The force exerted if 170.45 handguns were fired simultaneously at a single spot is applied to test the strength of pipes made by South Korean steel company POSCO Steel. TM Robotics teamed up with impact testing specialist Imatek Limited to deliver a robot system to handle the task safely. Nigel Smith, managing director of TM Robotics, explains how the steel giant can now offer its customers total confidence in the quality of its pipe steel.
The application uses Toshiba Machine SCARA robots to test large diameter line pipes that will ultimately be used for carrying fluids at high pressure. Preparation for the DWTT (Drop Weight Tear Test) process involves cutting a segment from the pipe, flattening it and putting a notch on one side as a crack initiator.
The segment of pipe is then immersed in liquid nitrogen, to cool it to as low as -160°. Placing the pipe in this cooling bath causes embrittlement before the specimen is then positioned on an anvil in a drop-weight testing machine and broken into two pieces.
The tricky bit is moving the pipe specimen from bath to machine and breaking it within the ten seconds allowed by the test method. “This poses a safety problem as an operator cannot be expected to put their hands inside unless the mass is fully supported,” explained Richard Sparks, who handles technical sales at Imatek.
“However, to achieve the test cycle time required by the standard, the mass instead has to be suspended, ready to drop. As a result we needed an automated solution, where the robot provided sufficient speed to transfer the pipe into position within the drop-weight tester within the required cycle time.”
With the specimen in position, a three-tonne weight is dropped from a height of 3.5 metres onto the pipe, striking the opposite side of the segment to the notch. This creates up to 100,000J of impact energy – about the same as firing 170, 0.45 calibre handguns at a single spot, or an average man weighing about 140lbs falling 180 metres! In order to ensure that the force is being applied in exactly the right place on the steel pipe, the robot needed to provide positioning accuracy of ±0.5mm – another reason for choosing the highly precise Toshiba Machine SCARAs.
“Positioning control for the drop mass was also difficult,” explained Sparks. “We overcame this by using a servo motor, which gives us precise control over the impact energy applied to the specimen.”
Usually the operator visually inspects the broken specimen in order to determine mode of failure. This drop-testing machine is fully instrumented ensuring a very repeatable test method. The initial velocity is measured using a proprietary laser system and force is measured during specimen failure to provide the user with force and displacement data from which the quality of the steel can be determined and how well it will operate under pressure. The end result allows the product to meet the demanding quality standards of the American Petroleum Institute.
“We believe this project’s instrumentation is unique,” explained Sparks. “Normally this test is carried out using very rugged, almost crude devices, without any instrumentation. This machine can measure forces up to 1.5MN and specimen deformations up to 100mm with very high precision. Also, it’s a very dynamic machine and minimising compliance issues were a serious consideration. Another important consideration was the foundation on which it stands. In the end we opted for a foundation made of Low-Q concrete to damp and absorb the shock pulse from the surrounding foundations. We were able to employ the customer’s expertise to make this decision. Transport and positioning of the specimen within the machine is a fundamental requirement of this test method and this presented challenges of its own – we tested countless suppliers until TM Robotics helped us choose the right one.”
“We selected TM Robotics and Toshiba Machine SCARA for a number of reasons,” explained Sparks. “Firstly, the robot only had to move in two planes, which is well within the capabilities of the SCARA. In addition, because SCARAs are so compact, the robot fits easily within the available space. We looked at pneumatic alternatives but the solutions on the market couldn’t achieve the positioning or repeatability we were looking for. We also looked at servo motion systems as an alternative but the price and performance of the robot solution was way ahead.”
The SR-1054HSP robot used by Imatek offers up to five-axes capability and has achieved traversing rates of 300mm in 0.63 seconds, carrying a 2kg payload. The robot’s reinforced arm can handle up to 20kg with reduced acceleration, or 10kg at full capacity. The SR-1054HSP is also extremely accurate, offering repeatability to within ±0.02mm. It uses parallel processing to ensure smooth, fast motion.
“Apart from the technical advantages provided by the product TM Robotics was also able to help with the application itself thanks to the company’s technical knowledge,” continued Sparks. “TM’s engineers came up with the right robot, trained us in its use and were able to ship at quite short notice. At Imatek we specialise in instrumented drop-weight testers – this includes a lot of work in the automotive and steel industries. However, this is physically the largest project we have done so far, even in this demanding field, so it was important to be able to rely on TM’s level of support.”
“I definitely plan to use Toshiba Machine SCARA robots again,” concluded Sparks. “For positioning control, compactness and value for money they come out on top compared to the alternatives every time.