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SAE Straight Sided Splines - External Spline Dimensions

SAE Straight Sided Splines - External Spline Dimensions

SAE Straight Sided Splines - External Spline Dimensions


After several years of referencing this website, I've decided to post a question I haven't been able to find an answer to.

I'm designing an air brake system for a railroad application which will use automatic slack adjusters which are typically found on over the road trucks/trailers. The slack adjuster acts as a lever arm to rotate a shaft which engages the brake pad to the rail wheel. The slack adjuster mates to the shaft by a 1.5" - 10C SAE spline.

I have a copy of the 28th edition Machinery's Handbook, but can only find dimensional information regarding the internal splines (specifically for broached holes). I can not find any references to what the external dimensions of the shaft splines should be.

SAE J499 makes reference to the shaft dimensions, but only states the following: "The shaft dimensions depend upon the shape and material of the parts, their heat treatment, and methods of machining to give the required fit. The method and amount of "breaking" sharp corners and edges also depend upon the conditions and requirements of each application."

I'm not really sure how to proceed.

I don't believe the application is severe. The slack adjuster can apply ~2,300 lb*ft of torque max to the shaft. The splined length of the slack adjuster is 1.00". The shaft will be ~12 inches in length.

The load cycles will be very low for this application as it is not a rotating assembly like a motor/pump.

I've tried looking for dimensional data on brake cam shafts on semi-trucks/trailers but haven't found anything worthwhile.

Does anyone have any experience with dimensioning SAE external splines?

Thank you,


thread404-287612: SAE parallel tooth splines:

RE: SAE Straight Sided Splines - External Spline Dimensions

SAE 10-C is for a sliding fit when not under load.
Using the recommended tolerances; there is a chance that axial movement may cease while the spline interface is under full load.

All dimensions are in inches:
1.5" External spline data:

Major diameter, D' = 1.494/1.495
Tooth thickness, W' = 0.230/0.231
Tooth working height, X' = 0.140

You'll need to speak to your vendor so as to determine if things like tooth tip chamfering will be required as it depends on the type of tooling that will be used and how the internal profile has been manufactured.

RE: SAE Straight Sided Splines - External Spline Dimensions

Gearcutter, Thank you for your recommendations. Are these dimensions referenced from any standard, or from experience?

mfgengear, Thanks for the attachment. This is the same information I've seen in the Machinery's handbook. It appears to only apply to the female spline, specifically for a soft broached hole. If I'm missing something, please let me know.

RE: SAE Straight Sided Splines - External Spline Dimensions


This specification is obsolete for a reason. sorry the best I can recommend is use the now ANSI B92.1-1996.
my best guess on the old spec is the male would be the same as the female except with clearance.
depending on the application. & type of steel. clearance required would be the precision required.
for the application. more or less backlash. tighter the backlash the more precise , but the more potential
for ware. slower the application..

RE: SAE Straight Sided Splines - External Spline Dimensions

Thank you guys. This has all been very helpful.

RE: SAE Straight Sided Splines - External Spline Dimensions

Just a minor issue I noted. The SAE J499 standard describes a type C spline fit "to slide under load", while the chart above describes a 10-C as a "sliding fit when not under load".

In reality, I don't think it matters much with the slack adjuster application being discussed. I watched a video of how to replace one of these brake system slack adjusters, and the tech had to use a jackscrew puller and impact wrench to separate the 10C spline joint connection. It was rusted solid and no longer had a "sliding fit" under any load conditions. Before installing the new slack adjuster, the tech simply wire brushed the old rusted spline shaft, smeared it with grease, and installed the new slack adjuster. I'm sure that if you measured the old heavily rusted shaft spline teeth, you'd find they are way out of tolerance compared to a new shaft.

The brake slack adjuster is located on the undercarriage where it is fully exposed to the corrosive effects of water, salt, etc. The splines are made from bare steel and there is nothing sealing the spline joint from the environment except the coating of grease applied when it is installed.

RE: SAE Straight Sided Splines - External Spline Dimensions

I am developing a drawing for a service part of a straight-tooth spline. I am not sure what info to put down on the print for "Spline Data." According to the Machinery's handbook (25) all is needed is the N=number of teeth. D=major spline diameter, d=minor spline diameter, T=torgue per unit length. I really could find no other info. This part has: N=69 D=6.9682 d=5.1984, for "T" I calculate as 185717 per unit length. What does "per unit length mean? Is this all the information I need. We have the tooth angle and required "R".

RE: SAE Straight Sided Splines - External Spline Dimensions

"T" is torque capacity in inch-lbs per inch of engaged length. This "T" value is based on a bearing pressure of 1000 lbs/in^2 at the spline tooth flank contacts.

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