Straightening
Straightening – Rotary and Two-Plane
David Jenkins Jr. – CPIM
Markin Tubing
Several years ago a top automotive executive referred to the new model year vehicles as “tortured sheet metal” for the outside appearance. In the sleeker, fuel efficient vehicles of today; you have to look at the undercarriage or under the hood to see the contorted tubular components supplying the fuel, power steering, transmission, coolant and lubrication to the vehicle. As the demand for lower weight, higher performance characteristics and lower costs increase, tubular components are increasing heavily engineered and scrutinized for purpose and value. In turn, the production of the component has to be defined in three critical performance areas: material, method and machine. This article will concentrate on the basic methods used to straighten tubular products for subsequent further processing.
The two basic straightening processes, driven rotary or two-plane machinery have not changed dramatically, however linking the machinery to a process line in order to produce a straighter tube at a lower cost is a challenge. Straightening itself is still somewhat of an art form even with today's sophisticated set up controls. There are those who would disagree with this premise, however positioning the roll tooling is not the biggest variable to be overcome in straightening. The incoming materials vary from production lot to lot and even melt chemistry. Straightening uses pressure (stress) to force the outside diameter of the tube to align itself within a given parameter. The pressure used to straighten also provides a stress relief from the forming forces used to make a welded tube. The controlled relief of internal stress while keeping a tube straight makes the final set up adjustments crucial and something learned from experience with repetitious processing.
Rotary straightening was a stand alone process from DOM tube production, but in todays tube process lines the rotary unit may follow the mill cut off machine with an integrated conveyor system. In the late 1990's the author designed a product specific process line for the in-line straightening of “prop shaft” for rear wheel drive vehicles as it was produced off the tube mill.
Two–plane straightening for coiled tubing was easily implemented into secondary cutting equipment and presented its own challenges as straightening was no longer continuous. In some machines, the straightener roll drive now served two purposes; serving as the feed up and length measuring system as well as the controlled relief of the internal stresses from coiled material. The change in material characteristics during straightening from stress relieving may cause length variations that the length measurement encoder is not capable of detecting depending on where it is mounted. Since the material will shorten or lengthen as internal stress is relieved if the encoder is taking its measurement based on the revolution of the feed roller, variation is not accounted for when the encoder is attached to the straightening rolls. Length measuring accuracy is improved if the encoder is located after the straightening rolls.
The second problem with integrated two-plane straighteners is stop –start operation. In rotary straightening internal stresses are allowed to find their own centroid or neutral axis along the entire length of the tube. Two-plane straighteners function only as well as the number of rolls that contact the tube over the desired finished length. The shorter the desired cut length the smaller the straightener roll diameter and the shorter the centerline distance between rolls. Knowing the product requirements will determine which machine of the variety available will produce the desired critical characteristics. Further discussion of straightening processes including detailed set up instructions may be researched by consulting other technical publications.
Markin Tubing has been producing the highest quality steel tubing for the heat transfer (refrigeration) and automotive industries for over 46 years. Sizes are currently 3/16” O.D. x 0.028” wall to 1.5” O.D. x 0.083” wall in low carbon steel welded tubing. Markin produces to several specifications, including SAE J-356, SAE J-526, and ASTM A513 type 2, ASTM SA 214 and A178. We flash copper plate and produce Galfan coated products for a variety of applications in addition to our plain carbon steel capabilities. As an ISO TS16949 and 14000-registered company, our mills produce proven high quality pressure tubing.
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