Washers: Using Them with Optomech

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Figure 2: Install washers before inserting bolts into slots to protect the slot from damage. The rounded, smooth side of the washer should be placed against the slot, and the rough, flat side should be in contact with the bolt head. The smooth surface is designed to translate easily across the anodized surface, without harming it. The BA2 base is illustrated.

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Figure 1: The diameter of the washer is 35% larger than that of the bolt head. This results in over a six fold increase in overlap area with the slot of a BA2 base. By distributing the force of the bolt over a larger area, the washer help prevent gouging of the slot.

The head of a standard cap screw is not much larger than the major diameter of the thread (Figure 1). For example, a ¼-20 screw has a head diameter between 0.365" and 0.375" and the clearance hole diameter for the threads is 0.264".

When the screw is tightened directly through the clearance hole to secure the device, the force is applied to the edge of the through hole, often cutting into the material (Figure 1).

Once the material is permanently deformed, the screw head will want to fall back into the gouged groove, thereby moving the device back to that location when attempting to make fine adjustments.

A device with a circular through hole is not meant to translate around the screw thread so the deformation is not expected to be a problem.

However, a slot should provide the ability to secure the device anywhere along the length for the lifetime of the part. Using a washer distributes the force away from the slot edge to decrease the chance of deforming the slot and extending the lifetime of the part. Figure 1 illustrates the difference a washer can make. The contact area between the slot of a BA2 base and a 0.27" diameter cap screw is 0.010 in². When a 0.5" diameter washer is used the contact area is 0.064 in², which is over six times larger.

When using a Thorlabs washer, there are two distinct sides (Figure 2). One side is flat and rough and the other is curved and polished. The curved and polished side should be placed against the device, which has an anodized surface.

As the screw tightens, the screw head can force the washer to spin against the anodized coating.

If the flat side is pressed down against the anodization, the friction created by the rough flat side can scratch the anodized aluminum. However, if the curved side is facing down, the smooth surface has less friction leading to less scratches and extending the visual appearance of the device.