By Josh Cosford, Contributing Editor
I’ve been in the fluid power industry for over two decades, and I’ve made precisely one hydraulic tube, which was an assignment for one of my hydraulic classes at Mohawk College. Despite my personal lack of man-hours bending tubes, every hydraulic professional should understand the principles of tube bending. I’m going to spare you the math here, and simply discuss some noteworthy intricacies of tubes and the process of creating them.
Let’s get the obvious out of the way — a well-made tube is reliable, safe, and leak-free. If you don’t know what you’re doing, the connections won’t be tight, the bends will be pinched, and the whole mess will be prone to ruptures. And if you don’t follow best practices, you risk introducing contamination into the lifeblood your expensive components need to survive.
The bend in a tube is the most critical aspect of running hard plumbing; otherwise you’d just be spanning lengths of pipe. The bend radius must be, at minimum, twice that of the tube OD. Notice I said “radius,” which means if you bent a circle shape, the ID of that circle would be four times the tube OD. If your material is thin, such as for return lines, you may want to increase the bend radius to prevent pinching or wrinkling. When in doubt, bend to the largest radii your machine or power unit has the real estate to manage, as doing so also improves laminar flow inside the tube.
Be sure to use only heavy-duty, commercial-grade bending equipment. Hand benders are perfectly fine as long as their dies are high-quality to ensure an excellent fit with the tube diameter you’re working with. Never bend a tube with an oversized or undersized die, as either option will encourage pinching or flattening of the tube material. A ½ in. tube, for example, requires a well-made ½ in. die to support the inside radius properly. We can generally leave CNC benders out of this particular discussion, since you wouldn’t even be reading this article if a CNC bender was on your shopping list.
If you’re having trouble with bending thin-wall tubing, you may need to consider a set of mandrels, although they can add quite a bit to your capital investment. A mandrel is a series of three or four thick discs attached to a cable that is inserted into the tube at the bend location to support the ID from collapsing during a bend. If you’ve ever seen vehicle exhaust tubes, the mandrel bends are smooth everywhere, while the traditional bends are rippled at the ID where material had to fold on itself. Just be aware that mandrels must match the internal diameter, so you may need to stock various sizes to accommodate different tube wall thicknesses.
Once you have the math and gear worked out, standardize your procedure with good preparation habits. Some technicians like to work from a longer piece of tube to avoid waste down the road, so be aware that the length you cut should include enough space for fittings, flares, sleeves, and clamps. Tubes must be cut square, deburred, cleaned, and then diligently inspected before they are even placed in the bender. You can purchase specialty deburring tools that take just a couple of easy twists of the wrist to shave off any burrs from the cutting procedure. Remember, you’ll need two such tools, which are generally size-universal but one each for ID and OD deburring.
Now that you’re ready to bend, remember that your top priority is to create a smooth, consistent bend with appropriate geometry. You’ll often be required to create compound bends — up, down, left, right and in any shape suitable for the space. Our eyes prefer perpendicular and parallel lines, so don’t leave your power unit looking like it was plumbed using copper tubes feeding water to the back of your refrigerator.
Also be aware where you run a length of tube — don’t run the cooler’s inlet and outlet tubes side-by-side, and avoid allowing tubes to hang out in free air where they’re prone to damage or encourage people to use as a ladder rung. Be sure that all the connections are located where mechanics will inevitably need access in the future for replacement or servicing.
It’s encouraged that you eyeball and test-fit frequently. Bend the tube, bring it to the machine to confirm accuracy, then rinse and repeat. You’d rather perform a visual test after every bend than make the full assembly at once and discover you’re off by inches. When the final piece is complete, give it a once-over to confirm there are no deep scratches, missed wrinkles or inadequate flares.
Tube-bending, like many other technical crafts, takes practice, patience and thoughtfulness. Practice on old tubes or scrape lengths as much as you can, ask for help and advice if you’re unsure, and take pride in your work. With this in mind, you’ll surely beat my lifetime tube-bending record of one tube.
Filed Under: Components, Engineering Basics, Hose & Tubing, Hose Assembly Tips
