16.5 to 1 ratio for Helical gears? 1

[mschodde]

Is a 16.5 to 1 ratio outside the acceptable range for a single reduction using helical gears. I'm just starting to look at a machine tool that has been designed by someone else (non-engineer type)?

Some specifics:
Helix Angle 45 degrees

12 Diametral Pitch

Pinion
12 Teeth

Gear
198 Teeth

Thanks,
Mike

 

[diamondjim]

Mike,
Using a 45 degree helix angle, you end
up with a contact ratio equal to 1.01
This would really be borderline and jerky.

Reducing the helix angle to 15 degrees,
contact ratio increases to 1.40. any number
larger than 1.00 indicates more than one
tooth in contact part of the time. i.e.
the number part greater than one as above
indicated 2 teeth are in contact 40 percent
of the mesh cycle. A 1.20 minimum contact
ratio is recommended.

Why the large 45 degree helix angle?
This would also produce a high axial load
on the interfaces.

 

[mschodde]

diamondjim,

Thanks for the response.

"Why the 45 degree helix?" I'm asking the same question to the Machinists who came up with this tooling design. The reality from my point of view is the configuration of the tool doesn't even warrant helical gears. Spur gears would do.

I also notice they are ordering the pinion from boston gear who seem to only have 45-degree helix helical gears.

Some follow up questions:

1. What textbook reference are you using for the equation to calculate the contact ratio? Or are you using some other method? Any recommended gear textbooks for sources? I'm starting to look at Dudley's Handbook, 2nd edition.

2. Is contact ratio the dominating factor when getting above 10 to 1 in a single reduction? Most of the textbooks state 10:1 as the highest gear ratio for a single reduction using spurs and helical.

Thanks,
Mike

 

[diamondjim]

Mike,
There is nothing wrong with gears having ratios
of 10 to 1 or greater. I have the Fairfield Mfg
Gear program that calculates spur and helical gears.
Mike Bull has some excell spread sheets free on his
site, but they are basically for module gearing. You cannot beat free software, but you must
understand how to manipulate the input to achieve
better than normal results. Much literature and
research indicates when the pinion is the driver, that
long and short addendum teeth are of great benefit.
50 percent long addendum pinions and 50 percent short
addendum gears seem to work fine for power gears.
To try to do all these by hand is a good idea for
understanding how the software works. ITF has a handbood
of the basic formulas for spur and helical gears. Dudley,
Buckingham, and even the Machineries Handbood should get
you over the hump. It is a slow learning curve. By having
someone make the gear and pinion for you, they should provide you with the basic input and output. Every Engineer uses a slightly different approach as to the Rating of Gear and applying all of the factors. The basic calculations for mesh and strength are fairly straight forward. I do not know your application, but any gear house would offer you free engineering in hopes that you purchase the items from them. They know that you are going to take their specs and cross shop for prices. I do not know your application, and assume you have parallel axis.
Helical gears are more complex than spur gears and I would
try spur gears unless you are convinced that you need
helical gears. For high ratio gears, the long and short addendum are the best approach to balance the strength of the parts and to get the smallest center distance. I work
with large gears and bearings. I have specced out helical gears to several bottling companies who demanded them.

 

[mschodde]

jim,
Which contact ratio are you talking about (total, transverse, axial/face)? And specifically how are you calculating it?

Note these gears have a pressure angle of 14.5 degrees.

Using Dudley's book, I'm getting some different numbers.

Thanks,
Mike

 

[diamondjim]

Mike,
I was using 20 pressure angle and was speaking
of transverse contact ratio. You may contact
me at work 330-562-4000 or at home 330-678-0226
or email j.geisey@juno.com I can fax you the
results of the fairfield programs. I just assumed
you would be using 20 degree pressure angle. It
is almost an industry standard especially if you
are using small pinions. Be glad to pump in 14.5
pressure angles. Do you have a fixed center distance?
Or is your question general at this time?

 

[diamondjim]

Mike,
I quickly ran a helical gear set as you
specified except used 60 percent long
and short addendum design to avoid undercutting
in the root of the pinion. Surprisingly the
contact ratio is 1.56 but the teeth are a little
pointed or marginal at the tips. The teeth could
be stubbed a little and you still would have a
high contact ratio. The SAP diameter of the pinion
is 1.0002 which relates to a .9523 SAP angle which
means unless you stubbed your gear, you would have
to use approximately the longer addedum pinion or
you will have undercutting in the root of the pinion.
Some is ok, but the contact ratios are reduced from
the theoretical ratios.

 

[BrianDengel]

If you are working in a parallel axis application, the only thing the 45 deg helix angle will accomplish is to increase the center distance. A lower helix angle will have a better contact ratio and less thrust loading. If this is a crossed axis application, then why not go with worm gearing?