Metrology
Metrology
What Good is a Parallelogram
EN-US
 
 

WHAT GOOD IS A PARALLELOGRAM?
The ABCs of Reed Spring Motion Transfer
George Schuetz, MAHR FEDERAL INC.

  Do you remember learning the names of weird shapes in elementary school and then later in geometry?  There were isosceles triangles, parallelograms, 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.  The two pairs of opposing sides are of equal length and are 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 frequently by engravers to reproduce an image outline to a user-definable scale, either smaller or larger. 

  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 connecting 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 and on-going repeatability, then you can use a simple height stand and sensor, and allow for one part after another to be continually slammed underneath it.  Or you can transfer the motion inside the gage in a way that protects the sensor and ensures repeatability.

Here is how reed springs can be used for this purpose:

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.  This is a simple linear application anyone might use.       

B.  Reed springs may also be used in 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 itself.  The reed spring protects the valuable sensor while its accurate frictionless motion results in an extremely accurate and repeatable micro inch measurement.

  Are there other ways to do the same things?  Certainly.  Precision bearings and slides come immediately to mind.  However, the reed spring is unique in that it 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, and being frictionless, the reed can sustain virtually millions of cycles without any noticeable damage.  Perfect, in other words, for harsh, shop floor environments.  The only downside is its limited degree of motion.  But we are talking about precision measurement here.

  So now you know what parallelograms are good for.  I am glad to be the bearer of the good news that at least a small part of your early education has not gone to waste.  Now, do your part.  If anyone out there has come across a practical application of the dodecahedron, I would like to know about it.