What Good Is A Parallelogram?

Do you remember learning the names of weird shapes in elementary school and then later in geometry? There were isosceles triangles, parallelograms and dodecahedrons. What good would come of all this bizarre knowledge in “real life?”

Well, it turns out that at least one of these shapes is very important to those of us who lay out gaging setups or select precision measurement tools. It’s the parallelogram, and it can make high precision measurements very repeatable and save a lot of money by minimizing wear and tear on expensive sensors. But before we get to the benefits, we have to talk a little bit about the principles involved.

A parallelogram has four straight sides. Each of the two pair of opposing sides is of equal length and is parallel. The unique properties of the parallelogram have been applied extensively in industry to accurately transfer mechanical motion from one place to another. Perhaps the best known application is the pantograph, a four-sided device used by engravers to reproduce an image outline to a user-definable scale.

In gages and gaging setups, simple devices called “reed springs” simulate the behavior of parallelograms to transfer motion from one component to another. One type of reed spring consists of two parallel blocks connected by two or more steel strips of equal size and stiffness to form a reed-type flexure linkage. One of the blocks is attached to a fixed surface. When a force is applied to the free block, the connection strips flex, resulting in a displacement of the movable block.

Some observers will note that when this movement occurs, the connecting strips bend ever so slightly and that, technically speaking, the parallelogram has been compromised. However, I’m sure you would not be such a nitpicker. What is important is that fixed and moving blocks are still parallel and that the moving block is not deformed by the contact. So nothing has been added or subtracted to the degree of motion transferred. For high precision transfer of motion involving a range of a few thousandths, reed springs can be “EDM’d” from a solid piece of steel.

So now that we’ve gone through all this, what’s the big deal? If you don’t care about damage to your sensor or ongoing repeatability, then you can use a simple height stand and sensor and allow one part after another to be slammed underneath it. Or you can transfer the motion inside the gage in a way that protects the sensor and ensures repeatability. Reed springs can be used for these purposes:

   A. In a gaging situation where it may be necessary to protect the gaging indicator, the reed will accept all the side loading and not transfer it to the sensor. So the reed switch itself takes all the pounding rather than the expensive sensor.

   B. Reed springs may also be used for gaging in situations where the contact point and sensor must be in different locations. Again, the reed absorbs the side loading as it allows for placement of contact at locations where the sensor may not fit, in this case a confined inside diameter.

   C. Finally, the reed spring can be manufactured into a micro precision sensor. The reed spring protects the valuable sensor while its frictionless motion results in a repeatable micro-inch measurement.

Are there other ways to do the same things? Certainly. Precision bearings and slides immediately come to mind. However, the reed spring is less expensive, and there is no moving contact between its components. This latter quality practically defies the laws of physics by resisting the onslaught of dirt and grease. Being frictionless, the reed can sustain virtually millions of cycles without any noticeable damage. Perfect, in other words, for harsh shopfloor environments. The only downside is its limited degree of motion. But we are talking about precision measurement here.