New to gear geometry, and need advice. 1

[jonnyp]

Hi all. First I would just like to say what a great site this is.

I work for a small jobbing shop doing gear cutting.
We have a job on at the moment for which I have the sample pair of helical gears. Usualy we put them on a profile and lead tester to determine the helix angle, but unfortunately they are too big. We know the centre distance because we measured over the shafts in the box, and also used feeler guages to determin the backlash.
When we used the information we know, Our span calculations came to more than the samples measure, which if we cut them to this size there would be excessive backlash. This I suspect is down to us being wrong on the calculated helix angle. I suspect profile modification is also a point I need to know about. Profile modification is something me and the boss have limited knowlege about and I wish to improve my knowlege in this area.
With the information I have from measuring samples etc, Can the helix angle be calculated to what the samples actually are, without using a lead tester etc ?
Many thanks.

 

[williedawg]

If you have another gear of known similar specs (smaller?), hold it in mesh it with one of these gears.
If the side faces of the meshed gears are parallel, then the helix angle would be the same; if not parallel, then some method would have to be devised to find out what it actually is.

willie

 

[gearcutter]

There is no way of finding the helix angle accurately other than mounting the samples on a gear cutting machine(hobbing or shaping), gear checker or gear grinding machine.

Using a gear cutter or grinder will allow you to check the sample's helix angle by tracing the profile with a dial indicator. We use this method regularly and (if the samples are in good condition) can get the result down to seconds of a degree.

Reverse engineering by calculations is also possible, however, no guarantee can be given that anybody will get it right unless you manufacture the gears as a mating pair.

Having said that, for the majority of the time this is the method we use and so far have been able to get it right as, to date, the helix angles have been easy to find such as 10, 12, or 14 degrees etc. This has been on older equipment only. Be very careful as most modern industrial gearboxes use lead modifications to compensate for things like shaft and housing deflections. Helix angles in this case are difficult to find by calculation alone.

I would highly recommend you purchase some kind of gear calculation software to help you do this. If you are running Windows XP of earlier I would suggest GearCad as the fastest and easiest to use for simple workshop calculations.

 

[spigor]

It is possible to measure the helix angle using a CMM (co-ordinate measuring machine), even if no gear measuring software exists in it.

 

[jonnyp]

Thanks for the replies everyone. Gearcutter, We have 3 fellows shapers, one is a 6a, but the gears are too big to go on this machine to check. But we have a couple of sunderland planers that we will be cutting the gears on. Could you explain more about your method of checking the helix angle with a DTI for me please, I dont quite follow. Is this possible on a sunderland planer ? Sounds well worth knowing. If it works for you then I'm sure it would come in handy for us.
Thanks again.

 

[mfgenggear]

Jonnyp

to make sure your reverse engineering is exact
use an other gear shop to measure the helical gears
lead & involute that uses fellows, m&m or hofler equipment.

to make sure of the Normal DP , Normal Pressure Angle & the helix angle.

otherwise it will be a hit and miss situation.
I question how the gears will be Inspected while Manufacturing?

MfgEngGear

 

[CageGear]

If you can forward me the measurements that you have taken, Iwill run some numbers for you and let you know what the helix angle "calculates" to be...

Also, could you let me know what the helical gears are used for? Some gearbox manufacturers modify the pitch of the gear to try and capture the customer for spareparts.