Telescopic Tube Fabrication Made Easy

The Need For Telescopic Tubing 

It would be nice if one size fit all. But the world is not quite like that. This is true for political leaders as it is for tubular assemblies such as a camera tri pod. I’m sure you can see it now. Clearly a tripod has a need to be positioned in an infinite number of positions. This is where the elegance of smooth telescopic tubing can come into play. 

For a great number of applications, a high degree of precision is needed when two tubes are expected to slide one within the other. Without a high degree of precision, the excessive clearance within the fit of the two tubes can cause the telescopic action between the two tubes to lock up. In an effort to prevent this, a quality sliding motion is needed. 


 

 

 

 

 

 

 

 

 

 

 

 

Clearly the allowable clearance between the two mating tubes is a function of the length of engagement. Thus the longer the engagement, the more radial clearance can be tolerated. This is true for two tubes that are expected to slide freely relative to one another (telescopic tubing) as it is for a bronze bushing sliding up and down a precision shaft. Spoken another way, as the length of the bronze bushing becomes shorter, the tendency of the bushing to self-lock on the shaft becomes greater.

The moral to the story is that radial clearance between two telescopic tubes is very important. This is where a precision end former comes into play. End forming machines that participate in the precision forming for telescopic applications can come in several types.They can range from less soup such as a ram type end former to the more soup type such as an I/O sizing machine. Also, segmented finger type machines such as the ones manufactured by Finn-Power can also product precision reductions. A segmented finger type machine really shines when the length of the telescopic region becomes longer that a ram type machine can form efficiently. 



The Precision Ram Approach 
Using a ram end former to reduce the outside diameter of a tube is a good idea providing the amount of reduction is not too server relative to the length of reduction required. Figure 1 shows a tube of diameter D reduced to diameter d for a give length L. 


 

 

 

 

 

 

 

If you try to reduce the diameter D too much in one shot, then the tube can buckle as would a long slender column would buckle under a compressive load. Other factors that come into play are the material and the tube’s wall thickness. 

As the amount of required reduction increases from D to d, then it may be possible to reduce the outside diameter of the tube in multiple steps. Therefore, by using a series of progressive reducing dies, a tube of diameter D can be reduced by a greater amount using a progressive approach. Nevertheless, at some point the segmented finger approach may be considered to replace a series of reducing dies. Here it is application dependent. 
In the case of telescopic tubing, it may be necessary to reduce the outer tube such that the inside diameter of the outer tube is smaller than needed and then come back with a secondary ram operation to slightly expand the inside diameter for a precision fit with the mating inner tube. 


About the Author

George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at gwinton@wintonmachine.com or 888.321.1499

About the Machines We Build 

All of our semi-rigid coax and tube fabrication machines at Winton are designed, manufactured, and tested in-house.  We have a large line of standard products as well as the ability to engineer the best solution for our customer’s needs.  Our experienced sales staff makes sure that our customers can justify their capital equipment investment by offering a solution that is exactly what they need in order to manufacture their parts.  Please contact us today to discuss your project.