Thirty years ago Bend Tooling Inc. developed and refined the only line of poppet links available for rotary-draw tube-bending. As consequence, when Bend Tooling’s competitors need to supply their customers with poppet links, they purchase them from Bend Tooling.

Poppet links replace the many varieties of split links, an older design that was developed after World War II. The split link design was a compromise of the link’s effectiveness with the limitations of metallurgical and machining technologies of that time. However, modern materials and computer-controlled machining have eliminated the need for this compromise by making possible the ideal design for links, the poppet link.

These are the advantages of the poppet link over the split link:

• The poppet link is much stronger than the split link. The simple reason for this is that the poppet link is solid not split, therefore, all of the link bears the load of tube-bending instead of just half of it.
• The poppet link is more durable than the split link. It is tougher and can absorb a shock better than the split. This is because the split link, to make up for the lack of strength in design, must be made of harder steels that are brittle and so more likely to fracture.
• The poppet link has a heavy-duty detent system compared to the split link. The detent system makes the mandrel balls snap back into a straight line upon retraction from the completed tube bend. Because the head of the split link is broken up into three separate pieces, there is no clearance for anything but a small detent system, which wears out faster than the detent system for the poppet links.
• Because of these three factors, the poppet link has a longer life than the split link.
• Because the poppet link is simpler in design, it requires less time to manufacture. Therefore, the poppet link is considerably less expensive than the split link.
• Longer life combined with lower price means that the per-bend cost of the poppet link is a fraction of that of the split link.
• Also, the poppet link is easier to inventory than the split link. Because of the lack of precision in its manufacture, each half of a split link must be matched with the other for proper assembly. If the halves are separated and mixed up with others, the split link is no good.

Beyond these advantages of the poppet link, the chief difference in its use from the split link is that a small arbor press is used to snap them together, which eliminates the need for special expertise to assemble a mandrel. 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 Bend Tooling 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.

The poppet link is the ideal design for rotary-draw tube-bending, and modern machining and materials technologies have now made it possible. Switching over to the poppet link is worth the effort, because doing so will save you time, money, and headaches.