Shear Pins for Hot Pressing Engineered Wood Products.

How can measuring principle stress on the neutral axis of bending help a $41 billion dollar industry keep ticking along smoothly?

Plywood is a veneer based engineered wood product manufactured by pressing materials together with adhesive under high compressive force.  The critical step in this process is called “Hot Pressing”.  The adhesive bonding the wood is cured under controlled heat and pressure. If the Hot Pressing techniques of force and temperature are not controlled, the plywood strength requirement will not be met and the product will have diminished value or be so flawed it’s thrown to the scrap heap.

The global plywood market reached $41 billion USD in 2018, growing at 7% during the 2011-2018 period.  The market is expected to reach more than $55 billion by 2024.

A worldwide leading wood products company came to Stress-tek for help in solving the force/pressure control of the engineered wood products Hot Pressing process used in their manufacturing plants worldwide.  The machines to create this wood product use four, 40 horsepower electric motors with a 300 to 1 gear ratio to generate 10,000 lb-ft of torque for creating the pressure in the Hot Pressing process.  The motors use linkages to engage pressure rollers which apply the forces.  In reviewing the processes and components already used on the machines in the field, Stress-tek custom designed a set of pins to directly measure the force applied to the pressure rollers giving the “Hot Pressing” step exact force control, minimizing loss and adding a metric for the overall procedure. 

The pins are subject to bending and twisting loads where bending loads are a maximum on the pins surface and zero on the neutral axis.  By measuring the principle stress on the neutral axis, bending stress and rotational shear stress are avoided and the principle stress equals both the maximum normal stress and maximum shear stress on this axis. 

A single grid shear Micro-Measurements strain gage sensor was used in each pocket utilizing a back-to-back gaging technique to optimize cross talk rejection.

Using those facts of casual, curious but concrete information and a few other generalities from Timoshenko – a shear pin detailed design was created and built.

The shear pins were custom made to be a direct replacement pin for the existing OEM pins.  This design approach promoted easy installation and allowed the OEM to have a “slip out the old, slip in the new” replacement strategy.  The existing drivetrain components in the machines (and there safety factors) remained unchanged – the easiest of all possible solutions.  The pins report applied “Hot Pressing” loads directly, in real time, so all four electric motors can be balanced allowing each motor to apply equivalent force magnitudes creating the ideal process control.  Embracing critical thinking leads to imaginative control with the implementation resulting in a superior product.

About Stress-tek

Stress-tek has extensive experience in the design and manufacturing of strain gage based sensors and load cells, applying strain gages to element bodies and providing stress analysis services. It has complete in-house capability in engineering, machining, assembly, and testing. 

Stress-tek was founded in 1978 to provide professional consulting services in the areas of strain gaging, transducer design, and experimental stress analysis. Since then they have evolved to designing and manufacturing of custom and high volume weighing, force measurement, pressure, and deflection sensors requiring accuracy and reliability. The result is an extensive line of shear beam load cells, bending beam load cells, shear pin load cells, tension and compression load cells, deflection transducers, hydraulic and air pressure sensors. Stress-Tek, also designs and manufactures digital, two-wire electronics to integrate with our load cells and sensors to provide complete solutions.

In 2015, Stress-tek became part of Vishay Precision Group, Inc. (VPG), an internationally recognized designer, manufacturer and marketer of sensor and sensor-based measurements systems specializing in the growing markets of stress, force, weight, pressure and measurement. VPG is a market leader of foil technology products, providing ongoing technology innovations in precision foil resistors and foil strain gages, which are the foundation of the company’s force sensors products and its weighing and control systems.