Our piano hinges act much like shears when under load. Each pair of hinge segments loaded in opposite directions and the hinge pin stops the movement. Real shears need a hardened edge to ensure the wire or item being cut fails first. In the case of hinges, the goal is not cut through the hinge pin, but it is worth noting the similarities.
Since the pin is not infinitely stiff, the forces will try to turn it into a slalom S-shape. This also can occurs with a cheap pair of shears, when the shearing blades are not held in contact with each other.
This slight deflection serves to concentrate the loading at the very edges of each hinge segment. If the hinge material is not able to withstand this local stress it will begin to deform. The local stressed are far greater than any value derived from using the average area of the pin and hinge. Thermoplastic bearing materials are particularly susceptible to this load concentration allowing a greater deformation. This in turn exaggerates the S-shape of the hinge pin.
After recent postings, I took a new sample of Carbinge and inserted a pin with a really tight fit- to the point where I was concerned of pushing the Nylon segments out the hinge. The hinge was really stiff. Pulling by hand, no movement could be seen. I loaded this sample into my 'hinge puller' and applied a relatively low cyclic loading (~10 lb/in). The resulting movement was rather large and is shown in the following clip.
http://www.n91cz.com/HingePlay/PressFit-100lb-per-in.wmv
Even though the hinge pin was essentially a press fit, the bearing material at the edges of each hinge segment allowed the pin to take on the S-shape described above.
As hinge segments attempt to shear the pin, it is forced into a very slight S-shape and the local stress at the edges go up. The Nylon is deformed allowing further deflection. This cycle continues until the forces balance.
Piano hinges are relatively simple devices, but if you require good load bearing capability and/or precision, the best results are obtained with:
1. Stiff bearing material - up to the stiffness and strength of the pin itself
2. Stiff hinge pin - i.e. largest possible diameter
Chris Zavatson
N91CZ
L360std