend what they call equivalent lubricants but in many cases they have fallen short of the gear/pinion lubrication design requirements. Some lubricants require much more frequent or even continuous applicationtion. There is a tendency for many operators to cut back on application frequency to save money but this can precipitate drastic failure. You can readily check the effect of misalignment using infra red temperature measurements. i.e. It is now common practice with large mills to install multiple permanent temperature probes above the pinion for checking this. The probes can provide indication of misalignment as well as lubrication failure. They can alert an operator to a potential problem before it becomes a major failure issue. Your girth gear is an expenive item and protection equipment such as infra red equipment is not expensive.
Re Girth Gear
Non symetrical girth gears (i.e differing rim thickness) can deform with thermal expansion and result in misalignment during operation. This has been known to occur and can become progressively worse. This problem may not be identified during a cold alignment check.
The girth gear cracking usually initiates from a defect in the gear. This can be an internal or and external surface defect. Competent ultrasonic examination, magnetic particle and dye penetrant inspections on a regular basis can identify potential problem areas and in many cases steps can be taken to prevent propagation of cracks and total failure. Before attempting repair welding of defects in girth gears it is most advisable to evaluate the risks and the options available very carefully. e.g. Can the defect(stress raiser) be removed by carefull grinding??. Will the weld itself result in a coarse grained structure that will readily crack. Will the weld be a full penetration weld or a partial penetration weld with an inbuilt crack that can propagate.?? Is the defect of a size and located in such a location that repair is not needed.?? Can the defect be monitored.?? Will the repair require changes to the way equipment is operated (loaded).??
If foreign matter or objects pass through the gear /pinion mesh it is very likely that damage will occur. Careful examination of broken teeth may reveal the root cause of any falure problem.