Specific sliding of external gear pair

[gearman1234]

Does any body know which standard is used for calcualting and specifying the specific sliding ratio for an external gear pair?

I know that this is a ratio of sliding velocity to the rolling velocity and is an indicator of the scoringg and noise generation.

 

[gearguru]

gearman1234,
specific sliding is a mathematically defined feature of two meshing gears. All standards - if they show the way how to calculate it - must lead to the same result.

 

[gearman1234]

I do not have a standard with me. I searched through the internet and the UTS website gave me the formula mentoned above. But there is no mention of what is a desired value for specific sliding ratio.

I am interested to know the range of value within which this ratio should be held.

 

[diamondjim]

I believe the value is 1.00.
Anytime the SAP Diameter approaches
the base diameter of the pinion
the values change dramatically.
If you are getting a very high ratio
on the pinion, you may want to consider
reducing or relieving the tips of the
gear.

 

[gearman1234]

Thanks Diamondjim

 

[israelkk]

The specific sliding should be as low as possible but "the rate of change" of the specific sliding value along the line of action should be as slower as possible too.

I am not familiar with a standard that specifies a desired specific sliding but I have found a correlation that shows that when I use my software that follows the AGMA 2001-C95 and AGMA 908-B89 standards to calculate the gear life for pitting it appears that when the specific sliding is lower the life is longer.

This may be explained by the profile shift I made and/or the change to smaller teeth resulting in larger contact ratio which lowered the speicific sliding values and rate of change.

 

[gearguru]

The specific sliding can theoretically reach the value from +1 to -infinitum. These are theoretical extreme values, Positive sliding is in the area above the operating pitch circle, negative below it. Zero sliding is when the teeth are in contact at the pitch circle. Negative infinitum would be at the base diameter.
The reason for calculating the spec. sliding is to decide, if the "correction" of the teeth addendum is necessary. The goal then is to "balance" the sliding on both gears (to make the sliding values the same on both, if possible). There are calculating methods in advanced gear literature for finding the addendum modifications for both gears to reach this goal.
Of course there are also another considerations for the optimal tooth design, the described procedure is just one of them.

 

[EnglishMuffin]

The trick (if possible) is to apply addendum modification such that the modulus of the slide/roll ratio is the same at the highest and lowest points of tooth contact - this results in the lowest possible maximum value for a given situation. For very high speed gears, the value of the slide/roll ratio should be minimized because it directly affects the local oil film temperature, which can lead to oil breakdown and instant scoring. And as gearman says, it supposedly affects noise as well. I have a Ray Drago program somewhere which calculates the slide/roll ratio and the flash temperature (from Blok's equation) for a series of points along the line of action - I think from memory that Drago's text book covers the subject quite well.

 

[israelkk]

The book On the Geometry of External Involute Spur Gears, T. W. Khiralla 1976 covers the specific sliding calculation and graphing issue in details.

It is sold for $475 on the net.