Spur Gears and Pinion

To check gears, we used stuff called Prussian Blue. It came in a squeeze tube like artist's oil paint. Paint a thin film on one gear and then mesh the gears. Where they contact, the Blue will transfer to the other gear.

If a gear is worn, it is done, no life left. To prolong a gear, use the factory recommended lube, keep it perfectly clean, and don't overload it. And it must be in perfect alignment and properly shimmed and be running on bearings with no play out of tolerance.

Thank for responding to my question. It really help me a lot.

Regarding the Prussian Blue.

In what degree the transfer of the Prussian Blue then the gear is considered as a good gear. Any Photoes can I review?

Magnaflux. If you are checking reconditioned gears to determine possible lifespan. You need to determine if there is any cracking.

That is a good idea. But where can I get more information regarding the magnaflux?

Really thank for the suggestion

1. magnaflux is a company Google magnetic Particle Inspection and you will get all details but then you will need a certified NDT man (at the very least level II) for the inspection. But as pointed out, it is necessary.

2. The used gears should have a typical contact pattern at least 70-80% of flank. In unused gears it is quite a bit less we usually get somewhere between 30-60% on these - note that these contacts are on no load.

3. The importance is the scatter of the contact along the axis as well as along the pitch line.

4. You must know the amount of wear out and that can be checked by measurement of the tooth thickness at pitch and also the back-lash at the ideal center. The theoritical values should be available as per module and application.

What else - as far as I can say check visually flank, for pittings or any other defects, scratches, poor finishing, abnormal surface etc.

Never been in the reconditioning- always buy new, so can not be of real help.

Re the blueing of the gear flanks followed by the roll test, each gear should be mounted on a dedicated shaft, and the alignment of the shafts, together with the shaft-to-shaft centre distance, must be correct and to specification for a worthwhile assessment to be made. A true indication of the condition of the gears, using the roll test, will not be obtained if one gear is loosely rolled, freehand, upon the other gear.

Additionally, if still possible, inspect the shafting of each gear, while still as originally assembled, to determine if there is any radial wear in the bearings at either end of the respective shafts. Excess wear will have permitted the shafts to take up a degree of un-intentioned out of parallelism which will be manifested in uneven flank wear over the length of the gear teeth. Such wear, if it has occurred, is undesirable and cannot be corrected by re-aligning the respective shafts; the gears would have to be re-machined, in which case it would be better in the long run to plan on new gears.

It really depends upon the intended tooth loading, the speed of operation, the design of the lubrication system, the expected lifespan of the intended system, etc, etc, etc!

If these gears will be critically loaded in their new application, the advise given earlier to have the gears non destructively crack detected will be worthwhile pursuing, but don't expect such inspection, if done competently, to be cheap, hence another possible reason to look at new gears right from the start.

The first examination will involve a visual inspection of the tooth surfaces. Be aware this is just an elementary inspection! Regardless of the method used to cut the teeth, there will be some evidence of "tool marks" left in freshly-cut teeth. In a used gearset these are often still evident on the unloaded flank of individual gear teeth. If the loaded side of the tooth is worn you should see smooth and even wear. If you see scuffing or "cupping" of the tooth flank, then either the alignment was poor, the lubricant was insufficient, or the gearset was not sufficiently hardened. Gear teeth should wear evenly from root to tip, and if the wear is uneven then the gearset should be replaced.

I suggest that you determine which tooth flank is this unloaded side and use its appearance to compare the loaded side. From that a trained eye can determine if the wear is present and even in its distribution. The type of gear can make this inspection difficult. Simple spur, helical, and herringbone gearsets lend themselves well to a visusl inspection, but bevel, spiral bevel, Zerol, hypoid, and worm gearsets are much more difficult.

A spur, helical, or herringbone gearset has relatively simple alignment requirements that are easy to verify. The others that I mentioned involve more difficult techniques to verify. Each gearset of these type has a recommended exact centreline placement that must be verified while they are held at right angles to each other. The right angle is the easy part - it is the relationship of pitch circles that is difficult to establish.

Prussian blue is commercially available as Dykem Hi-Spot Blue and is sold by any good machine-shop supply store. After alignment and backlash are correctly done, it is used to mark the teeth of the pinion and the gearset is rolled together for several revolutions. One is looking for a transfer of the blue to the center of the tooth face of the mating gear. This area typically appears to be a skinny ellipse of blue film and will show if the gearset is aligned.

Your main question asks how to determine the remaining longevity of the gearset. A properly-designed gearset has tooth proportions that are calculated to transmit a maximum horsepower at a certain speed with a certain safety factor. As far as the predictable life of such a gearset, I for one cannot help you there. My experience dictates that gearsets must be periodically inspected for tooth wear characteristics.

Rotary vibration analysis is a great tool to use for this if the oscillation characteristics of a new gearset are known, and if the gearset operates at a sufficent speed to produce measureable oscillations. But in order to make this method effective one must know the oscillation characteristics of a similar gearset during its useful life. I suggest that you speak to a specialist who uses or makes this type of equipment to see if this idea will work for you.

Best Regards,

Ing. Robert Forbus

Thank you to your guys giving me such good and impressive ideas. I really appreciate it.

Regarding Nova85-rebuilding. The rebuilding sound interesting. If I re-cut the gear/pinion, the dimension will definetely change. How come the gear can reused in the same place for same task? Any add comments?

I just woondering if there are any processes can I carry out to prolong the gear life. (Pls Give comments)

Kyle

Improved lubrication, filtering of oil if that is used, and a preventative maintenance program. What are the gears used for?

I am involving the recond vehicle starter which involved the gear, pinion and planetary gear. So that is quite difficult for us to inspect it frequently. After recond process then we will send it to customers and waiting for feed back.

the remachining of gears and pinions will result in increased back-lash and of course reduced tooth strength.

Increased back-lash will affect the vibration behavious especially in case of variable load. Since you are on planetary chain (and I suppose your gears mentioned are the planetary sun/planet/ring) the increased backlash also will have a bit effect on the location of the sun (assuming the system is fixed ring or the ring - fixed sun structure) since these are centered by the gear train them selves.

It all depends on the amont of metal you are removing by the flank correction.

If these factors are acceptable then you can go ahead. Any way you are going to test the mechanism at your works, check the performance with reference to vibrations and then decide.

If you're rebuilding auto starters, you are probably not doing the same starter all the time. For starters, I would be most concerned about the condition of the bushings, and any bearings. The gears will have to be visually inspected, and experience is going to be most valuable, so I would have that as a speciality task. Have one person concentrate on inspection and quality control, the rest doing disassembly, cleaning, etc. Auto starter gears are going to be very hard and durable, any flaws or wear will be visible. Have the inspector check gear lash and bench test each starter before delivery. Sometimes by the sound you can tell a good or bad starter.

You are correct that the dimensions will change. The decision to re-cut depends on the application. You can do it in most cases if increased backlash is not a concern. If the gears rotate in both directions than backlash may be a concern. Normally the material you take off has minimal effect on the tooth stress.

If the center distance can be adjusted you can also make up the difference by reducing the center distance between the gear and pinion.

If you have a very large gear (by large I mean 10 feet or more) it is more economical to recut the gear and manufacture a new oversized pined.

Another possiblity is to flip the gear. If the gear is symmetrical and the gear runs in one direction you can flip the gear on the shaft and the unloaded flank (which should be like new) will become the loaded flank.

In some cases gears and pinions can be re-cut or re-ground to re-establish the gear tooth profile and it does extend the life. Most gear manufactures will offer this repair service. I suggest you contact a manufacture that can handle the type and size gears you have.

You may need to familiarize yourself with spur gear calculations and the tools used for checking such. For example when checking tooth thickness at the pitch circle, a gear tooth vernier caliper is used.

Alternatively a good machinist should be able to do the required calculations for the gear dimensions and the required measurements.

If you are using these gears for making something private, fine.

But if you are going to sell this "object" and especially if there is a possible safety issue, you may be "Biting off more than you can chew!"