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Assembly of Mandrels With Poppet Style Links

by Bill Tingley, Vice President & General Manager, Bend Tooling Inc.

 

The industry offers two styles of links for mandrel assemblies.  Both perform in the same way:  Each connects the ball segments of the mandrel assembly to each other and to the body of the mandrel so as to allow the balls to flex in the direction of the draw of the tube-bend around the bend die.  The only difference is in the manner of assembly.  (See the diagram below for identification of all the mandrel components discussed in this article.)

 

The older style, developed a half-century ago, is the split link.  The center link consists of three separate pieces, two longitudinal halves and a key through the head of the link to align the two halves.  Similarly the mandrel link is also split longitudinally, but a key is not necessary because the mandrel screw automatically aligns the two halves.  By its nature, the end link, even in a split-link system, remains whole.

 

The split design of the center and mandrel links facilitates assembly of a ball mandrel strictly by hand.  With sufficient dexterity the operator of tube-bending machine or tool crib attendant can close the two halves of a center link around the head of an end link or another center link, slide the key through the head of the new link, and then hold all three pieces together while securing them with a mandrel ball.  In a similar manner, the split-link system can be disassembled when the breakage of a link occurs.

 

Reducing the frequency of link breakage is the rationale of the poppet-link design.  The fundamental drawback of the split-link design is that it weakens the neck and the head of the link by dividing it into the three parts.  Most manufacturers of the split link compensate for this inherent weakness by making the link out of tool steel.  However, hardened tool steel is more brittle than tough, so even a tool-steel split link is a compromise.

 

The poppet link is a single piece.  Therefore, by its nature it is stronger than the split link.  Plus it can be manufactured from materials that are tough, durable, and springy, which do not become excessively brittle when hardened.  Consequently the poppet link last longer on average than the split link.  Another benefit is that the poppet link, because of its simpler design, is less expensive.  When all factors are considered, a poppet link costs about one quarter of a split link on a per-bend basis.

 

Another advantage, incidental to the solid head of the poppet link, is that it can accommodate a heavier duty detent spring and ball to maintain the alignment of the mandrel balls during insertion into the tube.  Because the split-link design carves the head of the link into three small pieces, there is no clearance available for anything but a relatively delicate, therefore shorter-lived, detent system.

 

Beyond these advantages of the poppet link, the chief difference in its use from the split link is that a small arbor press is needed to snap them together.  The diagram below, using the example of a three-ball inserted mandrel assembly, illustrates the step-by-step sequence for assembly of a mandrel with poppet-style links.  (Alternatively, pre-made ball sub-assemblies can be ordered from the factory to eliminate the need for assembly altogether.)

 

 

Step #1:  Slide the end ball over the barrel of the end link.

 

Step #2:  Secure the end ball to the end link with a retaining ring.  Slip the detent spring then detent ball into the hole in the head of the end link.  Lightly lubricate the bore of the first center link, and then press it over the head of the end link.

 

Step #3:  Slide (or in the case of larger mandrel assemblies, press) the first center ball over the barrel of the first center link.

 

Step #4:  Secure the first center ball to the first center link with a retaining ring.  Slip the detent spring then detent ball into the hole in the head of the first center link.  Lightly lubricate the bore of the second center link, and then press it over the head of the first center link.

 

Step #5:  Slide (or press) the second center ball over the barrel of the second center link.

 

Step #6:  Secure the second center ball to the second center link with a retaining ring.  Slip the detent spring then detent ball into the hole in the head of the second center link.  Lightly lubricate the bore of the insert link (sometimes called the mandrel link), and then press it over the head of the second center link.  Note that significantly more force is required to snap the insert link into place than a center link.

 

Step #7:  Slide (or press) the mandrel insert over the insert link until the counterbore firmly mates with the shoulder of the link.

 

Step #8:  Slide the mandrel body over the insert link, and then slip the mandrel screw and washer through the bore of the body into the threaded hole of the insert link.  Tighten the screw until the mandrel sub-assembly is securely affixed to the mandrel body.

 

Step #9:  The mandrel assembly is complete.

 

(Click here for an instructional video on the assembly and disassembly of poppet-link mandrels.)

 

Bend Tooling Inc., 2005

Past Technical Articles

SPECIFYING MATERIALS FOR TUBE-BENDING TOOLS

THE WIPER FEATHERED EDGE

POPPET LINKS VS. SPLIT LINKS

THE "FORWARD MANDREL" SECRET OF TUBE-BENDING

ASSEMBLY OF MANDRELS WITH POPPET-STYLE LINKS

A DIFFERENT ANGLE ON WIPER DIES


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