Girth Gear Repair

This is probably a flaw in the design or material fatigue. To find a good solution you should make some pictures and post them here. It might find a solution.

Thanks for the info. Apologize for missing a point, the gear has been in operation since last 11 year and the problem observed since last few months only. The gear is with single web and cracking observed ahead of spring plate pin. We were unable to figure out reason of gear crack after joint bolt over tighten. And, how to operate the gear without problem.

What is the composition? Are the cracks affecting the teeth too? There are many different ways to build a girth gear. I need more details to propose a fix. A detail about the defective part could help. The direction of the crack might show a lot. What is the mass in motion?

I hope you can copy, paste and view this in your browser. With the data you have given there is still a big possible selection. If one pictured is about like yours, we can move on. Good luck.

http://www.google.com/images?hl=en&client=gmail&rls=gm&q=girth+gear&um=1&ie=UTF-8&source=univ&ei=a39OTMb5IoKB8gbrmNzJBA&sa=X&oi=image_result_group&ct

Reason for failure is not properly investigated. Reason for crack in gear may be increased tightening. To avoid bolt failure you opted for inceased tightenig (double the recommended value) which is wrong. You should have investigated reasons for bolt failure instead of increasing tention elsewhere. After all it is in operation since last 11 years. The weakest link to fail shifted from bolt to gear.

Repair of moving and load transmitting part like gears, couplings ect by welding is generally not recommended. But if it to be done in emergency when spare is not available, do properly. Selection of correct electrode is only first step. Please check whether you did the following:

- Thorough grinding of the cracked area, littile beyond the progation length of crack.

- Die-Penetration Test to see if cracks propageted further. If found, again grind inreased length & repeat DPT.

- Drill a hole at the end of crack (little away in the direction of propagation). This is required to contain the crack while welding, otherwise it will propagate further.

- Carry out welding as per required WPS. (check whether preheating of either elctrode or base metal or both are required).

- After first weld run grind the weld surface & do DPT. If found crack in weld, remove by grindig & proceed.

- Heat treatment, if any required as per WPS.

- Do DPT & radiography, check the result whether acceptable.

Still I feel that repair by metal-stich for such component would be a better option compared to welding repair

You do not say what the gear was made from so we must guess, you also do not say what electrode material or weld procedure was followed. You essentially dismiss any possibility that the repair procedure was faulty but yet clearly it was since the gear cracked shortly after being "repaired". By doing that you tie our hands and we can only guess at what may be happening. Cracking can result from a whole HOST of causes. A lot of times when posters don't give us much information it is because they either don't know the information and have made assumptions, or they are trying to cover for their lack of knowledge with bravado and don't want to admit that they don't know enough to be attempting what they are trying to do. Unfortunately the title "engineer" means different things in different parts of the world and one should not assume that because the person claims to have the word "engineer" in his or her title that they actually KNOW anything about engineering. In my time here I have seen some frighteningly STUPID questions about truly dangerous technology from people who claim to have been engineers for years but they clearly don't know their arse from a hole in the ground. I'm not saying this is the case here, but you REALLY need to give us more information to work with or the answer you receive will be no more useful than the information (or lack of same) that it was based upon. Be honest with us, if you don't know something there is no shame in admitting it. Ignorance does not equal stupidity, but obstinance in the face of it does. There IS shame in pretending that you do only to get yourself in deeper trouble. A man has to know his limits and should be willing to admit to them.

Now that I've vented my spleen a bit...

Two points immediately come to mind. The first being that the repair was not done correctly. I suspect that the crack extended far beyond where you believed it did and your welder did not grind the entire crack out before welding, leaving the leading edge of the crack to propagate further. Cracks can extend a long ways away from the end of the visible crack. Fluorescent dye penetrant can only show where the crack meets the surface, it cannot show how deep or how far under the surface it extends. you have to grind out every bit of the crack before you attempt to weld it back. therefore you must grind out everything visible and then check it again with dye to make sure you got it all before you start to weld. and you have to clean the area VERY VERY well too.

The second is the weldability of the gear material.

I suspect that the gear is a relatively high carbon steel (most gears are). any steel with more than about .30% by wt carbon content is dang near unweldable unless you keep it hot before during and after the weld and immediately shove it in a heat treating furnace, normalize and then re-heat treat. I doubt you would have done this since it would have un-hardened the teeth of the gear which are usually induction hardened. It is entirely possible that this gear is not repairable.

Another possibility is contamination of the crack/weld site with something that combines with an element in the material to form a low melting point eutectic. If the gear or the welding electrode has appreciable nickel in it and the area were contaminated with sulfur then this is a distinct possibility. Nickel and sulfur form an alloy that melts at a very low temperature and this can cause very rapid cracking of weldments. The sulfur could be from cutting fluids, coal dust, or any number of other things, or the steel itself may be resulfurized (which means it would not be weldable anyway.) for machinability. Resulfurized steels have very poor fatigue properties and should never be used for anything like this but they are used this way every day by people who either don't know any better, or just don't care. Honestly wish they would be taken off the market. They cause entirely more problems than they solve.

I wish to attach/ paste a picture of the gear with the cracks . Please guide how to do the same.

However Please find the Detail description of the gear-

Gear Face width-624

Module - 30

No. of Teeth - 218

Type of gearing - Spur

Normal Pr. Angle- 20 deg

MOC- IS 2644, CS-840, 248 BHN Min

Weight - 9925 x 2 = 19850 kgs

ID: 5025mm

OD: 6663.61mm

PCD: 6540nmm

simply click on the little green camera icon and browse to the file.

the bit you don't post is what kind of steel is it. we'll need the chemistry and it's history and environment. what is it drying? at what temp? the actual hardness (vs the min hardness) because the harder it is, and the higher the carbon content the less weldable it will be.

The Gear material is IS 2644 CS 280 i.e. High tensile casting . Hardness is 248 BHN Min.Gear outer dia is 6663 mm and is mounted on the dryer shell by spring plates . The dryer is for drying di ammonium phosphate i.e in DAP Plant. Initially four nos cracks were observed on the web of the gear just above the pins of the spring plates. These cracks were welded with 660 NH electrodes in root and Xuper 2222 later as per WPS . PWHT was also carried out at 330 deg temp. After a month new crack was observed about 75 mm away from the previous crack. Here crack is not a partial crack , but the breakage of the whole section of the web i.e 180 mm X 90 mm. As we go on welding , we observe a new crack after few days interval. This time we had welded round bars of 50 mm over the cracks , but they are also not withstanding. Please suggest , how to run this gear for another one year till we get the new gear.

I am not able to figure out actual assembly. But, I think, you should stress relieve the gear. Increasing torque limit beyond rated limit is not good practice.

Can you tell me about all technical data of girth gear?

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Muhammed, it is REALLY a bad idea to post your email address, and it is not allowed here anyway. all you have to do is sign up here at CR4 and the OP can contact you through CR4's messaging system without giving the Spammers anything to spam.