August 8, 2006
This article, Part II of a two-part series, discusses the different types of die set accelerators found in flying shear tube cutoff systems. It presents an overview of earlier accelerator technologies still in use today—cam link, assisted lift target, and air/oil units. It also discusses three common closed-loop die set accelerators that represent newer technology—hydraulic servo valve, servomotor belt, and servomotor rack and pinion.
Flying Cutoff System with Servomotor Belt Accelerator
The modern flying cutoff system consists of a high-strength steel mainframe, a die set accelerator, lightweight tools, and programmable logic controls. This article discusses die set accelerators (Figure 1).
Simply stated, a die set accelerator propels the flying cutoff die set from its rest position, quickly synchronizes the die set speed to the tube speed, signals the flying cutoff to operate the die set, and then returns the die set to its rest position.
Many technologies have been applied to flying cutoff die set accelerators. Some early technologies, such as cam link, assisted lift target, and air/oil, still are in operation today.
In a cam link unit, a die set cam is mounted in the cutoff machine head. As the press head moves down it engages the cam, which moves the die set off the rest position. When the die set operation is complete, springs return the die set to the rest position. Cutoff machine head cam impact is a major die set maintenance contributor.
With a cam link unit, the tube length is set manually with the tube mill shut down. Speed is about 100 feet per minute (FPM). In some cases, tube mill speed changes require cam angle adjustment with the tube mill shut down. Tube length accuracy is 1/2 inch or more.
An assisted target accelerator moves the die set off its rest position with a rod connected to the die set. A target is positioned at a point along the rod's length to establish tube cut length. The tubular product that runs through the tube mill and cutoff engages the target and moves the die set off the rest position.
Air cylinders move the target rod axially to sweep the target off the end of the tube as it is cut and return the die set to its rest position. This system does not work for low-column-strength tubing. The tube length is set manually with the tube mill shut down. Speed is about 200 FPM. Tube mill speed changes frequently require target assist adjustment with the tube mill shut down. The tube length accuracy can be 1/32 in. or better, as long as the system is set up and maintained properly.
Like an assisted target accelerator, an air/oil accelerator (Figure 2) uses a rod and target for length gauging. In this case, however, die set propulsion is achieved by a compressed-air-powered hydraulic oil cylinder. A flag switch located before the target operates the hydraulic oil cylinder via a four-way air valve actuated by the tube running through the tube mill and cutoff. Oil flow control valves are manually adjusted to synchronize the die set speed to tube speed.
Once again, the tube length is adjusted manually with the tube mill shut down. Tube mill speed is about 400 FPM maximum. Tube speed changes require a flag switch position adjustment with the tube mill shut down. Properly set up and maintained, this accelerator type can hold length accuracy 1/32 in. or better.
Modern flying cutoff die set accelerators operate as closed-loop systems in which a central processor controls all functions. They take length and speed data from a sensor (normally a measuring wheel-pulse generator assembly) that operates on the tube while it is running through the sizing section of a tube mill. Die set speed and position data are sent to the central processor via a sensor (usually a pulse generator-resolver) that operates off the die set carriage.
Hydraulic Servo Valve Die Set Accelerator
Die set movement and synchronization are controlled automatically by the central processor. The tube is cut to length from data programmed into the central processor. Multiple tube lengths and quantities can be programmed into the processor at any time. Tube length and quantity changes take place without stopping the tube mill.
Tube mill speed changes are tracked automatically by the system with no manual adjustments for length or die set to tube synchronization. Tube mill speed is 1,000 FPM maximum.
Typical length accuracy is 1/16 in. when tube speed variation is no more than 1 percent. Length accuracy increases with speeds over 500 FPM and lengths over 20 ft.
Three common closed-loop die set accelerators are hydraulic servo valve, servomotor belt, and servomotor rack and pinion.
A hydraulic cylinder operates the die set in a hydraulic servo valve die set accelerator (Figure 3). Oil delivered by a freestanding hydraulic power unit is supplied to the cylinder through a directional servo valve controlled by a central processor.
This system requires careful regular maintenance, including oil filter changes, power unit oil changes, seal replacement, and servo valve repair. First applied to flying cutoffs in the early 1970s, this accelerator type is good for speeds up to about 500 FPM.
In a servomotor-driven belt accelerator, the die set is driven by a well-supported shaft attached to a heavy-duty timing belt (Figure 4). A central processor controls the servomotor/gear reducer (Figure 5) that operates the timing-belt drive pulley. This system requires very little maintenance. First applied to flying cutoffs in the early 1980s, it has been used with systems operating up to 400 FPM.
Timing Belt Assembly
Servomotor/Gear Reducer Assembly
First applied to flying cutoffs in the early 1980s, the servomotor rack and pinion system is the workhorse of flying cutoff accelerators. In these systems, the die set is operated by a rack driven by a servomotor, gear reducer, and pinion gear assembly (see Figure 6 and Figure 7). Automatically lubricated, this accelerator requires very little maintenance when operated and maintained as specified. It is recommended for flying shear cutoffs operating at more than 500 FPM or for extreme-duty production environments. It has been applied to flying cutoffs that operate up to 1,000 FPM.
Servomotor Rack and Pinion Die Set Accelerator
Servomotor Rack and Pinion Die Set Accelerator at Final Assembly
Modern flying cutoffs using closed-loop accelerators can be significant contributors in today's high-yield tube mills. Part III of this series will discuss modern flyoff cutting system tools.
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