Straddle mounted double helical gearing

[MrClean]

I have been asked to design double helical gearing for a 2-stage reducer. Gears are to be hobbed, so a gap between helical sections is required. Axial space is limited, so a design with one stage straddling the other seems appropriate, because one of the gaps does not add to the overall axial width requirement, since it is occupied by the other stage. See photo.

In the photo, the HS stage straddles the LS stage. I would have thought having the LS stage straddle the HS stage would be better because:

1. The LS gears, with their higher loads, would be closer to bearings.

2. The one gap that contributes to overall axial width would be in the HS stage. The finer pitch of the HS stage would allow for a narrower gap.

What is standard practice for this kind of arrangement - LS straddling HS or HS straddling LS?

Another question - are there formulas for calculating the required gap width based on pitch, helix angle, etc?

 

[gearcutter]

Your biggest problem is going to be maintaining an even load distribution across the individual stage pairs.
The timing of the teeth on each shaft will be critical.
Very tight tolerancing of tooth-tooth alignment will be required.
Creation of the drawings along with the manufacturing process will have to be closely monitored.
Before going any further with your project; I would highly recommend that you first find someone that is capable of manufacturing these components and then go from there.
Good luck!

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

As gearcutter notes, unless your bearing system provides the correct degree of axial constraint at each gear shaft, then indexing precision between the mating halves of each double helical gear can be an issue with regards to load sharing. Normally, one of the gear shafts in a double helical mesh is allowed to float axially so that it will assume an axial position where thrust forces between the gear halves are in balance.

You mention that the valley width between the mating halves of your DH gears is a concern because you wish to produce them using hobbing. With hobbing or grinding the valley width must be sufficient to prevent the hob or grinding wheel OD from contacting the opposing gear teeth at the inboard limit of its travel. Sometimes it is possible to reduce the valley width slightly more by taking into account how the hob/grinding wheel tips align with the opposing gear tooth spaces.

If you want to minimize the valley width between mating faces of a DH gear, you could also shape them instead of hobbing them.

Looking at the photo provided, it would not seem possible to hob the smaller diameter inner gears on the center compound shaft due to their close proximity to the larger diameter outer gears, unless they are produced separately.

Lastly, if overall axial length of the gearbox is a concern, it looks like your design would benefit from some optimization of gear face widths, tooth proportions, helix angles, bearing sizes, shaft diameters, etc.

Hope that helps.
Terry

 

[Philrock]

Thanks Terry,

The HS gears will made separately then pressed onto the LS pinion shaft.

The HS and INT shafts will be free to float axially.

 

[gearcutter]

Is 'MrClean (Mechanical)' an alias that you use Philrock?
How many do you have when using the Mech Eng Tips forums?

If axial space is limited; have you considered generating the second stage pairs on a herringbone gear shaper........either a Sikes, Sunderland or MAAG? A gap wouldn't be required in that case.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[MrClean]

Ron,

Yes, I do have 2 identities on Eng-Tips. One day a few years ago I was unable to log on because I forgot my password. I could not see any way to get on besides creating a new identity.

I want to make the gears so they can be hobbed - this method is used by the largest number of vendors, and gives better quality than shaper cutting.

 

[tbuelna]

MrClean,

You can get excellent quality using shaping/shaving/honing. In fact, given the wide spacing between your bearings and the small shaft diameters, you may find your gears will benefit from some face profile modification to reduce edge loading.