Just Another Shaft Fit Question

[TomFin]

Hi,
I'm interested in properly specifying the bore for a keyed sprocket hub. The 2" sprocket will sit over a 3/4" drive shaft that rotates ~3rpm and ~1000 lb load. Everything has been sized except for the fit class.
I have just about exhausted my internet resources looking to find which class fit I should pick from the Machinery's Handbook.

"RC1 Close sliding fits are intended for the accurate location of parts that must assemble without perceptible play." -Machinery's Handbook 27th

RC 1 looks the most tempting as it's description is somewhat similar to the fit I experienced when disassembling the keyed crank shaft on my old ChevyBlazer, but then again that crank spins much faster and I'm sure delivered higher loads.

Any thoughts... Thanks in advance

Failure is a prerequisite of successful design

 

[hydtools]

The slip fit has more to do with how tight or loose you want the fit than with speed of application. Slip fit with no perceptible play is a very nice fit. The sprocket will have no perceptible wobble. It doesn't matter that the speed is 3 rpm. You have 1000 lbs. pulling on it, keep the fit close.

Ted

 

[Tmoose]

The timing sprocket on your Chebbie slipped on the crank snout, but it was sandwiched axially between the damper hub face and the crank shouder, and clamped by the damper bolt being torqued to 65 plus lb-feet. Many pounds of force.

Subjecting slip fitted keyed components to loads that vary in direction or force will result in micro-motion or larger motion, and fretting, wear, loosening and heart-ache.

That 3/4 inch shaft sounds kind of dainty.

 

[Cockroach]

Typical interference fit, 0.002 inches maximum.

I wrote several Excel spreadsheet using the University of Tennessee at Martin, mechancial engineering course. Google to that website and look for the lecture notes in the subject. It is a good presentation. Shigley does a fair presentation in his design textbooks, an alternate source suggestion.

Shrink fits are interference fits and follow the classical text on Thick Wall Pressure Vessel theory. Therefore the Von Mises-Hencky model would apply and be of value in your computations.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Tmoose]

Actually Chevy small and big blocks had interference fit dampers, at least thru the 80s, PLUS a hefty axial clamp via bolt. A few makes, maybe olds and buick had slip fit dampers and whopping bolt torques.

The original Mazda Miata had some issues with dampers eating crank snouts. Probably not coincidentally it Usually happened after repairs that could result in less than specified bolt torque. The factory upgraded to a larger full length damper snout with a larger bolt for harder clamping.