1. Real world theater problem

Batten Sleeve 1.1.jpg
Referring to the above drawing.
In professional theaters, we use long pipes to hang curtains, lights and set pieces on.
But since pipe comes in about 21' lengths, we usually have to splice them.

We don't use the standard couplers, because they make too big of a 'bump'.
So, instead, we use "internal sleeves" as shown in the attached drawing; this maintains the OD at a consistent size.

As far as I can find, there is no standard for how long the sleeve must be to effectively join one half of the long pipe to the other half.
Meaning, that the sleeve should perform as effectively as if the two halves were welded together.

The sleeves are typically joined to the pipe using 1/4" bolts, two on each pipe.
The bolts are offset to each other 90 degrees.

So... how do I calculate the length of the sleeve and the placement of the bolts, to create a rigid joint in this application?

2. My ~ guess is that at minimum 3 x OD or more full engagement into the pipe each end. The clearance between the internal sleeve and inside pipe diameter would be very important to achieve full strength. A snug but not force fit would optimal in that it would basically create just a thick higher area of inertia area.

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