Extend the Life of Materials Testing Equipment

May 5, 2003
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Is the performance of your material test equipment limited by aging components and outdated controls? If you have an aging mechanical dial, digital display or chart recorder, or an outdated computer control system, now might be the perfect time to upgrade your testing equipment. An upgrade allows you to retain your existing load frame, grips and load cells, while replacing older electronics and recording devices with a new controller and advanced testing software. The result can be a revitalized system at a fraction of the cost of new testing equipment.

Upgrading equipment enables the integration of your testing procedures with your business operation. Automated test control in-creases the repeatability of test results, while PC-storage, automated calculation and networking capabilities en-able statistical analysis and data archiving, leading to improved productivity and more reliable results.

Identify upgrade candidates
Before purchasing an upgrade, evaluate your system by performing a visual inspection of the frame, operating the machine, and gathering data about its capacity and transducers. If the frame of your testing system can't be calibrated, exhibits signs of backlash or has broken parts, retrofitting the system's electronics will not improve its performance.

The first step in any evaluation is to determine if the system is mechanically or hydraulically powered. Mechanical testers use an electromechanical drive system to supply torque power to the load frame.

A standard system consists of a servocontrolled drive motor, timing belts and two screw columns. Hydraulic systems use a hydraulic fluid and a pumping unit--often called a hydraulic power pack--to supply the forces necessary for tension, compression or cyclic testing.

On mechanical systems, all gears, pulleys, belts, chains and critical frame parts should be visually inspected before retrofitting the system. Start by walking around the testing frame and viewing the columns from all angles. Columns are often bent from off-center loading or incorrect lifting. If the columns are bent, machine alignment is jeopardized and uneven loading will occur.

Next, remove the cover that encloses the mechanical assembly and electronics. Look for worn gears, broken teeth, frayed belts and loose chains. If parts are severely damaged, consider the cost and availability of replacement parts.

For hydraulic equipment, screw columns, crosshead pockets and critical frame parts should be visually inspected before an upgrade is purchased. As with mechanical testers, start by walking around the testing frame to check for bent columns. Static universal testing machines may include stay plates to hold the columns in place. Check to ensure that these plates are not bent. Replacing bent stay plates may require complete disassembly of the frame.

If the testing frame includes in-head gripping pockets, they should be inspected for wear. Incorrect use of grips and filler plates can result in crosshead deformation. Severe deformation will affect the gripping and loading of specimens. A repair requires the crossheads to be removed from the frame so the pockets can be remachined.

Finally, look for hydraulic fluid leaking from the piston assembly. Machines that are leaking oil may not be able to reach capacity. This fix could be as simple as replacing the seal, or it could require a new piston and cylinder assembly.

Check for backlash
Whether a system is mechanical or hydraulic, backlash is another critical factor in determining the success of an upgrade. Backlash is "slop" or "slack" in the frame. Both mechanical and static hydraulic systems may have backlash in the screw column assembly.

To test for screw column backlash, insert a very stiff specimen and plot load vs. position. A flat spot--or halt of load--in the graph at the approximate weight of the cross-head indicates a substantial gap in the threads. This backlash will cause load and strain control problems if the frame is outfitted with new electronics.

If you do not have a plotting device, a bottle jack and dial indicator can be used to perform this test. Place the bottle jack in the center of the loading table and the dial indicator close to one of the screw columns. Slowly jack the crosshead while watching the indicator. A slight movement will occur as the frame responds to the pressure.

Exaggerated movement in the indicator as the weight of the crosshead is lifted is a sign of backlash in the screw column assembly. Move the dial indicator to the opposite column and repeat this procedure. Use caution and only lift the weight of the crosshead. Do not exceed the frame's capacity.

In addition, mechanical frames may have slack in the drive assembly. Examine this backlash by cycling the frame and plotting the controlling measurement (load, position or strain) vs. time. If there is substantial backlash in the mechanical drive assembly, a flat-topped waveform will be present.

If you do not have a plotting device, check for access to the motor shaft. Turn the shaft in one direction until the mechanical drive begins to move. Then, quickly reverse direction. The shaft turning before the system responds is an indication of excessive backlash.

As you evaluate a system for an upgrade, it is useful to locate the last calibration record. This re-cord may in-clude important notes from the service technician about frame operation, calibration and linearity of transducers. Materials testing service technicians can also assist in frame evaluation, and materials testing equipment suppliers are often the best source for answering questions about the availability of components or the cost to remanufacture them.

Checklist
If frame damage is identified, consider the cost and availability of replacement parts. Remember that simply retrofitting a system's electronics will not fix legacy frame problems.

If a system is a candidate for an upgrade, you should make a list of information for the upgrade sup-plier. Note the type of load weighing system the frame uses. Some older systems include a linear variable displacement transducer beam, a capsule or a pressure transducer in lieu of modern load cell devices. These systems should be examined carefully to determine if they are still in good working condition and if they will interface with new controllers.

For mechanical systems, determine if the frame has several speed ranges or clutches. If it does, note the number of speed ranges (clutches) and the capacity that each supports.

Finally, make a list of all transducers (load cells and extensometers) used on the system. Specify which transducers are used across various testing systems in your facility, as the upgrade will typically require a differ-ent connection or interface to the new controlling system.

Choose your upgrade package with an eye toward the im-proved functionality you desire. Most upgrade systems provide electronic data acquisition, PC-based data storage, automated test control, networking capabilities and computer-generated calculations. Resolution, control rates and data collection rate may vary among suppliers. Some upgrade packages include user control panels for manual frame control. Others allow you to retain existing transducers and even provide automatic recognition and calibration of these devices.

With an adequate pre-upgrade inspection and the right upgrade package for your tester, the end result will be improved operator efficiency and more accurate results.

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