This article was originally published in the February 1948 issue of Applied Hydraulics magazine. It was written by Lee B. Green, then a Consulting Engineer with Struthers Wells Corp.
The Tangent Bender is a basic production machine designed to bend sheet metal and to form rounded corners in a single operation. This is accomplished by wrapping a ram-type rocker plate die around a radius forming die against which the stock to be formed is clamped. The power arm of the machine is a swinging wing assembly to which the rocker plate is attached by a rack and pinion. In the loading position the rocker plate is horizontal and acts as a female die directly or as a base platen for the die fixture.
A heavy spring on the outer half of the power arm is adjustable and fits against a bearing block supporting the pressure roll. The rear end of the power cylinder is anchored on a clevis mount against the rear base of the machine, the front end is similarly mounted on a projection of the bearing block housing at the approximate center of the power arm.
The Tangent Bender was first—and still is—widely used for forming shells of refrigerator cabinets. Other production uses include washing machines, electric stoves, space heaters, vending machines, tubular chair frames, camera and small-appliance bodies. When originally introduced, the machines were air-cylinder operated. These units did—and still do—credible work. However, they had some limitations that were troublesome on some classes of work.
The application of hydraulic power to the machine was the result of an effort to find a power source that would correct and compensate for the commonly encountered production difficulties. The first difficulty was the interruption in production due to uneven power flow. The second problem arose from the need for greater power to form heavier sheets. A third difficulty was due to the fact that variations in the thicknesses of commercial sheets required sorting and interruptions m production to adjust the machine.
On heavy-duty jobs, cylinders had a tendency to pause until the air pressure could build up sufficiently to overcome the resistance of the metal sheets. When pressure did build up, the cylinders jumped ahead, resulting in a wrinkling or "orange peel" effect on the finished work. These irregularities, when severe, caused rejection. The salvageable work required considerable hand finishing.
The power source for air operation was often a problem. Plants with central station air facilities seldom have ample power for cylinder requirements. Plants without such systems or with a heavily loaded system have the problem of providing a separate air pressure system of suitable capacity.
The variation m the thicknesses of commercial sheets is now much greater than pre-war due to a number of reasons. This made the sorting of sheets into groups of similar thicknesses an extra operation which is slow and costly when hand sorted. In addition, frequent stopping and adjusting of the machine cut down production rates. A switch to hydraulic power was made with the introduction of the improved double-wing model (Fig. 1) by Struthers Wells after the recent war.
1. The machine illustrated at the top of this article shown here at the end of its cycle. The power cylinder has fully retracted, the power arm assembly turned through a 90-degree angle bringing the rocker plate on which the work is mounted on a fixture to a horizontal position. The unloading may be manual or by overhead conveyor.
Several important production advantages were gained. Of primary importance was a marked increase in the quality of the finished products. Since the hydraulic cylinder exerts a steady, continuous flow of power, the action of the rocker-plate die irons out the metal under a constant tangential pressure.
Full advantage is taken of the design of the die. Cylinder "jump" is entirely eliminated. With hydraulic power, the rocker plate die and the radius die meet at the same location on every cycle under the same pressure. Orange peel is practically forgotten.
One major refrigerator manufacturer has used the hydraulic powered machine in uninterrupted production for more than six months without producing a single shell that had to be rejected. Furthermore, the steady application of power throughout the die-forming action takes advantage of the plasticity of metal and causes it to flow around the changes in contour.
Because no stress is created, the radius section-of the metal is just as strong as the straight section areas. Hardly less in importance was the decrease in power requirement from 25 hp to 10 hp. This power reduction involved a floor space saving of approximately one third. Even with the power reduction, hydraulic operation made possible the building of machines of more rugged design, extending the range of work handled to heavier sheets and permitting much wider secondary die possibilities.
With the steadily gaining practice on the part of machine builders to incorporate as much of the operating elements as possible within the machine main housing, hydraulic operation is counted as a definite advantage. Elements necessarily outside the machine frame must be kept to a minimum in overhang of floor space and in size.
To illustrate the gain in power in the shift to hydraulic operation while also considering the reduced floor space, let us examine the power of two cylinders: first, the 5¾-in. clamping cylinder on the single-wing tangent bender (Fig. 2).
2. The hydraulic circuit of the basic single-wing tangent bender. Operation is by 4-way solenoid-operated valves. A sequence valve completes the cycling control. Note that all branch lines have sections of flexible tubing.
At 1,000 psi, this cylinder exerts a clamping force of approximately 26,000 lb. Through the leverage of the swinging arm assembly movement, the clamping pressure is multiplied to 52,000 lb before the die action starts. To accomplish the same result with air at 100 psi would require a cylinder with an 18¼-in. bore. Next, the cylinders that operate the wings have a bore of 3½ in., and at 1,000 psi, they exert 9,600 lb wing thrust on each wing, which has two cylinders. An air-powered machine would require cylinders with an 11-in. bore. Variation in stock thickness is not a problem with hydraulic operations. This is because automatic compensating-type dies may be used to address commercial variation in the gauge of the material. Therefore, production is continuous with sheets as supplied.
3. A single-wing Tangent Bender from the operator's side of the machine. The job is a "wrap-around" operation; four 1½ × 1½ × 3/16-in. angles are being formed simultaneously.
Hydraulic power is supplied from a compact, self-contained unit, designed and built exclusively for these machines by a leading manufacturer of hydraulic equipment. The unit is mounted within the machine housing. The diagram represents the functional elements of the hydraulic system for the single wing machine.
The hydraulic circuit in the double-wing machine functions as follows: With work loaded in the dies, the main ram is brought forward at rapid traverse; the automatic two-volume circuit shifts from rapid traverse to slow or high-pressure traverse as the work is contacted; the wing cylinders are then brought forward, either individually or simultaneously and perform the bending operation or operations; the wing cylinders and the ram cylinder retract in turn. A carriage cylinder turns the carriage 90 degrees, freeing the finished work for unloading. In the single-wing design (Fig. 3), the last movement of the ram retraction turns the swinging wing assembly through a 90-degree turn to accomplish the same purpose.
The change to hydraulic power in the Tangent Bender also involved several safety elements. The sound of escaping air in any pneumatic machine is often a source of uneasiness among plant workers. Momentary diversions increase the tension under which the operator works and tends to slow production or results in a laxness of normal precautions against mishaps which cause accidents and injuries. The quietness of hydraulic operation has done much to build the confidence of the operator. This is a production advantage as well as a safety factor-particularly in the case of inexperienced operators and nearby workers.