cONTROLLING INTERNAL SPLINE GROWTH IN 8620 FORGING

[gearman1234]

We are having a 8620 gear forging which we are gear cutting prior to heat treatment. Then we are cutting internal splines to ANSI class 5 and then sending it for carburise and hardening to 57-60 Rc.

The perfectly OK spline is increasing in size by almost 0.005" between pins. We purchased the broach to 0.002" smaller but again we are geeting the same results.

I think that the heat treatment needs to be improved. SO need inputs as to which microstructure is suitable for lowest distortion without affecting the mechanical properties. Will the plug quenching help to reduce the distortion in splined bore? If so does it require any soecial fixturing? How is it done?

 

[unclesyd]

Come back with some dimensions as this will help you get a better answer.

 

[gearman1234]

OD of the gear is 6.4", splines are 16/32 DP, 30 deg PA, 17 teeth. Maj dia. 1.151.
The forging has a hub dia 1.75". Gear width at teeth is 1.015" and in the hub area it is 0.88 and in the remaining area 0.515.

Gear data is not important in this case but it is 72 T, 12 DP, 20 deg PA, 15 deg hellical gear.
I checked the gear hardness at teeth and near bore it is 57-58 HRc.

 

[unclesyd]

Some more questions:
How is the forging treated prior to machining?
It probably should be annealed from forging and normalized prior to marching.

What is the hardness of the forging as received?

 

[gearman1234]

There is no annealing. It is normalised prior to machining.
In case decide to anneal ,which annealing process you recommend? Is it full annealing or stress releif annealing?

The hardness is not really specified on the forging dwg. But it should be around 20 HRC, Since it is forged and normalised. But I will find out.

 

[unclesyd]

Information on annealing 8620:
Anneal @ 1600°F; Furnace cool @ 30°F per hour to 1150°F, then air cool to RT
Normalize @ 1675°F; air cool to Rt.


I'm digging for may data on carburizing 8620 and the dimension changes expected and how to minimize them. You have two conditions, carburizing and HT, that are additive causing you problem.

Someone with a better reference set than I have jump in and help gearman1234. My reference material is in an unstructured stack of boxes.

How are you carburizing , salt bath or atmosphere?

 

[metengr]

A previous thread posted by carburize relates to this subject, it was in the gear and pulley engineering forum;

http://www.eng-tips.com/gviewthread.cfm/lev2/16/lev3/58/pid/406/qid/68845

 

[metengr]

One option that I did not see in any of the posts - what about nitrocarburizing the gear? We had some fluid drive gears made for one of our Fossil Plants some time ago and it worked very well.

Here is a web site for further review;

http://www.geartechnology.com/mag/archive/ferrit.htm

 

[EdDanzer]

We made some long gears that we did not want to grind the teeth after heat treat. To reduce distortion we rough machined, and mock carburized the part. Then we finish machined and case hardened the part. This method reduced the distortion to acceptable limit, .0005 to .001 TIR on a 12” long part 2 ½” diameter. There is a cost and processing time disadvantage to this method, but was far less than the cost of grinding 6” long gear teeth.

 

[gearman1234]

Thanks for response from all. On further investigation and comparison with other parts I think that the component geomtetry and how the parts are stacked in H/T is adding more distortion than what normally would occur. Working with the heat treaters to address the issue.

 

[metman]

gearman1234,
After carburize, if you reduce temperature and hold between the upper critical and lower critical temperature and then quench, it will stabilize the distortion. However this will tend to produce grain-boundary carbide network if you hold too long in the carbide forming temperature range. If your gear teeth are only modrately stressed in service, this should not be a problem. But if the teeth are substatially stressed it is a problem.

Jesus is THE life,
Leonard