PROS AND CONS OF ELECTROMECHANICAL tensile testers VS. HYDRAULIC tensile testers
In mechanical testing there are two different classes of tensile testing machines; Screw driven, also known as electromechanical, and hydraulic.
Electromechanical (screw driven) tensile testers have recently become popular over the older hydraulic systems. An electromechanical machine is often quieter, takes up less space, is easier to move around, runs on less overall power, and requires less maintenance when compared to a hydraulic testing machine using a Hydraulic Power Unit (HPU).
The electromechanical or screw driven test frames typically use an electric motor and two elevation screws to run the crosshead up and down. The pros of the electromechanical test machines are that they offer a large range of test speeds by changing the speed of the motor. They also provide for longer crosshead travel distances (stroke) of up to 100 inches or more. Screw Driven machines are known for speed and position accuracy.
Furthermore, on a screw driven machine, due to the load cell arrangement under the crosshead, it is easier to adapt a variety of tooling and accessories (including grips, custom fixtures, additional load cells, environmental chambers) Compared to a hydraulic machine that is more difficult to fixture due to the dual test space nature of their design. HZ-1003A Computer Type Bending Testing Machine.pdf...tensile testers Tension testing is performed in the top window (between top crosshead and middle crosshead) and compression testing is in the bottom window (between the middle crosshead and the table).
Cons of electromechanical are they are more expensive per force capacity compared to hydraulic machines (price per capacity). For example, a 60,000 Lbf (300kN) screw driven machine would be quite a bit more expensive compared to an equal force capacity hydraulic machine. They are also not as rugged and are more sensitive to debris and dirty environments than a hydraulic machine.
Hydraulic testing machines load frames are extremely robust and rugged. With today’s modern servo hydraulic technology, they have become very accurate in load and position control. These testing machines incorporate a single or dual direction piston to move the table and attached top crosshead to apply the force.
One disadvantage of a hydraulic tensile tester is the test travel is limited by the piston stroke, which is typically 6-9 inches. Furthermore, hydraulic machines are much more cost effective when the need arises for generating high forces, normally 60,000 (Lbf) and more. Many times hydraulics are the only option when testing above 120K (600 kN).
Frank Bacon Machinery computerizes both hydraulic and screw driven test machines, offering control with servo motors or servo valves controlling the machine’s test speed/rate. Both may have the capability to control testing speed as measured by load rate, position rate, and strain rate.
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