Span Measurements of Helical Gears

I'd like to help you, but I could not exactly understand your question.

Please 1) post (upload) an informative schematic detailing your problem (...perhaps the drawing you refer to in your post?) and 2) consider rephrasing your question.

I'll check back later for your clarifications and see if I can help.

I mean to say, I have given tolerance -0.037mm to +0.037 mm for gear span measurement 255.217 mm dimension on drawing but when gear manufactured then actual measurement came 255.020 that is minus by 0.15 mm to lower limit of tolerance.

So Now I have to decide how to adjust this dimension 0.15 mm to other mating gear?

So you want to match gears?, i.e. select gears that make up a correct amount of backlash? Don't forget that with that much variation in your actual tooth thickness you are looking at different working conditions for each matched gear pair so that your confidence in gear durability becomes comprimised. If you take that however on the chin then you have to look at the effective backlash. The backlash is needed so that any variation in gear tooth geometry, variation in centredistance and even operational deflections can be taken up without the gears potentially locking up which can prove disastrous (dual flank contact!). So your task is now to define your operating (required) backlash and to calculate the tooth thickness of the mating gears to achieve this.

Calculation of the center distance / backlash is given below but is an iterative process. To derive the tooth thickness from that you will have to calculate the addendum modification coef of the mating gear assuming all else (apart from Alpha_tw) is given. If you are good in programming you might be able to write some code to do this.

Really, 1 drawing is worth 1000 words here, you should have posted it- but I am in a helping mood.

Assuming that 1) what you report is a span measurement over n consecutive teeth, 2) it is more or less the same around the gear, thus indicating reduced tooth thickness and NOT pitch errors, 3) you are dealing with involute gears, 4) you measure at the normal plane and 5) with the nominal 255.217mm span you have a backlash of B (in mm, measured along the line of action) then:

0. The actual centre distance is imposed by your housing, regardless of above considerations. The presence of any backlash does not mean that you can actually reduce the centre distance, unless you modify the housing (not recommended). Negative backlash means that your gears simply won't mount, again unless you change the housing... A previous poster already said all that; just a reminder here.

1. The deviation you measured translates to an increase in backlash. This is good news. After some cryptic calculations (try to make them out from my pics appended below) the new backlash calculates to B+255.217-255.020. It being larger than B, you are obviously safe from interference problems. For spur gears you can find the angular backlash in rad by dividing this by the involute base radius. For helical gears you need to correct this by the helix angle.

2. If I guess correctly, you have not yet manufactured the mating gear and want to know what special span to prescribe for that, so that you can get the intended backlash (and not that calculated in point 1). Answer: If X is the nominal span, then increase it by the amount of the measured error, hence X+(255.217-255.020). That way, you'll still get your intended backlash of B.

3. However, point 2 seems like overkill: with such a large gear (at a 3-tooth span of 255mm, I don't even want to imagine your diameter) and span tolerance (quite loose), I smell a low tech application; it doesn't seem like speed, dynamics or positional accuracy upon reversal are design concerns...

So point 1 should mostly cover you: you are safe from interference and likely need not take corrective action. I wouldn't normally recommend going as far as point 2, unless your application is very special.

4. Incidentally, a +/- tolerance for the span measurement does not make much engineering sense (although it is not uncommon in drawings). As you can figure from point 1, any deviation on the + side can be potentially catastrophic for tight meshes. Much better to presribe it as +0/-x, where +0 corresponds to the smallest backlash you are willing to live with.

Hope this helps

And next time (for the 3rd time), do us a favour and please, you post the schematics!!

p.s.: Check the math once again, I have been known to make the occasional mistake when doing calculations after lunch...

Starting a numbering sequence at 0 is just plain silly. My two cents worth.

Are you saying: You measure the span of a helical or spur gear over an x number of teeth according to the drawing spec and the measurement you get is outside the tolerance specified on the drawing? If so then first you will have to find out if that out of spec measurement result is because the manufacturing process produces out of spec gears or whether your measurement method is flawed. If your measurement method is flawed (in other words you get incorrect readings) then you are stuffed as you do not have a frame of reference. Either way it is uncertain how you should interpret the tolerance on the mating gear as the same applies to that one. Span measurement is basically a measurement of the tooth thickness and the tooth thickness directly affects the tight mesh center distance of the gears. The real center distance however is defined by the position of the shafts the gears are mounted on. Therefore changes in the tooth thickness will give you changes in the amount of backlash (clearance) or lack of backlash (interference) in the gear mesh. Therefore you need to specify more clearly what you need to calculate and for what purpose.

GA

And also the span is affected by a wrong centerdistance too.

So you have to ensure the span error is due to

Tooth thickness error

or CD Error.

In either cases, you have a problem.

Finally why do you want a deviation from a standard ?

The deviation in CD is going to affect the performance of the Gear train in terms of the pitch line rolling (due to mismatched pitces of the mating gears)

The deviation in tooth thickness may have low backlash (and hence lube problem, less life) or high back lash (and hence less life, vibration)