Slewing ring drive gears 2

[Gerry45]

I am getting involved in discussions between the manufacturers of a slewing ring bearing with internal gear, and the drive pinion gear supplier. They have used the terms -

'addendum reduction',
'addendum modification'
'tip relief'

Can anyone please explain the relationship or the meaning of these terms. And on large internal gears, (7ft dia), is it normal to generate the tip relief with a file ?!

Regards,
Gerry

 

[diamondjim]

Gerry,
You might find these covered under long and
short addendum gears in some of the texts.
Because the pinion is inherently weaker that
the gear, this design was adopted. It also
allows smaller numbers of teeth in the pinion
to be used without having undercutting.
As to tip relief, it could be incorporated
into the cutter or hob design and not have to
be a hand operation. FMC Corp had specs
on a hob that had a 2.295 whole depth with
a full fillet radius and designed to do just
what you are asking for. This is the optimum
depth to incorporate a full fillet radius.
The long and short addendum increase the arc
of recess and is sometime called recess action
gearing.
Rather than tip relief, you might just want to
truncate the addendum i.e. increase the id of the
gear blank by 10 to 15 percent on the pitch radius.
Essentially you are therby removing also some
of the recess action and helping to reduce the
sliding velocity in the root of the pinion.
Sounds like good advice.
25 percent and 50 percent long addendum drives
are quite common in the heavy bearing industry.

 

[GearmanPE]

Expanding on reply from Diamondjim, there is a distinct difference in tip relief and addendum adjustment.

As teeth come in and out of mesh, the stiffness and thus the deflection change due to load being alternatly carried on one and then two teeth. The prevent premature engagment of the incomming tooth tip (remember, mesh is deflected, so the unloaded tooth is a little ahead of the perfect position), it is common to ease off material from the point on the tooth addendum where two teeth are in the mesh, to the OD. The amount is based on deflection, which is based on load and pitch. These are small amounts, from a few "tenths" to a few "thou".

The above is independent of addendum modification, aptly described by Diamondjim.

The tip relief can be applied to both mating gears, or applied to just one and done on both root and tip areas. The former is the case for most internals. The tip relief can be built into the cutting tools or included in a finishing operation such as grinding.

For gears to work well, and last long, they need proper tip relief as well as crowning. Tip relief is a tooth modification in the transverse direction ( root to tip) and crowning is a tooth modification across the face width. Crowning compensates for deflections and manufacturing tolerances. No crown leads to very high edge loading stress, while crowing relieves this. Too much crowning reduces the contact length and reduces load capacity. T get it right, ask you vendor for the engineering calculations or seek review by someone with the experience to provide it.

 

[Gerry45]

Thanks chaps, useful information.