Prop torque 1000 foot pounds

12000 inch pounds

shear force 2 inch diameter. 6000 pounds

1/4 28 set screw

Diameter of extended point is .156

Shear area of of extended tip is .156/2 X Pi

.156÷2×3.1416

= 0.2450448 square inches

Yield stress of EPSS alloy steel 150,000 psi

Sheer strength of 1/4 20 EPSS alloy steel

Alloy steel is 150,000 psi

.245×150000

= 36750

Two 1/4 20 EPSS shear strength would be

36750×2

= 73,500 pounds sheer strength

So in theory all we need are two 1/4 20 EPSS

Let's calculate the torsional strength of a 2 inch prop shaft with
a .125 wall heat treated and ground to needle bearing size.


1×1×3.1416−(.875×.875×3.1416)

= 0.7363125 square inches

Alloy steel is 150000 psi

so the strength is

0.7363125×150000

= 110446.875 inch pounds

110446.875÷12

= 9203.90625 foot pounds

Factor of safety with out the set screw holes is 9 shaft alone.

Lets try 4 set screws. Gives us a factor of safety based on set
screw shear only of 2

four set screw area is

four .156 diameter holes .08 deep

= 0.04992 square inches

0.7363125−0.04992


= 0.6863925 new area

alloy steel 150,000 psi

0.6863925×150000

= 102958.875 inch pounds

102958.875÷12

= 8579.90625

Factor of safety drops to 8.5

This surprised even me.

If we up it to eight set screws in two rows of 4 the factor of
safety based on set screw shear alone doubles to 4 while the shaft
gets no weaker in torsion. Still with a factor of safety of  8.5
shaft alone.

Check my arithmetic.

Paul Lamar

John Lynch, my hangar neighbour kindly did a SW FEA with animation
on the input shaft  extended point set screw coupling.

This would be used on the coupling to the Ford sun gear connection
and it is based on a 1.6 inch diameter sun gear shaft.

It is very very conservative as we figured 1000 foot pounds out of
the engine. It does not come close to that but we are erring on
the overkill side as it adds little or no weight over the old one
inch spline shaft. It is much much stronger and no back lash or
slop. Note the stress never exceeds 5600 PSI in the shaft which is
nothing for the Ford hardened alloy steel sun gear.

John is also machining the billet silicon rubber coupling for the
650 HP time to climb project.


Paul Lamar



Try the same analysis with only the four screws on the left.  Remove
the eight screws on the right.  They are not really doing anything
anyway. They're only along for the ride.

Mark LaPierre

It is possible but I don't own the very expensive  software so I hate
to ask John for yet another favour.

Don't forget this is only the stress in the shaft.
You must ADD ALL set screw sheer  area's to determine if you have
enough set screws.

Paul Lamar


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