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crowning
4

crowning

crowning

(OP)
Has anyone ever heard of crowming a metric module spline? My client is insisting on a crowned spline , but I am hesitant to accomodate. Unlike the ANSI 92.1 DP spline, the calcs for the metric module 92.2 include lead and profile deviations for machining based on the class. Also crowning is not discussed in the std as far as i can see.
Thanks  

RE: crowning

batter1979,

You can crown any external involute spline.  The crown would simply make it a modified spline, and it would mate with a conventional internal spline.  

Typically with a full crown, you would define the spline dimensions just the same as any other spline, but those dimensions would only apply at the crown profile apex point location, which you would define on your drawing.  The flank and tip crown radii would also be dimensioned on the drawing, as well as any tolerance for the crown location, or any special tooth edge breaks/reliefs that may be required.  The crown radii are usually defined using a gage point and "crown drop" dimension, with the actual spherical radius be listed as reference only.

A partial face crown might also be used.  This would be similar to the crown you typically see on gear teeth, where there is slight crowning only on the ends of the tooth flanks with a "flat" in the center.  This type of crown would help prevent tooth edge loading with limited misalignment, but would not help much with gross shaft misalignment.

Hope that helps.
Terry

RE: crowning

I think it would have to be a very short spline

RE: crowning

(OP)
the effective spline engagement is 90mm. 8750N-m 2800rpm

RE: crowning

batter1979,

I don't know if this is of any help to you, but I plugged your spline information into a little spreadsheet I already had for sizing crowned inch/DP splines.  Your last post gave an engaged length and torque, but I estimated everything else.  I used 35t, 90mm PD, 90mm length, and 8750 N-m torque. The .003" crown drop value equates to about 0.10deg angular misalignment.  Sorry, but I was too lazy to change my spreadsheet to metric module format.

A general rule of thumb for fretting contact stress limits with oil lubrication and heat treated alloys steels (ie. Rc 38-42) is about 12000psi.  So it would appear that your spline, with flank contact stress of 11,134psi, would have excellent life as long as my assumptions are correct.  If you require more angular misalignment capability (ie. a larger crown drop), the crown radius dimensions would become smaller and the flank contact stresses would increase.

Hope that helps.
Terry

RE: crowning

batter1979,
That spread sheet looks very useful.
Will you be marketing it?
If so, please contact me via the link shown in my signature as I'd be interested in purchasing a copy from you.
 

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
 

RE: crowning

batter1979,

That spreadsheet is just the equations from Machinery's.  

Attached is a reference from AGMA if you want another source.

Finally, here's a trick to getting better fretting life and lower frictions in your crowned tooth splines.  If you have an oil-lubed quill shaft (crowned splines on both ends), designing in a controlled amount of angular misalignment will cause an oscillation or wiping motion at the flank contacts during shaft rotation.  This relative surface motion will generate a bit of an oil film, and this will reduce friction and fretting.

Hope that helps.
Terry

RE: crowning

There are some poisonous Kluber assembly pastes that have proven pretty useful preventing spline fretting.

I have heard, and believe (despite no real A-B-A type testing) , The "misalignment" for improved life via lubrication theories for splines and needle bearing U-joints and CV joints from OEM sources for decades.

However the folks on vibration analysis forums generally insist misalignment must be zero. It may be fueled in part by third rate structures found in lots of pump and HVAC equipment, and the fear that vibration by itself always >causes< damage somehow.

Just sayin'

Dan T

RE: crowning

Tmoose,

Your comments regarding vibration are valid.  Most high speed splined shafts in aircraft use major diameter fits for that reason- to keep the shaft ends from "shuffling" around within the spline joint.  But these splines are also crowned, even if the installation is designed for coaxial alignment.  Maintaining accurate alignment between gearbox and turbine engine shafts in a lightweight, highly loaded nacelle structure is next to impossible.

The aircraft industry approach to minimizing spline fretting is to use very low flank contact stresses (typically 5ksi to 12ksi), and to design the spline joint so that it is kept flooded with a circulation of lube oil.  Attached is a relevant page from Sikorsky's helo gearbox design guide.

Good luck.
Terry

RE: crowning

(OP)
My application would be analogous to the aerospace industry. I design machine tool spindles. This one happens to be huge(60 taper). Unfortunately, there is no way I will be able to keep this shaft in an oil bath. I could use the Kluber grease from time to time though at maintainence intervals.
 

RE: crowning

batter1979,

A machine tool spindle would not be analogous to an aircraft drivetrain.  Aircraft drivetrain splines are designed to last a few thousand hours with a minimum of weight and large misalignment, with little regards to cost.  A machine tool spindle would be designed to last hundreds of thousands of hours, with no consideration of weight, very little misalignment capability, and great consideration to cost.

With regards to spline fretting, there is no "magic bullet".  The only proven solution is to keep flank contact stresses to very low limits.  The reason a spline oil dam with circulating flow works well is that it continually flushes away the metallic debris generated at the spline contact.  This process would not occur with a grease lubricated spline joint.

Good luck.
Terry

RE: crowning

Terry

Nice work

RE: crowning

(OP)
Terry,
Sheesh dont take it personal. Okay they are both very high precision applications. that is all i meant . But you might want to think twice before making the assumptions that cost, life and weight are basically opposite from your application. I can see the misalignment factor, but that is it. I have to design something that works same as you. whatever it costs is whatever it costs. And the customer drives the life of the product.. ie torque/rpm needed. And weight is a major consideration always.  

RE: crowning

batter1979,

Sorry if my last post gave offense, I didn't mean to.  

The point I was trying to make is that the approach to designing a spline can vary greatly depending on the application.  I have designed splines for high-performance, high-reliability, weight critical aircraft drivetrains.  And I have also designed splines for cost sensitive, extremely long fatigue life wind turbine drivetrains where weight and size were not critical.  

Each particular design had its own challenges.  And to be honest, designing a spline for a wind turbine drivetrain that had to operate trouble-free for 150,000 hours with no maintenance and within a modest budget, was more of a challenge than designing the aircraft drivetrain spline that had to last 5000 hours and had no cost constraints.

Having said all that, I would somewhat disagree with your comment "whatever it costs is whatever it costs".  The best example of "cost driving design" in splines is the typical spline used in automotive transmissions.  These splines are designed with geometry variables like pressure angle, tooth thickness, tooth count, root fillet, etc. that are selected specifically for ease of manufacture using mass production processes like roll forming.  The combination of fatigue life, quality and low manufacturing cost that these guys get with a mass-produced spline is truly impressive.

Regards,
Terry

RE: crowning

(OP)
good times Terry. We are all here for one reason to get and give help to and from those who know more, and less about a topic.

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