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2.25*Module -v- 2.35*Module whole depth interchangeability

2.25*Module -v- 2.35*Module whole depth interchangeability

(OP)
We normally specify spur and helical gears with 2.35M whole depth and 2.0M working depth to allow for grinding and shaving if necessary. We have a supplier who is proposing to change whole depth to 2.25M to suit their standard hobs. Everything else: working depth, centre distance, rack shift, pressure angle, helix angle, PCD, number of teeth, material & HT, face width etc remains the same. The gears are case carburised and used in a truck tcase.

My question is whether the gears with 0.25M clearance can be safely meshed with the existing gears with 0.35M clearance? The only problem I can see is that the 2.25M gears will be marginally stiffer in bending and therefore 2.25 and 2.35 teeth will deflect by somewhat different amounts under load, but I think the involute profile should take care of that.

If we can't interchange the new gears with the old then we run into logistics problems, both with controlling spares and because one of the gear designs is used in multiple gearboxes, which means changing all of them if we can't interchange.

RE: 2.25*Module -v- 2.35*Module whole depth interchangeability

2
There should not be any problem even the 2.35M works only in the 2.25M range. As you said the only reason for the 2.35M is to allow for extra tool clearance for grinding. The only problem I see is how the manufacturer is going to hob a case hardened carburised steel at >60RC hardness? If you intend to case harden it after hobbing then you need to grind for final dimensions which brings you back to the 2.35M design as before.

RE: 2.25*Module -v- 2.35*Module whole depth interchangeability


1MarcoD
just make sure the root radius does not go above the True Involute Form (TIF) and the mating gear does not roll on an non involute area.
At the required center distance analyze that there is no tip interference with the mating parts. there are equations out there. or at tight mesh the gear rolls correctly with a master gear.
if these are shaved gear the clearance will be required.
if these are finish ground gears just make sure the root diameter is not over size that there will not be to much stock removal and the effective case depth will be compromised.
depending on the grind stock allowance & case depth requirements.

I purchased custom hobs or shaper cutters to get the correct depth and tooth thickness with the required tooth thickness or tooth space.
there is no reason why, they can not do the same. stick to the original whole depth.

RE: 2.25*Module -v- 2.35*Module whole depth interchangeability

The biggest problem you're going to face is that the 2.25 'standard' profile will not have protuberance added to the hob's profile.
Without the undercut that the protuberance creates and without the extra clearance; grinding the profile will leave a grinding-step near the root. The step will become a stress raiser in an area that is usually already under a great deal of stress.
You could form-grind the full profile and this wouldn't leave a step but will leave the root area with a ground finish. On case-carburized gears, this is generally not recommended.

RE: 2.25*Module -v- 2.35*Module whole depth interchangeability

I forgot to add; using a standard 2.25M hob for pre-grinding operations will result in a clearance height that is less than standard (not 2.25 x module). The reason for this is because a pre-grind cut will require stock to be left behind for the grinding operation. This means that the hob will not be cutting at full depth and hence a reduced clearance.
I would recommend that you perform a profile analysis to ensure that there is no interference between mating components, particular if mating gears (both with the reduced clearance) are expected to mesh together.
This is why pre-grind hobs are designed for 2.35M.

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