Center distance between gears 2

[bbook1]

Is there any application where you would want the center distance between two spur gears to be greater than half of the sum of the two pitch diameters of the gear combination? I have a customer design where on one gear set in the gear train has a center distance of 0.177" and half of the sum of the pitch diameters is 0.175". A second gear train has a center distance of 0.2" with half the sum of the pitch diameters of 0.1969". The gear drawings specify no special dimentioning for addendum or dedendum or any other tolerancing changes that would change the tooth profile.

With the gears spaced this way, what problems can be expected, if any?

 

[israelkk]

What is the tolerance on the center distance?
What are the tolerances on the gears?

 

[bbook1]

The center distance tolerance is +/-0.0005", the gears are AGMA 8. The gears ride on pins 0.0394 +0.0003"/-0.0" and the bore of the gears are 0.0400-0.0407".

 

[israelkk]

Are you sure the gears are standard and not with some tooth correction (profile/rack shift)? Can you give more information about the gears: Diametral Pitch/Module, Number of teeth, Maximum and minimum testing radius? etc.

 

[gearcutter]

Could the discrepancy be due to a backlash allowance?
Theoretically; to add backlash, by plunging the cutter deeper and thereby thinning the tooth, would reduce the theoretical pitch diameter from standard.
In this case of course the working pitch diameter is not altered.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[bbook1]

The pinion1 is:
PA=20
PD=0.0875"
NT=14
FaceWidth=0.055" +/-0.003"

Mating Gear1 is:
PA=20
PD=0.3063"
NT=49
FaceWidth=0.035" +/-0.003"

This is the set that is on the 0.2" center distance. Both are AGMA 8. That is all the data on their drawings.

The second gearset has the same pinion and this mating gear:

PA=20
PD=0.2625
NT=42
FaceWidth=0.035" +/-0.003"
AGMA 8

I created a CAD model from their drawings and noticed the spacing issue when I tried to setup a simulation of the geartrain so that I could size the motor to drive it.

 

[bbook1]

All of the gears have "Normal Diametral Pitch = 158.75 (1/in)"

 

[israelkk]

The gears are smaller than the standard by 0.0025" to 0.0027" compared to the recommended of 0.0020" gear reduction for minimum backlash for AGMA 8 accuracy.

However, for the pinion the reduction on the pitch diameter is only 0.0005" which is recommended for AGMA 15 to AGMA 16.

The recommended tolerance on the center distance for AGMA 8 is +/- 0.0010" (not the 0.0005" which is for AGMA 13).

 

[israelkk]

This is not an optimal design. The pinion should have a positive rack shift to avoid undercut and to minimize sliding. The gears too should have rack shift (positive or negative to minmize sliding).

 

[tbuelna]

bbook1,

What are the relative CTE's for the materials of the gears and housings? And at what temperatures are they designed to operate at?

Thermal mismatch between gear and housing materials can produce gear mesh binding in extreme circumstances. For example, steel gears with limited backlash mounted in a magnesium housing and subjected to low operating temperatures.

Regards,
Terry

 

[bbook1]

The gears are mounted on steel pins that are pressed into an aluminum housing. The gears are made of 17-4SS. The operating range would be something like 0C to 30C.

 

[juergenwt]

bbook1 - Your info on the dia of the pin and the dia of the bore tell me you are looking at a metric gear. Once you determine the OD, Nr. of teeth and the module you can configure everything else. All the formulas are in your MH.
Also look at the previous post by "wratchet"

 

[bbook1]

juergenwt,

Thanks for the post, but I think this is a standard gear, atleast from the data that I have already posted. Everything has already been calculated by my customer, I know how to fix the problem, my original question was what problems I could expect with things the way that they are already dimensioned.

If my customer told me that I was free to fix this, I would change the center distances of the gears to remove the 0.002" and 0.003" spacing between the gear sets so that the PD of gears touch each other.

My feeling is that with the offset of the PD's of the gears, this device will make more noise and that the teeth of the gears will wear abnormally because they are not messing properly.

Brian

 

[ExRanger]

There is not necessarily any problem with a non-standard center distance. The involute form of spur gear teeth is not sensitive to center distance variation in that it maintains a theoretically conjugate (or kinematically correct) meshing action at any center distance. This is one reason the involute was chosen as a gear tooth shape.

Under load, you might increase the noise of the gears by using an increased center distance, but you might also decrease the noise. Without analyzing the design in detail and considering its profile modifications, its hard to say.

But generally speaking, involute teeth will mesh correctly at any center distance - of course, that is provided the center distance is not so small that the teeth jam or bind, and not so large that the contact ratio falls below 1 (or more practically, about 1.2).

The original design of your gearset might have had the non-standard center distance included to balance the strength of the pinion and the gear so as to improve the overall fatigue life of the gear pair.